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March 27, Gaia was disabled and sent to a solar orbit. I understand why ESA send it to a solar orbit (Kessler Syndrome), but I don’t understand why ESA had disable the spacecraft. Is there some risk of leaving Gaia with the subsystems still operating, is the science return so negligible that there’s no point in continuing to operate, and/or is there some other reason for shutting it down completely?

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One other consideration is the importance of disabling any radio transmit capability.

For instance, for the Rosetta mission:

ITU regulations require us to permanently switch off the craft’s radio transmitter at end of mission. Since Rosetta wasn’t designed to have its transmitter permanently off, we had to change the on-board software by patching it.

https://blogs.esa.int/rosetta/2016/09/29/how-rosetta-gets-passivated/

It is difficult enough to communicate with things in space at great distances. Having a dormant, tumbling spacecraft with a possibly malfunctioning local oscillator trampling all over the spectrum in the same solid angle as a functioning mission could be problematic.

Then, if the transmitter needs to be permanently deactivated, the options for any future productive uses of the spacecraft seem fairly limited.

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    $\begingroup$They do sort of mention this: "disabling the layers of redundancy that have safeguarded Gaia for so long, because we don’t want it to reactivate in the future and begin transmitting again if its solar panels find sunlight", so definitely a concern$\endgroup$
    – blobbymcblobby
    CommentedMar 30 at 11:21
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Is there some risk of leaving Gaia with the subsystems still operating, is the science return so negligible that there’s no point in continuing to operate, and/or is there some other reason for shutting it down completely?

A space vehicle that has lost the ability to control its attitude is for all practical purposes a dead vehicle. It is worse than a dead vehicle if the vehicle remains otherwise functional.

Without attitude control the vehicle will be unable to orient itself to collect sunlight and communicate with the Earth. It won't be able to collect data without solar power. If such a vehicle is not completely passivated, at some point in the future it might randomly become oriented so it does collect sunlight after having been powerless for a good amount of time.

Gaia had a reboot capability that had never been used. Suppose the vehicle lost power and later (perhaps much later) regained it. That reboot capability would kick in. Suppose it did work and started broadcasting to Earth. That's not a good thing. Suppose it didn't work quite right and mistakenly fired its main engine. That's definitely not a good thing.

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    The fuel supply of Gaia has been depleted now that Gaia's mission has been extended twice to early 2025 beyond its original intended operation from 2014 to 2019. Without fuel the spacecraft cannot be pointed anymore precisely - an absolute prerequisite for its operation in order to do the extremely precise measurements of the positions of all the stars it observes and also to ensure the instrumentation is in the shadow and the solar panels pointing towards the Sun. Thus with the last drops of remaining fuel Gaia has been moved away from the Lagrange L2 spot at which it carried out its operation in order to not litter the space there and keep it clean for future scientific missions.

    The amount of data gathered is so large however that data processing of Gaias data is still ongoing and we can expect the release of the last data releases somewhen in 2026 or maybe even 2027.

    See also ESA's website on the Gaia mission.

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      $\begingroup$Without propellant, Gaia would be unable to orient itself with one side (the side with solar panels) facing the Sun and the other side (the side with antennae) facing the Earth. Even worse, without propellant, Gaia would be unable to perform the burn needed to take it away from the L2 region. Gaia was not quite out of propellant when it was shut down after having performed that final burn. Some of the tiny bit of remaining propellant was needed to make that final burn controllable.$\endgroup$
      – David Hammen
      CommentedApr 1 at 10:44
    • $\begingroup$@DavidHammen isn't that exactly what I write ("cannot be pointed anymore precisely", "with the last drops of fuel...")? Anyway, I made it clearer that pointing is also necessary for energy supply, thanks for that pointer!$\endgroup$
      – planetmaker
      CommentedApr 1 at 11:11
    • $\begingroup$@blobbymcblobby Name one vehicle with fixed thrusters that have those thrusters so perfectly aligned so as to obviate the need for attitude control during a primary burn. Gaia, along with every other orbiting space vehicle yet made, is not in that fictitious group.$\endgroup$
      – David Hammen
      CommentedApr 1 at 13:29
    • $\begingroup$You're reading something i haven't said.$\endgroup$
      – blobbymcblobby
      CommentedApr 1 at 13:33

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