Showing posts with label Mars Sample Return. Show all posts
Showing posts with label Mars Sample Return. Show all posts

Wednesday, October 20, 2010

Thoughts on the Most Compelling Proposed Planetary Mission

At the moment, news for future planetary missions is scarce as the U.S. waits for the results of the Decadal Survey.  (Other nations continue their own planning cycles, but news is scarce there, too.)  The Decadal Survey has published a list of 25 missions it is considering for the next decade.  I thought that I would take the next few blog entries to pick the five missions from that list that I find most compelling.  I'm under no illusion that I will persuade anyone (especially anyone who influences government spending).  However, I find a well argued (and I hope these will be) argument to help me form my own opinions.  Please provide your opinions, too, in the comments.

There are many ways to decide on what would be a compelling mission.  One would be on the vicarious thrill of exploration.  On this basis, I would favor missions such as the Venus SAGE lander, the AVIATR Titan plane, and the Argo Neptune-Triton-KBO mission.  (All would also provide great science.)  However, nations chose to fund these expensive mission primarily on their scientific return.  So I have taken that as my criteria.  Which set of missions would most fundamentally advance our understanding of the solar system?

A recent paper in the scientific journal Astrobiology, New Priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life (subscription or purchase required), has got me rethinking the priority for a Mars sample return.  Mars has been the major focus of NASA's planetary program for the last 15 years.  With the recent cooperative agreement, it has also become a major focus for ESA.  Review board after review board for the last 30 years have concluded that the highest priority for Mars exploration is to return samples to Earth (see, for example, the 2003 Decadal Survey report).  The mission has been recommended for flight at the earliest opportunity, but NASA's budgets have never permitted the mission to begin development.

There have been scientists who recommend a slower approach.  Mars is highly diverse and we have explored its surface in only six places, and landing safety concerns limited our choice of places to explore.  Samples of past or present life are likely to be hard to find.  Instead of rushing to a sample return, these scientists argue, fly a number of landed missions, probably most of them rovers, to find the best place to return a sample.  This, in a nutshell, is the argument the authors make in the New Priorities in the Robotic Exploration of Mars paper.

At least based on presentations at meetings and the roadmap adopted by the Mars science community (principally through MEPAG), this appears to be a minority view.  Most of the community apparently has decided that we know enough about Mars to pick a very interesting site.  The samples returned from that site when analyzed with Earth-based instruments (most of which would be larger than the rover collecting the sample and some of which would be larger than the launch vehicle -- size and power counts for sophisticated study at the scale of individual rock grains) would greatly deepen our understanding of the early history of terrestrial planets.  Only Mars preserves that early history on a body that had both a significant atmosphere and liquid water.

Long term readers of this blog know that I am a skeptic on Mars sample return.  Not because I don't think the science is absolutely compelling -- it is -- but rather because I doubt that Congress will fund a $6B+ program for robotic exploration.  I've come to reevaluate that position, though.  Congress has funded the James Webb Space Telescope to the tune of over $5B  (but at the cost of the astronomy program foregoing many other missions).  Also, events seem to make this the time to move forward:

  • Fifteen years of missions by NASA and ESA to Mars have revolutionized our understanding of the Red Planet.  We may not be able to pick THE most compelling site on Mars, but we can pick A (and actually several) very compelling sites.
  • JPL has developed for the Mars Science Laboratory the entry, landing, and roving technology essential to carry out this type of mission.  (Of course, it also hasn't been flight tested...)  If NASA does not continue a program to use this technology, the key engineers will move on to other projects.  As it is, waiting seven years between the launch of MSL and the proposed Max-C sample caching rover is stretching the period over which teams can be kept together.
  • The opportunity to fly the caching rover with the ExoMars in 2018 rover is unique.  ExoMars can allow samples to be collected from up to two meters below the surface while Max-C collects samples at the surface.  This could greatly enhance the value of the returned samples.
  • ESA has agreed to partner with NASA on a sample return (exact roles and funding levels to be determined).  This offers an opportunity to share costs that may not come again.

So, I have come to decide that moving towards a Mars sample return with the 2018 Max-C rover/ExoMars mission is the most compelling mission to me on the list of missions under consideration by the Decadal Survey.  This mission depends on an orbiter being launched by 2016 to act as a communications relay; the Mars Trace Gas Orbiter is currently slotted for this role.  Together, the two missions would cost NASA probably $3B, perhaps as much as $4B with inflation and cost overruns (ESA would also make a substantial contribution for the orbiter and the ExoMars rover).  This would represent a quarter to a third the expected NASA planetary mission funding for the next decade.

There is the risk that the current estimates for cost will turn out to be wildly optimistic and the true costs will eat up a large portion of the planetary science budget.  After approval, politicians could cancel the mission to save money.  It happened with the Venus Orbiting Imaging Radar mission in 1981 (the eventual Magellan mission was less capable), the Comet Rendezvous and Asteroid Flyby mission (ESA's Rosetta eventually filled this slot), and almost happened to the Galileo mission.

Given these risks, I favor a go slow approach.  Fly the Max-C and ExoMars rover in 2018.  Wait until we know there is a compelling set of samples waiting on the surface for pickup before committing to the next mission in the sequence (see this blog entry for a description of the three missions needed to return a sample to Earth).  If the 2018 mission fails or is skunked, launch another rover to find and cache a compelling set of samples.  Under this approach, the earliest a sample could be returned would probably be 2028 (instead of the current strawman for 2026).

Going with a sample return mission isn't without its risks, both technically and politically.  However, the return seems to me to be greater than any other mission on the list.

I hope to hear your opinion whether or not you agree or disagree.

Thursday, August 5, 2010

Mars Versus Europa had a recent article on the competition between Mars sample return and the Jupiter System mission for the next large NASA and ESA missions.  (The article primarily focuses on the Mars mission, but I'd like to explore the competition.)  Both missions are large: The sample return is likely to cost $6-7B while the combined costs to NASA and ESA of the Jupiter Europa and Jupiter Ganymede missions will be ~$4.5B.  The backers of a Mars sample return would like to see the three elements of the sample return fly in 2018 (the ExoMars rover and the sample cache rover, MAX-C), 2022 (an orbiter to carry the samples brought up from the surface back to Earth), and 2024 (a lander and ascent vehicle to deliver the sample to Mars orbit).  (I believe that the 2022 mission would have to enter development around 2018 in this scenario.)  The backers of the Jupiter system mission would like to see that mission fly in 2020.

The article and others (see Decadal Survey Showdown) have suggested that the two space agencies cannot afford both sets of missions.  The U.S Decadal Survey is seen as the body that will decide between the missions.  In this blog post, I consider whether it might be possible for both to fly without consuming the budget.

The ESA elements for the 2018 ExoMars rover are already budgeted, while ESA's Jupiter Ganymede orbiter (if chosen over two other missions) would be funded out of its large science mission budget.  Funding for subsequent ESA contributions to the sample return, would according to the article, require more money than is currently being budgeted for Mars exploration.

On the NASA side, there is room in a ~$12-13B (in current dollars) decade budget to fly both missions.  Doing so would consume most of that budget, probably crowding out all but a handful of low cost missions to other targets.  (And if these two large missions experience significant cost overruns, even those small missions may have to go.)

However, I wonder if there might be a sequence of missions that allows both to fly.  The currently discussed sample return mission series assumes that missions fly as quickly as budgets could allow.  Another option would be fly the 2018 sample mission and wait for its results to commit to the subsequent orbiter and lander/ascent vehicle.  The 2018 rover might crash on landing, become stuck in the mother of all dust pits for eternity, or simply fail to find compelling samples.  Committing to flying the subsequent sample return elements before the results of the 2018 mission are known would seem to commit NASA's resources to a single large program predicated on success in 2018.

An alternative might be to fly the sample cache mission in 2018 and the Jupiter Europa mission in 2020 (hopefully with their ESA counterparts).  The results of the 2018 mission should be known by 2020 or 2021, and then if compelling samples are safely cached, development of the subsequent missions could begin with flights in the later half of the 2020s.  (I don't know whether a decade or more waiting on the Martian surface would cause the cached samples to degrade.)  In this scenario, the NASA would need to commit $6-7B between the two missions, leaving room for a couple of New Frontiers and Discovery missions.  The majority of the remaining funds needed for the subsequent sample return elements would come from the following decade's budget.

It's easy to be an armchair mission planner.  There may be many reasons why this possiblity may be infeasible or unwise and there may be better alternatives.  The Decadal Survey may also conclude that it wants to recommend just one (or no) multi-billion dollar missions in the coming decade, eliminating the competition.  However, I have not seen a discussion of the question of whether or not results from the 2018 caching mission should be known before committing to the next sample return element.  I hope that the Decadal Survey will consider that question as it reportedly is considering whether to recommend either or both of these two large missions.

Friday, February 26, 2010

Mars Sample Return and Cumulative Risk

I listened in to parts of the Decadal Survey's steering committee earlier this week.  A full morning was dedicated to the proposed Mars Sample Return (MSR) mission. (Presentations haven't been posted yet.)  In its current form, the mission would consist first of the delivery of a rover that would collect and cache carefully selected samples.  Four to six years later, an orbiter would be launched that would eventually receive samples from the surface and then return them to Earth.  Two years after that, a lander would deliver a small rover that would fetch the cached samples and an ascent vehicle that would blast off from the surface and deliver the samples to the waiting orbiter.  Total cost for the series of missions is current estimated to fall between $6-7B.  (See Bruce Moomaw's descriptions of the current plan part one and part two or read a recent presentation at

The most interesting part came in the discussion by the committee members after the final presentation.  The point was made that at this cost, MSR would be NASA's mission of the decade and therefore could not fail.  Another member described the proposed plan as too fragile because everything must work for the samples to finally return to Earth. 

These comments got me to thinking.  The MSR proposal involves three launches from Earth, four interplanetary cruises, three atmospheric entry and landings (on Mars) or recovery (on Earth), two rovers operating on Mars, a launch from the surface of Mars, and the transfer of the sample in Mars orbit from the Mars ascent vehicle to the waiting orbiter.  I did a simple spreadsheet that looked at the effect of cumulative probabilities.  The challenge is to know what probability of success to assign to each individual element.  Since I don't know, I did a sensitivity analysis that first assumed 99% probability of success for each stage of the missions, resulting in a respectable 87% probability that all would succeed.  But reduce the probability of success for each stage to 97%, and the overall probability drops to 65%.  The odds of overall success drop to less than half if the probability of each element completing successfully drops to 95%.  I also tried a version that assigned different probabilities to different mission stages based on a weak sense of how risky each one might be.

Editorial Thoughts: Unfortunately, assigning probabilities of success to mission elements is tricky.  The goal is to have overall mission success in the high 90 percents.  Those probabilities are modeled, not calculated from statistics.  Except for launches (of which there are many) these are one of a kind experiments.  (According to Wikipedia, the Delta II family had a 95% launch success rate over 300 launches.)  What if Spirit's early memory problems had proved fatal?  What if the fetch rover finds a sand trap it cannot get out of?  What if winds cause a crash landing?  A Mars sample return mission has at least two never tried before stages: launch off the Martian surface into an orbit matching the waiting orbiter and then successful rendezvous and transfer of the samples.  Not all elements of these complex missions can be fully tested on Earth.  There will be, for example, no full end-to-end testing of the Mars Science Laboratory's entry, landing, and descent until the craft reaches Mars.

Balancing this is the careful attention the mission designers put into trying to minimize the chance of failure partially by over designing and testing the systems and partially be operating them conservatively.

The most obvious solution to the problem of cumulative risk for MSR would be to duplicate each element.  That strategy, however, probability would raise the overall program cost to over $10B and perhaps even to $12B. 

An alternative strategy would be an adaptive approach.  Hold off on building the orbiter and Mars ascent/fetch rover elements until you know that the caching rover has succeeded.  Also have the caching rover leave duplicate caches. If the caching rover fails, build and fly a second copy.

Once you know that the samples are waiting to be picked up, fly the orbiter with enough resources to last in Martian orbit for many years.  When building the Mars ascent/fetch rover, buy duplicates of key parts.  Then if the first attempt fails (and possibly loses one of the cached sets of samples), figure out what went wrong and then build a second version.

Designing missions is not my area of expertise.  JPL's engineers know probability statistics far better than I do, and so I'm sure they are looking for ways to address the problem of cumulative probability of success.  They probably have a more clever approach in mind than what I've suggested.  So think of this blog entry as raising a key problem that we haven't heard the solution to yet.

Friday, December 11, 2009

New Mars Sample Return Mission - Part Two

This entry continues Bruce Moomaw's description of the new plans for a Mars Sample Return.  The first part can be found here.

Background: 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 and fast forward through the first three presentations (although the third is an excellent rational for doing a sample return).

The second theme that turns up in Fuk Li's recent presentation to the Mars panel of the Solar System Decadal Survey project regarding the design of a Mars sample return (MSR) mission is how much preliminary research has already been done on the possible design for it -- and the extent to which these efforts have already led to preliminary strawman designs for its components.  As Dr. Li says, some desirable technologies -- working rovers, the MSL's "Skycrane" landing system, aerobraking into a low circular Mars orbit -- have already been developed, and that  while "we do have remaining technical challenges, I believe we have identified them and that we have defined technology plans" to deal with them.  A few examples:

(1)  Li regards development of the Mars Ascent Vehicle as the single most difficult remaining task associated with the MSR mission: "We have not built and flown this rocket", and his chart rated its technical development difficulty as "high".   However, repeated studies since 2001-02 have already done a surprising amount of preliminary work on its design; three industry studies converged on a strawman design for it which was approved as feasible by JPL's "Team X" group and the Marshall Space Flight Center, and this has been confirmed by other studies since.  Li still stated that its final development will likely take another seven or so years: three years to recheck various possible alternative designs (some perhaps using liquid rather than solid fuel) before settling conclusively on a final one, and four years to actually develop the chosen system and flight-test it in Earth's upper atmosphere.

The strawman concept is a slim two-stage rocket, weighing about 300 kg (including a 40% margin), and standing about 2.5 meters high, with its two stages using already long-developed STAR solid motors and liquid-fueled attitude thrusters.  During its stay on the Martian surface it would be encased in a slim heated "igloo" to protect its propellant from the effects of Mars' cold.  It would lie on its side, allowing the mechanical arm on the Lander to remove the cylindrical sample container from the returning sample "Fetch Rover" and load it into the spherical Orbiting Sample Canister located on the MAV's top; and then it would be elevated into a vertical position to take off out of the opened top of the igloo.

(2)  The second-most difficult remaining technical challenge for the Mars sample return mission is the development of its anti-contamination systems, to avoid both "forward contamination" of Mars (especially the sampled material) by Earth contaminants on the landed vehicles, and "back contamination" of Earth by Martian material that is on the surface of the sample containers or has been released from a ruptured container.  Li's chart rated the likely technical difficulty of both phases as being in the "medium" range.  He stated that forward contamination will be avoided not by the difficult technique of sterilizing each entire landed vehicle, but by the technique used on the Phoenix Mars lander: sterilizing all the components on the MAX-C sample collecting rover, the Fetch Rover, and the sample-return Lander that are likely to come into contact with the sampled materials, sealing them after "clean assembly" inside "biobarriers" that will be removed after landing, and using less rigorous clean-assembly techniques to minimize the number of microbes or organic contaminants on the rest of the landed spacecraft.

Unfortunately, Li was vague on the likely techniques that would be used to minimize the risk of back-contamination of Earth by Martian material.  He did note that precautions against this will likely be built into both the sample-return Lander and Orbiter, and that the sample container removed from the Fetch Rover will be implanted in the Orbiting Sample Canister using a "double-seal" technique.

(3)  According to Li, a surprising amount of promising work has already been done on the seemingly difficult task of having the sample-return Orbiter locate and rendezvous with the tiny Orbiting Sample Canister that will be launched back into orbit around Mars from the planet's surface by the Mars Ascent Vehicle, and capture the Canister so that it can be loaded into the heat-shielded Earth Entry Vehicle capsule carried on the Orbiter.  The current plan is, in fact, to use cameras on the Orbiter to locate the Canister at a distance of up to 10,000 km away -- with the first-stage development of such a camera already carried on the Mars Reconnaissance Orbiter -- although the Canister will carry a simple radio beacon as a backup for long-range tracking.  After carrying out an automated rendezvous, the Orbiter will "dock" with the round Canister by scooping it up in a conical retrieval basket and funnelling it into the EEV capsule.  Li says that many of the systems and computer algorithms needed to carry out such a complex operation have already been tested to a considerable degree in Earth orbit by the DARPA military research agency's "Orbital Express" mission that carried out repeated automated rendezvous and docking between two unmanned Earth satellites in 2007.  In a chart of current estimates of the difficulties likely to be encountered in developing various technologies for the sample-return mission, unmanned rendezvous and retrieval was ranked in the "low to medium" level.

(4)  There is great confidence in the existing design of the Earth Entry Capsule, which underwent drop tests as far back as 2001.  It would not use a parachute, but would instead nestle the OS canister inside a thick layer of crushable material that would protect it against the full-speed crash of the EEV onto Earth's surface -- a design that both saves weight and actually improves protection from any rupture of the sample container by avoiding any risk of parachute failure.

(5)  The Returned Sample Handling Facility that would store the samples for protection and study -- as well as appraising them for possible hazards prior to releasing them to outside facilities for further study -- has already undergone competitive engineering studies by three different firms in 2003.  It would operate at a "Biosafety Level" of BSL-4, like some existing facilities, and apparently there are no great difficulties likely in designing of building it -- although the sooner the process is begun the better.

(6)  A few other notes:

            (A)  All three of the spacecraft involved in the current design of the sample-return mission -- the MAX-C rover, the Orbiter and the Sample-Return Lander -- would be launched on Atlas V 551 boosters.  (An error in my previous report: the SR Lander, under the current design, would be launched in 2024 -- only two years after the Orbiter -- rather than in 2026.  In any case, the MAX-C rover would almost certainly be dead by the time the Fetch Rover shows up, which will not stop the latter from retrieving its container of cached samples.)

            (B)  The MAX-C rover, the Fetch Rover and the Lander platform would all be powered by solar arrays, unlike the Mars Science Laboratory.

            (C)  The plan is to improve the computer algorithms for driving the MAX-C and Fetch Rovers beyond those on the Mars Science Laboratory, allowing them to drive a minimum of 200 meters per day instead of just 65 meters minimum.  This would be done by allowing the rovers to process their images for navigation purposes while they were actually moving, rather than forcing them to stop at intervals for such processing.

            (D)  Four studies conducted this year by different industrial groups instill confidence that a reliable way can be found for the MAX-C rover to extract rock cores and cache them in its sample container.  The tentative plan is for it to collect 30 10-gram rock cores, probably from three to five overall locations during its drive.

Finally, there is the burning question of the cost of this mission.  Dr. Li reported the results of three recent mission cost estimates, all of them in 2015 dollars.  Two were provided by JPL's Team X group and the the independent Aerospace Corporation, both of which tried to estimate costs down to about 100 million dollars.  The third estimate came from studies by analogy of the different main components of this mission with earlier spacecraft, such as MRO and MSL (although all the cost estimates made an attempt to incorporate the unhappy recent experiences with serious cost overruns on the MSL).  As one might expect, this third estimate was a lot rougher, calibrated only down to about a half-billion dollars. 

Team X and the Aerospace Corp. both estimated the likely cost of MAX-C at $2.1 billion; the study by analogy pegged it at about $2 billion.

Team X and Aerospace Corp. estimated the orbiter at $1.1 to 1.3 billion; the analogy estimate was about $1.5 billion.

Team X and Aerospace Corp. estimated the sample-return lander at $2.3 to 2.4 billion; the analogy estimate was a good deal higher at $3 billion.

Finally, separate estimates of the cost of the Sample Handling Facility pegged it at about $300 to 500 million.

The sum-total estimates by Team X and Aerospace Corp. estimated the total cost of this mission at $5.9 to $6.2 billion; the rougher (but perhaps more trustworthy) estimate by mission analogy put it a good deal higher at about $7 billion.  Repeats of such a multi-part mission would cost a good deal less, since so much of the first  mission's cost would consist of its initial design and development -- but it's clear that we are talking about the sort of thing that cannot be done more than about once per decade, and which during that decade would be likely to completely consume the costs of the Mars Exploration Program -- something that must be taken into account when deciding whether to go ahead and ask Congress to approve this ambitious mission.

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 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.