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Stickney crater on Phobos
Stickney crater on Phobos Phobos monolith
For both images, credits: NASA/JPL

The image above shows the Phobos monolith near Stickney crater, a sign or message to go there first ?

An example of an advantage is the possibility to apply the sandbraking technique for landing large payloads on Mars.
Another advantage being on Phobos could be the possible presence of water ice below the regolith. for producing rocket fuel there.

And finally could all the pros and cons be weighed together to come to an overall conclusion ?

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    $\begingroup$ Phobos is quite a lot harder to reach than Mars surface. It has too little gravity to be of any use for health or to aid construction.. I try hard, but can think of zero benefit to being in a specific 6000km orbit around Mars chained to a ball of dust, rather than either on the surface or in your own chosen orbit. $\endgroup$ Commented Dec 19, 2021 at 18:42
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    $\begingroup$ Kim Stanley Robinson explored a lot of the many possibilities brought by utilizing Martian moons in a Mars mission. It's pretty interesting and a lot of thought was put into it. Red Mars and Green Mars have the most moon action in them. Might let your imagination roam instead of looking for scientific answers? $\endgroup$ Commented Dec 19, 2021 at 19:06
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    $\begingroup$ The Phobos monolith is not a sign to go there first. $\endgroup$
    – J...
    Commented Dec 20, 2021 at 0:23
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    $\begingroup$ Presumably the disadvantages include the risk of accidentally opening a portal to hell... $\endgroup$ Commented Dec 20, 2021 at 2:01

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This NASA paper explores some of the rationales and implications of using Phobos as a staging point for human Mars surface missions. The conclusion is stated in the abstract:

Results from this analysis indicate that, if the objective is to conduct Mars surface missions, utilizing Phobos as an exploration destination adds little benefit toward the goal of exploration of Mars.

The paper includes some discussion of some of the key figures of merit associated with staging human Mars missions at Phobos compared to staging in High Mars Orbit (as has been proposed in several other mission architecture concepts). These are summarized in the table below:

enter image description here

The paper also outlines some unique challenges associated with landing on the surface of Phobos, which would not be present for Mars surface missions (these have also been addressed in other answers):

  • Increased crew time spent in the hazardous “free-space” environment i.e., increasing exposure to micro-gravity and radiation.
  • Increased delta-v costs for Phobos surface access.
  • Additional exploration systems and technologies are needed for transiting to Phobos, maneuvering and living the low-surface gravity, and performing mission operations with minimal disturbance to surface dust – these systems present little synergies with exploration systems needed for Mars surface missions.
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    $\begingroup$ Well-referenced answer! $\endgroup$ Commented Dec 20, 2021 at 9:48
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    $\begingroup$ Braking. Breaking would be undesirable. I'm not aware that anyone has seriously studied sandbraking, and it honestly doesn't look that promising. You need more propellant to get to Phobos to pick the sand up in the first place, and it's then more mass you have to drag back to Mars and through the hard part of reentry, not to mention the sandblasting-proof pusher plate. And it seems to be an attempt to solve the traditional Mars-landing "problem" of getting the vehicle subsonic, when SpaceX has been routinely demonstrating supersonic retropropulsion for years now. $\endgroup$ Commented Dec 20, 2021 at 20:05
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    $\begingroup$ @Cornelis - sandbraking is not covered in the paper. My opinion on this technique is that while novel and interesting it is not a practical solution. The main rationale for employing this technique may be to reduce launch mass. However, the technique itself would require additional propellant to access Phobos and additional system hardware (i.e. dry mass) to collect and release Phobos regolith. This additional mass could instead be spent on additional EDL hardware + propellant for conventional direct entry of large payloads without added operational complexity and risk of a Phobos rendezvous. $\endgroup$
    – Armadillo
    Commented Dec 21, 2021 at 7:49
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    $\begingroup$ @Cornelis - apologies, I didn't realise that was a locked version - I've updated the hyperlink now. Just in case, you can access the paper here : ntrs.nasa.gov/api/citations/20200001602/downloads/… $\endgroup$
    – Armadillo
    Commented Dec 21, 2021 at 13:20
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    $\begingroup$ @Cornelis - The paper does not directly discuss utilization of Phobos water-ice, however, the same arguments used against the sandbraking technique would also apply here, i.e. the delta-v and additional system hardware needed to access Phobos resources make it a hard sell if the objective is to eventually explore Mars. $\endgroup$
    – Armadillo
    Commented Dec 21, 2021 at 13:27
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  • Establishing a human presence on Phobos would require development of a vehicle specifically for that purpose. Such a vehicle would have a far larger fraction of its arrival mass devoted to propellant than a Mars lander would need. This likely means a lower energy transit and longer transit time, with the added risks that entails.
  • The habitats and other ground infrastructure would also be completely different. You're suggesting adding a whole new exploration program that would have virtually nothing in common with a Mars mission.
  • We have zero experience operating on asteroid-like bodies, so virtually everything involving Phobos would be involve greater risks and uncertainties.
  • The density of Phobos hints that it may either contain water ice or be highly porous, but its surface appears to be dry, making porosity the more likely explanation. Mars is known to contain vast reserves of water ice in near-surface deposits.
  • Phobos has no CO2 atmosphere, so if it does have water ice, ISRU could only fully supply return propellant for hydrolox systems, which have various cost and complexity drawbacks.
  • The lack of atmosphere also means Phobos has far harsher radiation, thermal, dust, and micrometeoroid environments.

A vehicle such as Starship is designed to land on Mars and would at minimum require major changes to reach Phobos directly from Earth. The same vehicle would have little difficulty doing a Phobos mission if loaded with propellant after landing at a Mars base. Putting a crewed base on Phobos first would delay Mars exploration by decades, and may actually delay Phobos exploration, which would benefit far more from a Mars base than vice versa.

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    $\begingroup$ No advantages for being on Phobos first ? $\endgroup$
    – Cornelis
    Commented Dec 19, 2021 at 17:42
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    $\begingroup$ pro: phobos can provide a lot of mass in orbit already, to provide primitive radiation shelter for ships that (for whatever reason) need to loiter near Mars for many months or years. Can provide a near-infinite inertial anchor for tether-slingshot type activity. But both of these are longterm things, not for first missions to the region. $\endgroup$ Commented Dec 19, 2021 at 18:46
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    $\begingroup$ @Cornelis If your goal is to land on Mars, Phobos is a substantial detour, and would add a great deal of cost and complexity to your mission. Even if you have a spacecraft that propulsively enters Mars orbit, it would be more reasonable to meet up with one or more orbiting stations which could adjust orbit ahead of time to match incoming spacecraft. There are things Phobos could be useful for, but this is not one of them. $\endgroup$ Commented Dec 19, 2021 at 22:03
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    $\begingroup$ "Such a vehicle would have a far larger fraction of its arrival mass devoted to propellant than a Mars lander would need" - nope, only slightly larger (just a small additional amount to raise periapsis out of the atmosphere following aerocapture and to transfer to Phobos - Phobos's gravity, and, thus, escape speed, is so low that the vehicle would be able to safely lithocapture without needing any propellant for the Phobos landing itself). $\endgroup$
    – Vikki
    Commented Dec 19, 2021 at 23:08
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    $\begingroup$ @Vikki if the orbital planes are perfectly aligned and your aerocapture maneuver results in apoapsis perfectly matching the altitude of Phobos, neither of which is going to be the case. Aerocapture will be even more sensitive to atmospheric conditions than direct EDL (and you will need propellant to accommodate the worst case, especially when carrying crew), and plane changes are expensive. And then there's the little detail of this requiring a maneuver that has never been attempted, even by spacecraft that just had to manage capture rather than targeting a precise orbit afterward. $\endgroup$ Commented Dec 20, 2021 at 1:11

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