During the first four weeks of Curiosity's mission, all of the 10 scientific instruments have turned on and returned data from the martian surface.
All of them, that is, except for one. While other instruments have received rounds of applause as the first viable data was returned ("I think this is a hoot-and-hollerin' moment!" mission managers would proclaim after each successful boot-up), the CheMin instrument team was biding its time, putting a good face on the fact that it was the last one chosen in this sandlot pick-up game.
But finally, on Sol 26 (Sept. 1, 2012), CheMin will have its moment in the faint martian sun. Its X-ray beam will power on, the detector will read the intensity, and Curiosity will transmit the findings back to Earth.
CheMin is an X-Ray diffraction instrument: the gold standard for identifying minerals based on the consistent and diagnostic spacing of their atomic constituents. While other tools onboard Curiosity like ChemCam yield elemental abundances - magnesium, iron, silicon, or oxygen, for example - CheMin will tell the team the precise ratios of those elements and how they're arranged, resolving those elements into, say, the mineral olivine.
If ChemCam were to vaporize a stowaway copy of Harry Potter, it would send back the number of "A"s, "B"s, and "C"s; CheMin would tell you the tale of a precocious boy wizard and his mischievous yet lovable sidekicks. Hijinks ensue.
The instrument has 27 available sample chambers, but for now, they remain empty. Sol 26's CheMin debut will examine a blank chamber, providing a negative control and a key point of reference for future samples. After all, the team has similar measurements of blank chambers on Earth, and the degree to which the results match up will inform the interpretation of future results.
CheMin co-investigator David Bish explains it this way: "It's like a plastic bag. Eventually, we will scoop some material into the bag and turn on the X-rays. Tomorrow will be like examining the empty plastic bag - it will tell us how our detector's working and how the X-ray tube is working."
At the day's final planning meeting, the Mission Manager checks on the status of each instrument, per daily protocol. "CheMin?" he asks.
"We're go," replies the instrument's representative.
"Great. Good luck today CheMin, it's a big day for you."
Update: CheMin's initial test went as planned, and is ready to examine martian samples as the mission progresses.