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Could a Mars rover go to Phobos or Deimos instead of going to Mars? The choice is made after launch, and no further modifications can be made to the rover. Could it land safely? Up to what point could it change its trajectory to encounter them? Is there any difference in safety and fuel between going to one of Mars's moons versus Mars itself?

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    $\begingroup$ The Beagle 2 mission could certainly have performed as well on either moon as it did on Mars itself. $\endgroup$
    – Starfish Prime
    Apr 8, 2022 at 20:01
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    $\begingroup$ @StarfishPrime: According to en.wikipedia.org/wiki/Beagle_2 , Beagle 2 landed safely on Mars surface. $\endgroup$
    – Eric Duminil
    Apr 9, 2022 at 19:34
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    $\begingroup$ @EricDuminil I too often find a joke is improved by explaining it! $\endgroup$
    – Starfish Prime
    Apr 9, 2022 at 19:37
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    $\begingroup$ @StarfishPrime: I didn't get the joke, then. Beagle landed on Mars, but would have crashed on the moons, right? $\endgroup$
    – Eric Duminil
    Apr 9, 2022 at 20:19
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    $\begingroup$ @EricDuminil: The joke is that sending back zero data from anywhere is easy, no matter how you land. $\endgroup$
    – Peter Cordes
    Apr 10, 2022 at 4:14

2 Answers 2

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No, for a lot of reasons.

  1. The Mars rovers slow down based on aerodynamics, heat shields, and parachutes. None of that is available on one of the Moons, meaning that the fuel requirements are much higher. Just to give you an idea, the spacecraft enters the atmosphere of Mars going at 12500 miles/hour (5.6 km/s). That speed in a spacecraft landing on Mars is slowed down by the atmosphere. Almost all of that speed would have to be slowed by a rocket somehow, which there isn't enough rocket power on a Mars lander. Some portion of this with a lot of work could theoretically be done via an aerocapture, but that hasn't actually been done, and would require really accurate models of the atmosphere. It would require at least 1 km/s delta-v to finish an orbit after an aerocapture, which is more than most rovers have when landing on Mars.
  2. Even if they somehow could be diverted and land, the landing sequence would need to be drastically altered, which would be hard to do even in 6 months that it takes to switch targets. This one could theoretically happen, but it'd be a tough thing to do from scratch.
  3. They likely couldn't move very well on the moon, as there isn't enough gravity.
  4. The power situation would likely be okay. The days are 8 and 30 hours for Phobos and Deimos, so that's likely to be fine.

Bottom line, it couldn't happen with any past or planned mission to Mars. If one wants to land on one of the Moons of Mars one should design a lander for that purpose.

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    $\begingroup$ @Starshipisgoforlaunch re 1 'slow down' actually is speeding up - the rover arrives on a transfer orbit and uses the atmosphere to get dragged up to mars orbital velocity, for the same energy as would be needed for launching it FROM mars TO earth - AKA a pretty serious rocket. Re 3 there the Phobos gravity is low enough for traction to be an issue, and if you do get up to speed to end jumping far enough the rover tumbles or rolls on landing. $\endgroup$
    – GremlinWranger
    Apr 9, 2022 at 1:53
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    $\begingroup$ @GremlinWranger a martian aerocapture into a low-velocity phobos- or deimos-intercept orbit isn't impossible of course. I wouldn't like to try and plan it, or persuade a budget committee that it would be a totally sensible and minimally risky use of their hojillion dollars, though. $\endgroup$
    – Starfish Prime
    Apr 9, 2022 at 9:09
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    $\begingroup$ @Starshipisgoforlaunch The lower gravity can be a hinderance, it isn't always better. Astronauts had a hard time moving on the Moon. Deimos and Phobos for all intents and purposes don't have any gravity, so the little gravity would make things very difficult. $\endgroup$
    – PearsonArtPhoto ♦
    Apr 9, 2022 at 13:57
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    $\begingroup$ Brings up the interesting question if you can capture from Mars directly into a low-velocity Phobos/Deimos encounter and subsequent capture that's within the delta-v budget of an otherwise parachute-assisted Mars lander. I'm imagining that you get it just right you might even be able achieve astoundingly low relative velocities. $\endgroup$
    – Dragongeek
    Apr 9, 2022 at 18:48
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    $\begingroup$ @Dragongeek, no. Any aerocapture into an orbit around Mars is going to produce a periapsis near the Martian surface. Any encounter with either Phobos or Deimos will of necessity have a highly-elliptical orbit, with a minimum encounter speed in the 1-2 km/s range. $\endgroup$
    – Mark
    Apr 10, 2022 at 19:58
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The skycranes don't throttle enough, a landing is impossible.

I note objections based on the EDL systems not having the delta-v to do it. This is a high hurdle but not actually a complete showstopper. If you could guide the vehicle through the atmosphere on just the right path (note that the path will vary based on atmospheric conditions, you would need something else to tell you where to aim) you could do an aerocapture, jettison the chute unused and use the skycrane rockets to circularize.

The showstopper problem is the skycrane is horrendously overpowered for the landing. I would also be surprised if the rovers could move once it's landed, it's not built to be so light.

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    $\begingroup$ A ballpark estimate is that you'll need at least 1000 m/s propulsive delta-V to circularize at Deimos (more for Phobos). Do any of the skycranes provide that much? $\endgroup$
    – Mark
    Apr 10, 2022 at 20:02
  • $\begingroup$ @Mark this early paper suggests the descent stage has ~400 m/s $\endgroup$
    – BrendanLuke15
    Apr 10, 2022 at 20:19
  • $\begingroup$ @Mark Huh?? I'm talking about circularizing after an aerocapture maneuver. Use the heat shield to drop your apoapsis to intercept Phobos, then burn to raise the periapsis. Deimos is farther out than I remembered, the skycrane can't do that one. $\endgroup$
    – Loren Pechtel
    Apr 11, 2022 at 22:26
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    $\begingroup$ @LorenPechtel, I'm also talking about circularizing after the aerocapture. You can (theoretically) capture into an elliptical orbit with any apoapsis you like, so long as you've got a periapsis within Mars's atmosphere. Assuming I haven't messed up the math (online vis-viva calculators never take inputs in sensible forms), raising your periapsis to match Phobos orbit requires ~590 m/s of delta-V; matching Deimos requires ~680 m/s. $\endgroup$
    – Mark
    Apr 12, 2022 at 0:10
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    $\begingroup$ @LorenPechtel, I don't think the difference in "capture to low orbit" versus "capture to Phobos-height orbit" is right, but even if it is, it's not a useful number: you can't aerocapture directly into Phobos orbit. $\endgroup$
    – Mark
    Apr 12, 2022 at 21:40

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