Making 1 + 1 > 2

Wags frequently blame designs that appear to be kludges to the deliberations of committees.  Camels and wildebeests have been held up as examples.  When it was first announced that ESA and NASA would deliver two separate rovers to the same location on Mars in 2018, many commentators on message boards would apparently concluded that attributing this to design by committee might be generous.

The basic deal was that NASA and ESA would combine their landing technology demonstration (ESA), Mars Trace Gas Orbiter (NASA), ExoMars rover (ESA), and MAX-C astrobiology and caching rover (NASA) missions.  The first two elements would fly in 2016 in a mission led by ESA, while the latter two elements would be delivered to Mars in a mission led by NASA.  To keep the 2018 mission within fiscal bounds, NASA would do a single launch and a single landing on Mars that would deliver two rovers to the same location.

 The rovers would have different missions.  The ExoMars rover would have a sophisticated analysis lab (a la NASA's 2011 Mars Science Laboratory rover) with sample delivered via drill from as deep as 2 m below the surface.  The MAX-C rover would have a suite of contact instruments to study the surface soils and rocks and would also collect a cache of samples for latter return to Earth.  Still, two rovers to one location seemed like a stretch of credibility.  I suggested that NASA's rover be delayed and the ESA rover could be enhanced with the MAX-C contact instruments.  (This blog entry also had a lot of background on the two rovers.)

ESA and NASA have moved forward to look at how the two missions could be combined.  The first question appears to have been whether or not a modified MSL entry capsule and the skycrane descent and landing system could deliver two rovers to Mars.  A team has looked into this question and concluded it could, although future studies will also look at airbag landings and landers with legs a al Viking and Phoenix.

The next question is how the two rovers could operate synergistically on the surface of Mars.  That apparently is proving to be a harder question.  To understand the tradeoffs, here are some basic facts about the two missions:

ExoMars

• Primary goal: Acquire and analyze samples from up to 2 m beneath the surface
• Be capable of operating for a total traverse path length of at least [3] km.
• Be capable of conducting Mars sample location selection, sub-surface sample collection down to 2m depth, and sample analysis operations at [6] different locations for at least [180] sols.

MAX-C

• Find locations of interest for sampling and characterize composition, mineralogy, and presence of organic materials to allow sampling decision at many locations
• Select, acquire, and cache at least [38] core samples ([2] caches of at least [19] cores each) from surface materials
• Traverse path length capability of at least [20] km.
• Conduct Mars surface sample selection and coring/caching operations for at least [500] sols.

Based on the design goals, MAX-C will rove almost six times further and operate almost three times longer than ExoMars.  It will also be able to use its instruments to investigate many more sites.  If the experience of the MER rovers with their 90 day planned lifetimes is any example, then both of these rovers may operate for many years and travel many tens of kilometers.

A team has been chartered to look at joint mission opportunities.  They used a couple of analogies, apple orchards and marriage to describe the options and issues.  First, the apple analogies were used to describe how two children in an orchard looking for apples might divide the work:

In the first option, “One looking for apples, the other picking them,” MAX-C scouts for locations that ExoMars will sample with its more sophisticated instruments.  This option makes use of MAX-C’s greater mobility, faster analysis capabilities, and larger limit on the number of sites it can analyze.

In the second option, “Have a 2nd opinion on your best apple,” the two rovers would operate independently, but would combine analytic capabilities when one finds an interesting “apple”.

In the third option, “Are the apples better on different trees?,” rovers separately examine the landing area to maximize the changes of stumbling on the best “apples.”

In the fourth option, “Give you best apple to your friend to take to town,” interesting samples collected by ExoMars would be transferred to MAX-C for caching and eventual return to Earth.

In the fifth option, “Scouting to help choose the best trees to pick,” MAX-C would receive additional instruments too allow it to more efficiently search for ExoMars sampling sites.

In the sixth option, “Make sure not to run into the trees,” the landing system would be enhanced so it could land in more geologically diverse and otherwise hazardous regions to better exploit the capabilities of both rovers.

The marriage analogy comes in as the costs of the two missions getting married.  First, operating together would require extra time and complicate mission operations for both rovers.  Second, finding a landing site that would meet the two rovers engineering constraints and science objectives becomes much harder.

The team did not make any final recommendations, but suggested that the most compelling advantages for using the rovers as a team would be for MAX-C to scout for locations to study, use their different instruments suites (MAX-C surface, ExoMars subsurface) to study interesting sites, and to allow ExoMars to pass samples to MAX-C for caching.

The recommended hardware changes to the missions to enable their cooperative exploration (and most of these would be considered major changes except as noted) would be:

• Improve landing hazard avoidance to allow landing at a site that better addresses both rovers’ goals
• Modify ExoMars and MAX-C sampling handling to allow transfer and caching of samples
• Extend ExoMars roving distance to ~ 10 km and double its planned lifetime (this might be a major change; not noted in the team’s report)
• Allow two telecommunications sessions per sol with each rover to reduce conflicts for available communications bandwidth (this might be a minor change; not noted in the team’s report)

 From 2 Rover International SAG presentation to MEPAG, March 2010

Editorial Thoughts: If all of this makes you worry that this is a mission designed by a committee and possibly a kludge, remember that camels and wildebeests are highly successful species.  I suspect that the combined efforts of these two rovers will be greater than the sum of their separate contributions.  And without this marriage, the Mars Trace Gas Orbiter would be a minimalistic endeavor instead of the highly capable mission now planned and ExoMars might not fly at all.   That's a "committee" decision I have grown to like.

Resources: The following presentations form the basis of this report and were presented at the March 2010 MEPAG meeting

2018 mission update

2 Rover International Science Advisory Group

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.

Resources:

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