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Several robotic Mars surface missions have a planned mission duration of 90 sols.

I would like to understand why this specific mission lifetime is commonly used by mission planners.

Below is a non-exhaustive list of a few past/present/proposed missions where this has been the case, where the original planned mission duration can be found in the associated hyperlink:

Interestingly, these missions do not have similar landing site locations or primary mission objectives.

Clearly missions that successfully land can often go on to have an operational lifetime far longer than this (notably the Mars Exploration Rovers, Spirit and Opportunity, which far outlived their original planned mission duration by several years).

The rationale for including a deadline to meet all mission objectives is obvious; mission operations are expensive. A mission which can complete all its primary objectives in a reasonable timeframe is likely to be far more appealing to the stakeholders funding the mission. Similarly, proposing a robotic mission which drags-on indefinitely when its primary objectives can be achieved in a short period of time is unlikely to get funded.

But why specifically 90 sols? Why do many missions consider the same primary lifetime? Why not 80 or 100 sols?

I believe the answer has something to do with the fact that all these missions listed above are solar powered. Curiosity and Perseverance are notable exclusions from the list – both use RTGs which are likely needed to achieve their respective mission objectives that cannot be achieved in a “short” mission.

Dust accumulation might be the reason, but I cannot find a source that states this directly. Furthermore, the Mars Global Dust Storm Season (GDSS) is commonly cited as occurring between solar longitudes Ls180 and Ls360 (i.e. from the start of NH Autumn to the end of NH Winter). The time between two potential GDSSs is ~372 sols, far greater than 90 sols.

Expected solar irradiation through NH Spring and Summer might be another reason, but mission duration could then be augmented to match the energy budget by varying solar array size – so the question remains, why 90 sols?

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    $\begingroup$ "But why specifically 90 sols? Why do many missions consider the same primary lifetime? Why not 80 or 100 sols?" – Is there any reason to think the answer is not simply "because 90 is a nice round number in an Earth calendar" (i.e. 3 months). $\endgroup$ Dec 23, 2021 at 12:46
  • $\begingroup$ @CuteKItty_pleaseStopBArking oh that's a really interesting bit of orbital mechanics! Please consider posting as a partial answer (e.g. "I haven't yet found a supporting source, but this certainly seems like a partial explanation that's hard to ignore:" or similar.) $\endgroup$
    – uhoh
    Dec 23, 2021 at 22:11
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    $\begingroup$ Doing a back of the envelope calculation (using numbers from memory which is dangerous), solar conjunctions are every 26 months. So opposition to conjunction is 13 months. Hohmann transfer is 7 to 8 months, so arrival is about 4 months after opposition. That leaves 8 months until conjunction which is a lot longer than 90 or 100 solstice. The solar blackout is only a few weeks nowadays. $\endgroup$
    – JohnHoltz
    Dec 23, 2021 at 22:31
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    $\begingroup$ @uhoh The comment from CuteKItty_pleaseStopBArking disappeared; what did it mention? $\endgroup$ Dec 25, 2021 at 4:13
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    $\begingroup$ @AlexHajnal I guess I spooked them somehow. Earth to Mars Hohmann transfer work every synodic period, which is 2.14 years. CuteKitty mentioned that about 100 days after such an arrival Mars would pass close to the Sun seen from Earth and there would be a communications blackout, and that might have something to do with it. This is my best recollection of what was there. (see also Are launch windows to Mars avoided if they result in landings during dust storm season?) $\endgroup$
    – uhoh
    Dec 25, 2021 at 4:27

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I believe I might have found an answer to this question in this interesting paper, which reviews the origin of duration and distance requirements for planetary mobility systems.

It appears the 90 sol value could tie its origin to the early studies for the Mars Exploration Rovers - Spirit and Opportunity. According to the paper, at the time of MER's early formulation it was reportedly in competition for selection against alternative mission concepts for the 2003 launch opportunity:

Specifically, a decision as to whether an orbiter or a lander/ rover would be developed for the 2003 opportunity was pivotal, and the anticipated scientific capability (and thus engineering requirements to support them) had to be attractive enough to merit selection.

In order to remain competitive, MER's mission design team were encouraged to extend the mission from an originally proposed 30-sol duration to be at least equal to the 86-sol lifetime of the 1997 Sojourner rover (Mars Pathfinder). According to design studies at the time, the team believed to be able to maintain a positive power margin up to 91 sols. The team then accepted a 90-sol mission.

As the paper states, it is an instance where the capability became the requirement.

It may be the case that several missions after MER have simply used this as a bench mark requirement.

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