Sunday, October 25, 2009

Two Strategies for Mars Rover Instruments

ESA and NASA are both planning small Flagship ($1-2B each) rover missions to explore Mars in 2018. The agencies have recently decided to merge their efforts to pool costs. As plans stand now, NASA will provide the launch vehicle and a skycrane entry-descent-and landing system. Each agency will provide its own rover, which will be simultaneously delivered to the same location by the skycrane system.

They idea of sending two rovers to the same location has caused a lot of raised eyebrows (to put it mildly) at Unmanned Spaceflight. In this blog entry, I want to explain why this isn't necessarily as stupid as it sounds and to discuss what I think may eventually happen.

In any mission, there's are fundamental tradeoffs that drive the mission design made to keep costs reasonable. For ExoMars, that key tradeoff was to acquire samples using a deep drill (up to 2 m) that would get beneath the level of organic sample degregation. Samples would then be processed by a very sophisticated set of instruments housed inside the rover. A fundamental tradeoff to this approach is that ExoMars will not have a robotic arm to acquire samples or place instruments in contact with rocks or soil. (The Mars Science Laboratory Curiosity will use a sample arm with a drill to deliver samples to its own internal suite of instruments as well as place instruments in contact with the surface.)
NASA's 2018 MAX-C rover has a primary task of selecting, acquiring, and caching samples for a potential future Mars sample return mission. The system to acquire, handle, and store the samples is quite complex and heavy (tens of kilograms). As a result, there is insufficient mass and space for a suite of internal instruments. Instead, the mission proposes to have a set of highly advanced contact instruments on a robotic arm. Unlike MER and MSL, these instruments will be able to study micro-variations in composition across the contact point much as might be done in a terrestrial laboratory with a sample. The instruments can examine the contact area in multiple spectra, measure elemental and minerological composition, and measure organics (if present). The instruments are potentially light, perhaps 15 kg (probably not including the robotic arm, although the presentations are not clear on that point). Currently, many of these contact instruments are in a low state of technology readiness, but with nine years to flight, there's time to address that issue.
ExoMars also will carry a ground penetrating radar and a power wide- and narrow-angle camera system. MAX-C presumably would also carry a capable imaging system and tentative plans have it carrying a spectrometer on the mast for remote identification of surface composition.

No single rover can do it all. If you want to sample deep beneath the surface, have a sophisticated laboratory of instruments inside the rover, have sophisticated contact instruments, and acquire and cache samples, you need multiple rovers. Flying ExoMars and MAX-C to the same location would provide complimentary, not redundant, measurements.

Editorial Thoughts: In a world with unlimited budgets, flying two rovers to the same location would be wonderful. In a world of constrained budgets, it seems unlikely to me to happen (but please, ESA and NASA, prove me wrong). So, if we are reduced to one rover, what should it look like? That depends on priorities, and your's, mine, and the scientific community's may be quite different. But here are mine. I think the idea of a deep drill with an internal laboratory is compelling, but the number of samples is likely to be limited either by drill life or the number of experiment chambers. (An ExoMars presentation states that there would be six sample acquisitions.) I also think that the idea of an infinitely reusable suite of contact instruments on an arm is powerful. (An arm also allows measuring locations on the sides of rocks, hillslopes, and rock/soil faces that would be challenging or impossible for a deep drill.)

Caching samples is less compelling to me. Before the flight of ExoMars, we won't know, for example, how important it is to acquire samples from deep beneath the surface. What happens if the site you dedicated a rover to sample turns out not to be the one you want to return samples from? And will Europe and the U.S. actually fund a $5-6B return mission? For me, the more compelling sequence of missions is to study several locations with science oriented rovers such as ExoMars and MSL. Then, if funding for a sample return comes through, fly a rover dedicated to acquiring samples followed by the return vehicle. (Note: The Mars scientific community would disagree with this and is willing to forgo a much more sophisticated suite of instruments on MAX-C to kick start the move towards a sample return.)

If budgets or landed weight limits restrict the 2018 to a single rover, what I think would be compelling would be to add an arm with micro-scale contact instruments to an ExoMars rover. The arm and its instruments would be relatively light (20 - 25 kg?) become a complete subsystem that can be developed and supplied by NASA, simplifying the development interfaces.

In the best of all worlds, I would advocate enhancing ExoMars with the MAX-C arm and instruments for 2018. Then I'd fly MAX-C in 2020 with its arm and instruments and caching to either the same site (if ExoMars finds compelling reasons to make it the site for a sample return) or to a new site (or possibly the Mars Science Laboratory site if it is the compelling site). All it takes is money.

As the following two slides highlight, the question of how to merge the two missions is one the two space agencies are wrestling with.


All images and slides are from the following presentations:

ExoMars: ESA’s Mission to Search for Signs of Life

Proposed 2018 Mars Astrobiology Explorer-Cacher (MAX-C) Mission


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