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I had this idea for a fictional scenario in which a forgotten satellite, like a communications satellite that was decommissioned because it was replaced by a newer model, was hacked into and then smashed into another satellite.

And then I realized I have no idea what happens to high-altitude satellites when they are no longer used. Are they completely shut down so that communication with them is no longer possible? I assume communications are secure, passwords and cryptography and stuff, but are they monitored so that hacking attempts would be detected? If they are decommissioned with some fuel left, are they still parked up there or will they be deorbited? Are any satellites, in fact, ever decommissioned before they run out of fuel? Nothing I've read has seemed to quite cover any of that.

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    $\begingroup$ Good question! Never considered it myself $\endgroup$
    – slebetman
    Commented Sep 13, 2019 at 6:49
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    $\begingroup$ Consider asking the same question on worldbuilding.SE where other creative suggestions may be sourced. Example, a lost asset instead of a decomm'ed one. $\endgroup$
    – Criggie
    Commented Sep 13, 2019 at 12:45
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    $\begingroup$ J002E3 is another "high" debris. $\endgroup$
    – chux
    Commented Sep 13, 2019 at 13:22
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    $\begingroup$ Bad code gets into orbit. If the hypothetical satellite lost communication for some reason it couldn't be parked or decommissioned. If the "bug" resolved itself somehow the satellite may start working again.... I'm thinking some kind of date or missed sentinel value that causes a race condition taking years to resolve... $\endgroup$
    – Ron Jensen
    Commented Sep 13, 2019 at 17:37
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    $\begingroup$ You could always take the shady government agency angle: an anti-sat "communications satellite" was parked in orbit for future use: "retired". $\endgroup$
    – TemporalWolf
    Commented Sep 13, 2019 at 18:12

3 Answers 3

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If the satellite is close to the Earth, a last bit of fuel is used to de-orbit it so that it burns up. If it is farther out, it is moved to a retirement orbit out beyond the used orbits.

The last thing done after moving it to the retirement orbit is to permanently disable the communication system so that it doesn't randomly transmit stuff and put noise on the communication frequencies.

Here is a Wikipedia article on GOES2 that mentions that the communication system was permanently disabled when the satellite was retired.

https://en.wikipedia.org/wiki/GOES_2

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    $\begingroup$ Okay, so deactivating communications is something that is done. It's interesting that the article says the weather satellite was "placed into storage" in 1993 and then reactivated as a communications satellite in 1995. I didn't know they could change careers like that. $\endgroup$
    – Greg
    Commented Sep 12, 2019 at 18:03
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    $\begingroup$ @Greg wow that's pretty cool! $\endgroup$
    – uhoh
    Commented Sep 12, 2019 at 22:55
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    $\begingroup$ Placed into storage is different from being placed in a graveyard orbit and retired. Putting into storage is done with the idea that the satellite might be restored to service. $\endgroup$
    – Ross Millikan
    Commented Sep 13, 2019 at 14:27
  • $\begingroup$ @RossMillikan that could be the basis for a good answer. The two existing answers focus on retirement (and seem to indicate the OPs situation is implausible), but the question refers to satellites "no longer in use" which could include some in storage, which may be hack-able and usable in his plot. $\endgroup$
    – Steven Jackson
    Commented Sep 13, 2019 at 16:51
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    $\begingroup$ It also sounds like when a satellite is placed in storage, it's still possible for it to receive communication signals and only outgoing communications are disabled. Otherwise, it'd be impossible to reactivate the satellite. So, a fictional satellite in storage could still be targeted by a fictional bad actor and hacked to be reactivated and potentially moved into a different orbit. $\endgroup$
    – Ellesedil
    Commented Sep 13, 2019 at 17:48
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A satellite that is retired ordinarily and not expected to reenter will be passivated. The aim here is to minimize the amount of energy stored in the spacecraft, ideally it will be a dead rock floating in space, far from anything it could interfere with.

This includes deactivating the comms to stop interference as zeta mentions, but also emptying the tanks to space and running down the batteries. This is important to reduce the chance of a break-up from pressure later which could poison adjacent orbits with debris. The risk from non-passivated rocket stages in particular is a serious contributor to space debris, especially in high-energy transfer orbits that cross a large number of other orbits and stay up for a long time

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    $\begingroup$ How universal is this approach of passivating unwanted satellites? Are there (current) entities putting things into orbit who don't follow this general rule? Were there times earlier in our space exploration history where this rule wasn't followed? $\endgroup$
    – dwizum
    Commented Sep 13, 2019 at 17:16
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    $\begingroup$ @dwizum, many of the early satellites failed rather than being passivated, and many of them didn't even have any provisions for passivation. There's a fair risk of something like Courier 1B spontaneously waking up again and starting to transmit. On the other hand, most of the early boosters have self-passivated by having any remaining fuel or pressurization gas leak out. $\endgroup$
    – Mark
    Commented Sep 13, 2019 at 21:40
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    $\begingroup$ As I recall, when you're building a satellite you need to have a piece of the spectrum assigned to it so that you can talk to it. Those pieces must get recycled again and again as old satellites go out of service and new ones come in. If you don't at least disable communications, I would think there would be incidents of "You talkin' to me?" $\endgroup$
    – Greg
    Commented Sep 13, 2019 at 23:29
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    $\begingroup$ @Greg Geostationary satellites (more or less) operate in the same frequency range (their transponders' band(s)), and have a defined beamwidth (IIRC, C-band is 2deg per slot and Ku is 0.5deg, but I forget what the guardband is). IOW, you lease the slot and not band. One satellite won't respond to another's control; that would be built into the onboard in case an uplink antenna is pointed at the wrong place. Turning off transponders is to prevent them from glitching on and washing out someone else's downlink. Non-geos can share spectrum, too, with coordination. $\endgroup$
    – mpdonadio
    Commented Sep 14, 2019 at 0:41
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When they are replaced with an updated satellite, GPS satellites are sometimes stored in a pseudosynchronous orbit. This allows them to be reactivated and returned into their previous orbital position with minimal delta-v. See Why end-of-life GPS satellites given orbits that seemingly still intersect active satellite orbits but with a different period? Recipe for disaster?

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