A few more details on the background and planning behind last week’s rare ISS deboost performed by ATV-5. This text was sent in by ESA’s  Eric van der Wal, based at Johnson Space Centre, Houston, Texas.

On a regular basis, the ISS performs reboosts for achieving the correct orbital phase angles in order for Russian visiting vehicles to successfully rendezvous with the ISS. Reboosts are also sometimes preformed to counteract natural orbital decay. These orbital manoeuvres are planned months in advance and are continuously adjusted based on solar activity. As Solar Cycle #24 approaches a minimum over the next few years, both Russia and NASA agreed that continuing to do posigrade phasing manoeuvres at our previous altitude may result in the ISS inadvertently climbing higher than the desired maximum altitude (due to much lower natural orbital decay rates between reboosts). Higher altitudes are required during solar cycle maximums in order to conserve ISS propellant and allow the propellant delivery vehicles to supply the ISS within their nominal capabilities. During solar cycle minimums, lower altitudes aid in minimizing micrometeorite and orbital debris (MMOD) risk, as well as, reducing crew radiation.

The manoeuvre on 28 January was planned as a retrograde burn to achieve the correct phase angle for the next Russian visiting vehicle, as well as proactively dropping the ISS altitude to one low enough not to increase propellant consumption significantly but also that would alleviate the risk of remaining at a much higher altitude during the upcoming solar minimum. This particular retrograde manoeuvre was originally planned to be a magnitude of -2.3 m/s. Over the last few months, the ISS was able to lower its altitude naturally by allowing atmospheric drag to decay its orbit, resulting in a deboost that was much smaller in magnitude that only lowered the ISS altitude by 1.2km. The difference in monthly propellant costs for maintaining the ISS orbit between the higher and lower altitudes is expected to be about 10-20 kg/month depending on the solar cycle activity. The orbit duration is effectively unchanged.