From: NASA Mars Exploration Program Analysis Group (MEPAG)
Posted: Friday, November 21, 2008
To: Dr. Michael Meyer, Lead Scientist, Mars Exploration Program, NASA HQ
From: Scott McLennan, SUNY Stony Brook (chair)
Date: October 9, 2008
Subject: Analysis of arguments for and against removing the sample cache hardware from MSL
The primary benefit of the sample cache on MSL is the potential increase in the likelihood of Mars Sample Return. The primary quantifiable risk of retaining the cache is in mission operations. The immediate concern is the impact of the cache on implementation of the sample observation tray and possible enhancements to the sample delivery system that may be needed for the MSL analytical instruments to meet their measurement goals.
1. At the request of NASA HQ, a group of Mars scientists, drawn from the community to represent divergent opinions, debated the pros and cons of retaining the MSL cache; their deliberations are documented in this letter report.
2. The group agreed on the following:
3. The group had a range of opinion on the essential risk-benefit issues:
4. A wide range of positive and negative reasons for retaining the cache were expressed, reflecting many different prisms of experience. The discussion was unfettered and two diverging lines of thought emerged. 1) Any cache on Mars will be a positive step towards sample return and the cache box should be retained unless it is demonstrated that the MSL objectives will be compromised; and 2) The value of this cache is likely to be low enough that it does not justify the possibility that MSL capabilities or operations will be compromised by its installation, and it should be removed.
In September, 2008, NASA asked for a community-based analysis of the pros and cons of retaining the MSL cache. This analysis was a follow-up to a letter from the MSL project scientist and MSL PIs (dated Aug. 11, 2008), and the MEPAG letter from Jack Mustard dated Aug. 15, 2008. The requested analysis was conducted by a committee consisting of: Scott McLennan (chair), Jan Amend, Dave Blake, John Karcz, Mike Malin, Dave Des Marais, David Mittlefehldt, Jack Mustard, Clive Neal, Dimitri Papanastassiou, Lisa Pratt, Chip Shearer, and Dawn Sumner. Chris McKay and John Grotzinger presented the benefits and risks for retaining the sample cache. (Michael Meyer and David Beaty participated in an ex-officio capacity). This letter report summarizes their deliberations.
Arguments to retain the cache
Arguments to remove the cache
Throughout the discussion, one consensus opinion of the committee was that if the presence of the cache negatively impacts the ability for MSL to achieve its launch target date, impedes its surface operations, puts at risk any of its primary goals, or impedes planning for the end-of mission, it should be removed. However, the major sticking point beyond this general view was how to evaluate what constituted significant impacts and how these could be quantified or objectively assessed. Proponents of removal of the cache argued that it had significant potential to put at serious risk both MSL development and mission surface operations that outweighed any potential benefit from samples that might be obtained. On the other hand, proponents for retention of the cache argued that there was no way to quantify this risk at the present and accordingly the cache should be retained at least until such risk was understood in some quantitative way.
The discussion of whether or not the MSL cache should be retained mostly focused on three issues, described in greater detail below (listed roughly in the order they were first raised during the discussion).
1. Impact on MSL Surface Operations
The potential impacts on MSL mission surface operations are summarized above and were re-iterated by several committee members during the discussion. Experience from MER suggests that the cache will affect surface operations in some material way. Although the Mars science community could recommend that this not happen, certain key decisions will be made elsewhere. For example, the community had little impact on the decision to include the cache in the first place and, in general, community involvement mostly influences long term programmatic planning rather than issues related to mission operations. Michael Meyer was asked if any guarantee could be given that the cache would not affect operations and his answer was that such a guarantee could not be given.
Cache operations are expected to consume 20-50 sols of time that would otherwise be spent working toward the L1 science objectives. Subtracting some non-commandable sols due to Earth-Mars phasing (but making the generous assumption that every other sol is productive), this is likely to represent 5-10% of the primary mission. While the value of each MSL sample analysis vs. cache sample is impossible to know, the time impact against the MSL's nominal science operations is an objective fact. It was suggested that caching of samples should not occur until the prime mission is over. This would alleviate the idea that time would be spent caching rather than analyzing. However, if the cache is a part of the mission, this does not seem plausible. If the cache is on the rover, its use would be persistently advocated.
2. Cache Sample Quality
Questions were raised about the scientific value of the samples that might be stored in the cache, and whether or not they would be attractive to a future MSR mission. The concern is that the cache samples would not be of sufficient quality to address our scientific goals related to aqueous history and habitability. These concerns were recently raised in both the Steele et al. (2008) report and the MEPAG ND-SAG (2008) report, and they are summarized in Mustard's letter to NASA. Concerns centered on issues of sample degradation, contamination between samples and between samples and spacecraft, unrepresentative nature of cm-size surface fragments, planetary protection, and the impact on MSR sampling priorities and strategies. On the other hand, proponents of the cache pointed out that the cache could increase sample diversity (the opposite view was also expressed) and may provide new insights and findings that are unanticipated, as was the case with lunar samples.
It was argued that a possible value of the cache, independent of whether or not it was ever collected, is that changes in sample characteristics could be monitored over time under Mars' conditions. This could have both scientific value and could inform us about MSR sampling strategies. This led to a discussion of whether the cache could be monitored during the mission, and by instruments adequate to the task of monitoring any change. There are two issues: 1) Only the outer samples of the cache, which would consist mostly of those added at the end, could be examined over time, and 2) The only instrument that might be brought to bear is MAHLI. In addition, there were mixed views about the scientific value of such observations.
3. Threat of the Cache to MSL Launch Date
The impact of cache removal on the launch schedule was discussed at some length by the committee. This is not a clean question--the cache is a part of the SA/SPaH system, and although the SA/SPaH engineering team is facing some significant challenges, it is not possible to determine the degree to which these challenges are attributable to the cache; there are too many interconnected parts. Many components of the SA/SPaH system are absolutely essential to MSL's ability to achieve its scientific objectives. It was suggested that perhaps a better way to think about this is that if the SA/SPaH engineering team is under serious schedule pressure, what is the risk that MSL will end up without all of the tools needed to complete its scientific objectives? Removing the cache would simplify the engineering trade space that the SA/SPaH team needs to reconcile, and this weighed heavily on the minds of the advocates for removing the cache. The advocates for retaining the cache argued that we do not know that deleting the cache will make a meaningful difference to the chances of SA/SPaH success.
In follow-up to this concern, Richard Cook, the MSL project manager, was contacted and invited to describe his perception of the engineering implications involved in the cache question and achieving the launch date. His response (Oct. 8, 2008): "MSL is engaged in an effort to improve our 'tolerance/ to material handling problems. The highest priority aspect of this effort is to add additional tools that can help mitigate the risk of clogging the sample processing equipment. The specific designs of these additional tools are still in work, but one major concern is likely to be the physical accommodation of these tools. The size and complexity of the Sample Arm turret makes it very difficult to access rover-mounted tools (particularly if they are to be used to physically interact with portions of the sample processing hardware). One of the key features of the current cache location is that it is the prime real estate where the turret can be readily positioned (which is why the cache was placed there to begin with). The SA/SPaH team has taken a brief look at whether other sites exist where these additional tools could be placed. Although the answer is still premature, it appears that there are no other spots which would be accessible without significant additional effort. A burden will be placed on the SA/SPaH team to accommodate these additional tools, and not having access to the cache location will make that burden somewhere between moderate and major."
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