Space maneuvers have often been described as an orbital ballet, but the European Space Agency's (ESA) Cluster II satellites are currently in a ballet where the dancers are moving blindfolded at
hypersonic speeds as they pass within a cosmic hairsbreadth of one another. That's because two of the Cluster satellites are flying within "touching distance" of one another as scientists try to learn more about the effects of solar wind on the Earth's magnetic field.
Launched in 2004, the Cluster II mission is the replacement for ESA's original Cluster mission, which was lost on liftoff in 1996 due to a software error. Consisting of a constellation of four satellites flying in a tetrahedral formation in highly elliptical orbits, usually spaced about 600 to 20,000 km (370 to 12,400 mi) apart. Their purpose is to produce a 3D map the Earth's bow-shock wave, which is produced by the solar wind hitting the magnetosphere; causing it to decelerate from supersonic to subsonic speeds as it flows around the planet. On January 7, Cluster satellites 3 and 4 were brought to within 6 km (3.7 mi) of each other to allow scientists to make very detailed observations of the shock wave. Meanwhile, the other satellites are keeping station 5,000 km (3,100 mi) away. "This enables us to study physical processes such as plasma heating and acceleration at the bow shock," says Philippe Escoubet, the mission’s project scientist. "Previous studies of the bow shock have
shown electron heating over distances as small as 16 km (10 mi), but we want to see what is happening using two satellites at smaller scales." Six kilometers may not seem very close, but satellites flying in formation aren't like aircraft or ships. The orbital spacecraft are in freefall and as they circle the Earth they speed up and slow down, so their trajectories have to be very carefully worked out so they don't collide with one another. In the present maneuver, the two satellites pass one another on each orbit with a safety margin of only three seconds. According to ESA, this is made even more difficult because the satellites have no way to track one another and must rely on ground stations to keep them apart. However, at 6 km, they are so close together that the tracking stations can't tell one from the other except by means of their beacons, which transmit at different frequencies. This has the advantage of allowing a single station to track the satellites because there's no need to triangulate.
"Adjusting the formation requires plenty of teamwork and coordination, not only among teams in Darmstadt but also with the Joint Science Operations Center in the UK and the Payload teams," says Cluster operations manager Bruno Sousa. "We also work to optimize fuel usage and eliminate any risk of potential collision, though this only becomes really significant if we go down to 1 or 2 km (0.6 - 1.2 mi) separation." The Cluster II mission is scheduled to maintain the close formation until March.