Cluster quartet move in step
21 May 2001Moving time has arrived for the four Cluster spacecraft. During the next few weeks, the satellites will carry out a series of manoeuvres that will increase their separation distances. Eventually, each spacecraft will have drifted to a distance of 2000 km from its partners, compared to the current figure of 600 km.
The first engine firings took place on the evening of 10 May, starting with spacecraft 3 (Samba) and progressing through spacecraft 4 (Tango), 2 (Salsa) and 1 (Rumba). Further adjustments of each spacecraft will take place until the heavenly dance reaches its finale on 3 June.
In order to reach their final configuration, the thrusters will have to be fired seven times on Rumba, eight times on Salsa and Samba, and five times on Tango. The longest engine burn - by Samba - will take about one hour. Not surprisingly, the lowest fuel consumption (2.7 kg) will be on Tango, while Samba will be gulping down the most (11.7 kg). Even so, there will be plenty of fuel left for the remainder of the Cluster mission.
"We will be using only 5 - 10% of the fuel reserves on each spacecraft during these manoeuvres," said Cluster project scientist Philippe Escoubet. "This leaves plenty to spare for future readjustments of the four spacecraft."
So why do the Cluster spacecraft have to perform this complex dance routine?
"At the moment, the spacecraft are flying in a tetrahedron (lopsided pyramid) on the flank of the polar cusp - the region where magnetic field lines reach the Earth's surface," explained Dr. Escoubet. "The satellites are close together because the magnetic structures we want to observe are smaller and more detailed data is required. Some fascinating new results have already been returned from these studies."
"The manoeuvres now being undertaken will enable us to rearrange the spacecraft so that the tetrahedron formation will be shifted to the Earth's magnetic tail region," he added. "In the huge tail, we will be studying much larger structures, such as magnetic waves and particle acceleration, so the spacecraft have to be moved further apart."
"The tail observations start in June and last for six months," he said. "This will be the first time that this region has been studied simultaneously by four identical spacecraft. By August, the spacecraft apogee - its furthest distance from the Earth - will be in the centre of the magnetic tail at a distance of 120 000 km from Earth. This will be a very exciting time for us."
Team work is vital if the orbit changes are to be successfully carried out. The current manoeuvres would not be possible without advanced planning by the flight dynamics team at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
The commands to fire the thrusters are uploaded to the computers on each spacecraft one day before a particular manoeuvre is scheduled to take place. The modified orbits are later checked as the quartet fly within range of the ESA ground stations of Villafranca 1 and 2 in Spain and Perth in Australia.
"Flight dynamics have done a tremendous job in planning the 28 manoeuvres, generating the commands and then verifying that everything goes alright," said Sandro Matussi, Cluster spacecraft operations manager at ESOC. "Engineers are monitoring the situation day and night when the manoeuvres are taking place, ready to intervene if necessary. Only eight of the thruster firings take place when the satellites are not 'visible' from Spain or Perth."
During this period of unusual activity, the temporary build-up of gases around the spacecraft means that Cluster payload operations have to be reduced. Some instruments, for example, FGM and WEC, can remain in operation, but others such as ASPOC, CIS, EDI, PEACE and RAPID, which operate at high voltages, have to be switched off or put in a standby mode. This prevents the possibility of electrical arcing that could damage the satellites and their instruments. These instruments are switched on again between 8 and 24 hours after each manoeuvre is completed and the cloud of gases around the spacecraft has dissipated.
This activity has inevitably created extra work for the Joint Science Operations Centre at the Rutherford Appleton Laboratory in the United Kingdom, since each of the 42 instruments on Cluster has its own operating schedule.
"Once again, this job has been perfectly carried out with the planning tools available at JSOC," said Philippe Escoubet. "Everyone has put in a great effort to make these orbital manoeuvres a success."
For more information please contact:
Dr. Philippe Escoubet, Cluster project scientist