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Suppose we want to do a lunar sample return mission. We can then launch a small rocket from the ISS that carries a small probe to the Moon. It will collect a sample and then return to the ISS.

Because the entire mission happens in almost a perfect vacuum, it should be possible to do this using miniature rockets and a miniature probe. In principle everything can be scaled down to perhaps the molecular scale, but with the technology available today, there may be some limitations to how small one can make the probe.

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    $\begingroup$ Have you checked out the Lunar X Prize? The contenders there have pared down their rovers as small as possible because they are on extremely tight budgets. And they are hitch-hiking on someone else's rocket. The gravity well of the moon, and the need to escape Earth's gravity, means the requirements from the ISS versus from the surface are pretty much the same. $\endgroup$ – kim holder Apr 29 '15 at 20:44
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    $\begingroup$ The question is severely underspecified: do you want to know the size of the smallest return capsule? If you have a railgun on the Moon, the return capsule does not have to include an engine. If you don't mind your sample heating up at reentry, you can simplify shielding to the minimum. You can also provide for lithobraking :) $\endgroup$ – Deer Hunter Apr 29 '15 at 21:32
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A starting point is the smallest sample return probe ever built, the Luna 24.

Luna 24 return stage accent

Massing 5300 kg in lunar orbit it was pretty much bare bones. To see where we can improve this, we must first split the mission it its separate parts. This follows the standard procedure of doing the mission backwards. First, I would not choose ISS as the target, as braking into orbit and then rendezvous and dock with it is a complicated task. To simply hit the Earth is much easier.

Of the 514kg for the return stage of the Luna 24, around 300kg was propellant. Perhaps a slightly more efficient engine exist today, but the technology used now for landing and ascent would still be hypergolic propellants. That is the most important limit for scaling.

The re-entry capsule was only 34kg, but you might still be able to shave off a few kilograms. The main savings are in the remaining 180kg though, including electrical systems, control systems, the engine and the propellant tanks. With the miniaturization of electronic equipment since the seventies, and some new lighter materials, you may be able to squeeze everything required into a dry mass of 100kg. That is about 220kg at the lunar surface, roughly halved. A similar miniaturization of the descent stage and drilling equipment yields a spacecraft of about 2 metric tonnes in Lunar orbit.

To transport the spacecraft to the Moon, the minimal solution is a spiralling ion-craft. That is within current technology, take for instance the engine powering Dawn. At the cost of a long transfer time, the total mass in Earth orbit is only going to be around 3 metric tonnes minimum. However, considering the relatively high drag at the altitude of the ISS, the craft must start from a higher orbit.

Additionally, it is more effective to combine multiple related goals, like a lunar rover, and on-site experiments, than to launch multiple minimal missions with high risk of failure.

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