say that the spacecraft must move 300 km above GEO soon. A damaged battery is no longer being used (for fear of explosion?) and it is running on purely solar power and will enter eclipse soon and therefore run the risk of becoming inoperable/unresponsive.

From Engadget:

The company already switched the batteries off and has been operating Spaceway-1 using power generated by its solar panels. However, it's expected to pass through Earth's shadow in late February where only batteries can serve as its power source. That's why DirecTV has to send it 300 kilometers above geostationary orbit, at a place where satellites go to die, before February 25th. "The risk of a catastrophic battery failure makes it urgent that Spaceway-1 be fully de-orbited and decommissioned prior to the February 25th start of eclipse season," the company said in a statement.

I was surprised to see "de-orbited" refer to something other than a forced atmospheric reentry, but that's not my question.

Question: Raising above GEO by 300 km means that any explosion debris with a velocity above 16 m/s can still intersect GEO periodically. Is the risk of damage to operational spacecraft in GEO by a battery explosion of Spaceway-1 actually reduced by this move? Or is the risk simply that control will be lost and it will wander aimlessly, and the move doesn't affect risk of damage due to the explosion itself?

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    $\begingroup$ Really interesting detail is that they also need to ditch as much propellant as possible before the 'eclipse season'. I guess, the intention is to place the bird on a graveyard orbit and start venting propellant only after that. Why not using all that excess of propellant to raise it even further beyond the graveyard orbit to mitigate risk due to debris generation to a higher extent? $\endgroup$ Jan 23 '20 at 13:32
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    $\begingroup$ @prop-a-gator I'm not an expert but I guess it is preferred to keep most debris in a known "location" instead of dispersing it. That way there is only one orbit to avoid, and the risk of collision is lower everywhere else. $\endgroup$
    – zovits
    Jan 23 '20 at 13:57

Apparently the battery is only prone to explode while used. So when turning the satellite off, the risk appears to be mitigated sufficiently (I expect the battery to be discharged. A battery storing no energy can't cause an explosion, as an explosion is simply the uncontrolled and rather sudden release of said energy).

This means the main risk of a GEO satellite is that it'll become unresponsive, which would mean it couldn't counteract the various changes to the orbit:

  • Earth Gravity-induced changes to the orbit (slight prograde/retrograde acceleration)
  • Solar/Lunar gravity changes to the orbit (tilting of the orbit)
  • Various other influences, such as solar wind

A satellite in GEO doesn't generally have enough fuel to de-orbit into Earth atmosphere. It is common procedure to raise a dying GEO satellite's orbit into a "graveyard orbit". In graveyard orbit, the risk of the satellite interfering with other (active) GEO satellites is nigh-zero.

So this is the expected and common action, protecting the geostationary orbit in general.

  • $\begingroup$ the battery won't explode, however the reason why it's also standard procedure to completely empty the prop tanks when retiring a satellite is that history has shown that years or decades down the line dead sats prop tanks have a nasty habit of exploding. Assuming there's no way to put it in an orbit that stays in sunlight for the next few months all the options available seem to differ only in how bad they are. $\endgroup$ Jan 23 '20 at 15:59
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    $\begingroup$ No accessible energy stored does not mean no energy stored.... $\endgroup$ Jan 23 '20 at 21:37
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    $\begingroup$ Could someone explain why not push it 300 km lower so if it breaks apart the pieces will eventually impact the earth? $\endgroup$
    – gwally
    Jan 24 '20 at 0:27
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    $\begingroup$ @gwally 300km below GEO is still almost 33,000km. The timescale for things to deorbit from those altitudes is longer than people other than geologists or astronomers mean by "eventually." It's basically up there forever. $\endgroup$
    – Erin Anne
    Jan 24 '20 at 2:12
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    $\begingroup$ @gwally as Erin Anne said, things don't really deorbit on any noteworthy timescales. Putting them to a lower orbit just means that there is a chance they'll interfere with satellites being brought into GEO or are there, either by being a potential cause for a crash or by just being in the way of signals. It's a completely unnecessary risk with absolutely zero benefit. $\endgroup$
    – Infrisios
    Jan 24 '20 at 6:37

Referring to moving to the designated graveyard orbit as de-orbiting is a little odd - perhaps autocorrect or a sub-editor had a go when they saw "re-orbiting"

Moving to the graveyard orbit significantly mitigates the risk of damage to other operational satellites from explosion, as it is highly unlikely that neither of the apsides of resultant debris will be in the geosynchronous shell. As a result debris will pass quickly through the occupied region, at a significant angle, and (because space is big) it is unlikely to hit anything (it'll create two cylindrical risk areas). In addition, the debris is likely to be accelerated in the binormal direction, so those cylinders are likely to intersect the occupied altitude out-of-plane with the main geosynchronus orbits

If it wasn't moved, then one of the apsides will lie in the occupied region, and be tangential to it, creating a large risk corridor tangential to and in-plane with the operational orbit.

Finally 16 m/s is a reasonably large acceleration, so only small (alas, likely untrackable) fragments will be accelerated by that much, so a strike from one will be survivable (it may take out a cell on a solar panel)

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    $\begingroup$ Well it's a de-orbit in the sense that they are taking it out of it's original nominal orbit, but I agree, using the term "de-orbit" when they are moving it into another (higher, graveyard) orbit is unusual $\endgroup$ Jan 23 '20 at 20:26


While I'm not smart (or knowledgeable) enough to actually do the math.... At current orbit debris flying in any angle within 180 degrees has a chance to damage satellites or spacecraft. By raising the orbit by 300km you're significantly decreasing the angle at which debris is potentially dangerous. At the very least you're decreasing the angle at which debris is continuously dangerous (stable orbits that collide with other objects).


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