The International Space Station (ISS) has been forced to fire the thrusters of ESA's Automated Transfer Vehicle (ATV) in order to maneuver the station and its crew out of the way of potentially harmful debris. Such instances are rare, making a catastrophic scenario highly unlikely. However, with each passing year, the amount of orbital debris increases, heightening the risks of a collision for mankind's only manned outpost among the stars.
There is currently estimated to be around 21,000 pieces of debris exceeding 10 cm (3.9 in) in size currently existing in low-Earth orbit (within 2,000 km or 1,243 miles of the Earth's surface). Whilst the majority of the debris is very small, some pieces travel at velocities of up to 15 km per second (9.32 miles p/s), meaning that despite their diminutive size, any impact with the ISS would impart a devastating amount of kinetic force.
Because of the potentially disastrous ramifications of an impact, orbital debris is constantly monitored from ground stations spread across the globe, and whilst potential impact events are very rare, collisions do happen. This is evidenced by the event that created the debris prompting the emergency maneuver of Oct. 27.
The debris emanated from a 2009 collision between Russian satellite Cosmos-2251 and the US-made Iridium 33. The impact resulted in a vast cloud of debris, which included the roughly hand-sized object that would pass within 4 km (2.5 miles) of the ISS, threatening both the station and her crew of six.
A mere six hours prior to the potential impact, the five space agencies tasked with administering the station agreed to undertake an emergency burn to lift the ISS out of danger. Ordinarily, in a scenario where there is less than 24 hours warning prior to a possible impact, the station would be shunted out of harm's way by the thrusters of a Russian progress spaceship used to bring supplies and science to the station, docked to the Zvezda service module. However at the time of the emergency, no such spaceship was present.
Therefore the task fell to the European-made ATV Georges Lemaître. At 18:42 CET the ATV executed a four-minute burn, successfully raising the orbit of the 420-tonne (463-ton) station by 1 km (0.6 miles). Having rescued the ISS and her crew, the spacecraft is due to be released from the station in February, burning up harmlessly in Earth's atmosphere a short time later.
Source: ESA
