Considerations on Mars Sample Return
Returning Mars rocks to Earth has always been viewed as the "holy grail" of unmanned, planetary science missions. But the challenges involved in doing so have necessitated a big budget and lengthy schedule for the mission, in comparison with other probes. Before the failures of Mars Climate Orbiter and Mars Polar Lander in 1999, a Sample Return was tentatively scheduled for 2011.
The number of design trades in the Mars Sample Return mission is astounding. For instance, consider the following:
--Do you have a separate lander and orbiter, or will the entire craft descend to the surface of Mars?
--If the craft separates into a lander and orbiter, will the two travel to Mars mated to each other, or will they be delivered by two separate launchers?
--Will the ascent stage of the lander be launched from earth fully-fueled, carrying hypergolic propellants? Or will it produce some or all of its fuel from the Martian atmosphere, as Bob Zubrin has suggested?
--Will the lander collect samples from just its landing site, or will it have at least one rover for collecting samples at different locations?
--If the ascent stage has to rendezvous with an orbiting stage that will return the samples to earth, how will the samples be transferred autonomously from the ascent stage to the orbiter?
--Will the Sample Return Capsule be recovered at sea, on land, or snatched from the air? What measures will be taken to quarantine the samples?
The functionality and complexity of the Mars Sample Return mission will be dictated by the capabilities of the launcher that is baselined for the mission. While Ares I or Ares V might be available by the time MSR launches, the conservative engineer and program manager will baseline a vehicle that exists in hardware form today, with a well-characterized vibrational profile and established performance levels. The only vehicle up to the task is Delta IV Heavy. And while it's tempting to launch two 22-tonne spacecraft and have them rendezvous after reaching Mars (such as an orbiter and lander pair,) conservative engineering practices would frown on making such a rendezvous so far away from earth (at least for initial missions.) Thus, I would not budget any more than 22 metric tons for the spacecraft and its earth departure stage on an initial Mars Sample Return mission. This would appear to rule out a separate rover on the first MSR.
Because MSR is so far into the future, NASA needs a proactive strategy for keeping Delta IV Heavy in production by the time MSR reaches hardware stage. Because Delta IV and Atlas V have common spacecraft interfaces, it might be possible to switch to an Atlas variant (either Atlas V Heavy or wide-bodied Atlas) if United Launch Alliance switches to an all-Atlas fleet. But doing so will also change the vibe profile, potentially affecting the spacecraft.
Beyond the fundamental challenges of mission architecture, there is the issue of how much MSR will demonstrate technologies essential to a human Mars mission. I see great potential for using the Sabatier reaction to produce ascent and Mars-departure propellant. But until the Sabatier reactor is proven in a relevant environment, there's no way the taxpayers will invest in a Mars mission (even an unmanned one) that relies on an unproven technology for mission success. In order for the Sabatier reactor to fly on MSR, it must be tested as an experiment on a near-term Mars mission. This was the plan for the Mars Surveyor 2001, before it was canceled and re-scoped as Phoenix. But here's a novel idea: why not test a Sabatier reactor as part of a Mars rover that could be put into hibernation and then re-activated to collect samples for MSR? It's a possibility, if the rover can be built robustly enough to survive an extended hibernation.
MSR is going to be a costly and expensive mission to pull off. It will be virtually impossible to pull off (and thus a waste of taxpayer money) if NASA takes risky gambles with immature systems. The extreme conservatism in design that the mission demands will probably result in an underwhelming science return for the cost of the mission. From a scientific standpoint, it might be more worthwhile to perform experiments in-situ on Martian soil. From an engineering standpoint, MSR will be a valuable step towards sending real scientists to Mars in the future.