"Everybody waltz!"
On Monday, 11 October 2010 at 14:00 UTC we entered uncharted territory. Around this time, the thrusters of TanDEM-X were fired to reduce the separation between it and TerraSAR-X from 20 kilometres to 500 metres. TerraSAR-X remains in its original, circular orbit, while TanDEM-X, moves on a slightly eccentric orbit in a plane that is rotated by a small angle with respect to that of its partner. The result is a 'dance of the satellites' – not in three-quarter time, but something more leisurely.
If one could fly behind the satellites, one would observe that TanDEM X moves around its twin, TerraSAR-X, once every 95 minutes – the duration of their orbit around the Earth. At the North Pole, TerraSAR X overtakes TanDEM X, as the latter, because of its eccentric orbit, is slightly higher and thus orbiting more slowly. At the South Pole, the situation is reversed; TanDEM X orbits lower and faster, and overtakes TerraSAR X. Of course, a collision must be prevented – 'touching' is not allowed! Viewed from side, the two orbits can be imagined as being like two links in a chain – one link round, the other elliptical – intertwined but not touching one another.
To ensure that this works in practice, it has been tested in numerous studies and simulations – made possible by colleagues at the control centre in Oberpfaffenhofen. Our 'Dance Master' – and a specialist on the flight dynamics team – Ralf Kahle, will report in the next blog post – as soon as his heart rate has returned to normal…
The circular orbit of TerraSAR X (red) and the eccentric orbit of TanDEM X (green) never cross, preventing a collision between the two satellites.
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