Cluster quartet go into eclipse
30 August 2000
Doom and gloom are currently affecting the Cluster project team, but there's no need for concern. It's just the start of the eclipse season for the four Cluster spacecraft. Over the next week, the quartetwill periodically cease to be illuminated by the Sun as they sweep through the Earth's shadow.For ground-based sky watchers, eclipses are fairly rare, though dramatic, natural events. Last summer, millions of people peered upwards at the sky in an effort to see one of Nature's wonders - a total eclipse of the Sun.
However, to spacecraft engineers, eclipses are something to avoid whenever possible, a potential source of problems for the precious satellites and their scientific payload. In particular, the lack of sunlight falling on a spacecraft's solar cells shuts off the normal power supply, so that back-up batteries are required to keep the satellite alive and healthy. Another potential threat comes from the sudden changes in temperature as the spacecraft is alternately bathed in sunlight and darkness.
In the case of Cluster, engineers have long foreseen these difficulties and prepared a plan of action for the occasional eclipses that take place when their orbits carry them into the Earth's shadow.
Each spacecraft carries five silver-cadmium batteries which provide the necessary power to survive eclipses lasting up to four hours. At the same time, power requirements are reduced by switching off all of the scientific instruments.
Small heaters have been built into the experiments and vital spacecraft systems, and thermal blankets have been carefully installed by hand around the main structure in order to keep the temperature within the required range.
"This will be a test of the capability of the spacecraft to live on battery power and will confirm the predictions of the thermal-vacuum tests done at IABG in Munich," said Cluster Deputy Project Manager Alberto Gianolio. "However, we do not expect any surprises!"
The current Cluster eclipse season began on Monday 28 August with a short penumbral passage for the Salsa spacecraft, when the light from the Sun was only partially cut off. No problems were reported.
The first total eclipse, lasting approximately 3 1 hours, was also successfully negotiated today (30 August) as, one by one, the Cluster flotilla flew in and out of the Earth's cone-shaped shadow.
First to enter the gloom at 14:32 CEST was Salsa, followed by Tango (14:37), Samba (14:47) and Rumba (14:48). Their emergence back into sunlight was timed at 17:46 CEST (Tango), 17:57 (Samba), 18:00 (Salsa) and 18:05 (Rumba).
Onboard telemetry data was recorded on the solid-state recorders of each spacecraft and played back after completion of the eclipse. Analysis of the data is under way at the European Space Operations centre (ESOC) in Germany, but preliminary checks show that all four spacecraft behaved exactly as expected throughout the event.
Two more periods of eclipse will take place in this current cycle. The next one will occur on the night of 1 - 2 September between 22:53 and 02:48 CEST. A shorter eclipse sequence will take place on 4 September between 8:05 and 10:39 CEST.
Meanwhile, the commissioning of the 44 scientific instruments has been continuing at a rapid pace. So far, at least one unit of each instrument on the four spacecraft has been started and undergone initial checks.
FGM has been commissioned on all four spacecraft and has already detected intense activity in the magnetotail. Commissioning has also begun on RAPID, CIS and PEACE. The first data are most encouraging. RAPID has detected high energy particles in the magnetotail, while CIS and PEACE have found low energy particles in the lobe regions.
The five wave instruments (DWP, STAFF, EFW, WHISPER, WBD) on Rumba and Salsa have been switched on and all units are functioning nominally. STAFF was able to measure a local magnetic field of around 60 nT, which is the value expected in the magnetotail lobes.
However, the first real data from the wave instruments will not be received until after the deployment of the 50 m wire booms, which is scheduled to begin on 5 September. It will take about four weeks to extend the booms to their full operational length.
