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