PARIS, June 26, 2007 - Satnews Daily - Four sturdy European science satellites past their prime continue to boogie in the solar wind they’ve been studying for the past seven years. The solar wind (the perpetual stream of subatomic particles given out by the Sun) can damage communications satellites and power stations on Earth.
The European Space Agency’s (ESA) four “dancing” satellites-- Salsa, Tango, Rumba and Samba – comprising its Cluster Mission are also studying the behavior of near-Earth plasma, an extremely variable state of matter, composed of ions and electrons but electrically neutral, spread over large distances. A key to understanding it and studying complex geophysical processes in different regions is to have space-based, multi-point observations and to be able to vary the distances between spacecraft, as these processes operate at different scales in nature.
The four Cluster spacecraft have spent several years passing in and out of our planet's magnetic field. Their mission will be to complete the most detailed investigation ever made into the ways in which the Sun and Earth interact.
This is why the four Cluster satellites are not always at the same distance from each other. The inter-spacecraft distances are varied depending on the type of phenomena under study. Close maneuvering is required to boost the timing and spatial resolution of scientific observations.
The 17 km separation between Samba and Tango reached only recently may seem safe enough, but is a mere whisker in operational terms. The two spacecraft are traveling at approximately 6 km/s with respect to Earth and now, almost seven years after launch, the batteries on both spacecraft are well beyond their nominal lifetime. Unpredictable battery anomalies have already led to unplanned velocity changes three times in the past.
Before the current maneuver campaign, Samba and Tango were separated by a distance of 450 km, following each other around the Earth in a polar elliptical orbit from roughly 14 000 to 124 000 km in altitude, every 57 hours.
In this initial configuration Rumba, Salsa and the closer Samba-Tango pair formed an isosceles triangle in space 10 000 km across. This was oriented roughly perpendicularly to the so-called ‘neutral sheet’. This is an area of Earth’s magnetosphere consisting of a thin electrical current sheet located within the magnetotail on the night side. The study of the physics of the current sheet is one of the main goals of Cluster.
At the request of the Cluster scientific community and through a series of very delicate maneuvers to avoid collisions, mission controllers modified this orientation and reduced the distance between Samba and Tango to 17 km. They eventually positioned the triangle of satellites roughly parallel to the equator inside the neutral sheet.
"In the new orientation it is possible to monitor very minute fluctuations in the thin 'neutral sheet' with a high spatial resolution, as we simultaneously perform two measurements very close together," said Juergen Volpp, Cluster Spacecraft Operations Manager at ESA’s European Spacecraft Operations Center (ESOC), in Germany.
“The inter spacecraft distance of 17 km is approaching the limits of what can be attained with Cluster, where the four spacecraft are operated independently,” says Detlef Sieg, flight dynamics engineer at ESOC. “Future missions will need inter-satellite communication systems to achieve even smaller distances.”
Philippe Escoubet, ESA’s Cluster and Double Star Project Scientist, said this space ballet is another major achievement of the Cluster flight control team at ESOC. Their contribution is key to the ongoing scientific success of the Cluster mission.
The Cluster mission continues investigating the small-scale structure (in three dimensions) of the Earth's plasma environment, such as those involved in the interaction between the solar wind and the magnetospheric plasma, in global magnetotail dynamics, in cross-tail currents, and in the formation and dynamics of the neutral line and of plasmoids. They are providing a detailed three-dimensional map of the magnetosphere, with surprising results
During periods of high solar activity (which cycles every 11 years), the solar wind can be particularly energetic. This can have a dramatic effect on human activities, disrupting electrical power and telecommunications or causing serious problems in the operation of satellites, especially those in geostationary orbit. Subtle changes to the weather on Earth also occur during these times. Watching the effects of this increased activity during these periods is one of the main tasks of Cluster.
Understanding the interaction between the solar wind and the magnetosphere and how the plasma levels of the magnetosphere are affected is important. Cluster will help us to prepare for the effects of sudden bursts of solar energy here on Earth.
The original operation life-time of the Cluster mission ran from February 2001 to December 2005. In February 2005, however, ESA approved a mission extension from December 2005 to December 2009.
