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Following the question on re-fuelling en-route to Mars, I am interested in exploring a mission architecture in which a crew may be offered different options for impromptu mission changes, for example in case they have to abort (definitively or conditionally) the nominal landing on Mars.

Within the re-fueling paradigm (for a discussion on "free-return" option, see Note 1 below), one of the possible options is to re-route to a "depot" where propellant and/or life-support amenities have been pre-positioned. What would be the best locations for such depots? Among the following possibilities (non-exhaustive): Mars low-orbits (MLO), Mars satellites, Mars-Sun libration points, a carrousel of cargos orbiting the Sun near to Mars's orbit, ...

"Best" is understood in terms of maximizing rescuing/recovering success, in particular as dictated by the time and energy needed to move from/to a "coasting" orbit (not necessarily MLO).

Assume for the time being that the delta cost to bring the goods to the different locations is "negligible".

Edit-Note 1: A free-return option would not necessitate re-fueling in principle. However, the round-trip journey can take between 2 years to almost 5 years, the faster the outbound to Mars, the longer the round-trip. This would entail carrying more food and water if such an option is included in the mission.

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There are a lot of variables in this question concerning exactly when an abort is required and what has caused the abort.

Some Mars mission profiles can use a free return trajectory. If this is used and for whatever reason it becomes necessary to abort, it is relatively easy to return to Earth by applying a small course correction. This will allow the vehicle to swing around Mars and be redirected back to Earth with little further propellant requirement. This option would be available during 95% of the journey to Mars. During the final approach the amount of propellant required to alter course would increase until it was no longer possible. Beyond this point the vehicle would be committed to Mars and aero braking or capture.

Once in LMO the best position for a depot would be in the same LMO to allow the vehicle to re-tank with propellants and return to Earth. However it should be pointed out that such a depot would need to contain a great deal of propellants and moving those propellants from LEO to LMO would be even more costly. And getting those propellants into LEO… but I digress as we are assuming the cost of that is negligible.

The safest place for the crew on such a mission is the surface of Mars. On the surface of Mars the atmosphere and Mars itself will provide significant protection from radiation and further “sand bagging” might be possible to provide even more protection. Mars also provides a gravitational field that is likely to be beneficial for the crew compared to zero gravity in orbit. And Mars may also offer the prospect of re-tanking the ship from processing local materials and might also provide water, oxygen and nitrogen depending on the equipment the crew have brought with them.

But assuming the vehicle cannot land the best option for the depot is whatever orbit the vehicle can reach. If the cost of placing the depot is truly negligible then it might be better to provide two or more depots to better align with the abort options. Perhaps one in LMO in case the heatshield works but the engines don't and one in a much higher orbit in case the engines work but the heatshield doesn't.

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  • $\begingroup$ Thanks! We can imagine the following reasons to abort: (1) detection of a malfunction that makes the descent unsafe; (2) detection of a malfunction that prevents the landing place to be near enough to the base camp; (3) the transfer orbit is not accurate enough for proper alignment with Mars at arrival; (4) bring back a sick crew member. $\endgroup$ – Ng Ph Jun 15 at 7:10
  • $\begingroup$ I understand that there is a low-energy option to abort Mars landing and head back to Earth (~ Apollo 13). Would it take the same time as the outbound leg? How easy (safe) is it to re-coast in LEO for a spacecraft designed to land on Mars (only)? $\endgroup$ – Ng Ph Jun 15 at 7:23
  • $\begingroup$ When an abort has to be decided? Just after escape from Earth and before the maneuver to enter Mars capture. $\endgroup$ – Ng Ph Jun 15 at 7:26
  • $\begingroup$ Bringing tons to Mars always has a cost, especially together with the crew. But it is not the same for ferrying goods to a depot. We will be using the cheapest means (even to LEO) because we can accept high failure rate, we do not have to take the fastest route, we are not bound to a launch window every 26 months. We will use it only rarely. So let's assume at this stage that the benefits are worth the "hassle". We will debate when all "realistic" options to implement depots are on the table. $\endgroup$ – Ng Ph Jun 15 at 7:46
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    $\begingroup$ Free return trajectories come in multiple options. Basically the greater the initial delta V the shorter the transfer to Mars but the longer the subsequent free return journey is. Some specific examples are quotes here: en.wikipedia.org/wiki/Free-return_trajectory $\endgroup$ – Slarty Jun 15 at 8:38

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