Why would extra fuel be needed to de-orbit the ISS (if it comes to that)?

Question

The BBC article The Astronaut Fighting to Save our Home in Space discusses “experienced astronaut, and astrophysicist Dr Michael Foale”'s exciting career aboard the Space Shuttle, Soyuz, Mir, and ISS, and now current efforts to find future backers for the ISS after its current mission (and funding) might come to a close.

But the station’s days are numbered. Funding by the various space agencies involved is only agreed until 2024. This means in just six years’ time, the most expensive structure ever built will be pushed out of orbit by a Progress spacecraft to disintegrate over the Pacific.

And the countdown clock is ticking.

“Year by year, Russia is launching the fuel to fill up the tanks of the ISS service module to enable the space station to be deorbited,” says Foale. “That’s the current plan – I think it’s a bad plan, a massive waste of a fantastic resource.” (emphasis added)

Since the ISS' large cross-sectional area including the solar panels already leads to continuous loss of altitude requiring regular orbit-raising burns, why would there be such a requirement for so much fuel for de-orbit? Has there a specific plan been developed to de-orbit (if decided) in such a way that it comes down over the Pacific? How much fuel is thought to be needed to do this?

Answer 1

The extent of Earth's atmosphere and thus the amount of drag on the station varies greatly with solar weather; the relationship between drag and time-to-deorbit is very nonlinear. Combined, this means that reentry time is not very predictable on the scale of hours, which means the ISS could come down anywhere on its ground track and we wouldn't know where it was headed until a short time before it hit.

By doing a substantial deorbit burn at a precise time, the reentry can be controlled much more closely, and the ISS can land in the middle of an ocean instead of in my back yard.

According to the 2010 NASA deorbit plan, a Progress spacecraft would be modified to be able to thrust while taking propellant directly from the station's tanks (the ATV version of the plan is obviously a non-starter now). Apparently the Zvezda module's thrusters, while more powerful, can't burn continuously for the durations envisioned here. A Progress would be modified with sustained-fire engines and used for deorbit burns. Zvezda can store up to 860kg of propellant and the FGB/Zarya module 5760kg. Using the rocket equation, we can calculate that this comes to something like 50 m/s of ∆v applied to the station. This is enough to drop from a fairly stable circular 270km orbit to something like a 270km apogee x 100km perigee, which will produce a prompt and predictable reentry. This version of the plan has a map suggesting an Indian Ocean disposal rather than Pacific; this gives the longest available safe ground track (about half of Earth's circumference!), passing South of Australia and NZ before coming back north to the West Coast of North America.

The plan seems to have been updated in 2012 to make use of 2 Progress spacecraft, but I can't find more detail on that version of the plan.