In my last couple of posts about the Stardust spacecraft, which is now basically out of fuel after a remarkably successful extended mission to comet Tempel 1, I've mentioned that it's soon to be decommissioned. A reader asked me: what does it mean to decommission a spacecraft? When you decommission, say, a naval ship, it's formally removed from service and usually then scrapped, its sensitive technology destroyed, its component materials turned to whatever useful purposes possible. It's easy to see how you can remove a spacecraft from service (you simply stop talking to it), but you can't scrap spacecraft that aren't returning to Earth -- so what else do you do when you decommission a space ship?
Stardust's project manager Tim Larson was kind enough to take the time to answer this question for me; the following are his words, with my comments in brackets.
When you talk about decommissioning a spacecraft you first need to consider whether you ever want to see it again.
If you think you might like to communicate with it in the future, the key is to design a hibernation or safe mode that allows the spacecraft to spend long periods of time on its own without any maintenance from the ground. This requires making sure the onboard parameters support orbit propagation and sensor calibration curves that are valid for long periods of time.
You also need to make sure the orbit is safe for the period of time the spacecraft will be in storage. [That is, that it's not going to run in to anything we know about, or get too close or too far from the Sun, heating or cooling the spacecraft beyond its design parameters. This is generally not a problem for spacecraft in solar orbit, but ensuring a "safe" orbit for a spacecraft in orbit around a planet, especially a giant planet or Earth, is a much bigger challenge.]
Then the last step is to configure the spacecraft to receive commands through its antenna with the widest possible pattern (usually the low-gain antenna) and turn off the transmitter. This allows the ground to wake up the spacecraft in the future and possibly use it again. [One spacecraft I'm aware of that was decommissioned and then reawakened is the International Cometary Explorer, though the capability to reawaken it was accidental.]
If the decommissioning is permanent, then the trajectory needs to be evaluated for a much longer time period to ensure the spacecraft will not make its way to bodies that are of interest in the search for life outside Earth (planetary protection) and will not make its way back to Earth for an unwelcome reentry. Then, depending on the spacecraft architecture, the spacecraft transmitter is turned off, and the appropriate steps are taken to make sure it does not come back on later (either disconnecting the battery from the solar arrays, commanding the spacecraft to an attitude that does not support battery charging, or, in the case of Stardust-NExT, let the lack of fuel allow the spacecraft to drift off sun and eventually completely discharge the battery).
The primary goal is to leave the spacecraft in a known configuration, particularly the radio transmitter. This is of concern since the X-band radio spectrum is shared by many spacecraft, and there are missions that share frequencies. Turning off the transmitter ensures that there will be no unintended radio interference in the future.
Some planetary spacecraft [such as Galileo at Jupiter, Magellan at Venus, and most lunar orbiters] plunge into the planet at the end of the spacecraft's life to make sure it is destroyed in a controlled way. Earth orbiters are usually boosted to a long-term parking orbit or de-orbited to be broken up on reentry. Spacecraft in interplanetary orbits are evaluated for long-term trajectory safety.
Thanks, Tim, for the explanation! On a related topic, I realize there's a very active discussion going on in the comments section of this blog entry concerning the distance Stardust traveled. I'm following up on that; stay tuned.