I just heard, via this article at Universe Today, that the sample cache on MSL has been canceled. First, some background. The cache was supposed to be a way to collect samples which would be retrieved in a future sample return mission. Problem is, the cache was sort of slapped on the rover as an afterthought. From what I understand, it was poorly designed, could not hold many samples, and did little to prevent cross contamination between the samples. In other words, it was basically useless. It was pushed through by NASA's previous head of space sciences, who resigned earlier this year after an unpopular move to reduce funding for the Mars Rovers.
The real problem with the box wasn't that it was scientifically useless though. It was that having such a sample cache would change the mission, and might actually be detrimental. People would constantly be thinking about whether or not to pick up a sample, and there would be the concern that, if the rover drove into a difficult to access location, that it would compromise the ability of future missions to retrieve the cache.
So, I'm really happy that this thing has been removed. It was never part of the mission and was forced through by NASA management, rather than by the scientists involved in the mission. Sample return is a great idea, but it should not be an afterthought on a pre-existing mission.
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