What modifications would be necessary to make to a Mars rover to let it land on Phobos or Deimos? In this question, I found out that a Mars rover would need a host of modifications to land on Phobos or Deimos. What would those modifications be? Assume that NASA is aiming to have the least amount of changes possible.
The very low gravity would require a total redesign. Phobos is the larger body and has a surface gravity of around 0.6 milli-g, or around 1500 times less than here on Earth. The newest rover, Perseverance, is 1050 kg. On Phobos, it would weigh less than a laptop!
I once worked on an asteroid lander. The gravity was even lower, but the problem would be similar. Just moving a sampling arm at a normal rate would push the lander over onto its side! (that's Newton's law of action and reaction at work!) Moving across the surface with wheels probably won't work either, because you need enough weight to keep the wheels from spinning.
Others already mentioned the major differences in slowing down and landing, and they are also so serious that the system would need to be redesigned from scratch.
The Russian Fobos-Grunt mission was intended to soft land on Phobos, collect samples, and return them to Earth ("grunt" means "ground" in Russian). You can read about how they solved the problem in the link below. In particular, the lander stayed put once it was down. Unfortunately, the spacecraft propulsion failed and it never left Earth orbit.
A partial list of subsystems that would need to be redesigned:
- Since the gravity is low, there's a high chance that you will flip over at some point. You will need to ensure that you can flip back, and that the flip itself doesn't damage your various appendages (e.g. antenna)
- Your wheels are going to be useless, and you need legs with active suspension in order to absorb your momentum at landing. So you might as well design the on-land propulsion as a jumping-around set of legs.
- There is no atmosphere, so the nuclear generator will have a different thermal behaviour. This is probably ok because space is cold, however that's not the only thing that will suffer from temperature. The whole probe will see its environment go from full sunshine to the dark of space several times a day, so you can expect faster thermal aging of each component.
- On the other hand, no atmosphere, so no sand/dust to infiltrate everywhere, so some protections against sand can be removed
- Phobos rotates on itself and around Mars, so the direction of Earth will change rapidly. The algorithms that point the antenna at Earth probably need to be rewritten from scratch, and you might need to change the antenna pointing servos as well.
- And finally there's the question of how do you dig for samples without exerting some downforce, the reaction of which will send you jumping in the air.