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Say we send a craft to Europa which would then launch a probe to land on the surface of Europa. Let's say for the sake of this question that the probe weighs more or less like the Opportunity rover. Given what we know about the moon's atmosphere and gravitational pull, how would a lander accomplish that?

Rockets like the moon landers? Parachutes like the Opportunity rover? Something else?

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    $\begingroup$ Very carefully. $\endgroup$ – Mark Adler Sep 17 '13 at 2:46
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    $\begingroup$ Great question. I imagine that the answer will be more complicated than one best method. $\endgroup$ – called2voyage Sep 17 '13 at 13:21
  • $\begingroup$ How about establishing a synchronous orbit over the equator and lowering the lander on a tether? I know that we are nowhere near being able to do such a thing on Earth, but with Europa's lower gravity the job must be a little easier. Anyone know what the synchronous orbital radius is for Europa? $\endgroup$ – user1292 Dec 18 '13 at 5:14
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    $\begingroup$ A synchronous orbit about Europa is outside its Hill sphere, and so is not stable. In fact, it's very difficult to find any orbits around Europa that you could call stable. At low altitudes, Europa's non-spherical gravity augers you, and at high altitudes, perturbations from Jupiter auger you in or throw you out. $\endgroup$ – Mark Adler Dec 18 '13 at 6:44
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The "atmosphere" at $10^{-12}$ that of Earth's is entirely useless for landing. It would simply be retrorockets as you would for our Moon. Many types of landing gear could be considered, such as traditional legs, airbags, wheels a la skycrane, pallets, crushables, whatever. But to get to that point at a survivable velocity, it's all about rockets.

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Europa's surface gravity is just a bit less than that of our moon (0.134 g versus 0.165 g), and as Mark Adler notes, the atmosphere is negligible. Russia and China have already put unmanned rovers on the moon via unmanned rocket-powered landers; both of these rovers are roughly in Opportunity's weight class (Yutu a bit lighter, and Lunokhod about 4x heavier).

While a Europa lander and rover would have to rely on RTGs rather than solar power, and would have to spend far more time on "standby" during the journey, and there would be countless other differences (particularly in thermal management), the overall approach to lander and rover design wouldn't have to be greatly different.

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