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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?

Starship - On Strike
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2 Answers2

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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.

PearsonArtPhoto
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    1): the rover have the extra fuel, it doesnt take much since phobos and demios have such little gravity – Starship - On Strike Apr 08 '22 at 23:36
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    2):Why is it so hard to change a landing sequence with 6 months notice. How long does it take them to create it orginally anyway? – Starship - On Strike Apr 08 '22 at 23:37
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    3):Wouldn't having less gravity make them move better, not worse @PearsonArtPhoto – Starship - On Strike Apr 08 '22 at 23:37
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    @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. – GremlinWranger Apr 09 '22 at 01:53
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    @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. – Starfish Prime Apr 09 '22 at 09:09
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    +1 This is a great answer, it could be useful to add @GremlinWranger's explanations to the appropriate points. – Dave Gremlin Apr 09 '22 at 12:08
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    @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. – PearsonArtPhoto Apr 09 '22 at 13:57
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    @Starshipisgoforlaunch see this question for Phobos gravity insights – BrendanLuke15 Apr 09 '22 at 17:34
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    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. – Dragongeek Apr 09 '22 at 18:48
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    @Starshipisgoforlaunch, to give you an idea of the problems with moving in low gravity, if you were to do a standing high jump on the surface of Deimos, it would take you almost 45 minutes to come back down. Similarly, if a rover were to hit a bump and bounce up, it might take a minute or more to come back down -- with a very real risk that it doesn't land wheels-side-down. Any practical surface exploration craft for Phobos or Deimos is going to be a hopper, not a rover. – Mark Apr 10 '22 at 19:55
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    @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. – Mark Apr 10 '22 at 19:58
  • @Mark I'm not quite so sure: Phobos and Deimos are very close to Mars. Phobos, specifically, has an orbital velocity of around 2km/s and an eccentric orbit that nearly scrapes the surface would have around 1.5km/s at periapsis if my back-of-the-envelope calculation is correct. Still a big difference, but not multiple km/s – Dragongeek Apr 11 '22 at 18:32
  • Add (5) Phobos and Deimos are very small moons (< 10 miles across) - too small to land a rover without overshooting the target? (Although I suppose JASA landed a probe on a smaller asteroid so not impossible.) – RobertF Apr 11 '22 at 19:50
  • @RobertF It's easier to land on a target without an atmosphere in a specific spot, so... – PearsonArtPhoto Apr 11 '22 at 20:05
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    @Dragongeek, now that I've actually done the math, you're sort of right: ~590 m/s for Phobos, ~680 for Deimos. Not what I'd call "astoundingly low", but a fair bit lower than my initial estimate. – Mark Apr 12 '22 at 00:14
  • Note that you also have to aerocapture into an orbit with the same plane as the target moon, within a degree or so of equatorial, Mars itself having about a 25 degree tilt with respect to its orbit. This is pretty much only going to be possible if the trajectory was designed to target the moons from the time it departs Earth, otherwise there will be a costly plane change on arrival. – Christopher James Huff May 09 '22 at 17:14
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    @Starshipisgoforlaunch parachutes and heat shields add little mass? BS. Please check your facts before making edits--or better yet, leave well enough alone. It's getting tiring keeping up with correcting you. – Erin Anne Mar 08 '23 at 23:40
  • I changed from no mass, so that is an improvement. @ErinAnne – Starship - On Strike Mar 09 '23 at 00:06
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    @Starshipisgoforlaunch ??? the side-by-side doesn't show "no mass" in the prior version. This edit is harmful. I'm rolling it back. – Erin Anne Mar 09 '23 at 00:29
  • I have fixed this and returned to mine as it made other improvements @ErinAnne – Starship - On Strike Mar 09 '23 at 00:40
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    @Starshipisgoforlaunch Your edits changed the answer to no longer answer the original question. I've kept the first few that you did do some useful editing, but rolled back to before you started making more drastic edits. – PearsonArtPhoto Mar 09 '23 at 02:18
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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.

Starship - On Strike
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Loren Pechtel
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    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? – Mark Apr 10 '22 at 20:02
  • @Mark this early paper suggests the descent stage has ~400 m/s – BrendanLuke15 Apr 10 '22 at 20:19
  • @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. – Loren Pechtel Apr 11 '22 at 22:26
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    @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. – Mark Apr 12 '22 at 00:10
  • @Mark My understanding was more like 200 m/s but I don't recall an actual value. The difference in the capture to low orbit vs capture to Phobos is well under 200 m/s. – Loren Pechtel Apr 12 '22 at 04:44
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    @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. – Mark Apr 12 '22 at 21:40
  • I once worked on an asteroid lander concept. The gravity was so low that whenever we wanted to move the sampling arm, it had to be veerrrrryyyyy sllooowwwllllyyyy or else the lander would tip over or maybe leave the asteroid entirely. And we weren't even trying to move. – John McCarthy Nov 22 '23 at 16:29