A new East-West scientific collaboration recently took a further step towards acceptance when a group of European Space Agency delegates and space scientists travelled to Beijing to meet their Chinese counterparts.

Under discussion was possible European participation in a dual-spacecraft mission known as Double Star. A key aspect of the proposed project is the inclusion of a number of instruments that have already been developed and successfully flown on ESA's Cluster mission.

Illustration of orbits for Double Star Program (DSP) satellites. This mission consists of two satellites, the equatorial satellite DSP-E, following a 550 x 60,000 km orbit, inclined at 28.5 degrees to the equator and the polar satellite DSP-P, following a 350 x 25,000 km orbit inclined at 90 degrees to the equator. Photo: ESA

"A number of Chinese co-investigators have been involved with Cluster for many years," explained Cluster project scientist Philippe Escoubet. "One of these was Professor Liu, a member of the Chinese Academy of Sciences, and in 1997 he proposed that the principal investigators (PIs) on Cluster should consider flying their experiments on two Chinese satellites. About half of the PIs eventually agreed to support this."

This approach by the Chinese was particularly significant since Double Star will be the first mission that country has ever launched to explore the Earth's magnetosphere - the magnetic bubble that surrounds our planet.

As its name suggests, Double Star will involve two satellites flying in complementary orbits around the Earth. This will enable scientists to obtain simultaneous data about the changing magnetic field and population of electrified particles in different regions of the magnetosphere.

The 'polar' satellite will concentrate on physical processes taking place over the planet's magnetic poles and the development of aurorae (Northern Lights). It will follow a 350 x 25 000 km orbit that carries it around the Earth once every 7.3 hours.

The 'equatorial' element will follow an even more elliptical orbit of 550 x 60 000 km, inclined at 28.5 degrees to the equator. This will enable it to investigate Earth's huge magnetic tail, the region where particles are accelerated towards Earth's magnetic poles by a process known as reconnection.

Each 270 kg Double Star satellite will carry a payload of scientific experiments - ten on the equatorial spacecraft and nine on its companion. Although the exact complement for each satellite has yet to be determined, it is envisaged that about half of the experiments will be provided by European institutes.

The proposed European contribution includes:

• FGM - the fluxgate magnetometer (Imperial College, London and IWF, Graz)
• EPS - the energetic particle spectrometer (IDA, Braunschweig)
• CIS - the Cluster ion spectrometer (CESR, Toulouse)
• ASPOC - active spacecraft potential control (IWF, Graz)
• PEACE - plasma electron and current experiment (MSSL-University College London)
• STAFF / DWP - spatio-temporal analysis of field fluctuation experiment / digital wave processing experiment (CETP Velizy / Sheffield University)
• NIA - natural atom imager (National University of Ireland, Maynooth)

Nearly all of these are flight spares from the four Cluster spacecraft, which are currently revolutionising our understanding of near-Earth space.

"This is important because we can provide the instruments relatively quickly and at reduced cost," said Philippe Escoubet. "It also means that there should be no problem in meeting the Chinese launch schedule. At present the satellites are expected to be launched in December 2002 and April 2003."

Following these recent intensive discussions in Beijing, the proposal to participate in Double Star will be discussed at the next meeting of ESA's Science Programme Committee in May.