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Oumuamua may be a hydrogen iceberg (yale.edu)
138 points by elorant 32 days ago | hide | past | favorite | 94 comments

This is great sci-fi fodder, imagine if you could land a tiny craft on that thing and then use the hydrogen as fuel without having to haul all that fuel into deep space (very unlikely to be practical but the idea is exciting).

Imagine you just land a tiny craft on that thing and let the thing take you to interestelar space without you having to burn no more fuel.

Unfortunately, if you can't catch it, you don't need it for that, as you'll be traveling faster that it to be able to intercept it and would just burn fuel trying to break enough to match its speed.

Rule 1 of orbital mechanics: if you're about to dock with something, you're already on the same trajectory as that thing.

Might make a good radiation shield, though.

Except this is not an orbit around any body. If you can do what the parent proposes, you can add the chemical energy of fusing the hydrogen to your kinetic energy and defeat the rocket equation

That's what my comment's GP proposed. I understand my comment's parent to be asking about just attaching yourself to an asteroid.

You could always impact/land violently with it without having to match its trajectory. If your instruments are designed to survive the impact , it could work.

You can, but you only save as much velocity as the difference during the impact. Can't think of the case where that is, at the same time, both a useful saving and not fast enough to melt your equipment from a high-velocity impact.

Well is it not cheaper(by payload mass) to have higher protection to your equipment than more fuel ? Any benefit is useful , and this could be substantial depending on the strength of materials used

I imagine that, mass for mass, it isn't (of the extra fuel you carry, each bit you expend means the rest has that much less mass to push). Also, I can't think of means of protection that would withstand a high velocity collision with space gravel, that wouldn't be at the same time prohibitively heavy.

I'm not into orbital mechanics at all, just an interested layman, but how does a space tug work in relation to your 'Rule 1 of orbital mechanics'?. What is the point of a space tug then?

One of the purposes of a proposed 'space tug' is to use slow low thrust but high delta-v, high efficiency engines to do things such as:

Raise the orbit of satellites in LEO which are affected by atmospheric drag. Or re-boost things like the international space station, proposed future Chinese space station, etc.

Extend the lifetime of geostationary satellites which are still electrically functional, but out of station keeping propellant. One such thing docked for the very first time with a satellite earlier this year. https://en.wikipedia.org/wiki/Mission_Extension_Vehicle

On unmanned missions, slowly move cargo from low earth orbit to destinations at the Moon or Mars. If you can use ion and hall effect type thrusters for your missions to move cargo around, you can establish a logistics supply chain for essential supplies consisting of unmanned craft.



An interesting example of using ion engines to maintain low earth orbit, through long continual thrust was this mission:


I try to follow interesting space missions, but I've never heard about either of the two you linked to here. Thank you!

Others have provided excellent answers explaining both regular tugs and gravity tractors. So let me add a justification of my "1st rule".

It boils down to the fact that in space you can, with a very good degree of approximation, determine the trajectory of an object not under thrust, knowing only these two things: it's position at a given time, and it's velocity at that same time. These two vectors give you a single trajectory through space.

It follows from two physical laws that you may remember from high school:

- Newton's second law, or F=ma

- Newton's law of universal gravitation, or F=GMm/r^2

If you put them against each other, you get ma=GMm/r^2, or consequently a=GM/r^2. The acceleration (and thus, future position and velocity) of an inert object in space is independent of its mass. So all you need to tell where it will go is to know where it is, and what is it's current speed and direction of movement. And, of course, what other bodies influence it with their gravity.

Of course, in practice, there are other considerations in which the mass may become relevant. For instance, solar radiation acts on the surface, impacting force that's scaled by object's mass. The same is true for collisions with various stray atoms, especially prevalent near planets with atmospheres.

But discounting above factors (and, again, you can go pretty far just ignoring them), if you have two objects very close to each other and not moving relative to each other, they'll just go together along the same path for a long, long time.

The space tug is there to change the trajectory of the other thing:

1. Tug gets in same position and velocity as its target, and docks.

2. Tug performs some kinda of burn to put both it and its target on the new trajectory.

3. Tug undocks and performs a burn to head off to where ever else it's needed.

If I understand correctly, it's really just a way to avoid putting engines and fuel tanks on things that don't need to change trajectory much.

The space tug and its load / target are on the same trajectory at the time of rendezvous. Once they're synced up, they're drawn toward each other, with the tiny gravity of each affecting the trajectory of the other. But while the load is passively traveling through space, the tug can reposition itself. This way, the tug determines the ultimate trajectory of the load while also having some control of its own trajectory, which it uses to continually bend the load's trajectory.

If you match its speed, sure you'd be already independently traveling to interstellar space at the same speed as it was.

But landing on it and using the free large fuel tank of hydrogen it has now became you can 1) accelerate more 2) decelerate at your destination 3) steer (at least a bit).

If you match it's speed you already burned all the fuel you'd have to burn to get where it's going.

Right, you want to wait until the next one is inbound and catch it while it's still coming towards you. You just have to design the probe to survive a "landing" at several kilometers per second. I'm sure we can figure something out.

You can imagine intercepting a thing like this (Oumu has left the building), and hooking elastic rope to it to get dragged along.

Seems like a real hard mechanical problem, but not fundamentally impossible.

Pfft, you can just lasso it

Yeah, though there's lots of water in the outer solar system you can get your propellant from. The term to google is In Situ Resource Utalization (ISRU).


But those are mostly just orbiting the sun.

If you ever wanted to send a probe to Proxima Centauri, and stop when it got there, accelerating the fuel you'd want to have available to decelerate up to solar escape velocity would be a tremendous energy expenditure, even if you can mine it in the asteroid belt.

If, instead, you got lucky and jumped your probe (fuel tanks dry) on an interstellar wanderer that happened to be going approximately in the direction you wanted, you could have hundreds of tons of rocket fuel moving at interstellar velocities.

True, but those interstellar velocities are mercilessly pitiful in respect to much of use there.

I think the theories of it being an artificial alien probe were definitely more sci-fi fodder.

I'd always imagined that an alien intelligence might have seeded every solar system in the galaxy with a probe to monitor for evolving intelligence - but after Oumuamua, I realized it would be far more efficient and reliable to have swarms of them coasting through systems periodically to check on them. ...or maybe both.

Then the problem becomes getting oxygen. As a hydrogen won't burn in a vacuum.

I could still be useful as a propellant for say, a nuclear thermal engine.

Unless you can fuse it into helium.

If you have the tech to fuse hydrogen the amount of fuel you would need for a nearly unlimited amount of energy is tiny, you would save very little by harvesting it off a comet/asteroid.

Depends on how much energy you need. If you are talking about turning it into an interstellar spacecraft by hooking up some kind of fusion powered thruster (probably using H2 as the reaction mass) to the back you may need all of its mass.

This is of course well beyond our current capabilities, but it's not something that's impossible to consider in the medium-distant future. Plus, even if you expend a colossal amount of energy accelerating you're still talking about thousands of years for the journey where you'll need to keep those reactors running for the entire trip (there is no useful solar collection in deep space) having a literal mountain of fuel to start with is important.

Unless you just want to colonize this hydrogen iceberg, and live on it for next thousand years, recycling all other elements indefinitely as you cruise through the galaxy.

You still need propellant mass. Energy in itself doesn't get you anywhere. But in practice you'd probably be better off electrolyzing water ice which is ubiquitous in the outer Solar System.

Insulating the fusion reactor from the solid hydrogen in Oumuamua could prove difficult.

Put Oumuamua at the front? It's not like drag is a concern.

Yeah, imagine you have whole lot of hydrogen and no oxygen to get it to burn. You are going to freeze to death while moving nowhere.

About the only good thing it could be used for is reaction mass. But you can use almost anything as reaction mass as long as you have a source of energy, you can exhaust any particles to propel yourself forward, doesn't have to be hydrogen.

If it's only hydrogen, you can still fuse it.

Well, not today, but we can't reach it today either.

Matching speeds with it for this operation would take much better rockets than we have available.

A Starship sized vehicle using the well tested NERVA engine would be able to add around 20 km/sec to the velocity it already has in earth orbit. So pretty close and gravity assists would increase that.

Once you dock with Oumuamu, you can run it’s material through at an 890 ISP and gradually accelerate to much higher speeds.

You underestimate the problem.

First you have to add something like 40 km/sec to go from Earth orbit to escape into interstellar space, and then you have to add another 20 km/sec to match speeds with Oumuanu. That's 60 km/sec, which is 3 times the speed and takes 9 times the energy.

Pioneer and Voyager got to interstellar space with gravity assists. But those work best when you barely get to a planet's orbit, then it picks you up and you go close to 180 around it. (Thereby adding 2x its velocity to your own.) Once you start getting to the velocities needed for Oumuamu, you just shoot by the planet and don't get much of an assist. (Assuming, that is, that there is an alignment between the planets and a random interstellar visitor. Which is extremely unlikely.)

I stand by my statement. We do not have good enough rockets for this maneuver.

It'd be fun to operate a nuclear-thermal rocket for decades, heating the ice and piping it through the reactor and out the nozzle.

Or just follow it.

"It has now passed beyond Saturn’s orbit and will travel another 10,000 years before exiting the system".

How big is our solar system? I always assumed it referred to everything between the sun and Pluto, but 10,000 years suggests it's far larger.

I'm not sure of what definition they used here, but one that is common is heliosphere, or in other words sphere around the Sun where solar wind is dominant. This is about 120 AU.

‘Oumuamua's incoming and outgoing speed is ~5AU/year. In 10,000 years that's 50,000AU, which is approximately the distance to the outer most region of the Oort Cloud: https://en.wikipedia.org/wiki/Oort_cloud

I always thought the Oort cloud was a scientifically proven thing but apparently it's "a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au (0.03 to 3.2 light-years)"

Thanks for correcting my similar misimpression.

It’s curious to note that the proposed outer limit of the Oort cloud (3.2 ly) is more than half the distance to the closest star system (4.4 ly) [1] though I’m unclear whether the cloud would be roughly spherical or have some sort of directionality.

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

Too faint to be proven with our current instruments.

But it would still take way less than 10000 years to cross the heliopshere boundary (it took 40 years for the Voyagers probes). It is probably referring to the gravitational sphere of influence of the solar system (marked by the Oort cloud) 1 to 2 light years away.

Could be the distance at which the sun's gravitational influence is comparable to that of the surrounding stars. This would vary over time, but as a rough estimate it's probably alright.

There are many definitions on the limit . Depends on how you see it, one definition not commonly mentioned is the size of the hill sphere [1] . Depending on the direction you look at the size of hill sphere changes .

- [1] https://en.wikipedia.org/wiki/Hill_sphere

I like the idea that these things could be potential star-seeds. It would be an interesting way for new solar systems to form.

Makes me wonder if you could slingshot around the sun towards relativistic speeds with thermal power. At the very least it could save fuel on the egress, if not the second half.

Can you aerobrake around a star?

"if you could slingshot around the sun towards relativistic speeds with thermal power"

Unfortunately, no. The boost you can steal from a powered flyby is only of the same order as the escape velocity of the massive object. For the sun, this is 617 km/s at its surface, or 0.002 c. The term you're interested in is "Oberth effect".

To get relativistic speeds on the table, you'd need exotic, compact objects whose escape velocities are relativistic. I.e. white dwarfs, neutron stars & black holes.

No one asked, but: Freeman Dyson pointed out [0] some astounding properties of a short-period white dwarf binary. (Particularly: there are two stars; both are extremely dense; and they are orbiting around each other at an extremely high relative velocity). If you could perform a close slingshot maneuver around one of these stars, you would be accelerated to relativistic speeds in mere seconds. What's more, this would be an entirely unpowered, propellantless manuevuer (stealing orbital momentum from the binary system). And finally, as an incredible example of the equivalence principle, human passengers wouldn't even notice the insane ~10^4 g's of acceleration: it's equivalent to free-fall.

[0] This is a PDF https://www.ifa.hawaii.edu/~barnes/ast242_s14/Dyson_Machines... (1963)

> Can you aerobrake around a star?

Not a physicist, but it seems like you should be able to do something similar to aerobraking (not one-to-one since the Sun's "atmosphere" is different from a planet's). Maybe using a solar sail of some sort directed at the right angle?

I went on a wiki-binge on this topic a few days ago and so I recognize this as a magsail[0]. It would be an attractive addition to an interstellar craft since it obviates the need to carry propellant for deceleration.

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

That's really cool. Definitely helpful in order to drop deceleration fuel requirements if we ever manage to put together a relativistic interstellar spacecraft.

In a bit over 1M years another star is expected to pass around 1000 AU from the sun. Chemical rockets should be able to get there in a decade or two. It's a huge stretch, but we have a million years to prepare.

Oh wow. Had never heard about that. Wouldn't that pass within the range of the Oort Cloud? Seems like we might have some issues from the gravitational effects if so (although that's assuming humans manage to keep ourselves going for another million years).

Something makes me doubt that humans will still be around at that time.

With the way we’re going, maybe not. But maybe we’ll shape up and some version of humanity will make it.

Can't do the maths at the moment but you'd probably need a lead spacecraft to get close enough. Getting close to the sun also means you'll be going ridiculously fast e.g. Parker Solar Probe will hit something like 200km/s and it's still millions of kilometres from the sun

Thermal power requires a reservoir of cold temperature; your car's engine would do nothing if the incoming air was just as hot as the post-ignited air.

I'm sure this is true in practice, but I can't wrap my head round why it's correct in theory. Let's say I hook up my exhaust to my intake, I can still squirt some gasoline into the engine, make it explode and expand and thereby drive a piston, at the cost of having even hotter air the next time round.

If I repeat this long enough, my engine will melt, but that's a minor matter of materials science. Maybe I can win my drag race before it melts. Is that the limiting factor or is there some other reason I'm missing that we need a heat gradient?

If you heat it more each time, yeah, the old air can act as a cold reservoir and make things run.

If you do draw it with enough detail, you'll notice that if you do not heat it more at each turn, you won't be able to get any pressure difference at your "explosion".

Yes, I was envisioning solid hydrogen fuel powering by outgassing as envisioned in the article.

> Can you aerobrake around a star?

If you can keep your cool, why not? Matter is matter.

What's better, you could technically radiation-brake around a star. I don't know how big of a solar sail you'd have to present to get a meaningful force from it.

> slingshot around the sun

You can only slingshot around the sun if you were already in a hyperbolic orbit w.r.t the sun. An object in a closed helio centric orbit cannot gain any momentum by "slingshotting" around the sun.

It can if it loses mass, correct? Energy conservation would result in increased momentum.

Yes. Your mass in low Sol orbit has a much higher specific orbital energy, so thrusting a rocket or ejecting mass while at or near perihelion makes more of a difference than doing so in deep space.

Brown dwarf stars can be relatively cool and can have atmospheres, so you could aerobrake around those.

Magneto-braking maybe?

Is there an element or compound in the universe that will remain a gas at 0K? Does everything become solid at that temp?

To be pedantic, it's impossible to get to 0K due to quantum fluctuations. Even empty deep space is over 2K. In laboratory settings, we've gotten way below 1K. I don't think anything qualifies as a gas at that temperature. It'd be a solid or a BEC.

This is true, but doesn't have anything to do with quantum mechanics -- the third law of thermodynamics is basically a statement that you can't cool things to absolute zero.

Your input caused me to google a video about this:


Very intereting!

Helium does not solidify down to the lowest temperatures that have been achieved (at standard pressure). Though it does become a superfluid, which is a distinct phase of matter from a gas/liquid.

0K implies no Brownian motion at all (no thermal energy), so there would be no force to keep the molecules apart.

Actually, in some substances the molecules are further apart in the solid than they are in the liquid phase.

Intuitively I would think so since 0K is a motionless state. By definition everything would be solid because nothing would be capable of e.g. filling a given volume, or for that matter, doing any action at all.

But this isn't my field of expertise.

Complete lack of motion would violate Heisenberg uncertainty principle ("no motion" == "I know both momentum and location with zero uncertainty" and that's not allowed)

No, you can have zero momentum. However it comes along with having a position wavefunction that fills the whole universe.

> However it comes along with having a position wavefunction that fills the whole universe.

I love that this sentence exists. Because it's absurd and enlightening at the same time.

The universe kindly requests that you do not divide by zero.

Because it does not want to be Bourne Again.

Hey let's probe the boundary conditions of the universe.

The boundary condition is that at some arbitrary point god smacks you and tells you to knock it off

Have you ever played The Talos Principle? This is pretty much what happens if you try to walk outside the playable game area: https://www.youtube.com/watch?v=vWjUzvLk58o

Sorry God!

Careful now, you don't want to crash the simulation.

It already slows down when you are moving too fast.

Precisely. But that doesn't work for OP's question since the whole universe isn't at 0K.

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