Cluster: ESA
Spacecraft Flying Closer Than Ever for Better Science
(21 June 2007) 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.
The 16 July 2000, at 14.39 CEST, a Soyouz-Fregat launch vehicle provided by the French-Russian Starsem consortium lifted off with FM 6 Salsa and FM 7 Samba, the first pair of Cluster II satellites. ESA's Cluster II mission, consists of four identical spacecraft flying in formation between 19000 and 119000 km above the Earth. There, they will study the planet's magnetic field and electric surroundings in three dimensions.In particular, they will be looking at the effects of the solar wind, the hot wave energy produced by the Sun, which buffets Earth's protective magnetosphere. This wind often breaks through the magnetosphere at the poles, producing auroras. Cluster II will examine this and many other phenomena associated with the solar wind. (courtesy: ESA)
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 scientific observations.
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.
Orbit of the Cluster constellation in June 2007 within the Earth’s magnetosphere. Two of the four spacecraft, Tango and Samba, reached the very close distance of 17 Kilometres from each other – the closest ever achieved in ESA’s spacecraft routine operations. This configuration will provide new scientific results about Earth’s magnetosphere. (courtesy: ESA)
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 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 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 smaller distances."
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".
(source:
ESA)