WISE: Deployments coming soon

It looks like the WISE observatory is happily working through the early checkout phase. During the first few days/weeks of a mission, the engineers verify the basic functionality of a newly launched spacecraft, essentially making sure nothing broke during the extreme environment associated with launch.

During launch, the observatory experiences extreme vibration, acoustic, thermal and space (vacuum) environments either simultaneously or in rapid succession. Prior to launch we test each of these individual environments as best we can, but nothing can really prepare for the intensity of the launch. As a result, the post-launch checkout is a very slow, careful process that exercises each bit of spacecraft functionality one tiny step at a time. That way, if anything has broken, it can be identified early, troubleshot, and either recovered, or changed to a redundant piece of hardware. NASA prefers for all spacecraft to be fully redundant (that is, have a spare bit of hardware for everything in case the primary one breaks) but the reality is that full redundancy is cost prohibitive.

Instruments are often not fully redundant. Almost every single instrument on a NASA mission is one-of-a-kind hardware, designed specifically and solely for the science goal of that mission. Instrument designers do their best to provide redundancy where possible. But there are always parts on the instrument that, should they fail, will end the mission for that instrument.

In addition, many missions have hardware that must be deployed. For example, a solar array may need to be driven from a folded to an extended position, or a door may need to be opened to provide a sun-shade to an instrument. For WISE, the sole instrument on board has a cover protecting the cryogenically cooled interior. The cover keeps out water that would freeze to the interior surfaces, possibly obscuring the detectors. This water originates on Earth, but is carried into space on various surfaces of the observatory, such as within the thermal blankets protecting portions of the spacecraft. Once the observatory has had sufficient time in space for the water to have sublimated away (outgas time), the cover can be removed. In this case, the cover deployment currently scheduled for December 29th will be carried out by firing three explosive fasteners.

Should a deployment fail to occur, it can often mean a mission-ending situation. There is no redundancy in the WISE cover (i.e. you cannot remove an alternate cover and get science data). As a result, this is a very critical activity. In addition, ground testing typically does not exercise the firing of pyrotechnics. Rather, the signal used to release the nuts can be tested prior to installation, and the response of the nuts to such a signal can be tested on duplicate hardware. The upshot is that this deployment is likely the first and only time the entire system has ever been tried all together.

It may seem like this is a foolish method, but unfortunately some systems are simply untestable on the ground. Spacecraft engineers work to test such hardware as thoroughly as possible. However, when deployment time comes, we are all holding our breath until the pyros blow correctly and the cover comes off…


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