There are proposals and maybe serious plans to send astronauts to asteroids or Mars' moons, objects which are a few kilometers or just a few meters in diameter. I call them "milligravity" objects since their gravities are operationally relevant, but maybe medically equivalent to microgravity. Some of those objects should be loose rubble piles, never before encountered by humans.

Wouldn't it be a valuable preparation to launch 20 tons or so of such a rubble pile, a bunch of sand and rock loosely bound together, to (a short lived) low Earth orbit and have astronauts test out handling technologies and operations on it? The structure of the specimens would be completely controlled by design. It could have sensors planted inside of it. The operations could be performed in relative safety. No time consuming transport of some boulder on some NEA would be needed, as with the ARM mission. A boulder which could be picked up from a NEA won't be an exotic rubble pile anyway, I suppose it would be more like some meter sized meteorites found in museums on Earth's surface.

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    $\begingroup$ Are you looking for actual cases where this has been considered, or just ideas? (Personally I think the idea would be far too expensive in launch mass to even consider.) $\endgroup$
    – Andy
    Mar 21, 2016 at 15:12
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    $\begingroup$ @Andy I've never heard it considered, other than jokingly to make fun of ARM. I'm looking here for some good reasons I cannot now see. Why should an Atlas V launching a 20 ton rubble pile (in a bag) to LEO be more expensive than an Atlas V launching a multi-year SEP robotic mission to a NEA to tow a boulder to lunar orbit, be more expensive? The astronauts could be launched only days later to visit this artificial and controlled rubble pile in LEO. $\endgroup$
    – LocalFluff
    Mar 21, 2016 at 15:47
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    $\begingroup$ 20 tons is going to have pretty much 0 gravity. IMO that's different enough from the gravity generated by an asteroid a few km across to make it not worth bothering. $\endgroup$
    – Hobbes
    Mar 21, 2016 at 16:17
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    $\begingroup$ Probably a non starter just from a PR perspective. "NASA spends billions launching rocks to space ... like there weren't enough there already!" $\endgroup$ Mar 21, 2016 at 16:27
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    $\begingroup$ @OrganicMarble +1 But maybe it could be framed as a revenge? Hitting back against that polluting nightness which respectlessly throws its garbage at us. Like Xerxes whipped the Bosporus as punishment after a storm on it had destroyed his navy, Besides, recycling is very popular these days. From space to space, from ashes to... $\endgroup$
    – LocalFluff
    Mar 21, 2016 at 17:12

4 Answers 4


In short: yes, it could be. But it is needlessly expensive, so they will not do it.

First you have to get the junk up there. Then you need to get the astronauts up there, and essentially put them in the same situation as they need training for to deal with. Then get them down again.

The same could easily be achieved close enough in a pool or in a cable-robot for a tiny fraction of the cost.

Also: this question, and its answer, are relevant. In short: at 80 tons and 10 meters out, the Space Shuttle exerted almost 10 times less gravitational pull on the astronauts than did Jupiter.

So you are going to need to put a LOT of junk up there before you have anything even remotely useful for training in milligravity.

  • $\begingroup$ The question does include the idea that the rubble be 'loosely bound together', which I take to mean it is in a net or something. I don't think it then imitates a milligravity environment in that case, but the idea is there. $\endgroup$
    – kim holder
    Mar 21, 2016 at 17:03

There is already too much junk in orbit which could collide with satellites and spacecraft, adding 20 tons of loose garbage would be extremely risky - if it got loose it could be a serious hazard.

To get the benefits of any training you would need to build a very, very big pile of rubble. 20 tons would not be enough, you would need more on the order of 2000 tons. Launching that much mass would be incredibly expensive, far more than any mission to go to a real space rock in the first place.


Could we launch a rubble pile simulant to LEO...


...for astronauts to practice on before visiting natural milligravity objects?


20 tons or so of such a rubble pile, a bunch of sand and rock loosely bound together...

With a density of half of SiO2 (1.3 g/cm^3) due to porosity, this would (at t=0) be a 3 meter diameter sphere.

However, it would immediately begin to experience fictitious pseudoforces in its center of mass frame, and probably disintegrate and disperse within hours.

Despite Parallel orbits around the Earth - effectively? and its answers, bits on the "left" and "right" (across-track) sides would have opposite inclinations; they want to at first converge as they go around, and once they rebound they will want to diverge for a half-orbit before re-converging and bouncing again.

Since the final states after collisions of randomly shaped objects are also random, they will also knock themselves to slightly higher and lower orbits, which will have different periods and naturally de-phase over time.

And that's besides the immediate de-phasing due to tidal forces (in the center of mass frame); the particles at the "bottom" (nadir) side will want to orbit faster than those at the "top" (zenith).

If they were all charged, coulomb repulsion would be another problem, and the gravitational attraction is so darn small compared to these pseudoforces that I think it's hopeless.


What would be the mass of a small body with a diameter of 10 km and a density of 1.3 Mg/m^3?

I calculate some examples:

      1m radius, mass 5.445          metric tons, gravity 363E-12 m/s^2
     10m radius, mass 5.445 thousand metric tons, gravity 363E-11 m/s^2
    100m radius, mass 5.445 million  metric tons, gravity 363E-10 m/s^2
   1000m radius, mass 5.445 billion  metric tons, gravity 363E-9  m/s^2
    10km radius, mass 5.445 trillion metric tons, gravity 363E-8  m/s^2

So your 20 tons are enough for a very tiny body of 1.54 m radius only.

Unfortunately the gravity is only proportional to the radius, but the mass is proportional to the third power of the radius.


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