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NASA is working on an asteroid retrieval mission. A small asteroid will be moved into Lunar orbit where it will be visited by space walking astronauts. Since it will be only max seven meters in diameter, I prefer to call it a meteoroid. The one which exploded over Chelyabinsk last year was about 20 meters in diameter and hence about almost nine times more massive. The retrieved meteoroid will be too small to pose any danger to Earth. Also, I suppose it will have a lower speed relative to Earth, than would an average interplanetary asteroid. They will look for a low delta-v asteroid.

Wouldn't it be easier to visit it in low Earth orbit and maybe use the ISS to investigate it?

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It would take vastly more $\Delta V$ to get it to a low-Earth orbit. The targets selected are close enough to Earth's orbit about the Sun that it only takes around $200\,\mathrm{m/s}$ to get it into a distant retrograde orbit about the Moon. To get the thing to a low Earth orbit would be around $3\,\mathrm{km/s}$. The tyranny of the rocket equation makes that infeasible.

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  • $\begingroup$ Very interesting! That sounds like a potentially very good explanation. But could you please explain a bit about what basically makes it so? The Moon orbits Earth at only 1 km/s, that could intuitively explain 1/3 of the difference in your numbers. But why would near Earth asteroids have lower than that delta-V relative to the Moon than to Earth? Because they could be pushed into trajectories which pass by the Moon several times, maybe? $\endgroup$ – LocalFluff Jan 28 '14 at 14:17
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    $\begingroup$ Because the Moon has very nearly escaped from the Earth. Going the other way, it takes about $3.1\,\mathrm{km/s}$ to get from LEO (low-Earth orbit) to an orbit that just reaches the Moon. From LEO, it takes $3.2\,\mathrm{km/s}$ to escape Earth's gravity completely. Only $100\,\mathrm{m/s}$ more. So in terms of $\Delta V$, the Moon is much closer to the rest of the Solar System than it is to Earth. $\endgroup$ – Mark Adler Jan 28 '14 at 15:19
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    $\begingroup$ @LocalFluff Gravity well of the Moon is shallower since it's only about 1.23% the mass of the Earth. It's also non-negligible that it orbits the Earth, which can potentially also reduce required delta-v, depending on how the captured object moves relative to it. Since we're talking of NEO asteroids here that would already be at orbital velocities close to those of the Earth-Moon system, that difference wouldn't be small. What Mark answers in numbers, while I don't have any doubt is correct, also feels right. $\endgroup$ – TildalWave Jan 28 '14 at 15:19
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Safety of our blue planet. Eventually, gravity anomalies would cause even a perfectly orbited object (a moonlet?) to preces and hit the body it orbits around. Since orbiting an asteroid means reducing large fraction of its momentum to bring it closer to celestials it naturally orbits (NASA's plans involve capturing a near-Earth asteroid, or NEO, as part of its Asteroid Initiative program) it wouldn't impact with such velocity as the Chelyabinsk meteor did (required LEO velocity is only about 8 km/s, which would have to be reduced to deorbit, while the Chelyabinsk meteor entered Earth's atmosphere at an estimated velocity of 18.6 km/s), which caused it to disintegrate to smaller fragments in the atmosphere due to friction and aerodynamic heating. So it would still pose a significant threat to Earth, even if it was much smaller than the Chelyabinsk superbolide was. I'm myself much more in favor of this happening on the Moon than on the Earth.

It also wouldn't make much difference for the safety of the astronauts studying this asteroid (while it's much bigger than about 1 m in diameter, it can't be considered a meteoroid) or much more difficult to reach, since it couldn't be orbited in Low Earth Orbit (LEO) due to still sufficient atmospheric drag decaying its orbit, and where astronauts would still be somewhat protected from solar wind and high-energy proton particles of cosmic radiation by the Van Allen radiation belt, or they could be reached easier or sent supplies by currently available orbital launch vehicles in case of something going sour.

It is also easier (smaller required delta-v) to later haul the rock back into a safe orbit in cis-lunar space once you're done studying it, if it's orbited around a smaller mass body that's already a significant portion away from the Earth's Hill sphere.

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Multiple reasons exist.

Weapon of Mass Destruction

Any large body in orbit is a potential weapon of mass destruction. Drop a 20-ton rock from orbit, and it may not survive, but if you do it right, it creates a crater some 100m across.

While NASA is not planning on using deadfall ortillery, the possibility is a political issue.

Ownership and access

Legally, all natural bodies in space are common property of all Earth's peoples.

Putting it in LEO would make it accessible to a variety of states that are seen as politically untrustworthy. The key example is North Korea - they have demonstrated the ability to launch SRBM's, which are precursors to a space program as well as to ICBMs. (The initial US space launchers were in fact developed as part of ICBM programs.) The concern that these nations may reach and deorbit a body as a form of terrorism is a real and present (but low probability) threat.

A Lunar Orbit location has far less access to everyone, however, it's a near total bar to most nations. The US, Russia, China, India, and the EU have the capability to get there, as evidenced by the ability to land lunar rovers and install lunar orbiters. It's still close enough for most real-time experimental controls.

More margin for error

The lack of atmosphere ➀ allows for a miss to not result in slowing and impact, nor to result in surface melting of the body.

Lower ∆V

Less overall energy is needed to put items into a stable Lunar orbit than a stable Earth orbit. The speed change needed is less, and thus the duration of acceleration and total thrust can be less.

Less to impact

Fundamentally, inserting anything into Earth orbit involves hitting the mark at the right time and speed to not hit anything else in Earth orbit. There are huge numbers (thousands) of tracked items, and hundreds of useful items, including a manned facility, that might be affected.

Lunar orbit is far less crowded, and no lunar orbiting equipment is essential infrastructure.

Much less impact on satellite orbits.

If I recall correctly, the intent is to move an asteroid of sub-kilometer diameter - but preferably over 100m diameter. These are objects large enough to have a noticeable effect upon other orbiting bodies. The cumulative effect could be catastrophic. The impact is tiny, but would be persistent and affect all other orbiting structures notably.

Less tidal stress

Tidal stress being the difference in pull between the ends of an object in orbit around a more massive object. Lunar orbit results in far less tidal stress than Earth orbit, due to the much lower lunar mass. (Note that Earth tidal stress will still exist in lunar orbit, but will be considerably less than even geosynch.

Proving the tech doesn't require a Earth orbit.

The technology in question is the ability to put asteroids into desired orbits. The principles are exactly the same whether that's Earth, Lunar, or Martian orbit. Success will result in proving the technology.

A base for future science

A lunar orbiting asteroid is a good base for further lunar and deep space oriented science. Being closer to the moon, it provides relay access for far-side rovers, as well as a location outside the van allen belts to test various practical radiation shielding approaches without having to land on the moon itself.


➀ Lunar atmosphere is present, but is at so low a fraction of a pascal at surface as to be safely ignored for orbital purposes.

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  • $\begingroup$ Thanks for your answers. Low deltaV is the only valid reason, I think, and it might be enough. But the terrorist paranoia is an illness and planet safety is a joke. The max size will be 8 meter with a mass of maybe 3 tons. A solar ion thruster could never do anything with a real asteroid for planetary defence or of economic importance. The meteoroid retrieval mission has obviously been designed only to use the ion thruster, SLS and Orion. Not for science, security or resource extraction. I read online that many are of the opinion that this crazy mission will never happen. $\endgroup$ – LocalFluff Feb 16 '14 at 11:00
  • $\begingroup$ The initial proposal I read about was for a 100m asteroid - and using a mass driver, not an ion drive. The ion drive is more readily fundable. And NASA has done crazier things. The Kittinger jump comes to mind. $\endgroup$ – aramis Feb 17 '14 at 4:23
  • $\begingroup$ For small-asteroid science, they could go to Chelyabinsk instead. They have half a ton pieces of a 20 meter asteroid highly accesible right there on the ground. That'd save a number of billions of dollars. But what to use the SLS+Orion and ion thruster for now? Cancellatiooon... $\endgroup$ – LocalFluff Feb 18 '14 at 11:14
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    $\begingroup$ @LocalFluff you're utterly missing the point of moving an asteroid into orbit - not the least of which is determining if 0.01G is significantly different physiologically and psychologically, from freefall. A 20m asteroid is worthless for those purposes - a 100m asteroid, as the early proposals were, was sufficient to start looking at the effects separate from freefall. $\endgroup$ – aramis Feb 19 '14 at 7:22
  • $\begingroup$ A 100 m asteroid would be different, yes. For planetary defence, for futuristic mining visions, and perhaps scientifically if they generally form quite differently than 7 m asteroids do. But now, 7 m it is! (Wouldn't it be much cheaper to put the largest recovered part of the Chelyabinsk meteorite in Lunar orbit instead?) :-p $\endgroup$ – LocalFluff Feb 19 '14 at 11:42

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