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Water found on a potentially life-friendly alien planet (nationalgeographic.com)
605 points by Leace 6 days ago | hide | past | web | favorite | 382 comments

I got into the Wait Calculation[1] a few weeks back and made a Jupyter notebook out of it.

At our current max speed (692,000 km/hr, stated max speed of Parker Space Probe), it would take about 173,000 years to get to that planet.

We could instead choose to Wait and grow our tech. By picking a constant annual growth rate and then doing some calculus to find the minimum, we can calculate the shortest possible time it will take for us to arrive there.

One common recommendation for annual energy growth rate is 1.4%, and then taking the square root to get velocity growth rate since velocity has a square root relationship with energy.

By plugging that in, we can minimize our time by growing for 1020 years, and then traveling for 144 years, for a total time-from-now at 1164 years.

Another paper[2] estimated an annual velocity growth rate of 4.72%, quite a bit faster. Plugging that in, it says we should wait 195 years for a travel time of about 21 years, or 216 years overall. This is of course incorrect since it assumes being able to travel FTL. So if you instead look at how long it would take to get to light speed travel at that rate, you're looking about about 159 years, or arriving at the planet at a time-from-now of about 270 years.

Of course, if you're seeking to minimize time-from-now from the perspective of a traveler, maybe you'd take off sooner. Kind of a tradeoff - less time to wait for the traveler, more time to wait for the home planet. I haven't figured out that part of the math yet.

[1] https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1m... [2] https://arxiv.org/abs/1705.01481

Yeah. Well. That's a fun math game and not much more. Other view on "energy growth rate": https://dothemath.ucsd.edu/2011/07/galactic-scale-energy/

Energy growth is due to end sooner or later.

First, in the real world, any long term exponential growth is actually the upward branch of an S (that plateau at some point) or of a bell curve (that falls down after a peak).

Second, with such an energy growth rate we'll boil the ocean from heat dissipation long before we reach the stars.

There is also the problem that as you get away from the sun you receive less energy. Making your own portable mini sun with a fusion reactor is essential for interstellar travel.

Mostly I agree. But: we can keep our energy curve growing for a bit longer, if we branch out into the local space around us. No worries about boiling the ocean, when the energy stays in orbit.

Does that include slowing down and stopping, or would you be at max speed by the time you got there?

I always thought that was a very well thought out aspect of The Expanse: spend half the trip accelerating, then flip the ship around and spend the other half slowing down (aka accelerating in the opposite direction).

Disclaimer: Just an arm-chair Space Engineers player here.

> I always thought that was a very well thought out aspect of The Expanse: spend half the trip accelerating, then flip the ship around and spend the other half slowing down (aka accelerating in the opposite direction).

With current technology you would not do this, since that means using extra fuel that weighs a lot and thus increases the force required for the same amount of acceleration as you'd get with less fuel and less burn.

Of course that changes drastically if the fuel required for more acceleration (and its container) is very light-weight. I would assume fusion or fission based thrusters would be better in this regard, however I think currently those produce very little acceleration in a vacuum compared to combustion thrusters.

Any impulse / reaction engine is subject to the Tsiolkovsky rocket equation. It's just that with higher specific impulse and reactant velocity the total mass is reduced.

Since the rocket equation is a function of the effective velocity times the natural log of the wet/dry mass ratio, changes in exhaust velocity matter more than changes in propellant mass: you're better off with heavier propellant moving faster, than lighter propellant moving slower.

Ion drive engines tend to use xenon (a heavy nobel gas) for this reason. Unfortunately, xenon is rare even on Earth, and extraordinarily rare in space -- tanking up on a road trip would be difficult.

This completely dismisses any tangential factors, of course. Going earlier might be an advantage, as new ways to kill each other (or kill Terran ecosystems) emerge.

Going later to optimize for time is one aspect of deciding how to spread throughout the galaxy. Another question might ask, which is the most risk reducing strategy? And the answer might be completely different.

That's a lot of hand-waving. Reminds me of Drake equation. Why not just admit we have no clue yet? Also the Parker Solar Probe speed is due to orbital mechanics of basically free-falling towards the sun and you'd have to do things very differently if you wanted to have that speed going out of the solar system.

As a startup founder, I learned the hard way to not assume progress will ever be exponential.

As a guy who lives in a finite world where energy comes mainly from fossil fue in a way or another, I do not assume that any constant progress in energy supply is something achievable.

No, and traveling near lightspeed is likely hardly possible. However, fusion will likely become a reality within our lifetimes, and if we can build fusion powered spaceships, we could likely reach a fraction of the speed of light, say, 0.2c. I don't think it's that crazy to think we could do that a few centuries from now. That still means it would take over 1000 years to reach that planet.

Personally, I think that if humanity progresses that far technologically, without destroying itself, we probably won't care that much about habitable planets or planets with water. We'll just live in space and harvest the energy and resources that are available nearby. There are a lot of other planets much closer to us than K2-18b. We could just build bases in orbit around those stars, harvest asteroids for minerals and hydrogen for fusion power.

Alpha Centauri would "only" take 22 years to reach at 0.2c (plus some time for acceleration and deceleration). You could leave earth and live to see that solar system, and you could travel there on a ship that is basically a large city that will just park itself in orbit around the star on arrival, acquire resources, and start building something bigger.

Besides fusion I think hibernation will also become a reality within this century.

This will make the 22 year trip to Alpha Centauri more palatable.

1000 year trip in a single lifetime will require some life extension as well

Although it seems unlikely now, imagine if we could ever reach 0.9c or maybe even 99.9% of the speed of light.

Perhaps not ships containing humans, but some kind of other tiny ship containing a replicator machine and DNA for humans.

For 0.999c, it would only take around 71 days to reach Alpha Centauri from Earth (as seen from the ship). That's pretty mind boggling.

Besides the incredible amount of energy you would need for 0.999c, which is physically difficult (impossible?) to bring on a ship, there's other physical constraints that make this difficult, such as the fact that the vacuum is actually not that empty. The faster you go, the more you have small particles hitting your ship at near lightspeed, micrometeorites, etc.

As soon as you get fusion the game drastically changes. And fusion is only 30 years away.

Isn't it always 30 years away?

No. It's mostly a number of Euros away and we haven't been funding it very well.

Maybe we should update it to "always 30 billion Euros away".

Why do you think that no state put significant effort into fusion research? You would imagine the first country to figure it out would dominate the world for decades to come.

I don't think fusion power makes you dominate the world. It's not like building fusion plants is free.

Could you give some sources for that or explaing where exactly the problems in fusion technology lie and how it could be solved by throwing money at it?

ITER is planned to be completed in 2026 and it'll generate a net 0 of electricity at 300MW in/out.

It seems the issue with fusion is that it actually works much better at scale. But the same could be said about nuclear reactors - it obviously is much harder to build a compact one.

Does it work better at scale? The sun definitely produces a lot of energy in total. However, "the Sun's "power density" is "approximately 276.5 W/m3, a value that more nearly approximates that of reptile metabolism or a compost pile than of a thermonuclear bomb".

If, overlooking the details, we were able to build a cubic meter sized "fusion box" that outputs 276W, then it would take about 2 million of them to produce a reasonable output for a power plant of 600MW. And it would take up about half the volume of NASA's vehicle assembly building, ignoring any support structure needed. That's not unimaginably huge, but it's pretty large compared to other types of power plants, I would think.

So assuming we solve all the technical problems to capturing the power source of the stars using handwavium or perhaps generously donated alien widgets, I wonder if it might still be uneconomic.

This is already a few years old, but pretty informative:


Over any timeframe, the rate of progress between the beginning and ending of the timeframe can be phrased as exponential for some exponent. It's just a matter of accurately estimating what that exponent is.

I take your point though - hypergrowth cannot be sustained - and estimating that exponent is fairly valueless if volatility is too high. This is all just math fun that won't be valuable if we bump along for hundreds of years and then all of sudden make a massive discovery.

Thats true considering only two datapoints. It gets more interesting when more datapoints follow an exponential rule, say subsequent disruptions, no of transistors on processors or stock market indices.

As others have said, E=mv^2/2 only holds at very small fractions of the speed of light. I believe the equation you need is E=mc^2/sqrt(1-v^2/c^2)-mv^2.


Can you explain this further? It seems this is relevant more for something other than the Wait Calculation. The original Wait Calculation doesn't factor in mass at all. I'm having trouble understanding what this impacts. Is this more about relativistic time passage, like from the ship traveler versus the observer?

Most scenarios I've explored - low velocity growth rate, nearby planets/stars - the Wait Calculation tells you to stop researching and start launching long before your tech gets to significant fractions of light speed.

Honestly, I made that comment without fully thinking through how to implement my suggestion, and now I realize that I've stumped myself. However, let me explain what I meant anyway:

First of all, you're right that this won't really impact your answer at insignificant fractions of light speed. You mentioned that using the 4.72% growth rate, the equation tells you to wait until you've passed the speed of light, and I thought it might be interesting to more accurately model the energy required at relativistic speeds.

So the same way that you used the classical mechanics equation for kinetic energy, E=mv^2/2, ignoring mass and solving for v, to get v=sqrt(2E) and approximating to v=sqrt(E), I thought you could manipulate the relativistic equation similarly.

Now, having gotten a solution from WA, I'm starting to think that I overestimated the effect on accuracy that changing the equation would have. I want to approximate the solution by ignoring some terms or changing an nE to an E^2 or something, but I think that might negate any gain in accuracy.

So to answer your questions more directly, I was attempting to address the error you get when the calculation tells you to wait until you can travel above or near the speed of light, and I only included mass in my equation to try to communicate it to you more accurately, with the assumption that when actually using it you would ignore the mass.

Anyway, I hope I at least clarified my previous comment, even if it turned out not to be very useful! If anyone has a better understanding of how to better model relativistic speeds I'd love to hear their explanation.

Btw, if you measure the time as experienced by the traveller, the classic equation will give you exactly the right answer:

Pouring more energy into acceleration won't make you move faster (even subjectively) but it will shorten the way. (From the outside, it looks like time dilation.)

Progress in physics was much faster in the early 20th century when it wasn't so fascinated with deep space. Yes, you needed to observe eclipses to verify relativity, but that was a tool, not the end in itself.

Cosmology involves distances that are so great that it seems like it's pouring a whole bunch of smart people's efforts down the drain in an ultimately futile waste of brain power that will never amount to anything much at all. Besides, when we get faster than light travel we can just pick up exoplanet research where we left off and it will actually be practical and probably far more efficient with the computers and such we will have developed by then.

> Cosmology involves distances that are so great that it seems like it's pouring a whole bunch of smart people's efforts down the drain in an ultimately futile waste of brain power that will never amount to anything much at all.

I think it depends on what one wants to get out of the research. If the goal is a commercially realizable product on a shortish time horizon (say < 50 years), then cosmology may not be the best approach. But the justification for cosmology and much of astrophysics is typically that it is a probe of fundamental science and the acquisition of knowledge for its own sake. In which case whether we can ever travel to other planets or galaxies is moot, since it's the physical understanding and knowledge that's the goal.

Many before me have used the example of relativity, which when proposed, seemed to have little practical value. But GPS wouldn't work without relativistic corrections. 100-120 years ago one could've made a similar statement about fundamental physics and work on relativity. But if we'd abandoned it because of a lack of immediate relevance then we wouldn't have workable GPS today. The benefits of fundamental research (in many areas, not just astrophysics) are often quite difficult to forecast.

> when we get faster than light travel

FTL requires numerous things to be true about our universe, that all signs say are not.

The speed of light is just a compute constraint imposed on the physics simulation that is our universe. If we can just get the gods who are running that simulation to give us a little extra compute time in our neck of to woods then we can all be galactic explorers.

Just for the sake of argument, let's run with that.

If we're in a physics simulation, we don't experience at the rate that the simulation is processed. One "tick" of the simulation could be processed in one second of the host universe, or one hour: and we wouldn't feel the difference. We are processed at the same tickrate as the rest of the universe, so we experience the passage of time at the same rate that the simulation flows.

The speed of light isn't just the speed of light, it's the speed of causality. It just so happens that light moves pretty well at that "speed limit" (in a vacuum). We could ask our computational hosts to increase or decrease c, and we still wouldn't be able to travel any faster than it.

We could ask the host to update their algorithm to let c be a property of objects and then just do spaceship.c = 1000 * c.

Would probably be a heck of a refactor to get it working without bugs though.

Just leave the bugs in for the speedrunners to use.

speedrunners can restart the game if they crash it.

@mesid asked in a now dead comment:

> What does speed of causality mean? And how is light so close to that?

Take a look at this video by PBS Space Time on the topic:


You may need to go down the rabbit hole and watch the earlier videos on relativity and related topics.

The short story “Dust” (1992) by Greg Egan explores these ideas in a pretty entertaining way.

Here, I found a copy: https://emperybooks.com/wp-content/uploads/2018/06/Greg-Egan...

Why would time outside of the simulation have any bearing on time within the simulation?

Only via side-channel attacks.

Maybe there are glitches possible.

...crushing all my dreams of being a starship captain, thanks.

IMHO we will still get a space opera-esque future, just within the Solar system, not outside of it. There is plenty of space, energy and material around for potentially trillions of people on billions of space stations and space ships.

And we can still spread out from there. It is a well-thought out element of the Hyperion Cantos (occult SciFi): They do have some kind of Stargate to insta-travel, but building those Stargates requires people to travel for decades if not hundreds of years. These guys are put into stasis for the duration of the trip, and get to go home every couple of decades to talk to their grandchildren/distant descendants.

Leaving SciFi aside, our communication technology will be up to the task of relaying information comparatively quickly (light speed or faster), and parallel societies could be built in neighboring solar systems.

I am reading big space operas right now.

What we know of this reality is so limiting compared to what we can imagine.

Good dreams. Many of us have them.

I also like to dream the knowable remains much larger than the currently understood too.

Funny thing is that the imaginary worlds in books are way less weirder compared to what reality has to offer. Apparently because it's harder to develop the story.

Any book in particular you'd recommend?

"reality is so limiting compared to what we can imagine"

This is actually a central theme of the Zones of Thought novels by Vernon Vinge where a character in the far future describes our current time as the "Age of Failed Dreams" (GAI, nanotech etc.) - turns out that he is wrong but for rather neat reasons....



Anything in the Culture series by Iain M. Banks;

The Expanse series by James S. A. Corey;

The Commonwealth and Void trilogies by Peter F. Hamilton, and the Night’s Dawn trilogy also by Peter F. Hamilton;

The Dune saga by Frank Herbert.

I'll add the Ancillary (Imperial Radch) series by Ann Leckie to these good suggestions.

Hamilton’s The Reality Dysfunction was the stupidest book I’ve ever read and took hundreds of pages to expose itself. Are his others any more grounded (like your other suggestions)?

Yeah the Night’s dawn series was a bit out there compared to the genre average. The Commonwealth series is considerably closer to baseline, and his best books IMO. The void trilogy (and the newer trilogy whose name escapes me) follow on from the Commonwealth books, and while good, don’t hold up to the first three.

Thanks for the heads up! I liked his writing style so I’ll put them on the list.

did you read the same book as me.. it was pretty awesome

I'm a huge fan of Hamilton's books but I have to agree this one was.. different.

I think I liked it in the end, though not as much as his other books, but can absolutely understand people would dislike it.

The reincarnation of Al Capone?

Foundation Trilogy is not really an Opera, but definetely up there with the big guys. Also an incredibly quick read.

Neal Asher's Polity Series as well.

It's the culture with more detailed worlds and villains.

Is Pandoras Star worth the effort?

Definitely. I still have some series to read, especially the dark tower from Stephen King which I already started, but I think I'll re-read the whole commonwealth saga in the near future.

Yep, the Commonwealth series got solid 4s out of 5s from me, which is very good — I very rarely give a book a 5.

The Baen Free CD has always been a great source of free sci fi books for me

> Progress in physics was much faster in the early 20th century when it wasn't so fascinated with deep space.

I don't think measuring progress is tivial enough to make that assumption. It's also unfair to blame a fascination with deep space for any slower progress, things are more complicated than that.

Getting humans onto anther planet asap is one of the most important things one can pursuit for humanity. The earth is a giant single point of failure

> when we get faster than light travel we can just pick up exoplanet research where we left off

if we get FTL, we'll be able to pick up that research before we left off ...

> it seems like it's pouring a whole bunch of smart people's efforts down the drain in an ultimately futile waste of brain power that will never amount to anything much at all.

Well, at least they're not making people click ads ...

we might some day discover that we are roughly coaxially located between 2 civilizations exchanging knowledge, and get up to speed with their knowledge.

or observe civilizations broadcasting knowledge in a loop: motivation? perhaps the faster they can get others up to speed, the faster others might contribute knowledge back which may some day save their civilization.

Why don't you stick to the energy growth rate (instead of the 4.72% velocity growth rate) and then use the relativistic formula of the kinetic energy (in which the relationship between energy and velocity is not quadratic — the quadratic approximation is valid only for small velocities)?

Why is annual energy growth rate or industrial growth rate assumed to correlate to the speed of space travel?

Just because we build more power plants or sell more tractors doesn't imply a corresponding improvement in spacecraft technology.

The Wait Calculation seems to make even less sense than the Drake Equation - which is at least correct in theory even if the actual variables have so much uncertainty it is useless.

> taking the square root to get velocity growth rate

This is your mistake. It's an approximation that works well only at low speeds.

So our only hope to see a planet different from ours is that some advanced alien population comes to "see" us, with all the implications this has...

History does not favour the civilization which is visited.

Our history doesn't.

Given what we see in non-human nature, the tendency of visitees to get rekt by visitors is much, much more likely due to the dynamics of competition for life than to some human-specific variable.

> One common recommendation for annual energy growth rate is 1.4%

Where does this come from? Does it even hold water when compared to past data? Looking at some Wikipedia data for the past 20-30 years, it seems that the increase in speeds is much higher:


Does this take into consideration that your mass increases as you travel closer to the speed of light?

No, I'm not sure what that would suggest - how would that need to be accounted for? Maybe some fraction of light speed would need to be the upper bound, low enough that mass increase wouldn't have a material impact? I'm having trouble understanding what impact the mass increase would have in the first place.

It's a cool project! Just as others say, once you get to a notable fraction of the speed of light, you need to account for relativity.

does this take into consideration that you would need to start slowing down halfway there?

It wouldn't need to be halfway. At 3gs acceleration, it would take about 115 days to accelerate up to the speed of light. You'd then travel ballistically for the majority of the journey before having to decelerate for another 115 days.

Given the total journey would be ~40,000 days the 230 days of acceleration probably isn't going to impact things too much. Even if you brought it back to 1g acceleration, it's still only around 700days out of 40,000.

I just read somewhere in this thread that 'ship-time' at the speed up time is zero. So that would mean 230 consecutive days of acceleration.

No, it assumes you start and end at whatever that maximum velocity is that we've been able to achieve.

That'd be fun to add, though. I'm not really sure what human-safe acceleration is - people here assume it's in the 1g-3g range. (That seems like a lot to me though, particularly for a long period of time - I think anything more than a fraction of g would be wildly uncomfortable.)

1G would be great as it would remove any unexpected physiological impacts of living in zero or low gravity.

Having said that, the distance and duration means that only a small portion of this journey would be under acceleration, the rest would be in zero g.

Spending a century in zero g would probably have a bunch of strange side effects and almost certainly mean that the people alive when they arrived wouldn't be able to stand the planets gravity.

This exact topic is what about 80% of The Expanse is about.

Not quite true. You'd spend half of the travel time under 1g accelerating, then the ship does a 180 degree flip in 0g, and then you spend another half of the travel time under 1g decelerating.

Edit: Of course, at constant 1g acceleration, at day number 354 we reach 1c. Don't know if we should start coasting before we reach 1c or just the hell with it and see what happens if we keep accelerating.

So many people in this thread are acting like constant acceleration is possible. Unfortunately the faster you get, the more energy it takes to maintain constant acceleration.

> Don't know if we should start coasting before we reach 1c...

Don't worry about it. Your rocket would need to expend an infinite amount of energy to accelerate to c. The energy stored in your finite-sized rocket is finite. At a certain point it becomes futile to keep trying to accelerate -- though I suspect that the impacts of space dust will eventually cause a significant amount of drag


"Unfortunately the faster you get, the more energy it takes to maintain constant acceleration."

I'm not sure what you mean by this. You could also say "the slower you get, the more energy it takes to maintain constant acceleration". Constant means ongoing, so as long as it is constant, you are going to have to expend more energy.

Unless you mean that somehow, you could determine your absolute speed by how much you accelerate for a given expenditure of energy, but wouldn't that violate relativity?

Can you alternate accelerating and decelerating? I mean that's a heck of a lot more energy, but at least you're not risking reaching light speeds.

Wouldn't 1g be perfect? I've been enjoying it for a few years now on this planet.

I took it as 1g of acceleration. I don't know. Wouldn't plain old 1g mean no acceleration at all?

Edit: Oh... I guess not. :) I got confused with the extra g's we'd need to escape earth, but that's so short-term that it can be ignored. Yeah, I think 1g would be perfect.

When you sit in a car and it accelerates forward you can feel your back being pushed into the seat. When you lie in bed gravity will also push you into the bed. In both cases it's a force acting on your body. So being pushed in space in a ship accelerating at 1g will feel like normal gravity with the 'ground' being the back of the ship.

Some caveats per Marina Koren https://twitter.com/marinakoren/status/1171873631808438273

1. This planet probably doesn’t have a solid surface

2. Being in the “habitable zone” doesn’t mean a planet is habitable

3. The detection is of water molecules, chances are the water only exists as vapour in the atmosphere of this small gas giant.

4. If you ask 10 astronomers about this you will get 11 opinions

Years ago as a second year computer science student, I mentioned to a grad student I knew how disappointed I was about a certain algorithm. "It's O(n), but the constant is enormous! There's no point unless you have billions of elements"!

She said to me "Yeah sure, but who cares? What it really tells us it's that it's possible to do this in linear time at all. That might not have been true, and now we know we might find a faster linear algorithm."

(All of this is paraphrased because it's been over a decade).

The point is: we have found another planet with water. We now know this is possible! We know that it's probably not super uncommon (or else we wouldn't have found one so soon). That's what's amazing about this.

So what if it's not perfect, it's a great discovery!

We already knew it's possible to have a planet in the habitable zone with water (and life, incidentally).

Whether it's common is still unclear, given that so far there's a big bias towards detecting large exoplanets that are close to the host star.

The point is finding planets aside from our own.

Sometimes N=2 is extremely different from N=1. Specifically when we have N=1 and we suspect that it might be unique, or extremely rare to the point that you wouldn't expect to find instances.

Sometimes N=2 is not that different from N=1 though. Specifically when you don't think the situation is necessarily that rare, you just don't have a big population to choose from. That's the situation here.

Astronomers expect to find other Earth-like planets. We suspect rocky planets aren't that rare, we know planets at comparable distances from the star are not that rare, and we know H20 is plentiful in space. The problem is one of detection. The smaller the planet, the harder it is to detect water, so most of what we've found so far is gassy giants.

A good analogy is the fact that I don't know anyone who shares my birthday. That doesn't mean I would be amazed if I were to find someone did share my birthday, cuz I have no reason to suspect my birthday is particularly rare. I just don't know that many people.

In this case N=2 is very different from N=1. The probability of detecting further Earth-like planets is not influenced by the existence of Earth, because what we're actually measuring is the conditional probability of detecting Earth-like planets given the existence of Earth, which already factors Earth in. In this case N=1 is a lot more like N=0 and N=2 is a lot more like N=1.

That's silly. The type of world we're interested in is ones that could support life as we know it. We knew one of those exists. We're trying to find more.

Calling it silly seems a bit harsh, it's just statistics. We are interested in knowing the frequency of earth-like planets existing. If we assume that the existence of earth is a pre-condition for life existing and performing the measurement, then the existence of earth tells us nothing about that frequency. Regardless of the frequency in question the count of earth-like planets starts at 1, because in scenarios where the count is at 0 the measurement is not being performed. There could be millions of earth-like planets in the milky way, or there could be only one, or earth could even be the only life-supporting planet in the universe. There is no mechanism given only the existence of earth to reason statistically about the distribution of earth-like planets.

Whereas finding a second one tells us a lot. So that's why I say we are in more like the N=0 case, and that finding a second earth-like planet puts us in the N=1 case.

Just as an aside, Facebook is useful to demonstrate the Birthday Paradox. Even if you don't know that many people.

N=2 means that we now know how to detect more than the one we are standing on, so it says a lot about how many we might expect to find soon.

> Being in the “habitable zone” doesn’t mean a planet is habitable

True. Being a spooky near twin of Earth isn't necessarily enough.

The air inside popcorn factories is basically identical to the Earth's atmosphere, but breathing in diacetyl, a butter flavor you'll find in that air, will kill you a few months or years later.

Popcorn lung really undermined my naive view that we would one day be able to run a scan or a sniff test on a planet and then just breathe without assistance.


You have an identical twin to Earth with slightly different dust composition and it could shred our lungs. We are highly, highly tuned to our home.

Which doesn't mean we can't go anywhere. But it won't just be advances in travel speed that will get us off world. It may also require drastic modifications to the human organism.

That'd be something, people travel for 30 years to a distant planet, die four days later because something in the atmosphere we didn't account for and did not filter properly, or a crystalline fungus that eats away our suits.

Happens a lot in (bad) science fiction, and parody thereof.

People land on a planet, go "I wonder if the atmosphere is breathable?", then open their helmet and take a deep breath.

But I think by the time anyone is arriving on a planet around another star, it will be with biotechnology that allows adaptation to a rather wide variety of environments.

> 2. Being in the “habitable zone” doesn’t mean a planet is habitable

Related question:

Does "habitable" mean "habitable for humans"? I thought it didn't, but after reading some dictionary definitions, i believe it does. On the other hand looking at the planetary habitability wikipedia page it's clear that it means "life-friendly". No wonder many people are confused.

Of course a planet with 8-9 times the mass of earth is not very friendly for humans, ignoring all the other possible issues (pressure, chemical composition, radiation, flares etc)

People have commented that a planet a bit larger than earth (I think 8-9 times qualifies) would have gravity too strong for rockets to ever be practical, and thus whether you were a marooned earthling or evolved there, you would never ever be able to get into orbit.

So, there could be "super-earths" with life similar to ours, even intelligent, and they would never be able to participate in a space-faring society.

Never? What if you build a spaceline moon elevator on their moon and dangle it into their gravity well to assist them...

The “habitable zone” is the theoretical range from the Star that allows an Earth-like planet to have mean temperatures suitable for Earth-like life.

It’s almost entirely arbitrary, but roughly based on luminosity of the star.

Also it just seems to be water, they’re obviously not 100% sure it’s not something else that’s reading as water.

How easy is it to have different mixtures of substances that lead to similar enough absorption apectra that they are indistinguishable with the available resolution and sensitivity?

There’s no reason I can see to assume we’ve discovered even 1% of the elements out there... we’re also just seeing a trace of something that was there hundreds of years ago right? We’ve never even gone out there to see if our assumption that only water cause this kind of reflected light is even correct.

Not very.

I think the keyword here is "potentially habitable"

It isn't habitable even if it's a super Earth because the gravity would be 8g if it's 8x the mass of Earth.

That's not how it works. The moon is 1% of Earth's mass, but has 16% of our gravity.

I'm not entirely clear on the deciding factors, but it's probably mostly density.

It depends on the mass of the planet and the diameter. Since planets are usually sphere shaped, it effectively means that the gravity grows with diameter (for same density) and vice versa (for same diameter, grows if density grows). Moon is about 4 times smaller (for same density, gravity would be about 4 times smaller) but Moon has lower density as well (about 60% of Earth) so it comes down to those 16%.

Rough napkin formulas below. Apologies for formatting)

F=ma F(g) = GMm/(r^2) ma=GMm/(r^2) a=GM/r^2

Volume of a sphere = (4/3)pir^3 mass of a planet = density volume M=ro(4/3)pir^3

a=(Gro(4/3)pir^3)/r^2 a=Gro(4/3)pi*r

given that G, 4/3 and pi are constants, it comes just to density multiplied with diameter.

Higher density allows the surface to be closer to the center of mass, so the gravity there will be higher. Stand on a hovering platform 6300km from the Moons center and the gravity will be just 1% of Earths, since distance beeing equal, the only deciding factor will be the mass.

That's why the gravity on the surface of neutron stars is so high, even when the mass is the same as our Sun.

The earth doesn't have uniform density at all depths - not sure about the moon. I forget exactly, but I was just reading about how if you could travel to the earth's core, because the density increases a lot, you would continue to experience around 1G something like halfway to the center.

I am amused by the idea of a "small giant".

Neptune is ‘only’ 57 times the volume of Earth, whereas Jupiter is 1321 times the volume of Earth. The size range of ice/gas giants is bigger than the gap between Earth and the smallest ice giants.

Does it make sense to compare both Neptune and Jupiter to the Earth? I would think the relative numbers for each pair are a more useful comparison.

If Neptune is 57 times the volume of Earth and Jupiter is 1321 times the volume of Earth, then Jupiter is 23 times the volume of Neptune.

So the relative difference between Neptune and Jupiter is less than half the relative difference between Earth and Neptune.

As another illustration, let's take a different trio of objects: a golf ball, the Moon, and the Earth.

Using very round numbers, it is safe to say that the volume of the Moon is about one zillion golf balls.

The Earth is about 50 times the volume of the Moon, so the Earth is 50 zillion golf balls.

Clearly, the difference between 50 zillion and 1 zillion is much greater than the difference between 1 zillion and just 1 golf ball, by a factor of nearly 50.

Or is it? If you ask anyone on the street, they won't have to calculate anything, they will tell you that the Earth and Moon are much more similar in size than the Moon and a golf ball. That's because they are using relative sizes, not absolute.

With a planet this large, visiting it would be a one way trip due to the "The Tyranny of the Rocket Equation" [0]. I'm looking forward to the day we start finding exo-planets that are closer to Earth in size and which could potentially have space-faring races (and which we could leave if we were ever to visit them).

[0] - https://www.nasa.gov/mission_pages/station/expeditions/exped...

It's 111 light years away. For all intents and purposes, that's a one way trip right there; irregardless of the rocket equation. Even at relativistic speeds, you can't come home anymore.

Theoretically, you can. At 0.99c 111 light years take about 15 years of ship time to travel.

You can make the trip arbitrarily small from your perspective (in theory at least :) ). The issue is that by the time you've made it back home, it's 222 years later. Hence "you can't go home anymore". :)

If you put some money in an interest-bearing account you could at least buy a new home when you got back.

> If you put some money in an interest-bearing account you could at least buy a new home when you got back.

If you do it in a country that doesn't allow you to be declared legally dead when you are out of contact for a couple centuries, and if the institutions involved don't collapse, and if the interest on the account outpaces inflation, sure.

What percentage of the banks that were around 222 years ago have not failed?

From [0] we can see that there are 10 banks founded before 1700 and 24 from 1700 to 1800.

So it might not be that far fetched, assuming that the banks keep your accounts open after more than 100 years without use.

[0] https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

10 banks out of how many founded in 300 years?

I don't argue that it's probably a tiny fraction.

I was just pointing out that it might be somewhat possible for banks to exist for a span of time big enough for this.

As pointed out by other commenters: a non zero number have survived. A small investment made with many different institutions would substantially increase your chances of a massive return upon... return.

That's not the right question, though, because the expected rate of failure isn't constant throughout the lifetime of a bank. See, e.g., https://en.wikipedia.org/wiki/Lindy_effect

Seems like your best bet would be to park your money with several very old banks: https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

Also might want to think about purchasing bonds: https://en.wikipedia.org/wiki/List_of_oldest_banks_in_contin...

2nd link was supposed to be https://en.wikipedia.org/wiki/Perpetual_bond with information about a bond from 1648 with the issuer still paying annual interest.

Bitcoin! :)

What if they release Bitcoin 2.0 the year you leave?

Probably tiny?

Lloyds bank founded 1765

Barclays founded 1736

I doubt any banks in the new world are as old.

Berenberg Bank founded 1590 (429 years ago)

Monte dei Paschi di Siena 1472 (547 years ago)

So.... All western European then?

You know where I can find one of these interest-bearing accounts that can outpace housing cost inflation?

While not an interest bearing account, investing in an index fund of global stocks should outpace housing. Investing in a REIT should roughly keep pace with housing costs.

Better do it in different countries and in different currencies and hope none of them get confiscated or have hyperinflation.

Not if you have negative interest rates. I heard on the radio that some European banks want to introduce this.

Or maybe a meal at a nice restaurant?

Wait, is this correct? Doesn't 111 light-years mean even light takes 111 years to travel through? And light travels at 1c.

Light does not experience time at light speed - it is stuck in a static moment. So from our stationary perspective, light takes 111 years to come from a 111 light year distance, but from light's perspective it takes an instant.

Every time I think I’m closer to understanding matters of time and space...

Here's a quick analogy to help (loosely adapted from Brian Greene):

You're on a grass field, sitting on one of those riding lawnmower thingies, with a broken throttle. It's moving at a fixed speed of 1mph. You can't ever change its velocity. But you can steer it. If you are going precisely east-west, then it means you're not going north-south. The more you go north-south, the less you'll be going east-west. If you're going precisely north-south, it means you're not going east-west at all. One direction is traded against the other.

Pretty straightforward, right?

So here's the analogy: that grass field is a "dimension" in the same way that "spacetime" is a "dimension". The two "directions" of spacetime aren't "east-west" and "north-south", but "space" and "time". These are inherently traded against each other. The more you're moving through one, the less you're moving through the other.

So what about that constant-velocity rideable lawnmower? That's "c" -- the speed of light. You're always traveling at this velocity. If you are sitting still in space, then you are nonetheless moving through time. Your rate of movement through time is "c". But as soon as you start moving through space, it means you are moving less through time. This is exactly the same tradeoff as moving north-south vs. east-west. If you devote 100% of your "c" to moving in the direction of the "space" axis, then it means you're not moving on the "time" axis at all.

(This is basically all it means for something to be a "dimension": different axes that are traded against one another.)

This analogy can be used to understand quite precisely how movement relates to time dilation. (It also helped me understand e=mc^2. Why is "c" there? What does the speed of light have to do with the embodied energy of matter at rest? Answer: nothing is ever at rest; all static matter is moving through through time at the velocity of "c", and obviously that movement must have kinetic energy.) But it's not a completely perfect analogy. Weirder relativistic effects like length contraction and frame dragging need much weirder analogies.

I like this analogy, but I think an even bigger problem than distance contraction is that in fact the lawnmower can go east-west on one axis, but only towards the north on the other axis, towards the future, at least as far as we have observed in macroscopic systems.

This may not be a problem for some definitions of time, but for the notion of time which goes from past to future, I don't think the analogy holds very well.

Actually the analogy is a bit poorer than that, but also more consistent. You can only go one direction on either axis. Any movement in 3D space advances you along the "+space" axis, to the the detriment of your default movement along the "+time" axis. But just as there's no "-time" axis, there's also no "-space" axis.

Let me put it another way: from any point in space, we know you can move to any other point in space, in a finite amount of time (disregarding the accelerating expansion of the universe).

As far as we have observed, the same is not true with (the common-language definition of) time - I can't go back to the moment I was born, for example.

I think your comment about disregarding the accelerating expansion of the universe is a key point. We can't actually move anywhere in space; reacheable space is constrained by our reference frame relative to the reference frame of some other part of the universe that we're accelerating away from. At a certain point, the parts of the universe become unreacheable, because we would have to accelerate faster than the speed of light in order to reach them.

If my understanding is correct, this same condition exists inside the photon limit of a black hole. Technically you're still in navigable space -- not inside the singularity yet -- and can move in any direction. But to actually escape the black hole would require accelerating faster than the speed of light.

Again, if my understanding is correct -- and I'm definitely not a phycisist by any stretch of the imagination -- our movement through time is exactly the same phenomena. We can slow our velocity through time (by moving through space instead), but we can't escape our local reference frame without moving faster than the speed of light. If we could exceed the speed of light, then we would be moving into spatial regions which are otherwise causally inaccessible to us; in other words, we'd be going backwards in time.

So: if we could go FTL, we could escape from black holes, visit parts of the universe beyond the locally-observable limit, and go backwards in time. I think (IANAP) that these are all describing precisely the same thing.

Dunno if this helps. The lawnmower analogy has definitely broken down by this point.

Also, it makes me think: if our experience of time is navigationally equivalent to the experience of space for someone getting sucked into a black hole, does that imply the existence of a higher-dimensional universe where ordinary, non-accelerating time is as fully navigable as our ordinary "non-accelerating" space? And in that higher-dimension universe, are we living near the surface of some kind of singularity? Do the inhabitants of that universe wonder about how sad it must be for poor creatures like us, forced to live on a time gradient which inexorably slopes in just one direction, the way we might commiserate the fate of those sucked into a black hole?

So, actually, like, it's actually really intuitive I feel like:

From the perspective of an object accelerating, newtonian physics works totally intuitively. If you had a rocket that could accelerate at 1g indefinitely, you just go faster and faster and faster and you get to any destination you want (even far away!) pretty quickly. And it would be a rather comfortable trip! You'd have Earth-like gravity the whole way.

It's really only the observer's perspective that things get confusing. When an observer watches something accelerate, they see it never going faster than the speed of light, no matter how fast it "actually" goes.

The trick is time. Time for slow things passes faster than time for fast things. A clock on a very fast rocket ticks much more slowly than clocks on (relatively) stationary things. That's how the paradox is solved.

Let's say you wanted to visit the Andromeda galaxy, which is around 2,500,000 light years away. If you had a rocket that could travel at 1g indefinitely, you'd get there in a comfortable 29 years! However, observers on Earth would see the trip taking around 2,500,000 years.

If you'd like to play with these numbers yourself, feel free to check out this neat calculator (not made by me)


There is nothing intuitive about relativity unless you understand the physics and math behind it. Intuition can only be as good as your knowledge and experience.

Hey, look at this guy over here who's never been accelerated to a relativistic velocity here! But in all seriousness, you don't need to know all the math behind how relativity works to have a general idea of its effects.

Go watch a Carlo Rovelli lecture on time and loop quantum gravity.

Time might not even exist in and of itself.

Just view light as like being in line at the DMV.

Yeah, the experience of time to travel a distance approaches zero as your speed approaches the speed of light. It's really crazy to think about.

It is worth adding for the sake of clarity that light has no perspective or frame of reference because photons are non-inertial.

>but from light's perspective it takes an instant.

And for that reason they don't experience distance either. So the term 'sun-kissed' isn't actually that far off...from the photon's perspective the sun IS giving you a kiss.

Super fun. Great one. Actual replying to save this thread so I can go back and check some of this out. Looks neat.

i'm pretty sure the assertions means that the 111 years needed would only age the traveler by 15 years

It takes 111 light years from our reference point to travel that distance. If photons could perceive things, it would not take them 111 years by their perception.

accelerating at 1G to the mid point, then decelerating at 1g, will mean you’ll get there in about 15.8 ship years, or 115 earth years.

Spend twice as long on boats though (32 years) and you can do about 10,000 light years.

A constant acceleration of 1g would cause the ship to hit c in about one ship year.

The ship will never hit c.

If the ship travels tens of billions of light years will it eventually be traveling away from it's starting point at greater than c due to the dark-energy expansion of space itself? Or will the ship just red-shift more and more but never disappear from an observer with a very good telescope who stayed home?

And what if you drove your ship straight into a super-massive black hole?

One theory of the far future (Big Rip) is eventually nearby galaxies will be receeding faster then light, then stars, planets, then even atoms.

There's a short story about the Big Rip


So you can survive the trip but even if you come back home, it’s not home anymore? Imagine leaving 222 years ago and coming back now ...

When the French invade Fishguard, Wales in 1797 our hero Gruffyd Rhys Llewellyn goes to get help in the only place where help may be forthcoming. The Heavens!

I should make a kickstarter, but I am already in my jammies.

I'd like a pint of mead and a stout woman! /s

Many european cities would still exist and the mountains I grew up in front of too.

But granted, all people I know would be dead and some I could find in the history books maybe..

> and some I could find in the history books maybe..

Where you would also be, at that stage.

I would recommend reading Noumenon by Marina J. Lostetter if you'd like to scratch that theoretical itch playing out one specific way.

I mean that sounds pretty awesome if you invested your funds well. I'd definitely be willing to be the first person who did that.

Where can you invest that would still recognize those funds as yours after 222 years?

You'd have to form some kind of trust, the "Swizec is in space for 200 years trust" to handle your investments for you with a constitution that lays out how long it is expected for you to be gone.

You might come back to discover that after several generations without oversight your trust has invested in bombing children and enriching the fund managers. I dunno.

You still get in trouble I think. Have any political regimes been stable enough in the past 222 yaers for the trust to survive?

All of Europe has gone through multiple revolutions, USA was only 30 years old back then so you wouldn’t have considered it ... that leaves what, the UK? Any parts of Asia that survived since 1797?

With travel time that long , you have to consider technological advancement at home. Just imagine that you look out of your window after travelling for 70 years and there is this other ship overtaking yours with ease.

Going down in history as "First to leave, last to arrive". Brutal

The colony of New South Wales was founded in 1788 and there is a fairly direct lineage of laws from there to modern-day Australia, and going backwards also a direct lineage of laws in Britain.

Nothing is risk-free, diversify! The likeliest outcome aside from coming back to nothing is you come back to an institution that is nothing like what it started.

It's been done actually. Ben Franklin established a trust that lasted over two hundred years by design.


I admit I have some bias here being British, and just assuming the banking system won’t go under. I think if I was actually going to execute on this I’d end up finding a bunch of countries with well-established trust law and spread it between them.

And how much time will it take a person to reach that speed at human-safe acceleration? And how much energy would it take?

It will take longer that people here have calculated. As you approach c the energy required to accelerate quickly rises. To reach c you need infinite energy.

Maintaining 1g of acceleration for a useful amount of time would require an extraordinary amount of propellant.

All of those estimates are tongue-in-cheek and are accurate if your energy expenditure is actually unlimited.

Just under a year at 1g (according to google. I'm on mobile and would have to derive from F = ma on paper to solve). Energy is variable. I'd assume energy isn't relevant, as it's currently impossible, so we're taking that for granted.

Not really that long.

Not a physicist, but I think it might not be that simple. From our point of view, a ship with constant 1g acceleration increases its speed by 9.8 m/s every second, but people on a ship moving at a significant fraction of c take more than one of our seconds to experience one of their seconds. During the time they experience one of their seconds, the speed increases more than 9.8 m/s, so they must experience greater than 1g acceleration. I haven't done the math but it would be interesting to work out how the acceleration in our reference frame needs to reduce to ensure constant acceleration in the ship's reference frame, and how long it would really take to reach .99c without squashing the passengers.

Err, that isn't a correct understanding of physics, I believe. The ship wouldn't need to reduce its output as it gets faster relative to some other object in the universe. It can happily keep accelerating at 9.8 m/s for as long as it likes (or has fuel). No passengers would get squashed.

If the ship's means of propulsion is set to impart a constant force, then I understand that the passengers could experience a constant acceleration indefinitely, but that constant acceleration would be with respect to the ship's reference frame, not ours. A constant acceleration of 1g in our reference frame would mean by definition that the speed reaches 2c in two years, which can't be right. Subject again to the disclaimer that I'm not a physicist and haven't done the math, the only outcome I can picture is that in our reference frame the speed asymptotically approaches c in some interesting way.

Oh I see! 1G from our reference frame! Fascinating! That’s a perspective I haven’t seen before, I didn’t realize that that’s what you meant. Indeed, it isn’t possible. We’d see the ship slowly approach but never reach the speed of light, no matter how hard or long they step on the accelerator.

Isn't a second now officially defined as a single period of atomic decay of a certain atom (I think Cessium?)? So this theoretical near-lightspeed craft could adjust it's accelerators based on an onboard atomic clock that would not be based on seconds as we tend to perceive them, but on SI seconds. I assume these would be getting longer as velocity approached 1c. Unless radioactive decay is also affected by relativity (obviously I am not a scientist), in which case of course that wouldn't work.

One of the consequences of the theory of relativity is that there is no "absolute time". The atomic clock will perceive time just as you will–that is, slower than an inertial reference frame.

Length contraction is something outside observers would see; the people on the ship would see themselves as normal (and hence not be "squashed".)

This paper has some interesting ideas. They want to manufacture really tiny black holes (radii of "a few attometers") and use Hawking radiation (which is inversely related to mass) to drive the vehicle. The black hole serves as both engine and fuel tank.


I calculate it to take about a year, with 1 g acceleration, but don't take my word for it, am curious to compare with a more authoritative answer.

Which would put you in your 50s to 60s if all you did was go there and back.

Heck, I managed that without interstellar travel.

And? Seems unlikely that we would send a small number of people on a 30 year trip. Given 30 years in a tin can with little to do, I'd expect the age range on departure to be ~18+ (with some people knowingly making a one way trip), and the age range on arrival back at Earth to be 0+, due to new additions to the crew mid-flight.

We're talking about just escaping the surface gravity, not returning to Earth.

If you can get there from 111 light-years away, you can probably also solve the higher surface gravity. While the former problem is theoretically solvable in a straightforward way, both are impractical with our current understanding of physics and material science.

But that's assuming you're traveling at the speed of light, right? In reality it would take much much longer than 111 years.

The fastest man-made object: https://en.wikipedia.org/wiki/Parker_Solar_Probe

Travelling at that speed (0,00064c - no relativistic effects) would take ~170,000 years.

The fastest spacecraft humans have produced is the Parker Solar Probe which when it zips around the sun will reach 430,000 mph. At that speed it would take 173,000 years to reach this planet.

It is also the slowest spacecraft. Speed is relative. To get it going that 'fast' when close to the sun the other half of its orbit is correspondingly slower than earth. When launched it accelerated away from earth at speed, but from the perspective of the sun is was actually decelerating, and was moving much slower that it was when attached to the earth.

Great! , do that gravity assist a few more hundred times around the sun and we are there in 500 years.

You can't do a gravity assist around the sun once you've reached solar escape velocity, because you won't be visiting the sun again.

If we could pull that trick so easily, we already would have.

Well... you could wrap around another star, or two, and come back to this sun for another assist a la Rendezvous with Rama.

At 111 light-years, we need to create a vehicle that can sustain several generations.


> I'm looking forward to the day we start finding exo-planets that are closer to Earth in size and which could potentially have space-faring races (and which we could leave if we were ever to visit them).

I just realized I have no real concept of how many stars there even are within, say, a 100 light year radius of our sun (I guess that's a more realistic thing to find out than the number of planets).

A quick search provided some estimates and they're kinda... disappointingly low, at around 20000 stars. That's a number where some "1% of 1% of 1%" kinda filter quickly ends up in a scenario where a planet fitting all our criteria might simply never be in reach. For something more "realistic" (I know, heh!) like 20 light years, there are only 150 solar systems. I've seen different numbers and have no idea how they're calculated but for the usual astronomic scales which quickly go into "billions" territory, it seems we're kinda stuck with a comparably small list of candidates.

> That's a number where some "1% of 1% of 1%" kinda filter quickly ends up in a scenario where a planet fitting all our criteria might simply never be in reach.

It might cheer you up to think that's the only reason the human race happens to be the one in our neighborhood that made it into space, without being stepped on by an Old One.

I think people are beginning to realize, that just as there are more asteroids than planets, and more space junk than large asteroids, there are a lot more free floating planets than stars.

It could well be the universe is filled with life, but the dominant mode is underground chemo/radiotrophic microbes on planets without stars.

On the plus side there are more moons than planets and many of them may be habitable. We've barely started looking at extra solar Jupiter like planets because of the much longer orbital periods.

It might cheer you up to to think we have yet to prove there is a star with no planets.

I made it well into this decade before I was made aware of this fact. It's kind of a shock, still. At some point your planet is massive enough that you can't get into orbit with chemical rockets (even, I think, by flying them up like Burt Rutan?).

The implications for the Drake equation are pretty big.

Rockets without any promise of ever being able to break orbit are good for what, war? Would you keep developing them? Would you give up dreams of the stars? Would you look for intelligent life you couldn't ever possibly meet?

"At some point your planet is massive enough that you can't get into orbit with chemical rockets (even, I think, by flying them up like Burt Rutan?)."

Nuclear rockets don't seem to be very hard. They're somewhat dangerous if they explode, but they aren't very hard. Fairly solid prototypes were built decades ago and there's little to suggest they couldn't have been made production-grade [1]. We'd have them now if we didn't find the risk/reward to be too highly slanted to the "risk". Other species and other ecosystems may come to different conclusions, e.g., an ecosystem already more exposed to radiation and evolved to deal with much higher levels of it may judge it much less "risk" for some radionuclides to be scattered across the landscape in case of failure.

What can be more of a problem is being in a place where you have no obvious access to technology at all. However smart our cetacean buddies may be, it is not clear even at this point in the 21st century what path to technology they could possibly have from their starting point. "The literature", a.k.a. "science fiction" has hypothesized breeding programs to develop various tools, but it's still not entirely clear how they'd get from "breeding useful jellyfish" to, well, anything like technology as we know it. It's possible we're just not solving this problem because we don't have to, maybe there's some easy path with the right development path, but it's still not clear what that would be.

[1]: One of my markers for "the space age is truly here" is when we lift a nuclear rocket into space, sans fuel, and fuel it with space-sourced radionuclides. Earth-bound citizens will still complain, because "NUCLEAR BAD!", but their complaints will be ignorable at that point.

Do solids settle out of air on a high gravity environment faster? The air would be thicker. Does gravity or buoyancy win than tug of war?

I spent a day once trying to figure out what the Bronze Age would be like for marine creatures. Oxidation is less of a problem but galvanic action is huge. Fire pretty much doesn't work, which blocks a whole bunch of precursors like ceramics.

Atmospheric density it a little more complicated: Venus has the same gravity as Earth but a thicker atmosphere, while Titan is a lot smaller and still has a surface pressure 50% more than we do at sea level.

Couldn't marine creatures learn to collect air bubbles underwater in order to use fire?

How do they know that will lead to fire?

The big problem with marine technology isn't that it's totally impossible, it's that there's vast gulfs between various achievements and little sign that continued progress on some matter will lead somewhere. You could raise jellyfish to be transparent and lens shaped and build some telescopes, but how do you figure out that's a thing that might be a good idea? You might be able to turn an ocean vent into a forge, but how do you figure out that's a good idea? We had a path where we noticed certain rocks in a fire ooze useful metal, for instance. We didn't deduce from first principles the Periodic Table, guess the properties of metals from logic and maybe our interactions with (very soft!) silver and gold, determine it was likely that some of those colorful rocks are metallic salts, and then determine they might be useful to mine. We found they were useful to mine, then after thousands of years of civilization built on top of the resulting tools, only then figured out the why of a lot of those things.

This is one of those places where it's really a good idea to understand that despite the pretty Just So stories where science pre-dates engineering, in reality, engineering extremely frequently has predated science, at times by centuries. How are water-bound creatures going to figure out enough engineering to even get science going?

Certainly, as I said, they can breed things, but how do they even know where to try to go? How do they maintain the discipline to breed things over hundreds or thousands of generations? How do they get to genetic engineering?

There may be answers to this question but they sure aren't obvious.

So long as you're carrying all the fuel on you, right? If I can launch 1000 auxiliaries that resupply you and a 1,000,000 auxiliaries to resupply the 1000 auxiliaries I can go up farther.

So that's purely for a chemical rocket, right?

Is there any combination of tricks that can realistically push the envelope there? For example can we use a space elevator to start higher/faster (or, I don't know, balloons? a catapult or railgun or something?), laser power delivery from the ground, so we don't have to carry all the fuel, and an orbiting way-station for refueling, etc.?


From the reference article:

> Travelling from the surface of Earth to Earth orbit is one of the most energy intensive steps of going anywhere else. This first step, about 400 kilometers away from Earth, requires half of the total energy needed to go to the surface of Mars.

Which means that if we use something like a balloon/blimp in the first stage, it would be a lot more energy efficient.

Anyone knows why it's not done that way already?

Also, whatever happened with the plane+rocket Virgin Galactic project?

The Pegasus rocket used to launch satellites from a modified 747 for between 1990 and 2016 [1]. It just doesn't work out well in practice because the cost of the modified plane far exceeds the cost savings from a first stage unless you're doing half a dozen or more launches per year.

[1] https://en.wikipedia.org/wiki/Pegasus_(rocket)

Virgin Orbital are doing something similar - satellite launching from a 747.

most of the energy spent getting to that 400km low earth orbit is spent accelerating to very very fast lateral speeds. If you use a balloon to get your rocket up as high as you can before launching it you limit the weight of the rocket you can lift up there, and most of the weight of rockets is used to gain that lateral speed. The plane + rocket idea has been shelved for now, as reusable rockets have decreased the price much more than launching a small rocket from 40,000 feet.

Isn’t that orthogonal to interplanetary travel though?

If you go straight up far enough you’ll be out of Earth’s gravity well.

You’ll still need acceleration to escape the solar gravity well, but you’ll never need horizontal acceleration necessary for Earth orbit.

You want to accelerate lower down in the gravity well, however not too low due to atmosphere losses. It’s more efficient that way.

Launching from stationary altitude doesn’t save much at all

>If you go straight up far enough you’ll be out of Earth’s gravity well.

While you could theoretically do this by accelerating directly up, You still have to accelerate somehow.

Understood, but somehow the discussion ended up around achieving orbital velocity instead of escape velocity (the one relevant to the original sub-thread topic), two very different numbers (though escape velocity is ultimately a higher number.)


Sure, but they're not orthogonal at all. One is just a lot more delta-v than the other. It's the same basic thing. Point the rocket nozzle in the opposite direction that you want to go and turn it on (modulo the fact that the "direction you want to go" might not be intuitive because orbital mechanics).

Orbit isn't a height, it's a speed. 7 km/sec for low earth orbit. A balloon floating you higher while not making mach 20 isn't much of a shortcut.

How poor of a shortcut is it? I'm not really clear on what the math would look like. Would it just be better by approximately the ratio of gravity's strength at the surface vs the height of the balloon?

Better than the miniscule gain in potential energy (height x mass x gravity) because you also save a lot of aerodynamic drag not punching through the lower atmosphere at high speed and some "hovering losses" (rise time x mass x gravity, I'm sure there is a better term for that). But still not worthwhile due to the difficulty of floating a full size rocket. If you had a very very high mountain or tower, moving that full size rocket to the peak could be worthwhile. Theoretically, a civilisation trapped in a gravity well too deep for a solution to the rocket equation could dig themselves out by reshaping their planet from a roughly spherical geoid into a disc or rod (or into a torus for the really adventurous)

I doubt such a planet would be hydrostatically stable, though…

> "hovering losses"

Gravity drag

Basically just saves what you would've lost to wind resistance and gravity while getting to that height, while adding the complication of accelerating from 0 to orbital before falling back into the atmo.

Space elevator ideas usually have the hop-off point all the way out at geosynchronous orbit to solve the velocity problem. Which is.. a really tall elevator.

Because the problem with space is not at all the altitude necessary to reach it, but it is the speed necessary to remain in it.

So a balloon/plane/blimp/very high building will only help very marginally.


Launching from a mountain is a possibility ( I think China does it)

Realistically, but not plausibly unless its an emergency, Thermonuclear bombs:


> For example can we use a space elevator to start higher/faster (or, I don't know, balloons? a catapult or railgun or something?), laser power delivery from the ground, so we don't have to carry all the fuel, and an orbiting way-station for refueling, etc.?

These methods will all help with the first 1% of your problem, getting off of the earth.

But you need so many orders of magnitude more energy to reach the kinds of velocity needed to get to another star in less than a million years. It's just an unfathomable amount of energy per kg. Put simply: if you can get to another star, getting off the planet is nothing.

Nuclear bomb rockets would probably do https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

Solar sails might be able to beat it.


Aren't solar sails just for propelling yourself once you're in space? I can't imagine how you could launch off the surface of a planet with a solar sail.

Yes, and even then they're incredibly low thrust devices. A light sail (whether solar or laser powered) may well be viable for sending a tiny unmanned probe to another star system (see [0]). It's really unlikely that it will ever scale up enough to take an average communication satellite the same distance, much less a manned craft.

[0] https://en.wikipedia.org/wiki/Breakthrough_Starshot

M2P2 was going to test out magnetic sails, but Wikipedia is telling me they generate less thrust per kilowatt hour than ion thrusters. Explains why I haven't heard anything further about it.


Solar sails are good for inter-planetary travel, but they aren't going to move the needle for launching off of a planet with 2g gravity! You need something that is compact and would give you a big impulse.

As a KSP engineer would say, it "needs more boosters" https://i.redd.it/zuymxc5bb7s21.jpg

There's also the Wait Calculation to consider:


That's interesting in theory, but as far as I know our interstellar propulsion technology hasn't advanced significantly at all in the last 50+ years. You can't do theory in a vacuum forever. I'd argue that unless we launch an interstellar something, we're never going to see any technological advances in the field.

The wait calculation implies that the returns from economic growth will eventually be converted into rocket fuel. A fun equation, but economics doesn’t translate well on time scales where depreciation of capital is 100% and becomes another expense.

Hey, I like that idea.

Maybe it implies that every interstellar mission is just an in-flight rescue mission.

Surface gravity will be around 2g, so even if it has a rocky surface it's not going to be a very comfortable home from home.

So a 150lb person will weigh 300lb. Sure that's gonna suck on day one but there's plenty of people who weigh that much who get by. Without all the health complications from high body fat it wouldn't be that bad. You'd probably die young but I don't think that would bother people.

You're not the only thing that weighs twice as much; everything else does too.

I would imagine the atmospheric pressure would be the most noticeable consequence of this.

The article notes that the planet is substantially larger:

> K2-18b is very unlike our home world: It’s more than eight times the mass of Earth, which means it’s either an icy giant like Neptune or a rocky world with a thick, hydrogen-rich atmosphere.

And the Wikipedia article for "Super-Earth" mentions something relevant:

> a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar

For comparison, the atmosphere on Earth (sea level) is approximately 1 bar.

It looks like the apparent atmospheric pressure on such a planet might be similar to being approximately 200 kilometers below the surface of the ocean on Earth. For additional perspective, the Mariana trench is (I believe) the lowest point on the planet, and is only like 11 kilometers deep.

So I guess what I'm saying, is that the apparent doubling of one's weight would be an insignificant concern in the grand scheme of things.

Not sure the two are comparable. 300lb obese person's bone structure weighs the same, their heart and lungs weigh the same. I suspect 2x the weight on every organ is going to be seriously detrimental for sustained periods.

Not to mention you'd go from years and years of near 0G gravity to twice the gravity of earth. You wouldn't even be able to walk.

Compare to the difficulty of getting there in the first place, getting spin gravity to work right isn’t a big problem.

I would assume if we can figure out accelerating to near the speed of light, we'd also be able to make a viable rotating spacecraft to use centripetal force to solve that problem.

If you're making this trip in any reasonable amount of time, you're probably accelerating during the journey at least half a G anyway.

Given the published figures for size and mass, it will be closer to 1.5 - 1.6 g. Still tricksy to walk for unmodified people, but not impossible, especially given how long it will take to reach it. Plenty of time to work out how to change people.

I wonder if we could work around the stronger gravity by using a spaceship as the anchor for a space elevator or skyhook? Both of those technologies are pretty far off, but so is getting humans to another star system.

currently we have no known materials that would actually be strong enough to support their own weight at the length of a space elevator on earth. double the gravity and the length will also at least double (possibly quadruple? not sure on the math here) so it may also be impossible to build a space elevator on one of these planets, at least without the use of active suspension (possible? theoretically.)

Currently we have no known materials that would actually survive a 111 light year trip to another solar system, so that fact that we can't actually build the space elevator when we get there seems irrelevant. They're both roughly the same order of magnitude of impossible with our current technology.

Sure we do. Chuck a rock in the right direction, it'll still be a rock 111 light years later.

Okay, I should know I have to be 100% pedantic when discussing these kinds of topics, so that's my bad. I amend my claim to the following:

Assuming we could generate a sufficiently focused laser or other communication mechanism to communicate over 111 light years (which we can't), currently we have no known material that we could build the comm device out of that would survive a 111 light year trip to another solar system intact enough for the device to actually function.

Still a bit wishy-washy since it involves magical technology that we have no idea how to build, and if we had the communications technology we would probably have the materials science too, but hopefully that gets the point I was originally trying to make across.

Thanks for the clarification :)

If the rock is moving at a relativistic velocity then it will be gradually ablated by collisions with interstellar particles. That's why any realistic design for a starship requires some kind of bow shield.

As gravity increases the tensile strength necessary for the elevator increases. We haven't even figured out how to mass produce materials that would suffice for an elevator on earth yet.

I mean, we're already talking about getting humans to another star 111 lightyears away; I'd assume by the time we're ready to do that we'll have figured out how to mass-produced nanotubes or something.

I mean, as far as I am aware, that problem is due to the propulsion method. Would a nuclear based rocket not be able to solve this issue? My understanding is that it would.

A nuclear rocket still needs propellant, doesn't it?

Yes, but significantly less of it by weight. You get significantly more energy per kg of nuclear fuel than you do traditional chemical propellant.

Or we could shift to an other side of the universe by "inverting" the mass of the ship where the speed of light is much higher[1].

Cosmology is so cool... too bad we do not have time for that: we cannot even cure the common cold!

[1]: https://januscosmologicalmodel.com/pdf/2014-ModPhysLettA.pdf Cosmological bimetric model with interacting positive andnegative masses and two different speeds of light,in agreement with the observed acceleration of the Universe

> Special Relativity can be summed up in the sentence: “We live in a spacetime which is an M4 manifold with a hyperbolic Lorentz metric of signature (+−−−)”. General Relativity can be stated accordingly: “The Universe is an M4 manifold with a Riemannian metric of signature (+−−−)” which is a solution of the Einstein equation:Rμν−12Rgμν+Λgμν=χTμν.

It sounds like you get anti-gravity for free along the way to getting superluminal travel.

No because no "run away" effect. "inverted" mass means that the mass goes on the other side... gravity works the same there.

I couldn't find anything with its approximate radius (it mentions about-earth sized and 8x the mass but that could mean anything) to make an evaluation of the likely gravitational force at the surface in order to make the evaluation of whether you could get off the planet cheaply.

> With a planet this large, visiting it would be a one way trip

Assuming chemical propulsion and no refuelling at the destination.

The amount of fuel at the destination doesn't change the rocket equation for being able to get into orbit from the planet. From the provided link:

"If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. <snip> That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport."

If it's chemical propulsion, you're dead when you get there. If there's no refuelling (and you need refuelling) with some other travel mechanism, you can't get back.

So we have an "or" assumption, not "and", with an additional and assumption about refuelling. That's how I read it.

I think what the GP refers to is that there isn’t enough energy in chemical rockets to overcome its gravity, even flight might not be even possible albeit that’s also dependent on the density of the atmosphere to some extent.

Flight would actually be easier on such a planet. The advantage of increased atmospheric density is greater than the downside of the increased gravity.

That is of the atmospheric density is higher, we don’t know the composition and temperature don’t forget that at higher pressures the boiling point of water is higher so it might not actually have water vapor in the atmosphere.

The atmosphere can also be much more shallow than earth.

Also I was more referring to winged flight than balloons balloons might be a problem of their own if the pressure at ground level is too high for them to inflate normally.

Between earth and Venus there are a lot of options so if the atmosphere is similar to earths sans the water vapor I’m not entirely sure flight would be actually easier I can probably do some napkin maths over the weekend for this.

So... if one made a very aerodynamic craft, and launched at a more extreme lateral angle, do you think it would supplement the chemical rockets enough that escape would be possible?

not really going to orbit is about velocity not altitude the gravity at low earth orbit is pretty much the same as at sea level.

The savings you get when launching from say an aircraft at 40,000 feet mainly come from not having to go through max-q at sea level the relative amount of propellant you’ll need to get to orbit is the same you can just use a smaller rocket but it doesn’t help to overcome the rocket equation trap.

But at altitude you do get higher ISP, which is a greater limiting factor than drag/max-Q. The Saturn-V would not even get off the pad in an even slightly denser atmosphere. But lift it up to where the air is thinner, where its engines can generate more thrust, and it might have enough DeltaV to get something into an orbit.

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