21 June 2007
Cluster: ESA spacecraft flying closer than ever for better science
After weeks of manoeuvres, Samba and Tango, two of ESA’s four Cluster
satellites are now orbiting in formation, separated by only 17 km. This is
the closest two ESA satellites have ever been in routine operations and will
enable new scientific discoveries.
Cluster, ESA’s mission comprising four identical satellites, relays the most
detailed information ever about how the solar wind affects our planet, and
is the first mission to study the Sun-Earth connection in 3D.
This is done by studying the behaviour 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.
This is why the four Cluster satellites – Salsa, Tango, Rumba and Samba –
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
manoeuvring is required to boost the timing and spatial resolution of
The 17 km separation between Samba and Tango, reached on 20 June 2007, may
seem safe enough, but is a mere whisker in operational terms. The two
spacecraft are travelling 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 lead to unplanned velocity changes three times in the past.
Before the current manoeuvre 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
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 manoeuvres to avoid collisions, ESA’s missions 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,” says Juergen
Volpp, Cluster Spacecraft Operations Manager at ESA’s European Spacecraft
Operations Centre (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
In the words of Philippe Escoubet, ESA’s Cluster and Double Star Project
Scientist, “This space ballet is another major achievement of the Cluster
flight control team at ESOC. Their contribution is key to the on-going
scientific success of the Cluster mission.”
For more information:
Philippe Escoubet, ESA Cluster Project Scientist
Email: Philippe.Escoubet @ esa.int
Juergen Volpp, ESOC
Email: Juergen.Volpp @ esa.int
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