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With decaying orbits, the point of reentry depends heavily on solar activity, and is very hard to predict or control.
When there are things that could survive reentry and potentially be harmful, or humans and equipment you want to recover, you would wait until you reach the right spot in your orbit, then fire your thrusters against your orbit to slow you down enough to head back towards the earth in a controlled manner, roughly where you want to be.

Could this be achieved with ion propulsion?
Is the typical amount of ion thrusters mounted on satellites sufficient to deorbit in a timely enough manner?

What about controlled decay, could you use it to simulate a constant atmospheric drag, by compensating for solar activity?

If left to decay on its own through atmospheric drag, the moment reentry happens could vary by a few hours due to solar activity.

My question is about choosing the general point of reentry (a specific region or zone), not about aiming the spacecraft on a small ground based target.

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Probably not.

To control the point of reentry, you need to be able to adjust from a perigee high enough to not promptly reenter (i.e. above 200km) to one low enough to promptly reenter (i.e. below 80km) in significantly less than the time it takes to complete a single orbit -- otherwise, the unpredictable effects of drag in the variable density upper atmosphere will screw up your timing. Regardless of whether you're starting from low circular orbit or from an eccentric orbit with apogee at geosynchronous altitude, this perigee adjustment takes roughly 100 m/s of ∆v to achieve.

Typical stationkeeping ion thrusters on geosynchronous-orbit satellites produce a couple hundred millinewtons against several tons of satellite mass, which yields something on the order of 2 x 10-5 m/s2 of acceleration.

100 m/s divided by 2 x 10-5 m/s2 requires weeks of constant acceleration, so you can't do it in the time available.

A satellite starting from hundreds-of-thousands-of-km apogee and 200km perigee, optimized for ion-thrust-to-mass ratio and nothing else, could probably do it, since its orbit would spend several days at high altitude, but I don't think any existing or practical satellites could.

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  • $\begingroup$ Assuming that a GEO bird doesn't need to be several tons for this type of demonstration, is it possible to estimate the mass order of magnitude where this would be possible given a ∆v of 100 m/s? $\endgroup$ – aranedain May 18 '20 at 19:56
  • $\begingroup$ The highest delta-V achieved by an ion thruster to date was from the Dawn spacecraft with a delta-V of 11.5 km/s. Dawn was 1217kg at launch, of which 425kg was propellant (xenon). To achieve ten times the Delta-V you'd need something 1/10 as heavy, right? And that wouldn't leave you enough weight for propellant. So this is ruled out strictly by the delta-V requirement, no matter how much time you give it. The "thrust it fast" requirement is totally separate, and ion thrusters really can't do it. $\endgroup$ – Ross Presser Feb 2 at 16:03
  • $\begingroup$ @RossPresser You need 100 *meters*/s, not 100 km/s. Total delta V isnt the problem, it's thrust. $\endgroup$ – Russell Borogove Feb 2 at 17:23
  • $\begingroup$ @RussellBorogove D'oh! Sorry for making such a dumb error. $\endgroup$ – Ross Presser Feb 2 at 17:33

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