Hacker News new | comments | show | ask | jobs | submit login
Seven earth-sized planets discovered circling a star 39 light years from Earth (nature.com)
2256 points by ngoldbaum on Feb 22, 2017 | hide | past | web | favorite | 704 comments

My favourite part of the press release:

> The planets also are very close to each other. If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth's sky.

> In contrast to our sun, the TRAPPIST-1 star – classified as an ultra-cool dwarf – is so cool that liquid water could survive on planets orbiting very close to it, closer than is possible on planets in our solar system. All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun.


> The planets also are very close to each other. If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth's sky.

this reminds me of one my favorite films ever. it is just four minutes long


Have you seen Reid Gower's Sagan Series?


It similarly combines HD cinematography with Carl Sagan's narration. It's not CG like this, but the overall quality is extremely high (at least after the first chapter).

If you liked this, you'll probably also like MelodySheep's Symphony of Science series.


This brought me to tears, literally... There is a beauty to the words that is hard to describe other than to say it gives "meaning" (for lack of a better word) to life on this rock.

Another good video is Sagan's "Pale Blue Dot" https://www.youtube.com/watch?v=4PN5JJDh78I&feature=youtu.be

It reminded me of the end (spoiled by the poster, of course) of the (somewhat obscure) NZ sci-fi flick "The Quiet Earth":


Wanderers is indeed excellent.

See also: the timelapse footage of the sky from the VLT, Paranal. Just thinking about it gives me vertigo.


I just watched this twice. It made me smile, hopeful, and happy. Thank you.

It reminds me of Land and Overland by Bob Shaw:

"The setting for the trilogy is a pair of planets, Land and Overland, which orbit about a common centre of gravity, close enough to each other that they share a common atmosphere."

They don't share a common atmosphere, but I guess the delta required for interplanetary transfer is really low.

I watched this 10 times in a row. It gave me shivers every time...

Thank you, this is truly amazing. For all the people interested in any way in space - watch this :)

Thank you for this. Awesome.

It actually reminded me of Rick and Morty Season 2 last episode.

That was amazing

Wonderful and inspiring. I loved the narration from Carl Sagan. One of the greatest.

I was very stressed and watching this calm me down. Thanks

I get a little choked up every time I watch this.

I was very skeptical but pleasantly surprised.

Reminds me of the movie Melancholia.

Great film!

So, basically, this is cooler than just about any world that's been depicted in sci-fi.

I guess J.J. Abrams is somewhat forgiven for this, then:

http://vignette4.wikia.nocookie.net/startrek/images/2/29/Vul... [edited for direct .jpg]

I always thought that Firefly's scenario of a bunch of Earth-like worlds in close enough proximity to have regular trading ships flying between them was unrealistic. Now, not so much. This system even fits in with the "Abandon Earth en-mass, move to new set of worlds" backstory.

So, what did J.J. know, and when did he know it? And for that matter, how did he know?

Just making sure, but I hope you know that JJ Abrams had literally absolutely nothing to do with Firefly and wasn't even tangentially related to its creation nor production.

Also, I'm relatively positive that JJ (in the sci-fi works he actually, well, worked on), himself, knew nothing (see Armageddon for sources) and that his script supervisors dealt with any discrepancies with reality JJ had.

Man, I feel dumb. Yeah I meant Joss Whedon. Too many J's in sci-fi I like, I guess.

Corrections aside, it is an interesting observation that Firefly is less outlandish with this discovery (not a criticism, Firefly is optimized for fun, not scientific plausibility). Still, assuming light speed travel, Firefly's inhabited planets are still a heck of a lot closer, since the crew doesn't seem to noticeably age between the half dozen or so star systems that they visit, and don't appear to have any kind of tech to reversibly[1] halt their metabolisms.

[1] the reversibility it the hard part. Tech for halting metabolic processes irreversibly has been around for quite some time ;)

> Still, assuming light speed travel, Firefly's inhabited planets are still a heck of a lot closer, since the crew doesn't seem to noticeably age between the half dozen or so star systems that they visit, and don't appear to have any kind of tech to reversibly[1] halt their metabolisms.

Closer than what? This system is significantly smaller than Firefly's. Or do you mean closer than this system is to Earth? Don't worry about that, since in Firefly it's one super-system with five main stars. http://i.picresize.com/images/2013/02/06/BMhCv.jpg It's about 55 light hours across.

I never realized Firefly was in a super-system. Gravity would rip that thing apart in an instant.

They actually did numeric simulations of that system, and it seems to be stable for several millenia at least (their entire simulation time).

> Firefly's inhabited planets are still a heck of a lot closer, since the crew doesn't seem to noticeably age between the half dozen or so star systems that they visit

The 'systems' in Firefly are really all one star system. The system includes a central star orbited by planets, four other stars, and a host of "protostars" (they really mean brown dwarves). I've tried, and can't imagine a scenario where, even if such a system could form, that it could be configured in this way. The barycenter of the masses of this system can't possibly remain in the central star indefinitely, for starters. And the whole thing sounds hostile to planets - I imagine them being thrown out of the system constantly. But if you accept that, and you accept travel at some fraction of c, then you wouldn't expect them to age much like they would traveling between systems.

Proxima Centauri have a earth mass planet : https://en.wikipedia.org/wiki/Proxima_Centauri_b

The Firefly system is one in which stars are orbiting stars are orbiting stars, and among these stars are many many planets and moons. Presumably the distances involved here can be traveled in a matter of days or weeks at some fraction of c. So, they're fairly close.

In contrast, Proxima Centauri orbits from a distance of about a quarter lightyear, and we've found a single planet orbiting very tightly. We've also found a single planet around Alpha Centauri B, also orbiting very closely. There may be more we haven't found, but probably not nearly as many as in the Firefly system. Firefly seems to use the idea that more stars mean more planets, but more stars and planets during formation means more chaos, more things to swallow up or rip apart planets, and more mass to throw them out of the system.

I don't think the Firefly system is necessarily impossible. You can even construct models such that the white star is at the focus of all other orbits (you know, if you're the Alliance and you need to do that for political reasons). (An Earth-centric model of our solar system needn't necessarily be wrong in terms of its ability to predict the position of bodies in the solar system - It's just needlessly difficult and convoluted.) I just think the Firefly system is highly improbable.

> assuming light speed travel, Firefly's inhabited planets are still a heck of a lot closer, since the crew doesn't seem to noticeably age between the half dozen or so star systems

If you travel at light speed from your own point of view you arrive instantly due to time dilation, so even if it takes years according to an outside observer (because the distance you're travelling is light years) you wouldn't age at all.

From a photons point of view, it is emitted and absorbed by its destination instantly.

Ah, yes. Forgot all about time dilation effects. In my defense, it was a pretty off the cuff response. Also, I would assume they aren't meant to be literally traveling at exactly light speed, but I don't know the relevant equations to figure out how that impacts my comment. Clearly, in this area, I'm a dilettante at best.

>Still, assuming light speed travel, Firefly's inhabited planets are still a heck of a lot closer, since the crew doesn't seem to noticeably age between the half dozen or so star systems that they visit

My understanding is that Firefly does not have FTL travel at all, in fact that makes it rather unique among spaceship sci-fi. Also, they're in a single (though very large with multiple stars) system. They get around quickly because their ships are a lot faster than our slow-ass chemical rockets, but they're still traveling at sublight speeds, which is why it takes days or weeks to get places, instead of mere minutes (for example, at lightspeed, it would take less than 6 hours to get to Pluto). Notice that, in the show, they took paying passengers, and it took a fair amount of time to get between the "outer worlds". It was a lot like cargo ships of yesteryear, rather than modern airplanes, as far as travel time. They don't have tech to halt their metabolisms for the same reason we don't: they haven't invented it yet.

Joss Whedon is the name you're looking for, not JJ Abrams.

The worlds in Firefly were explicitly described as having been terraformed to be Earth-like (masses aside).

Except bigger, with multiple planets in the sky at once.

Firefox tries to download this file

Apologies, sloppy link-copying. Should be pointing to the .jpg now.

It's a webp. Firefox doesn't support those.

What? Worked just fine for me.

In which Firefox version?

51.0.1 32bit. Don't ask me why I'm running 32bit FF, this is news to me as well.

I'd be more inclined to ask why it worked for you in that exact version, but not for me in the very same one. (Mine might be 64-bit, but I can't see that making a difference here.)

I went to investigate for curiosity's sake but OP changed the link on us!

Worked for me in Firefox for Android (53, Aurora build).

Maybe it's a prerelease feature, or maybe Firefox is falling back on a capability that at least some desktop OSes don't have.

works on Arch Linux FF

It doesn't work in Microsoft Edge either (yup I use it).

Is this taboo? I also use Edge and find it great.

I don't see many people using Edge. I mostly don't have any problems now that it has extensions for ad block and lastpass. Sometimes opening a new tab just mysteriously doesn't work (it sits with the tab open doing nothing) or the website barely works without Edge freezing up (CarMax.com).

The battery life is really good though.

Doesn't work on IOS either (chrome specifically).

One of the games I'm working on (a sci-fi mech and space combat action rpg) actually is set in a solar system like this. Cool to see one in reality relatively close by! I think it provides a lot of interesting situations allowing for basically a "space opera in a solar system".

*Hollywood sci-fi

You get a lot more crazy worlds in written sci-fi. You can start with the Culture orbitals, some of the Commonwealth Saga worlds or Alastair Reynolds' inhibitors. If you count a neutron star with life on it, you get dragon's egg which is a must-read.

Sounds like you haven't read The Three Body Problem (which is a shame, you should totally check it out).

I was essentially thinking the same thing. I wonder if an elevator could be built between all of them?

They are still in separate orbits so the straight line between any two varies greatly in length... not to mention intersecting the star regularly.

Regular shuttles for interplanetary travel timed with periodically favorable orbital positions?

At that distance, gravity must be really fun. Tides are not only for water, such tides can also change the height of mountains, the balance of high-rise buildings, infer on volleyball rules, make you fart because of changes in atmospheric pressure, or... be leveraged to generate electricity. Those guys have simili-infinite energy at human scale.

Would they? I mean, we can see geological features on our moon, and our tides are orders of magnitude away from being rock-tearing events. I mean, the moon is small, but... how close do these planets approach, anyway?

The moon is not that small, it is 27% of earth's size. Any other satellite in the solar system is way smaller compared to the planet they orbit. I dare to say that earth and moon are twin planets instead of a planet and a satellite.

Unless you are excluding Pluto deliberately for not being "a planet", its moon Charon has half the diameter of Pluto. They are even both orbiting a common center of gravity outside Pluto.



> The Moon is about 2% of Earth's mass, not 27%.

2% of mass, yes, but also 27% of the diameter.

You are both right.

My first reaction was WTF?

Then 0.27^3 = 0.0197

And then, only 60% as dense (no core) so 0.0119 :)

One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth.

And only just barely! You get to around 99% of the Earth's mass with the rest of the inner solar system.

All of the other rocky bodies in the solar system give you another 10% of Earth's mass out to Pluto and another 10% beyond.

>One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth.

Is that correct? Venus alone is about 90% the size of Earth, and Mars roughly 33%. It really seems like all those bodies put together should be greater than 1 Earth mass (but less than 2 Earth masses).

That's size, not mass.

Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible.

In the outer solar system, it's basically all the Galilean Moons + Titan.

ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?

Yep, I was roughly equating mass with surface gravity, which I see now is incorrect.

I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.

The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius.

Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low.

The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)

You just blew my mind ;)

What do we know about the gas giants' cores?

Gas giant cores are weird. Based on models of solar system formation, we're like, pretty sure they started with rocky (or icy) cores that were dozens of times bigger than Earth. But we're not sure they're still there.

Due to the intense conditions inside a gas giant, we're not sure if they still have a rocky core, or if the cores have liquified or even convected away to mix with the gasses.

So the answer could be "there are dozens of times the Earth's mass worth of rocky material inside gas giants" or "there is no rocky material inside gas giants", or anywhere in-between. We need more detailed study of the gas giants before we can answer that question.

Models of solar system formation also suggest the Kuiper belt started with dozens of times Earth's mass, but we only see about 10% there now, and we only have pretty good guesses about what happened. (I mean, it almost certainly got ejected, but the "why" is harder.)

Thanks. I asked because searching seemed to be telling me that we have only theoretical arguments about gas giants' cores. That surprised me at first, but then I realized how little data we probably have. Basically, mass and its distribution, plus spectroscopic data about surface composition, right?

Juno is hopefully going to tell us more :)


Close enough that the perturbations these planets induced in one another's orbits allowed the team to estimate their masses at between 0.4 and 1.4 Earth-masses. In astronomical terms, that's very close.

So it possible to see Avatar like suspended rocks ? Somewhat orbiting some lagrangian point, which is very close to surface.

The Moon is about 2% of Earth's mass, not 27%.

One of the callers (a biologist, so not a crackpot) on TV suggested that given how close the planets were, he wondered if it were possible for there to be cross contamination between the planets given how close they are to each other. He referred to them being part of the same "ecosystem." I'm not sure how feasible that is as a non-astrobiologist, but it is an interesting idea.

Very likely, since this happens between Mars and Earth:


Fascinating! Would vira or bacteria beable to survive travelling from Earth to Mars in or on a rock or vice versa?

Tardigrades do survive in space: https://en.wikipedia.org/wiki/Tardigrade

Roughly quoting Neal de Grasse Tyson: Since so many bacteria survive outer space there is a good chance it's because of - yes - survivor bias. If only because early earth was bombarded so often that fragments of the planet with microbes took of in a meteor strike and returned home after a while in space.

Short answer: almost certainly yes, although i presume this is an improbable event.

It's not that improbable. They've found meteorites on Earth that have been identified as having come from Mars based on their mineral contents.

Sure, I was referring to the part where the microbe survives the initial blast, the passage through space, reentry, and landing, and happens to land in a spot where it can survive.

According to some research, microbes survival in meteorites is very likely (if they exist)


I know enough biology to be skeptical of papers like that (although, I admit I was skeptical when papers from the 50s predicted interplanetary mass transport, and didn't change my mind until we found Martian meteorites). I'd like to see actual evidence of this.

I actually think the rock getting into space with the microbe embedded in it is not that unlikely (many rocks in the earth are laced with bacteria), nor the space transport. I think the excessive heat of rentry, the force of impact, and the difference in target ecosystems are what are going to kill the microbes.

Possible, but cross-section for collision is way way smaller in the Earth-Mars system. Cross-section goes as r^-2 so the closer the bodies, the 'way better' it is.

IIRC, that is a serious hypothesis for how life began on earth, seeded from elsewhere (maybe not Mars).

But certainly someone here knows more than I do about it ...

The theory is called Panspermia


Considering the time scales involved, it wouldn't surprise me if multiple planets around nearby stars happened to be seeded with earth based bacterial/archaea life.

One of the presenters is an old friend from college. Folks were offering him money to mention "Zod, our Galactic Overlord" on the air. I guess that didn't happen?

No unfortunately, I don't think they did. That would have been hilarious.

The survivability of several organisms has been actually tested https://en.wikipedia.org/wiki/List_of_microorganisms_tested_...

There has been and continues to be cross-contamination between Venus, Earth, and Mars, so it seems likely that planets much closer together, and passing each other more frequently would have more 'communication' of materials.

Continues to be...? Outside of human action, what's the vector?

My understanding is that it is mostly due to solar wind, meteorite impacts, and various other infrequent events.

powerful meteor strikes spreading debris out of orbit.

And the debris isn't heated enough to be sterilized.

Gene Wolfe mined this idea for some of his science fiction works, except it was alien life making the jump ... Fifth Head of Cerberus & Blue's Waters/Green's Jungles.

Even if bacterial material were being transferred all the time, I think it'd be a stretch to call them the same "ecosystem" unless they were actually reliant on each other. But I'm not an biologist either.

I imagine their relative distances change frequently, so probably not.

Check out "Rocheworld."

Uh, Mongo when it flew by Earth?

edit: Mongo from Flash Gordon

Anyone else immediately think that we'd discovered Krypton when they read "red sun?"

As these planets are most probably TIDALLY LOCKED, making it hard for LIFE to thrive.

> All seven of TRAPPIST-1's planets orbit much closer than Earth orbits the Sun. A year on the closest planet passes in only 1.5 Earth days, while the sixth planet's year passes in only 12.3 days

>Tidally locked planets likely have very large differences in temperature between their permanently lit day sides and their permanently dark night sides, which could produce very strong winds circling the planets, while making the best places for life close to the mild twilight regions between the two sides.

> Another important consideration is that red dwarf stars are subject to frequent, intense flares that are likely to have stripped away the atmospheres of any planets in such close orbits.


On the other hand, tidal locking can stir the pot, possibly creating more opportunities for the chemistry of life to form.

One of the astronomers, Triaud has been quoted saying this is a relatively calm red dwarf that doesn't produce a lot of flares.

> If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds...

Makes me think of the film Another Earth


Melancholia by Lars von Trier is another great movie about the earth colliding with another planet.


An excellent, incredibly beautiful film, beyond the topical plot point.

Beautiful, but super, super, boring -- with an extra helping of boring on top.

> Beautiful, but super, super, boring

Let's just say that it's definitely an art-house film. Some find that sort of thing boring, but not everyone.

The challenging thing about Melancholia isn't "boredom" but the shaky camera work in the first half of the movie. I turned it off in disgust after 30 minutes or so. No idea why I went back and watched the rest of it, but I'm glad I did. One of those films that sticks with you.

You obviously haven't seen The Turin Horse. Melancholia was action packed compared to that.

Yeah, but Another Earth's main character is fascinated by astronomy and gets accepted to MIT

Which is on Netflix for streaming. Thanks for the tip.

Except with multiple planets?

I was gonna say this.

Other awesome facts, the closest star in the habitable zone might have a "year" of a day and a half, and the furthest out in the zone, a "year" of 20 days.

Well, in Lexx, there was Fire[0] and Water[1], which shared an atmosphere[2] which could be traversed with balloons.[3]

0) http://lexx.wikia.com/wiki/Fire

1) http://lexx.wikia.com/wiki/Water

2) https://goo.gl/images/xmxoJ9

3) https://goo.gl/images/oyVNF0

> the TRAPPIST-1 star – classified as an ultra-cool dwarf

I propose we name it "Dinklage."

Out of interest , although dimmer, are there any potential hazards simply due to the proximity of a dim star?

> are there...hazards simply due to the proximity of a dim star

Tidal locking [1]. Similar to how our moon shines at us ass first all the time, the planets around TRAPIST-1 would be expected to have one face locked to their star. Instead of an equatorial belt bookended with loops of temperate zones, as we have on Earth, we should expect, climatically, an "equatorial" face and a "polar" face separated by a tropic-to-temperate transition area.

[1] https://en.wikipedia.org/wiki/Tidal_locking

Of course, #askNasa didn't win me a question, but could any astrobiologists comment on this question: Tidal locking means the planets would always have a side facing the star, leading to very different thermodynamics in terms of climate and weather on the planet, not to mention radiation. Doesn't that very much hurt the chances for life on those planets?

The debate rages. The problem is that one side of the planet is essentially exposed to space and thus tends to be at cosmic background temperature, and the other side exposed to the star. It is thought by a lot of people that anything volatile enough to be a gas, ever (so, including water), would rapidly end up migrating over to the frozen side, so even if the planet started with the right mix of water and whatever other volatiles are needed it would shortly not have any in the temperate range. Some have objected that the planet could wobble and periodically unfreeze some of that stuff, and maybe life could exist in a resulting narrow temperate zone. My personal feeling is that it would still far-too-rapidly simply freeze out again, this time out of reach of the wobble, so the parched zone would simply grow to include everything ever touched by the sun rather than forming a stable temperate zone.

But we won't really know until we go look at a planet like this, because we don't have one locally to look at. (Plenty of tidally-locked bodies, plenty of bodies with volatiles and enough energy to keep them moving, I'm pretty sure nothing in the Solar system with both.)

Oceans can move a lot of heat around a planet, so the cold side could stay well above freezing. Remember it's T^4 so cold areas radiate far less heat.

That said, if there where minimal atmosphere then the effects your talking about become a larger issue. Atmosphere is largely a function of solar wind and volcanic activity which relates to the isotopes in a planets core making this hard to predict.

But the planets are close enough to interact gravitationally. Perhaps that would accomplish more than just a slight wobble.

If water freezes on the dark side, moisture in the atmosphere would probably move from warm to dark side and fall down permanently as a snow gradually removing liquid water.

If the oxygen and nitrogen freezes and snows down in the dark side (-220 C or so), the warm side can't have atmosphere either.

Unless there is some way tidally locked planet can have stable air or sea convection between the sides. I don't think they have.

If there is enough gravitational pull, the planet itself might be warmed by geologic flexing like Io.

I think one idea is large mountains on the dark side and the water slides back down as a glacier that melts, and eventually vaporizes, and back in a circle.

You have to watch out though - make your mountains too high, and you shift your center of gravity, and now your mountains are no longer elevated.

So if the water and gasses migrate to the cold side, I wonder if that would eventually make the cold side heavy enough to be rotated towards the star, causing the planet to periodically(probably over several millenia) reverse the side facing the star.

You're thinking of Earth, where the polar ice builds up perpendicular to the sun, causing tidal effects to wobble our axis a bit.

In this case, the ice is building up in such a way to make the planet even longer in the direction that tidal locking already had elongated it. The effect of which is that tidal locking would get stronger; not weaker.

I don't think it matters too much that it'd be icy on one side and rocky on the other; I think it matters more that it's oblong. But even supposing it does matter, if your intuition is that the more-dense end would want to fall towards the star, then that'd be the rocky end; not the icy end. Yes, the icy end got heavier supposing you measure from the rocky planet's center. But if you shift your frame of reference to be the planet's center of mass, then both halves of course kept exactly 50% of the mass; the densities are what shifted.

What about tidal and geological heating? or a greenhouse effect similar to Venus?

Any heat source exposed to space has the basic problem of its volatiles running away and not coming back. On geological time frames, this effect is probably nearly instant, so it isn't very helpful that maybe a new volcano could pop up somewhere and melt lots of stuff or something, because all that does is put a hole in the permanently frozen layer. Any heat source not exposed to space might harbor life, but that's not particularly true of these planets; we already have things like Europa locally with the same possibility.

The issues with valuable resources running away and not coming back is a serious one even for Earth, which I think might surprise some people. The geological carbon cycle [1] in particular is one that may surprise people who have not encountered it before, or not thought about it in terms of extraterrestrial life. On geological time scales it's surprisingly easy for entire important elements to go find themselves a low-energy configuration they like and disappear from any putative biosphere. The issues with a tidally-locked planet providing such solutions to "all gasses liquids" is merely an extreme example of the case, and really makes one wonder how it would be possible for such a planet to stay "stirred" enough for life to have access to what it needs to develop.

[1]: https://en.wikipedia.org/wiki/Carbon_cycle#Geological_carbon...

The link you provided suggests that one of the main ways for carbon to enter the atmosphere or ocean is through volcanic eruptions and geological hot spots. Isn't increased geological activity one of the results of orbital resonance?

If the 'central' planet has 6 close neighbours of equivalent size in resonant orbits would this not provide a very large amount of 'stirring'? These planets are also much more massive than Europa which is only 0.008 times the mass of the Earth compared to ~.6 and ~1.3 Earth masses for the ones in the habitable zone. They may have a more substantial mantle and core, possibly global magnetic fields and definitely more gravity to hold on to any fugitive gasses.

Considering geological timescales, this system is also billions of years younger than our own, potentially only a little over 500 million years old. The processes you draw concern to may simply not have had time to play out yet.

I'm also not convinced that the atmosphere would be cold enough to freeze out gasses on the night side if there were significant oceans or enough atmospheric circulation to transfer heat from the day side.

That sounds like a fun simulation to run

I know people have tried. I believe you can get both results if you put the right numbers in. Personally I consider this one of the canonical examples of where a simulation is a waste of time; with absolutely no ability to validate the model against reality, the model is in my humble-but-strong opinion utterly useless, which I mean in the strongest metaphysical sense possible. There's more ways to be wrong than to detect that you're wrong. (That's not just a flippant cute turn of phrase; I mean that as a rather fundamental issue with the entire idea.)

Apparently what would hurt the chance of life in a tidally locked planet the most is that due to being tidally locked the planet would have a reduced magnetic field to protect the atmosphere, and therefore there is a good chance that the planet would lose it's atmosphere over time.


Agreed, but there are other variables involved, including strength of stellar wind, density of atmosphere, whether a magnetic field is induced by the stellar wind, the mass of the planet (strength of its gravity), etc.

Does atmosphere include the ocean? Because life developed in the oceans first here, and some has never left.

Without an atmosphere you can't maintain a surface ocean, because liquid water can't exist in a vacuum. It's possible for the ocean to exist under a crust of ice though. See Europa for an example. So it's still theoretically possible for life to exist in a subsurface ocean, but I think it's a lot less likely compared to a planet with an atmosphere and temperatures that would support liquid water on the surface.

> It's possible for the ocean to exist under a crust of ice though. See Europa for an example.

That's exactly what I was thinking of. :)

> I think it's a lot less likely compared to a planet with an atmosphere and temperatures that would support liquid water on the surface.

Sure, but we have little information on how common life is. We're not even sure there's no life on Europa. If Europa supports life, it may be that (basic) life is somewhat common given certain criteria, and while I agree it's probably more common on planets with atmosphere and your statements were not incorrect, it may be that life is actually fairly likely on a planet like that (which is what I, possibly incorrectly, interpreted your statement as ultimately trying to convey).

I remember reading about 55 Cancri e (1), another tidally locked planet (2) featured in NASA's Galaxy of Horrors as "The Twilight Zone" (3), where life in the boundary seems unlikely.

1. https://exoplanets.nasa.gov/custom_planets/2

2. https://exoplanets.nasa.gov/resources/149

3. https://exoplanets.nasa.gov/alien-worlds/galaxy-of-horrors

I don't see why. If anything, life might be even easier if you're constantly getting energy from above, rather than intermittently. In any case, "different climate and weather" doesn't seem to me to imply "hurt the chances for life."

If they're in the habitable zone, then they're getting similar radiation as an organism at the far north latitudes of Earth are getting during the summer. I don't know if it makes that much difference to a short-lived bacteria whether it's full sunlight for several weeks at a time, vs full sunlight forever.

I'm not a biologist, but here's a fascinating article on the subject: http://m.nautil.us/blog/forget-earth_likewell-first-find-ali...

TL;DR is that there may exist a band around the planet where the two zones meet that is habitable. I think it's a reasonable supposition that most of the rest of the planet would be uninhabitable.

I'm not an astro-biologist, but I assume that it wouldn't be conducive to life as we know it, but that doesn't rule out all life.

I would be very interested in seeing comparative weather simulations for a fictional tidal locked planet to Earth and the other Sol planets.

Maybe but I like to think there are creatures living in the temperate rim of a tidally locked planet somewhere debating whether life could possibly evolve on a rotating planet.

"I mean, there'd be temperature fluctuations of tens of degrees every day! It would be dark half the time! How could life survive in such an unstable environment?"

And don't forget: there will be long-term cyclical trends of dozens of degrees due to geometric properties of the orbit and hypothesized tilts!

It's not a given, though. We used to think Mercury is tidal locked, but that turns out not to be the case. These planets, as I understand it, are in similar orbits, so they may not be locked either.

Well, Mercury is in a tidally locked orbit of sorts. Its rotation is resonant with its orbit -- it experiences two days every three years, and this isn't a coincidence, it's a relatively stable outcome of its tidal forces with the sun.

Fascinating! Also – off topic, but – this description reminded me of Twinsun[1], the planet in the game Little Big Adventure. It remains one of my fondest game memories of that era, despite all its weird bugs. I've thought about finding a copy and playing it again at some point, but I fear it might spoil my good memories of it..

[1]: http://lba.wikia.com/wiki/Twinsun

Dammit, why did you have to remind me... Both LBA and LBA2 stand out as extremely memorable from my childhood. It does look like there's a revival of game-styles from that era, given Thimbleweed Park[0], Pathway[1], and apparently 2Dark[2] from (one of?) the authors of LBA. Any more?

[0] https://thimbleweedpark.com [1] http://www.pathway-game.com [2] http://www.2dark.cc

Wow, those look amazing! Thanks for the tips!

You can pick up Relentless on GOG :] https://www.gog.com/game/little_big_adventure

And apparently there's a macOS version too – SOLD! Thanks buddy! :o)

For what it's worth I did this a few years ago and was not disappointed - great to hear other people grew up with this game too!

Great to hear! I just bought LBA and LBA2 from GOG, from the sibling commenter's tip. Now I just need to find a few free hours!

This would be my first thought as well, but the original article says that they are near-orbitally resonant. It seems possible to me that the regularly passing nearby planets could apply enough of a torque to counteract the tidal locking---though I don't have a good sense of the order of magnitude of the forces, so maybe not.

Fascinating thank you

Depending on the spectrum of the star, I would be concerned about non-visible radiation doses when in close proximity to the star.

IANA astrophysicist, though.

Actually, adding onto that, I'm wondering whether orbits this tight would result in a noticeable centripetal force -- that is, that you'd feel lighter on the night side of the planet than on the day side.

Good point, this is in TFA:

"Although at least some fraction of each planet could harbor liquid water, it doesn't necessarily follow that they are habitable. TRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have."

But it's complicated. For example, apparently the energetic radiation can help by stripping away the H/He atmosphere. For more, see:


Michael Okuda has raised the question on his Twitter feed of whether the planets would be close enough to the star that tidal force could be an issue---hard to say at this point (in the sense that they're not loose clouds of rocks, so tidal forces haven't sheared them apart, but I'd assume it's unclear whether they might have interesting geological activity, such as frequent earthquakes or unexpected extra heat due to internal friction).

Like others have pointed out, the combination of the planets being tidally locked so only one side gets sunlight and the strong X-ray radiation and flaring from the host star make it considerably un-Earth-like, and harder for life as we know it to develop.

Actually they are fewer. A dim star has less high energy radiation and a lower probability of harmful solar flares.

That's backwards. Red dwarfs have extremely powerful flares relative to their luminosity. So much so that it's unlikely that life would survive on the surface of an orbiting planet. (Though life would probably survive underwater.)



That's not quite true. These planets are very close to the star and can receive large doses of the little bit of harmful radiation that the star puts out. Additionally, these planets put out a lot of infrared, but not a lot of higher energy photons, and life-as-we-know-it needs those higher energy photons. That infrared is also absorbed by water in any atmospheres or oceans that are present, leaving even less for life. Finally, these types of stars tend to have rather variable output in their spectrums, which may make it difficult for life to adapt.

But the danger of both of those things surely decreases with the square of the distance to the star?

More radiation = more mutations = healthier/faster evolution!

Life is more than capable of evolving mechanisms to increase or decrease it's mutation rate. On Earth Eurkaryotic cells with large genomes have developed nuclei to reduce their mutation rate and bacteria are more of a mixed bag with some even deliberately uping their mutation rate above what would be cause by radiation, etc. So in the long run eukaryotic life would just have to waste more resources on mechanisms to resist the effects of the radiation.

That doesn't follow, at all. If we're talking about Earth-like life, then any such life would need protection from radiation to be able to live long enough to reproduce. If such protection wasn't provided by the planet's atmosphere, then life there wouldn't move out of the water. In any case, the mutation rate would need to be extremely low, as it is with Earth life.

Then the surface of stars ought to have more life than planets.

Just as interesting is a recent news story about a plan to build an interstellar space ship, propelled by giant solar sails to travel about .2c (20% of the speed of light).

In other words, we could get to the Trappist-1 star system in about 145 years.. practically in the blink of an eye.


Nuclear pulse propolsion... Project Orion... if only we could figure out the darn problem with the fallout and treaties/bans.

Tsiolkovsky is unforgiving. You'd need an amazing amount of fuel.

And they all seem to rotate around their sun in 1.5-20 days.


Revolve around. Sorry, had to.

They're more-than-likely tidally locked, so... you're both right.

Importantly, that means there is basically no way that a civilization that developed on such a planet would not at some point endeavor to travel to those other planets.

> no way that a civilization that developed on such a planet would not at some point endeavor to travel to those other planets.

That seems like a very human statement.

One could argue for a civilization to succeed it must be content with living on its home planet. And as a result, it would take good care of it.

You must be new here [on planet Earth].

Alternatively the parent considers the human race to either not have succeeded or to be in the process of changing to take good care of it

You could also argue that a civilization has not succeeded until it is able to completely sever ties with its home planet and venture out into the galaxy on its own. Leaving the nest, so to speak.

I'm wondering since these planets are so close to the parent star and to each other would it induce any gravitational instabilities?

maybe them aliens made the system up, so that it doesn't fall apart under its own gravity. Makes sense: just increase the habitable area by bringing in smaller planets into the goldilock zone - that might come out cheaper then to go interstellar.

Maybe lots of planets that circle the same star over the same orbit would be a better indicator of alien activity than a dyson sphere.

Is this an orbitally stable arrangement?

Yeah, they are all resonant.

So this means all the planets are all a volcanic mess because of the tidal forces?

Perhaps some of them, especially the innermost. Considering the orbital periods and resonance it seems like this system might be similar to the Jovian or Saturnian systems.

The planets may be tidally locked but there is also a chance they could have a slow rotation similar to Mercury. That planet has also shown us that a magnetic field is possible without a fast rotation so there's a chance of these exoplanets having one as well.

Tidal forces can be a boon to planets with synchronous rotation since they provide an additional source of heating for the side of the planet that faces away from the star. If there is liquid water or a thick atmosphere they may also drive tidal and weather patterns that could help circulate heat from the warm side to the cold side.

Geological activity also 'stirs the pot' when it comes to precursors for life. All this extra energy in the system could mean that even TRAPPIST-1g (outside the habitable zone) might have an atmosphere and oceans.

Those cool stars are also very durable and will not burn that easily.

Wonder how tidal forces from these planets would affect [possible] oceans on their neighbors. Would imagine it would make for quite a perturbed environment.

My thoughts exactly. The coolest skyscapes ever! On edit: This also reminds me of U. LeGuin's system in the dispossessed.

Sounds like tidal forces would be extreme.

I am wondering how it could affect our psyche if we were able to gaze on a mirror world with our own eyes in broad daylight. This is the first VR application I would think of: display an Earth like planet in the sky and see how it affects us.

That's exactly the premise of the (good) movie Another Earth: http://www.imdb.com/title/tt1549572

I wonder what planets that close together would do to tides on one another.

And the name of the star is shared with Trappist beer!

A colleague of mine has an insider info that the telescope was named after beer on purpose.

Yes, the project was born in Liège University in Belgium.

The name they chose for their programs are quite funny with belgian references: SPECULOOS[0] and TRAPPIST[1].

There are 11 trappist beers in the world (of which 6 are brewed in Belgium). Speculoos is a Dutch/Belgian cookie.

[0]: http://www.speculoos.ulg.ac.be/cms/c_3272698/en/speculoos-po... [1]: http://www.trappist.ulg.ac.be/cms/c_3300885/en/trappist-port...

I'm blown away the planets haven't torn each other apart or all collided by now...

And I was just thinking, if there were ever or ever to be oceans on these planets what kind of brutal tides would be experienced?

I wonder what kind of tidal forces these planets exert one one another.

Kinda like in No Man's Sky game.

To get a spacecraft there it would take:

Accelerating at 1g the 1st half, and decelerating at 1g the 2nd half, the traveler would experience 7.3 years of time. For observers it would take 41.8 years at a max speed of 0.998c

If you had a near perfect hydrogen -> helium fusion engine, it'd take about 6 million tons of fuel (about the mass of the Pyramids of Egypt or 2,000 Saturn V rockets)

It would take 37 days at warp 6 (assuming we're using the new warp factors established in The Next Generation). I have no idea how much dilithium crystal would be required though.

> I have no idea how much dilithium crystal would be required though.

Depends on the plot needs for the ~~episode~~ journey.

Particularly in Voyager. They were either running extremely low early in the show or had a virtually infinite supply later in the show. No explanation given.

Which is particularly troublesome given the plot of the Equinox episodes. Since on that ship they were harvesting creates to extend their dilithium crystal supply allowing them to travel faster/less efficiently.

But even throughout those episodes they never discussed why Voyager rarely had issues with supply after the first couple of seasons. Just hand waved it away with Voyager having a bigger crew, like that magically solves it.

Voyager spent most of the first season looking for deuterium, so that they could keep the ship moving forward.

Presumably, at some point around episode ~15, someone in either engineering, or the writing team realized that it is the third most abundant element in the universe.

Well.... the most abundant element in the universe is "Nothingium." Just because hydrogen is common doesn't mean it's always around when you need it, especially in heavy form.

It's been a long time since I laughed out loud at an HN comment! Well played. Thank you for that.

It doesn't just have to be common, it has to be worth harvesting.

It could be that it took them this long to locate and get to such a supply, or that they had to repair / construct equipment that normally wouldn't be expected to be used.

Their engines don't consume dilithium. Rather, it's a catalyst to the matter-antimatter reaction.

Most likely several hundred quatloos' worth, I'm guessing. :D

Edit: Oh, HN, you are so humor impaired sometimes.

Voting is a mechanism to signal whether the comment adds anything to the discussion. Star Trek references really don't add anything to an actual scientific discovery. I would much rather read actual engineering rather than entertainment driven thought.

I think you've just proven his point.

Yeah, I know, I might be occasionally too cynical.

I took the post as commentary on the engineering feasibility of the parent post.

Simply: this is not reddit. Voting happens based on the content of information added by your comment. Not a joke or a wordplay, unfortunately.

I had to downvote you because we're not supposed to compare HN to reddit. ;)

Edit: Oops, I cancelled my downvote.

That's a misreading of the guidelines:

If your account is less than a year old, please don't submit comments saying that HN is turning into Reddit. It's a common semi-noob illusion, as old as the hills.


That actually sounds... more plausible than I would have expected. Like, doable with technology I can imagine being within reach in the next 50 years even if we don't achieve any currently-unforseen breakthroughs.

Which begs the question, why haven't we at least even tried to have done anything like this already?

Do we really need to repeatedly spend comparable amounts of money on yet another dramatic TV special about humans traveling to Mars, as narrated by a favored celebrity? We really are running out of time. And it almost seems like it's pre-determined to happen. :\

The engineering issues are really tough, though surmountable. Currently, we just can't engineer the parts that will last that long in hard vacuum. Mostly because we can't test in that environment easily. Also, what do you do when all the light bulbs have burned out or gone haywire, what does that fault-tree look like for the billion and one parts on the craft? For a short journey of ~7 years, it's not a very big deal, it's just engineering issues. When you get into generation ship discussions then it becomes a human rights nightmare to trap newborns in a ship for their entire life and the lives of their descendants.

With a round trip parts delivery time OF a generation, and a communications delay of almost a decade (two for duplex), I don't see any reason not to classify such a mission as a short-range 'generation ship' / complete colony effort.

Getting our 'eggs' out of a single cosmic basket is really important, however I think we need to at least take the baby-crawl to establishing an outpost in the asteroid belt. That would be far closer, have far more reasonable communications/parts delays, and if we can actually send up some robots to build things ahead of us, might actually be a good manufacturing base.

I feel that advancements in quantum mechanics will solve the communication problem.

No, it almost certainly won't. Our current understanding of quantum information theory simply doesn't allow transmitting classical information faster than light. There's no room in what we don't understand that could plausibly contain FTL communication. This is a pretty fundamental result of quantum information theory. https://en.m.wikipedia.org/wiki/No-communication_theorem

> it becomes a human rights nightmare to trap newborns in a ship for their entire life and the lives of their descendants.

Which is exactly why we should get some people off this ship, and colonizing others!

Oh, you meant the generation ship, not the Earth; nevermind.

Yeah I'm to be honest just trying to awaken people to the priority being that we need to establish a self-sufficient colony somewhere other than Earth.

Honestly, as long as humans stay stuck to earth, losing all humanity on earth won't hurt anyone. As long as the extinction is instant. If we have colonies, those colonies need to suffer through the knowledge that we lost earth.

Establishing colonies is only imperative if the continuation of the human race is. I'd argue that humans have value only due to other humans. There is nothing inherently valuable about human life from a non-human perspective.

That is not to say I don't support space colonization, just that I don't think its a moral imperative.

"There is nothing inherently valuable about human life from a non-human perspective."

How would you know?

It's a perfectly reasonable assumption until there's some evidence there is. Given the vastness and scale of the universe, it seems quite likely that our disappearance would never be noticed.

Going unnoticed is not the same as inherently not valuable. Imagine aliens finding us extinct, wringing their little tentacles in dispair at the lost probing opportunities. :)

Well, HN is not the most optimal place to do that. There are not many of us on here and we all already know these things. 1600 Pennsylvania Pl. is a better bang for your buck.

I think you probably mean 1600 Pennsylvania Ave. (not Pl.), Washington, D.C.

Though I don't share your assessment that that is a productive place to direct citizen input on this (or any other) issue.


Elon Musk is working on that.


And you're certain enough that he's going to be able to do it in time and that he's doing it properly enough that you don't have to do anything about it. And you'll continue to go about doing the exact same things. Got it.

In no way attempting to deflect from my own regrettable inaction, I'm curious to know what you're doing about it?

Based on his previous achievements I feel like he is likely to move the field forward by big steps. And if not him the people after him. I'm not sure if pushing myself into field I don't understand much would help the progress. Scientists do science, not activists.

Are you a space activist of some sort?

The launch cost of launching the mass of the pyramids of gaza into orbit would bankrupt the richest economies. No your hypothetical engine won't be 100% efficient. This is completely unfeasible.

Low mass spacecraft are our only real options. A nano-sized solar sail craft powered by the sun or lasers from Earth can hit up to 20% lightspeed theoretically and its launch costs would be affordable, if not relatively cheap. We could have a spacecraft at Alpha Centauri in merely 20 years. Or simply launch them by the dozens or hundreds to various locations. Stephen Hawking has proposed this system in the past and we've had trial launches of solar sails via the planetary society and others. Its a solid and most likely doable concept at scale. Yes, your kids or grandkids will hear the results, but not you. Is that such a problem, especially when the alternatives are never launching a probe to those systems?

To be fair, the proposed Centauri mission would be within a person's lifetime. 20 years to get there then another 4 and half to get the data. So if we launch when you're in your 40s or even 50s, you'll still see the data.

We can even use this concept on heavier ships in the solar system to get to Mars quickly. How quickly? Three days quickly for a 100kg craft. This is a bit more pie-in-the-sky of course:


>The launch cost of launching the mass of the pyramids of gaza into orbit would bankrupt the richest economies. No your hypothetical engine won't be 100% efficient. This is completely unfeasible.

Not necessarily. You don't need to launch the mass of the pyramids from Earth, you just need to find a source of fuel somewhere else in the Solar System that's in a shallower gravity well, like on the Moon. This is why a lot of people want to work on mining asteroids, and building mining/refining/manufacturing infrastructure offworld.

You can't really just extrapolate the current cost of launching materials by the kilogram. A project of that magnitude would generate economies of scale and probably some innovations along the way that would bring down the cost. The "nearly perfect" fusion engine is really the hard part.

You are assuming that this mass will be launched using today's chemically powered rockets. But using nuclear propelled rockets will bring down the launch costs by orders of magnitude.

A Russian billionaire is trying: https://breakthroughinitiatives.org/Initiative/3

I'm talking about establishing a self-sufficient colony somewhere other than Earth. We have a major crisis on our hands. And few are even willing to consider let alone research and confirm that we're talking about an extinction event much sooner than we thought, maybe on the order of just a couple years out. Problem is, we don't know. Bigger problem is, we're not doing anything about it.

If its affordable to send people to an off world colony then its affordable to bomb it when war on Earth breaks out. Fleeing isn't going to help in a wartime situation and the idea it will not be targeted by world powers is a little naive.

I said nothing about war..?

There's no other conceivable extinction event that could occur "a couple years out". If you're talking climate change, then I agree that's a major problem, but on the order of at least decades.

Plus climate change would have to get pretty incredibly bad for, say, Mars to start looking easier to live on than Earth.

That's the problem with colonization as an Earth back-up—even Antarctica and the Sahara Desert are more livable than any other body in the Solar System, by a long shot. Another Snowball Earth or a heavily desertified Earth would still be preferable to Mars. Throw in a fair amount of radiation, even—still better.

Cuts down significantly on the sorts of events for which a Martian back-up world is preferable to a shelter-in-place strategy.

But if you could go out now and terraform a planet so that when in future you have problems here you have a place to go to that matches the original environment back on earth. And you learn a lot of things about "geo"engineering, the things you cannot try safely at home, which can come in handy to fix any problem back on earth.

We have no ability to terraform anything, let alone a planet, let alone the economics of it ever being feasible. Life isn't a star trek episode. We can't just go and 'terraform now' as you say. We can barely land anything on Mars without a 50% failrate right now. We're talking technology way, way into the future. Hundreds, perhaps thousands, of years.

This is precisely what we don't know. If you're talking averages, then sure it takes decades to observe a meaningful change. However climate change of the sort we're anticipating now isn't about averages. It's about peaks, it's about records, and possibly major shifts in equilibrium.

Basically what we witness so far and will keep on seeing are spikes in all directions of the spectrums (e.g. record high temps, but also record lows; rain in the Death Valley and droughts in formerly wet areas). More critically, what little we've been able to gather on how fast climate eras shift from one to the next, it appears that whereas in-cycle change happens rather slowly (change within an era, because averages), climate era shifts could happen very quickly (because chain of events). We're talking going from a warm era to a glacial one within a few years, sometimes months even. A high speculation is that some thongs might upset the Gulfstream and you'd observe climate evolve from year to year.

What about a comet impact? That's extinction level and comes with very little warning.

Given our limited sky coverage, we can't really rule out killer meteor on the time scale of "a couple of years", or even a couple of hours, though the probability is extremely low (like, sub-1-in-100 million, for the couple of years time frame.)

Still probably higher than extinction from war in the same time frame.

> There's no other conceivable extinction event that could occur "a couple years out".

Life on earth could end in the next five minutes. :) https://www.youtube.com/watch?v=RLykC1VN7NY

The resources and practically to gently shoving a comet or asteroid out of the way are probably a billion less than building self-sustaining colonies elsewhere. Worse, now we have two planets to defend, not one.

I don't see the "we must migrate now" types saying "lets put migration away for a while so we can better build anti-asteroid solutions and detection systems."

Arguably, we're just a couple launches and deployments of mass drivers away from fixing this issue. This is all known tech that could be deployed relatively quickly. A self-sustaining Martian colony is probably hundreds of years away considering the work it would take to terraform the planet.

The resources involved in preventing a GRB from hitting us, on the other hand, are . . .

. . . actually I have absolutely no idea how you would practically do that at all.

He appeared to be speaking of a known, ongoing crisis, not a minute possibility of a killer meteor.

Haha! My link doesn't go to a video about meteors. Your point is taken, though. I was just being cute and/or hoping you hadn't heard about this.

But I have heard the possibility of a GRB discussed seriously as a motivator for expanding, not just through the solar system, but through the galaxy.

Lots of good reasons to establish a space colony, fleeing the earth is not one of them. By this sort of argument you actually discredit the idea of space exploration.

> fleeing the earth is not one of them

Why not? If a big asteroid is going to hit earth (for example), fleeing is a pretty reasonable response. It's also a reasonable defense against technobiological hazards.

The impact would have to be absolutely massive to cause enough damage to the Earth that it is less liveable than Mars.

To compare, the Triassic–Jurassic extinction event created an Earth still more liveable than Mars. Even if we go way back in time, to when there was no land-life on Earth and a poorly oxygenated atmosphere, the Earth is still better off than Mars for supporting human life.

If the goal is to save the human race, then it'd be better to have a few high orbit or lunar habitats, then if something wipes out the majority of civilisation on Earth, we immediately begin recolonisation of our home planet.

Let me ask you this question: Would you consider the Titanic, oh, I don't know -- a success story?

Because that's basically what this sort of thing comes down to, at the end of it. There's basically nothing that is going to be existentially, immediately threatening to earth on a timescale that Mars is actually a more livable environment.

Where else in the Solar System could we even get the amount of water we would need?

Isn't Mars full of water? Source: I read the Mars trilogy by Kim Stanley Robinson.

The chance that an asteroid hitting Earth that actually matters is extremely low. It is even lower chance that we can detect it on time. It is much higher chance that politicians will start a nuclear war that wipes out most of humanity from the face of Earth.

I'm not certain what you mean by "fleeing", but we sure as hell aren't going to move seven billion people. A bunch of engineers and rich people may be able to get out, but that's about it. To save the rest, we'll need cheaper methods.

Isn't part of the problem that any ship capable of interstellar travel is also capable of being turned into a WMD? I imagine a lot of people would get really nervous if someone for instance parked a bunch of radioactive material in low earth orbit, and even with fusion there would be the fear that someone could turn the ship around.

To be fair kinetic bombardment is stupid easy to do once you can put stuff into orbit. Put a few, 20 foot long tungsten rods into orbit and you can effectively nuke, without radiation, any location that you orbit over [1].

Sure this is a problem but it's one that already exists today, not necessarily only when we start building space craft in orbit.

[1] https://en.wikipedia.org/wiki/Kinetic_bombardment

Contract SpaceX to bring the rods into orbit, then bill per strike. KBaaS, disrupting warfare since 2017.

Back of the envelope calculation: rods with length of 6m and radius of 5cm weight little over 900kg. Falcon 9 can bring 22.8 tons to LEO. So we can get up about 24 of these in one go. If we want to use the USAF dimensions of 6.1m and radius of 0.15m, then it's only 2 of them, maybe 3 with some adjustments to the rocket.

I always wondered how to de-orbit those rods. It's not like you can just 'drop' them as they are already in free-fall. You'd need to cancel quite a bit of orbital velocity to get the things to de-orbit. Aiming'd be even more of a bitch.

> ...a 6.1 m × 0.3 m tungsten cylinder impacting at Mach 10 has a kinetic energy equivalent to approximately 11.5 tons of TNT (or 7.2 tons of dynamite).

Well, that's a bit underwhelming. Sounds a bit like Russian polonium tea, that is, less like a matter of efficiency, and more like showing off the sheer extravaganza of killing somebody in the most colorful, expensive way.

Kinetic bombardment is silly and inefficient science fiction. Conventionally armed ballistic missiles can strike any point on Earth almost as quickly at a far lower cost and with greater survivability.

Well today sending a rocket up, putting these things in orbit and then dropping them would probably be a bit obvious and inefficient. But in the near future when we're mining asteroids? As long as they can be captured closed enough to Earth I would imagine kinetic bombardment would be pretty effective. They would be rather difficult to shoot out of the sky as ballistic missiles can be blown up prior to reaching target but kinetic would require an much, much larger amount of energy to destroy enough so as to not hit and damage the target.

Asteroid mining on any sort of commercial scale is far future, not near future. Even with fully reusable rockets, asteroid mining wouldn't be profitable; anything we need can be obtained cheaper on Earth.

Small asteroids are too irregularly shaped to be useful as precision-guided projectiles. As soon as they hit the atmosphere they're going to veer off course.

Kinetic bombardment satellites — whether fully artificial or asteroids guidance and propulsion strapped on — would be in known orbits with limited maneuvering capability and thus highly vulnerable to antisatellite weapons. It takes very little energy to kill a satellite; you don't have to vaporize it, just break any critical part in the communication, propulsion, or guidance systems. Satellites are generally easier targets to hit than ballistic missiles.

> Asteroid mining on any sort of commercial scale is far future, not near future. Even with fully reusable rockets, asteroid mining wouldn't be profitable; anything we need can be obtained cheaper on Earth.

It's certainly in the future, it just all depends on what is far to you. Many companies who want to get in on this are hoping to attempt within the next 10-15 years (obviously commercial mining would come some years later after a successful attempt). If it can be successfully navigated it would have the potential to be crazy profitable!

It's too expensive to bring materials up but if you can mine materials in space and deliver them to the various space companies / government departments is where you'd make your money. Mining water, in theory, should be crazy cheaper as long as you can capture the asteroid efficiently enough. NASA has already said they would love to see more work in this space so they can purchase materials cheaper while in space and they're even hoping to capture an asteroid in the mid 2020s as a type of test for this scenario.

> Satellites are generally easier targets to hit than ballistic missiles.

This misses my entire point: in order to destroy the kinetic projectile you would have to launch a preemptive strike against the satellite. There is no way around it. You won't be able to defend yourself against a large projective being precision dropped onto your location without a lot of energy (so depending on what country you are maybe you can knock several into a few direction or vaporize with a high enough yield but you only get one chance at destroying it). Meanwhile a ballistic missile, while faster, can be destroyed with a hot enough laser.

I understand they're not practical and if you want to use an asteroid itself that's even more awkward. But when we start mining asteroids, in my opinion, it's going to become crazy practical and cheap.

That's not how it would work. Incoming kinetic projectiles are just as vulnerable to defensive systems as ballistic missiles. Hit it hard enough and you can knock out the guidance system. They would need very precise and sensitive guidance systems to hit anything from orbit.

Space enthusiasts constantly underestimate costs and schedules. Just because something is theoretically possible doesn't mean that the engineering problems can be solved in an economical way. If there is large-scale asteroid mining in my lifetime then I'll eat my hat.

> Incoming kinetic projectiles are just as vulnerable to defensive systems as ballistic missiles. Hit it hard enough and you can knock out the guidance system

Not true at all. Kinetic projectiles would have no guidance system. It's just a big, dumb piece of tungsten. That's it. You would have to hit it hard enough to make sure that, when it hits, it won't cause damage. This means near complete vaporization. That's very energy intensive.

Ballistic missiles, however, simply need to have their payload exploded at almost any distance away from the target to reduce its power to near nothingness.

> If there is large-scale asteroid mining in my lifetime then I'll eat my hat.

Not sure how old you are but if you're under 40 and not planning on dying sooner than average then, in my opinion, you should start looking up hat recipes :D. At least I hope but it depends because the primary customer for asteroid mining is going to be space companies and government agencies like NASA. A disruptive political system that prevents said purchases could hamper progress significantly.

You seem to be hilariously ignorant about the realities of aerodynamics, rocketry, and orbital mechanics. Due to atmospheric turbulence and normal thrust variations in the rocket motors that would have to be used for de-orbit burns, any unguided projectiles would have such as huge CEP as to make them militarily useless. Unguided projectiles launched from aircraft today can't reliably hit anything at distances over a few miles; satellites are much farther. A (sensitive and fragile) terminal guidance system would be an absolute necessity. Contrary to what arrogant programmers might think, DARPA isn't run by idiots and they gave up on such nonsense schemes for good reason long ago.

As for asteroid mining, hope doesn't count for anything. There is no shortage of essential raw materials here on Earth. No one is going to commit the hundreds of billion $ necessary to do it on more than a trial basis. There's simply no economic incentive nor is there political will to spend that money. Sorry to burst your bubble.

Well I assume is the USG let alone SpaceX decided to put some 20 foot tungsten rods in orbit a lot of people would start raising objections pretty quickly.

If anyone is interested in kinetic bombardment, read Heinlein's "The Moon is a Harsh Mistress". Much like how the Fosterites in his "Stranger in a Strange Land" were a blueprint for Scientology, TMiaHM is a guide on how to plan and conduct a revolution, although in this case it was the Moon against Earth.

Jet planes can also be used to destroy skyscrapers. People don't seem to get really nervous about them.

We've created a giant security apparatus based on Jet planes. They are not something we just let anyone buy or fly and if you deviate from your flight path these days you stand a good chance of getting shot down. People definitely get nervous about them.

Are you implying that rockets capable of transporting several hundred kg of tungsten to orbit are -not- tightly controlled? I imagine if you assembled anything close to this size without seeking extensive permissions you would be quite severely punished

No, I'm just addressed the analogy of jet airplanes and pointing out those are tightly controlled. I understand that current rocked are very tightly controlled, my point is just that any sort of interstellar rocket would be much more dangerous and thus have to be subject to much higher controls. A ship that was capable of reaching a significant fraction of light speed would be the most dangerous weapon ever created though, and existing controls might not be sufficient.

NASA is not standing still. Europa just went to the next stage for exploration.


It just takes a lot of time to plan and build ships for space travel.

You're completely missing my point.

We've known about climate change for a long time. Since the 60s/70s, no? But nobody has been able to realize what it really is, because scientists do not understand gravity yet. I've been begging people to confirm this for themselves instead of believing what others say about it. If people really knew what global warming was they would have started working on ships decades ago.

My point is, people on stable self-sufficient colony outside of Earth is priority number one. Not sending a probe to Europa. But somehow I get the feeling you have no interest in this reality.

You sound a little unhinged to be honest. If you're really worried that climate change on Earth is going to cause big problems for humans (which is not an unreasonable concern; climate change threatens to cause significant sea level rise which will cause big problems for people living in sea-level coastal cities), building self-sufficient colonies offworld is a rather extreme answer to that. A simpler answer is to simply move inland, and make sure your structures can handle extreme weather. That's a lot easier than trying to build an offworld colony.

The real reason to build an offworld colony is because you're worried about a big asteroid strike. But here again, it's a lot easier to just build observation systems to spot these threats, and weapons systems to handle these threats, than to build an offworld colony that's truly self-sufficient.

Finally, offworld colonies have gigantic problems with them: 1) people don't handle radiation well, and no place in the Solar System protects us from radiation the way the Earth does, so we'd probably have to burrow underground to mitigate it, and 2) we really don't know how well the human body can handle low-gravity conditions, but we do know that the human body does not handle zero-g well and that it causes massive health problems. There's no place in the Solar System with close to Earth-normal gravity, except for Venus which is a hellhole hot enough to melt lead (on the surface). Mars is only 1/3g, and the Moon is only 1/6g, and pretty much everything else is even less except the gas giants which obviously aren't livable.

Now what climate change and offworld colonies (in this system at least) have to do with our lack of understanding of gravity, I have no idea.

So all we have to do about climate change is build structures to handle extreme weather? Climate change is and will continue to affect our whole food chain that supports the some ~7 billion people on earth. It's not just the weather we will be dealing with.

And you think it's easier to just pack up and move to Mars than to deal with the climate change? You think it'd be easier to figure out how to grow food on Mars than to mitigate climate change-caused problems with the food chain here?

There's lots of stuff you can do to grow food here even with climate change: greenhouses, indoor/vertical farming, etc. Good luck with all that on Mars.

Scientists understand gravity very well, pick up any book on general relativity.

> We've known about climate change for a long time. Since the 60s/70s, no? But nobody has been able to realize what it really is, because scientists do not understand gravity yet.

We know exactly what climate change is and it has nothing to do with gravity.

What does gravity have to do with climate change?

If you're curious on more than a surface level, this book is an excellent resource from an ex-astronaught on why things are lagging. "Safe Is Not An Option"


It sounds like you have a startup idea on your hands : )

How would this generate returns on investment? Monetary returns, that is.

I mean if I somehow magically become a billionaire I intend to throw money at the problem with no hope of ever making a return because I believe humanity's last hope is an offworld colony, but I doubt I'll convince any VCs to join in.

You've no idea. But for the first step, I need to find one or two very truthful physics grad students / electrical engineers who feel some responsibility for Earth or saving themselves. You wouldn't happen to know someone?

Why don't you become one?

Don't worry your pretty little head. I went to top schools.

I think you mean "Post-truthful"

Politics, specifically the PTBT[1]. Otherwise Project Orion[2] would be a reality now.

[1] https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treat...

[2] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Project Daedalus seems more appropriate for interstellar travel:


EDIT: My math was totally wrong. 10 trillion dollars, 0.1x world GDP. (Still a lot of money though) Leaving the old text below so the replies make sense.


To move 6,000,000 megatons of fuel into low earth orbit would take 111,111 Falcon Heavy launches, at a total cost of $10,000,000,000,000 dollars. (10 quadrillion dollars, 93x world GDP)

Something that takes the total global economic output of the planet a century just to lift the fuel into orbit doesn't read as plausible in the next 50 years to me.

You wouldn't gather that fuel from earth.. you'd gather it from an icy asteroid or comet or moon (Europa).

I'm not sure if your error is in the number or in the words, but the number you wrote down is 10 trillion dollars.

Note to mention that Falcon Launches would get significantly cheaper if we have to do 100,000 of them.


Is it plausible for anything we'd build to travel .998c and survive the journey?

Just going that fast isn't really risky at all (assuming you don't hit anything, which isn't actually a safe assumption). The tough part is making something capable of a constant 1g of acceleration for a 7 year trip. Most of the craft we make only accelerate for the first small percent of a trip. We can do accelerations that high, but not for very long, and we can't even manage acceleration for such a long duration even at a small fraction of 1g.

You have no idea how scary traveling at near-lightspeed actually is. Isaac Arthur has a great video covering it. https://www.youtube.com/watch?v=s6BQSgidbmc

If you go fast enough doesn't any incoming radiation just get blue shifted into gamma rays? Just running into cosmic dust at the speed of light would not be fun.

>Just running into cosmic dust at the speed of light would not be fun.

That's why you need a navigational deflector dish. http://vignette1.wikia.nocookie.net/memoryalpha/images/b/b4/...

Isn't that the whole relativity theory: that if you emit a photon while at celerity 1c, it looks like it goes away at 1c both from you and from a still reference? hence blue is still blue at 1c?

The spectrum of light depends on the frequency of the vibrations in the electromagnetic field, not the velocity of the wave. For both red and blue light the wave is traveling at 1c but the peaks of the wave are at 700 nanometers or 400 nanometers depending on the color. And under Special Relativity distances distort so that you do in fact see light shifted to higher frequencies/shorter wavelengths in the direction you're traveling.

I'm talking about radiation from other stars or what ever. Imagine if every star in the star field in front of you shifted from visible light to gamma.

I'm sure there is a study out there that describes the exact amount of radiation exposure you would receive at reletivistic velocities due to blue shifting.

The speed doesn't change but the wavelength does. That's why we see light from other galaxies redshifted.

> The tough part is making something capable of a constant 1g of acceleration for a 7 year trip.

Isn't that just a building with an engine in the basement? Seems like one of the harder parts would be making sure it can survive freefall at launch, turnaround and arrival.

> Isn't that just a building with an engine in the basement?

No, it turns out that the requirements of generating thrust (which require generating energy and using it to toss something out the back of your ship) end up running into hard limits in the delta-V you can carry based on the available thrust-generating technology. Surviving 1g of constant acceleration in easy (well, I mean, it requires a certain degree of structural strength, which as mass gets large--which it will even with something delivering a minute payload--becomes challenging because of square/cube issues), but building something that can actually generate 1g of constant acceleration for 7 years is a non-trivial engineering challenge.

> Seems like one of the harder parts would be making sure it can survive freefall at launch, turnaround and arrival.

No, surviving free fall is a non-issue. Literally, that means surviving not having external forces acting on it. There's nothing to it.

My layman's reading of physics-for-layman books has taught me that if we were to travel at the speed of light, we'd convert into energy. However, I'm not so sure about 99.999999% the speed of light. That's really OK, as far as we know?

You are technically travelling at 99.99999% of the speed of light right now, relative to something.

You are actually traveling faster than the speed of light relative to some distant galaxies. The "speed limit" in special relativity refers to an inertial reference frame (in general relativity, this would be a reference frame with no/negligible spacetime curvature). As soon as you have significant gravity/acceleration relative to things with mass, the answer becomes much more complicated (and not fixed).

What determines my speed limit? Is it set at 0 here on earth? What if I travel to another galaxy, this one was going at .5c relative to me when I was on earth. Then after trillions of years, inflation causes all of the galaxies to separate beyond sight from me in this new galaxy. Now, the centre of this galaxy appears at rest. Does my velocity get "reset?"

Special relativity says that no matter what your velocity is relative to Earth you can proclaim that you're actually at rest and it's the other guy that's moving. So your velocity is always continuously being "reset". And in each of the frames of reference of everything in existence nothing else is traveling faster than the speed of light.

The 1905 Special Relativity Theory (1905 SRT) version of reciprocity implies that the other guy in a different inertial frame than your own will be the one that is moving. The consequence of this implication is the "Twin Paradox" (i.e. both twins in different inertial frames have been accelerated and are both aging slower than the other twin). A different version of reciprocity that implies that the accelerated twin is moving and the apparent movement of the twin that has remained stationary is only an aberration (or mirage) is axiomatic because movement first requires acceleration.

Assuming spacetime is not expanding, let's say 1 galaxy is moving away from us at .9c. I leave our galaxy and catch it by travelling at .99c. Then a citizen of that galaxy, at rest, accelerates to .9c. So his velocity relative to earth is .89c? I think I need to go watch a youtube video on special relativity. Intertial reference frames are really weird.

inflation/expansion of space is not limited to the speed of light.

I know. That understanding is already illustrated in my question... I don't see what that has to do with answering it?

As far as we know, as long as you don't actually reach light speed (which is currently thought to be impossible anyway) you can pretty much get as close as you want and be fine.

Thanks. It's quite counter intuitive to think about. You'd be going along at 99.99999999999999999999999999999999% and then hit that last bit and poof now I'm photons.

Well, achieving that "last bit" takes an infinite amount of energy, so it's not surprising that that has some effect.

Yes, as long as you don't hit anything or care about the time dilation you will experience.

Also, and I'm not an astrophysicist, but I believe going fast _relative to Earth_ doesn't make it any more or less likely that you're going to hit anything. Moving at .998c relative to Earth and hitting something that is stationary relative to Earth is the same thing as not moving and being hit by something going .998c relative to you.

EDIT: Right, as chris_va pointed out, we're all traveling at .998c relative to something out there.

> Also, and I'm not an astrophysicist, but I believe going fast _relative to Earth_ doesn't make it any more or less likely that you're going to hit anything.

OTOH, given that it is (like most of the stuff around it) orbiting the center of mass of the Milky Way Galaxy, shouldn't Earth be moving relatively close (within a very small fraction of light speed) to the speed of most of the interstellar dust, etc., in its immediate galactic vincinity, such that moving at 0.998c relative to earth is also moving at a very similar fraction of c relative to lots of other things you are going to potentially hit on a journey from earth to a nearby star system?

Of course. Thanks. This is why they don't put me in charge of our next interstellar trip.

>doesn't make it any more or less likely that you're going to hit anything.

I believe that would be wrong. Also not an astrophysicist but I would guess that most of the stuff in our galaxy is moving at speeds which are, on average, relative to our galaxy, and moving at some average rotational speed around it. Of course there could be stray bits moving quite quickly on through.

So moving at c relative to the planets and solar systems swirling around our galaxy probably would make you more likely to hit something.

Not a physicist, but wouldn't a spacecraft standing on Earth's surface already feel 1g because of Earth's gravity (https://en.wikipedia.org/wiki/Equivalence_principle)?

You can look up Project Daedalus's report online. They considered all these issues in great detail and came up with an affirmative answer.

> Is it plausible for anything we'd build to travel .998c and survive the journey?

The first half is actually a bigger problem than the second half.

The problem is far easier to state than implement.

All about funding. NASA, for example, is about 35 years behind schedule by my math, solely from funding.

Now whether throwing money at a public agency would have made advancements in lensing, transistor along with propulsion technology happen quicker. Thats a tougher question that leans towards the "unlikely".

Funding is not the only issue. Their planning was also a disaster. The Space Shuttle wasted so much money with very little return. If NASA continued with Project Orion and NERVA a Mars colony would have been well established by now.

The Space Shuttle was fundamentally flawed because NASA compromised with DoD to get it funded. Had NASA been fully funded for space exploration alone they would have made better choices.

The rocket equation is the biggest block. Getting a constant thrust of 1g for 7.3 years is hard.

It takes an incredible amount of fuel, and an incredible specific velocity.

> about...2,000 Saturn V rockets

Or about 670 BFRs [1].

It would take 6 cycles of tripling size and/or performance to get a single system with enough thrust to meet your 7/42 year schedule. The Saturn V first flew in 1967 [1]; SpaceX plans to launch its Interplanetary Transport System (ITS) in 2022 or 2024 [2]. That's an expected 60 years to triple performance.

[1] http://waitbutwhy.com/2016/09/spacexs-big-fking-rocket-the-f...

[2] https://en.wikipedia.org/wiki/Saturn_V

[3] https://en.wikipedia.org/wiki/Interplanetary_Transport_Syste...

Though that kind of performance is probably not going to come about from improvements to chemical rockets but moving to fusion. There are some promising developments on fusion reactors (see Breakthrough in Nuclear Fusion? - Prof. Dennis Whyte https://www.youtube.com/watch?v=KkpqA8yG9T4)

Guess something like that with the 100 million C gas doing the propulsion maybe?

Running a direct output fusion rocket in the atmosphere would be tough-- all the reactors we know how to build, or can vaguely guess how to build, run at small fractions of one atmospheric pressure.

If you have fusion engines, you would just be doing all the construction in orbit anyway, rather than boosting stuff from the surface of the orbit.

There may have to be some clever system where you don't take a lot of fuel with you.

Imagine a type of particle accumulator and aggregator - call it cosmic flypaper. If you're going say 100 km/s, things seemingly insignificant and rare you may be able to collect a lot of pretty quickly.

It sounds like you're talking about the Bussard Ramjet. It collects interstellar hydrogen gas for use in fusion engines. The problem is that it's unlikely there's enough hydrogen floating around for it to really work.

Are there places that would have high concentrations of floating hydrogen? You would just need to "fly" through those.

> Are there places that would have high concentrations of floating hydrogen?

IIRC, a Bussard Ramjet is at least potentially viable at the typical density in the galaxy, but unfortunately for any use by humans to get away from our solar system, or local region of space (to a distance, IIRC, on the order of 1000 light years in any direction) is a pocket of relatively low density.

Sure: stars. I don't think flying through those would work out well.

There's also nebulae. But that's only going to be useful when you're actually in a nebula, which isn't most of the time.

To put that 60 years into perspective: It was 66 years between the first powered airplane flight and landing people on the moon.

These kinds of realizations make me sad that life is so relatively short.

It's also exciting, isn't it? That every generation is likely to experience some unique achievement?

Don't forget about space debris. Hitting anything at the posted max speed is just gonna dril a hole straight through your ship. If it's big enough, it'll add some nice shockwaves to really mess up your ship.

Note that GP was talking about the mass of two thousand Saturn V's, not the payload. It would take 20,000 BFR launches to put 6 million tons in orbit.

For anyone (like me) who wants to adjust the parameters while not having to deal with calculus, this[1] "Space travel calculator" works great.

[1]: http://nathangeffen.webfactional.com/spacetravel/spacetravel...

Honestly that amount of mass seems doable (the hard part being a near perfect fusion engine of course).

Just curious about your calculations though. Did you take into account the fact that you could eject spent helium and therefore lower your mass in transit? Assuming most of the mass is fuel, I would imagine by the time you're arriving there would be very little mass left to decelerate.

Upvote for doing the math, but we're going to have to ditch traditional chemical rockets for this kind of trip. Fortunately, we're making progress on alternatives.


Note that he's not giving a figure for a chemical engine, but a fusion engine (sometimes termed a "torchship" due to the absurd power involved.) That would be strictly more efficient than VASIMR or similar. The figure would be even more ludicrous with a LOX/LH2 engine.

(I did not check his math, however.)

At first I was confused about if the star is 39 light years from earth, how come it would take just 41.8 years from the observer perspective (assuming me looking out of the window). But with pen and paper I see that you would be accelerating the first year up to speed of light, then 40 years at almost speed of light and then one year decelerating. What is the efficiency of some current hydrogen -> helium fusion engine?

What's the math behind your estimations? Doesn't sound so unfeasible.

I didn't get the same result. I used the relativistic rocket equation [1], and the change in rapidity from 0 to 0.998c is about 3.5, which means a total change of rapidity of about 7 (static to full speed and then back to static). Assuming perfect hydrogen->helium fusion engine (specific impulse=0.12c) the ratio between initial mass and final mass is 10^25. For comparison the mass of the earth is about 0.6*10^25, or in other words, if you want to send only 2kg of load to this star, you need fuel as much as the mass of the earth.

[1] https://en.wikipedia.org/wiki/Relativistic_rocket

2000 saturn v rockets sounds possible

That's 2000 chemical rockets. The example talks about a perfect Hydrogen-Helium conversion.

the fusion bit not (yet) so much though

If we had a near-perfect hydrogen -> helium fusion engine, why would we want to stop at 2,000 saturn V rockets of mass? Hydrogen is cheap AF.

Also why would we only want to stop at helium, why not fuse any further?

Also why would you want to stop at 1g acceleration - why not 1.5g or 2g, say? Surely living in 2g environment is okay for that period?

However presumably there is no good reason for us to go there. Just because there are planets doesn't make it worth the trip.

What if we used engines powered by nuclear energy (fission or fusion)? Could we reduce the amount of fuel, since the energy density is so much higher?

The GP already posits a theoretical perfect fusion drive.

Ah, yes. My bad.

This estimation was already assuming a fusion engine.

You still need reaction mass.

I don't understand how it could be less than 39 years for the traveler. Time dilation is a thing, but being less than 39 years would mean the traveler observed the planet approaching faster than the speed of light, which should be impossible.

Or does space contract as well?

That's correct. You wouldn't be able to do the roundrip expedition in less than 78 years from an observer's standpoint so you still can't transmit information faster than the speed of light, however from the explorer's point of view it might take a lot less time to do that as you get closer to c.

If you travel at the speed of light (assuming you're a photon or something) then time stops "existing" altogether and you are simultaneously at the point of origin, destination and everywhere in between. At least that's my understanding of it.

I find it both exciting and depressing at the same time, it means that you could potentially make a rountrip to the center of the milky way and back within a human lifetime but by the time you'll be back everybody that remained on earth would be long dead and nothing would be the same anymore. If those types of travels become common then society would stop existing on a single timeframe. It's pretty mind boggling.

How much fuel would it take to get 6 million tons of fuel off Earth into the interstellar ship?

You have a path from earth to a planet 40 lightyears away. There is a planet every K meters that can house a fuel tank to serve as a checkpoint. Let F be the amount of fuel to take a ship between two adjacent planets. Let 0 be the first and L be the last checkpoint.

Suppose a rocket can hold 3F. Go to A, deposit F, and return 2 times. Next rocket goes to B, stopping at A each way to take F -- do the B trip two times. Now you have two tanks at B and none at A. Repeat like binary addition till you fill L-2 with F, then repeat for L=L-3 to L=0. Now each checkpoint except the last and next-to-last has 1F. A final ship can make the trip to the planet, build a civilization, find a fuel source and return with pics.

Are we willing to use a railgun, with a rocket burn at apogee? (So one-half orbit from the starting point: without the burn, the object will hit the ground when it completes its orbit). If so, a lot less than is needed by pure rocket burns, where you have to carry all of the mass used to accelerate to orbital velocity.

Step one, find an asteroid with ice...

The best way would be to use a space elevator pipe for this. But it's still not feasible.

Better off Enceladus. A ship that size probably would be built in space.

If you make the fuel on Mars it will take less effort to get it into orbit. It's also a bit further up the suns gravity well.

Is there any reason to accelerate to the half way point? Accelerating to .99c is much faster and would use much less fuel. It also wouldn't make the travel time much different, to the outside observers anyway.

If you accelerate beyond the halfway point you're not going to be able to stop in time.

I mean you can stop well before half way. 2 years at 1g will get you to .97c and drastically cut the fuel requirements.

Is that just for the rockets? We would definitely need much more than that considering we would also need to carry at least a city's worth of infrastructure, supplies, settlers, etc.

I'm pretty sure he meant a probe. Not a good idea to send a bunch of people to a planet we don't know is habitable (and probably isn't).

We don't have any spacecraft that can go that fast, do we?

No, but we probably could with enough fuel. The bigger issue is space dust. Hitting it at relativistic speeds would be like flying into a cosmic shotgun for the entire journey.

According to this paper, you can travel around 0.5 c before the radiation caused by colliding with interstellar hydrogen kills you: http://www.scirp.org/journal/PaperInformation.aspx?paperID=2...

"Going slow to avoid severe H irradiation sets an upper speed limit of v ~ 0.5 c. This velocity only gives a time dilation factor of about 15%, which would not substantially assist galaxy-scale voyages. Diffuse interstellar H atoms are the ultimate cosmic space mines and represent a formidable obstacle to interstellar travel."

Couldn't you put some kind of "snowplow" on the front so it pushes the oncoming detritus away to the sides?

Could that cause accidental fusion?

Assuming this [1] is reasonably accurate, very much yes. It proposes fusion occurring at speeds as "slow" as 0.9c.

[1] https://what-if.xkcd.com/1/

A large mass of water ice would make a decent ablative shield.

If I understand our capabilities correctly we can't even escape the solar system without using gravity assist boosts from other planets, let alone sustaining thrust for a long period.


"Capabilities" seems a little ambiguous to me. Is it what we have done or what we could do? Using those boosts is a way to achieve the same results, only cheaper. Yes, it's slower than building a big ass rocket, but it seems people that sign the checks are in no hurry.

Also if I'm not mistaken, Voyager probes were not even intended to live so long or leave the system.

If there were a reason to send a probe to another star as soon as possible and no matter the cost, there would be a way, maybe not fast, maybe very expensive, maybe something weird, but it would be done.

I've read about laser systems for sending unmanned crafts at high speed: http://www.space.com/29950-lasers-power-tiny-interstellar-sp...

I think we do if you look at it in terms of acceleration. As I understand it, In space there is almost 0 friction so "fast" is more about acceleration than "top speed". In this view, 1g isn't that fast.

About as "fast" as a helicopter at a stable altitude, or to fit with the no-atmosphere bit, a stationary jetpack or lunar lander (on Earth).

Doesn't energy required to accelerate keep exponentially increasing, though? ke = mv^2

Energy is relative to a frame of reference, and IIUC 1g acceleration of the space vessel is in The vessel's frame of reference, not earth's.

No. If I'm not mistaken we're in the order of magnitude of 0.0001c.

At 0.125g/cm^3 of density, this amounts to a cube of liquid helium about 350 meters per side (43.5 million cubic meters).

For others who don't know how much an American ton is, 6 mil of them = (5.44 * (10^9)) kilograms.

What do you consider a spacecraft though. What is the payload mass ?

How many Saturn V rockets have we launched in total?

Thirteen: the last was Skylab.

wouldn't it be better to plot a course where pitstops could be made at gas giants?

That actually sounds pretty achievable. Difficult, but all within the boundaries of known physics and existing human lifetimes.

Isn't 0.998c unrealistic?

> you had a near perfect hydrogen -> helium fusion engine, it'd take about 6 million tons of fuel (about the mass of the Pyramids of Egypt or 2,000 Saturn V rockets)

That is assuming constant energy consumption to sustain 1g, but unfortunately that's not the case, the closer you get to the speed of light, the more energy you'd need to sustain 1g, and this becomes unrealistic long before you are anywhere close to the speed of light.

Its probably more clear to say that a constant 1G acceleration in the local reference frame takes constant fuel, but produces a progressively smaller acceleration (asymptotically approaching zero as speed in the fixed frame approaches c) in the fixed reference frame you were launched in (or of the point you are trying to reach, the logic is equivalent) because relativity.

Your speed doesn't affect the laws of physics you experience.


Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact