Spitzer telemetry, angular momentum, and scheduling

I am at the Oversight Committee meeting for the Spitzer Science Center, which is, by the rules, not research. However, I learned a lot about scheduling and telemetry and pointing. The latter is done with reaction wheels. In low-Earth orbit, you can use a combination of reaction wheels and magnetic torques, and therefore control the reaction-wheel revolutions and speeds. But Spitzer is on an Earth-trailing orbit, very far away; it is not in a magnetic field that is large enough. This means that the reaction wheels are doing lots of revolutions, and they are known empirically to comprise one of the systems most likely to fail during the Telescope's latter years. The Earth-trailing orbit is also a challenge for data telemetry, because it relies on sending data bursts to enormous radio antennas in the Deep Space Network (and even this can only be done at certain satellite Earth-angles).

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