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The International Space Station is constantly losing orbital energy due to atmospheric drag. How does station-keeping work for the ISS? What sort of propulsion system is used to keep it in the desired orbit, and where are these thrusters located? I imagine it could be as simple as pressurized nitrogen.

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I think, pressurized nitrogen has too low exhaust velocity... – osgx Apr 12 '14 at 20:58
There's a plan to try reboosting with an ion thruster as well, possibly in 2015. – Russell Borogove Apr 13 '14 at 0:27
In November they said in three years, so perhaps late 2016. Since such claims from a company seeking investment funds are always optimistic, more likely 2017 or later. – Mark Adler Apr 13 '14 at 13:05
up vote 13 down vote accepted

Biprop attitude control thrusters using UDMH and N2O4 in a docked Progress vehicle are used to reboost ISS using about four Progress vehicles a year.

More recently, ESA's ATVs have also been doing reboosts using MMH/N2O4 thrusters.

On the order of 2 m/s per month is required.

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How much delta-v does ATV or Progress have for themself? How much delta-v get ISS from them (0.5-2 m/s)? – osgx Apr 12 '14 at 20:57
For the ATV: "From the 7 tonnes of available propellant, approximately 2.3 tonnes is available for free flight manoeuvres and approximately 4.7 tonnes is available for manoeuvring the space station at intervals of 10 to 45 days." From that, the Isp of 2650 m/s, and the mass of the ATV and ISS, you can compute the $\Delta V$'s from the 2.3 and 4.7 metric tons of propellant respectively. – Mark Adler Apr 12 '14 at 21:01

In addition to Progress and ATV vehicles, docked at the aft end of Zvezda, using their main engines to boost the orbit, the Zvezda module also has engines that can be used. In fact, the Progress cargo vehicles can refuel the Zvezda module.

From Wikipedia:

The two main engines on Zvezda can be used to raise the station's altitude. This was done on April 25, 2007. This was the first time the engines had been fired since Zvezda arrived in 2000.

The US plans to send a VASIMR engine by Ad Astra, to the station to test its ability to provide thrust to the station for orbital maneuvers.

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