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I wonder how practical it would be for a spacecraft to visit both moons of Mars on the same mission. Areostationary orbit lies between the two moons, and GEO is demanding to reach from LEO.

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Assuming coplanar and circular mars orbits, going from 9378 km radius to a 23459 km radius orbit takes about .75 km/s.

Tack on a little more since the moons aren't quite coplanar. Also tack on a little more for rendezvous delta V. So maybe .8 km/s.

The Vis Viva equation can be used to to determine circular orbit speeds as well as the periapsis and apoapsis speeds of the Hohmann transfer ellipse.

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  • $\begingroup$ If one goes to Phobos anyway, wouldn't one have to pay for those 0.75 m/s in any case? Isn't it "free" to land on Deimos when passing it on the way in or out or both? (0.05 m/s is insignificant). Maybe it requires more time, but the distances are small. $\endgroup$
    – LocalFluff
    Aug 1, 2015 at 10:26
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    $\begingroup$ @LocalFluff We can't stop at Deimos "on the way" without expenditure of delta V. Soft landing on Deimos would entail circularizing orbit at Deimos' altitude. $\endgroup$
    – HopDavid
    Aug 3, 2015 at 4:40
  • $\begingroup$ I see. 0.8 km/s is just for going between the moons after having landed on one of them. And how much is added for approaching the first moon depends alot on how it is done. Is it correct that there's no libration point landscape at Mars useful other than just at the surfaces of the moons, and that the transfer between the moon is too slow to think of using Mars atmosphere for aerobraking? 0.8+ km/s extra at Mars is not for free, but it buys a second moon(let). $\endgroup$
    – LocalFluff
    Aug 3, 2015 at 9:29
  • $\begingroup$ Phobos is 5700 km above Mars' upper atmosphere. So it's not possible to use aerobraking to circularize at Phobos' altitude. Not sure what you mean by "libration point landscape" $\endgroup$
    – HopDavid
    Aug 3, 2015 at 14:48
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According to this delta-V map it requires at least ~1.5km/s to get from one to the other through an intermediate orbit, so as HopDavid notes, there's a shorter direct path.

That's a substantial maneuvering investment, but less than getting a lander to the surface of Mars (let alone back up), and it could be done with a very lightweight lander.

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  • $\begingroup$ Between Phobos and Deimos there's "Deimos Transfer Orbit" as well as "Phobos Transfer Orbit" on that delta V map. Not sure what is meant by those two intermediate destinations. A direct route would be a Phobos to Deimos Transfer Orbit. This a 9378 x 23459 km ellipse. Periapsis speed is 2.56 km/s, apoapsis speed is 1.02 km/s. Phobos speed is 2.14 km/s. Deimos speed is 1.35 km/s. $\endgroup$
    – HopDavid
    Apr 21, 2015 at 15:15
  • $\begingroup$ Guessing the intermediates are Mars-to-moon orbits? I defer to your better-informed answer, anyway. $\endgroup$ Apr 21, 2015 at 15:38
  • $\begingroup$ Your 'delta-V map' link throws a 403 error. $\endgroup$
    – Mast
    Apr 21, 2015 at 16:00
  • $\begingroup$ Found a different one. The 403 seems dependent on whether you've visited the Project Rho web site previously or are deep-linked in. $\endgroup$ Apr 21, 2015 at 16:11
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I'm sure others will provide Hohmann numbers, but if you happen to be using low-thrust propulsion that requires spiraling down, the delta-v approximately equals the difference in Phobos and Deimos's orbital velocities, so about 800 m/s.

This is a theoretical limit for very low thrust and very long transfer times, many hundreds or thousands of orbits during the spiral, as discussed in a little more detail in this answer.

However, while Deimos' orbit is nearly circular, the orbit of Phobos has an eccentricity of about 0.015, so some additional details need to be considered - was the starting orbit a circular flyby at Phobos' periapsis, or a matched ellipse for example. These would have a small additional impact on the delta-v needed.

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  • $\begingroup$ Can you find a supporting link for that? It might be found within another answer within this site on a similar topic, or a separate site. Welcome to stack exchange! It iss a little different than other sites. If you take the tour you'll see that answers with factual statements should usually contain supporting information with links or references. $\endgroup$
    – uhoh
    May 25, 2017 at 0:10
  • $\begingroup$ Actually I'll just go ahead and add it to your answer in this case - I remembered where one was. $\endgroup$
    – uhoh
    May 25, 2017 at 0:18

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