Mars 2020: What are the advantages of doing two missions to return samples?

Question

The Mars 2020 plans to create a cache with samples for a future mission to grab and bring back. What are the advantages of doing one mission to create the cache and one mission to bring the cache back to Earth, rather than doing a single mission that collects the samples and brings them back?

At first, I thought that it might be more convenient to have one rover gather everything into one spot. Then you have all the samples in a single spot, so that you just have to land a second non-roving spacecraft, grab and take off again. But from further reading, it appears that the caches will not be all left in the same place. So you would still need a rover to roll around the surface to pick the caches up.

What are the advantages of separating the sample return mission into two missions ?

Answer 1

I think the reasons for caches, and for multiple caches are twofold.

Firstly, a fairly likely end-state of a rover is that it gets stuck somewhere horrible, and in particular it gets stuck somewhere you really do not want to be landing something near. That means that the rover needs not to carry its samples with it: if it does, then if it gets stuck, then those samples are lost. So it must cache the samples. Caching them has the additional advantage that you know that the rover can both get to and leave the cache, unless it gets stuck at the first cache, which risk you can minimise but not eliminate.

Secondly want to leave multiple caches because you don't want to keep coming back to the same place: since it takes you ages to get anywhere, if you have to keep returning to some central point you reduce the amount of science you can do as you waste more time coming back.

The reason for doing it in multiple missions is, I think, simple: we can make, today, a rover which can land on Mars & collect & cache samples and do other scientifically-useful things, but we can't make, today, one that can do all that and leave. So we either do that now, and wait for some later sample-return mission, or we don't fly a rover until we have the sample-return one sorted out, which may even be less capable in other ways than the one we can fly today because it has to devote more resources to the sample-return bit. So they've chosen to fly the rover they can fly rather than wait for years for one that may never exist and may be less capable in scientific respects when it does.