Tuesday, December 8, 2009

New Mars Sample Return Plan

At the last meeting of the Mars panel of the Decadal Survey, a detailed plan for conducting a Mars sample return.  The slides have not been posted, but Bruce Moomaw acquired a copy and wrote the entry below.  If you are interested you can watch the presentation, although video and audio drops out fairly frequently and at some key points.  Go to http://nasa-nai.na6.acrobat.com/p26625026/ and fast forward through the first three presentations (although the third is an excellent rational for doing a sample return).

Bruce's entry follows:

At the second meeting of the Mars Panel of the current Decadal Survey project on Nov. 4 to 6, JPL's Mars Exploration Program manager Fuk Li described the current status of studies of the design for a Mars sample return (MSR) mission.  Two themes stood out, which I'll describe in two separate entries.

The first is the recent change in the favored design for the mission.  Previously it was conceived as consisting of two launches at an interval of about four years.  The first would be an orbiter, the second a lander.  The lander spacecraft -- besides the underlying platform -- would consist of two components.  One would be a Mars Ascent Vehicle capable of launching a small canister containing about half a kilogram of Mars samples into a low orbit around Mars, after which the orbiter spacecraft would carry out an automatic rendezvous with the canister, capture it, and then blast itself out of Mars orbit and back to Earth to fly by our planet and drop off an Earth return capsule carrying the canister.  The other main component of the sample-return lander would be a long-range rover capable of using onboard instruments to analyze the Martian surface and identify promising sites for sample collection, and then drill up small rock cores that would be cached by the rover in a collection container.  The rover would then drive all the way back to the main lander and load the sample container into the canister on top of the Mars Ascent Vehicle, which could then launch itself.

However, Dr. Li reported that there is now a consensus developing -- which he himself has come to strongly agree with -- for the Mars sample return mission to be split not into two component launches, but into three.  The sample-collection rover would be launched separately first -- preferably in 2018 -- as the "Mars Astrobiology Explorer and Cacher" (MAX-C).  This 300-kg rover would drive as much as 20 km across the surface during a lifetime of at least 500 Sols (Martian days), carrying out its analyses and sample collection, and would end up back in the center of its small landing ellipse (with a radius of about 6 km). 

Four years later the orbiter would be launched, and then four years after that the lander would be launched.  This lander, however, would instead carry a smaller, simpler "Fetch Rover" -- a bit smaller than the current Mars Exploration Rovers (around 155 kg) and somewhat simpler in overall design, but designed to drive faster and farther (a minimum of 12 km).  Its sole function would be to hustle across the surface to the MAX-C rover, retrieve its sample container with an arm simpler than that on MAX-C, and return directly with the container to the lander -- which would have been targeted to land as close as possible to the center of the MAX-C landing ellipse.  (The fact that it too might land as much as 6 km off target explains the need for a 12-km total driving capability for the Fetch Rover.)  The rest of the mission would follow in the same way as the two-component mission design.

Li explained the multiple reasons for the design change.  To begin with, it should be kept in mind that the rover intended for the two-component version of MSR would need virtually all the same instruments as MAX-C in order to identify good collection sites for its small collection of precious samples.  Why does the new mission design fly this rover separately?

(1)  The original mission design, after landing on Mars, had to work against the clock.  As Li said, "The Martian surface environment is not very friendly" -- particularly to the Mars Ascent Vehicle, with its large propellant supply that may be sensitive to Mars' low surface temperatures -- "and we do not want to wait a long time for the rover to collect a sample."  The new design allows MAX-C to explore a wide range of Martian surface features and carry out its sampling operations in a completely leisurely, scientifically well-designed way over a period of 17 months.  By contrast, the Fetch Rover is planned to carry out its sample-retrieval round trip and return to the lander in only about three months, allowing the MAV to blast off from the surface after that short period (although the lander and MAV will be designed to operate reliably on the surface for up to 12 months, allowing a long safety margin).

(2)  The new MSR lander, with its simpler rover, is lighter-weight -- lightweight enough that it can be carried to the surface by the same landing system (heatshield, parachute and "Skycrane") used by the 2011 Mars Science Laboratory and MAX-C.  "The main thing I learned from MSL," Li says, "is that developing the Entry, Descent and Landing system is a major big deal.  The technical challenges and money needed are just painful.  We should capitalize on what we've already developed."  MSL's EDL system can land a payload of about 1000 kg on the Martian surface -- and if the sample-return lander carries the smaller Fetch Rover, the total mass of the lander (with a safety margin of 40%) is indeed estimated to be about 1000 kg.  But if the lander carries the heavier MAX-C type rover, its total mass could end up at about 1200 kg -- requiring a major new development effort for a new landing system.

(3)  The new sample-return mission design, by spreading out its components over time, avoids much concentration of both technical problem-solving effort and spending at one point in time.

(4)  The new plan has more flexibility to deal with problems.  If MAX-C finds out that its selected landing site is less scientifically interesting than expected and would serve as a poor place from which to return samples, a second MAX-C can be launched and the sample-return orbiter and lander can be simply delayed.  Putting the MAX-C rover on the actual sample-return lander would allow no such flexibility in choosing another sampling site.  (Incidentally, the arm used by the sample-return lander to remove the sample container from the Fetch Rover and load it into the MAV's sample canister for launch can also be used to collect an emergency contingency sample of rock fragments and soil from the lander's immediate vicinity if the Fetch Rover fails to return with its sample.)

In the second part of this report, I'll describe the second theme that struck me about Li's presentation -- namely, the rather surprising extent to which design and development work for this mission is already underway and has led to a detailed preliminary design.