Preventing the Space Station from plunging into the atmosphere …
The International Space Station (ISS) circles the Earth in a circular or slightly elliptical orbit at an altitude of about 360 km. But the orbital altitude of the ISS steadily declines as a result of atmospheric drag. This ‘orbital erosion’ is around 160 metres per day, or five kilometres per month. To prevent the ISS from plunging into the atmosphere it is therefore essential, approximately once a month, to perform a ‘reboost’ manoeuvre to maintain altitude.
This operation consists of slightly increasing the speed of the ISS by the ignition of appropriate engines, which causes it to rise.
What is more, space debris circling the Earth sometimes crosses the orbital path of the space station and threatens to hit and puncture its inhabited sections, putting the lives of the on-board crew at risk. The re-boost function makes it possible to perform escape manoeuvres to avoid such debris in the interest of crew safety.
To execute an orbit-raising or collision-avoidance manoeuvre, the ISS has two options. It can use its own propulsion system, incorporated in the Russian Zvezda service module, or the propulsion system of a ‘passenger’ vehicle docked with the Station – such as the Russian Progress vehicle and now the European ATV. The use of passenger vehicles is preferable in order to preserve the Station’s own fuel supply. The ATV was therefore designed to remain docked with the ISS for six-month periods, and carry up to four tonnes of propellant for executing ISS operations.
How does it work?
The ISS is travelling at a speed of 7,690 m/s (27,700 k/h). A variation of 1 m/s in the Station’s speed modifies the orbit semi-major axis by about 1.75 km. Modification of the semi-major axis by five km would consequently require a speed increment (Delta V) of the order of 2.85 m/s. Given the mass of the ISS, around 280 metric tons, this speed variation would consume about 260 kg of propellant.
Once docked with the Station, the ATV is switched nominally to the dormant mode with its propulsion system shut down. To initiate a reboost, the propulsion system is activated by the Toulouse Control Centre, responsible for operating the ATV and monitoring its correct performance. During the docked phase, the ATV acts as a propulsion ‘service pack’ for the ISS – it is the Station’s onboard computers which instigate initiation of the ATV’s thrusters to carry out the reboost manoeuvre. The reboost operation is performed with the same propulsion system used to manoeuvre the ATV itself. This ‘double use’ was taken into account from the outset of the project of the European Space Agency and incorporated in the design of the propulsion system, both in terms of the engines employed and the amount of fuel required.
Lifting 280 tonnes 4.7 kilometers!
The ATV Jules Verne performed its first reboost manoeuvre on 25 April 2008, just three weeks after docking with the ISS.
This was a major first for the ATV, which, in so doing, placed its propulsion system directly under the control of the ISS computers. The computers took over control of the ignition of two of the four ATV main thrusters, generating a total thrust of 1,000 N (Newtons). In human terms, this equates to lifting a mass of 100 kg. The boost manoeuvre lasted around 13 minutes. The desired Delta V was achieved with a 3 cm/s accuracy. By generating a speed increment of the order of 2.67 m/s, the reboost increased the orbit altitude of the ISS by 4.7 km.
On 19 June 2008, the ATV was used for the second time to reboost the Station’s orbit. This manoeuvre raised the orbit of the ISS by almost 7 km.
On 27 August 2008, the ATV enabled the ISS to perform its first space debris avoidance manoeuvre. In the current ISS configuration, only the ATV could be used to perform this: known as a ‘retrograde’ manoeuvre.
Altogether, the ATV Jules Verne’s propulsion system was used 14 times by the Station, either for reboost manoeuvres or for adjusting the attitude of the ISS.
A special dossier full of informative articles and videos has been created about the ATV ‘Johannes Kepler’.