Boom time begins for Cluster
7 September 2000The Cluster mission will be booming over the next six weeks. During a carefully planned series of operations, 16 wire booms, each almost 50metres in length, will gradually be released from the spinning satellites. Once they are fully deployed and begin to sweep out a giant circle aroundthe spacecraft, these booms will provide a flood of data for the five wave experiments on each Cluster satellite.
"It's rather like reeling out a fishing line," said Cluster project scientist Philippe Escoubet. "The centrifugal force generated as the spacecraft rotates causes the wires to slowly extend outwards, away from the spacecraft's main body."
The lengthy deployment sequence began on Tuesday, when the first pair of wire booms on the Salsa spacecraft were released. The deployment started with the firing of pyros to open the protective covers on the four booms. Pulling against a small motor which braked their progress, the wires edged out of their housing at about one centimetre per second.
Although one hour was allowed for the deployment to 15 metres, the actual extension only lasted about 30 minutes. Almost immediately afterwards, the second pair of wire booms on Salsa were also slowly extended to a length of 15 metres.
"We can check the rate of extension and the distance reached by counting the number of times the wire spool turns," explained Philippe Escoubet. "It's most important that everything is symmetrical and each pair deploys the same length at the same time."
Meanwhile, the change in momentum caused by the sprouting of these wire extensions caused the satellite's rotation rate to slow from 15 rpm to less than 12 rpm. Engineers sent instructions for the spacecraft to spin-up from 12 to 22 rpm in order to get the best possible electric field measurements.
The same procedure will be followed later today (7 September) when the wire booms are extended to 15 metres on Rumba.
After a brief interlude to switch on the particle instruments on all four spacecraft, stage two of the wire boom deployment will begin on 12 September. This time, the first pair of booms on Salsa will be allowed to grow to 35 metres in length while the other pair will be restricted to a length of 20 metres. This will allow the scientists to get data from different distances and to check that everything is working as it should.
The booms will remain like this for nine days, before the longer pair are extended to their full operational length of 43 metres. A similar procedure will be employed for Rumba a few days later. Samba and Tango will follow the same protracted sequence in early October.
By the end of this painstaking procedure, each of the spacecraft will resemble rotating drums with four wire spokes, each 43 metres long, capped by a small sphere. The way will then be clear to receive the first real data from the five experiments that will use the antennae to study electromagnetic fields and waves in the Earth's magnetosphere.
"This will be a major step towards the start of our unique scientific programme," said Philippe Escoubet.