ESA has recovered a crucial science package on its Cluster mission, using a series of commands invented for the occasion.
Since 2000, the four Cluster satellites have been orbiting Earth in tightly controlled formation. Each carries an identical payload, designed to investigate Earth's interaction with the solar wind – the stream of charged particles pouring out from the Sun.
Of each satellite's 11 instruments, five comprise the Wave Experiment Consortium (WEC), which measures electrical and magnetic fields. The sensors on all four satellites need to work together to make carefully orchestrated observations.
But on 5 March this year, the WEC package on Cluster's number 3 satellite, Samba, failed to switch on, and the instruments refused to give any status information.
Ground controllers at ESA's European Space Operations Centre, in Darmstadt, Germany, immediately triggered a series of standard recovery procedures - none of which succeeded.
"With no status data and no response from the instrument, we suspected either that the device's five power switches were locked closed or a failure caused by an electrical short circuit, one of the most dangerous faults on any satellite," says Jürgen Volpp, Cluster operations manager.
After several weeks' work, the team eventually made use of some onboard software that had been dormant since just after launch over 10 years ago to diagnose the problem.
The conclusioin was that the five power switches were locked in the 'closed' position. Nobody had ever considered how to deal with this eventuality, which meant a recovery procedure had to be developed from scratch.
"The solution was based on a 'dirty hack' – jargon referring to any non-standard procedure – but we really had no other option," says Volpp.
Finally, on 1 June, the new series of commands was radioed up. It worked: the the power switches flipped back to 'on' and the WEC came back to life.
Cluster has since returned to normal operation and measures are being taken to prevent this failure from happening again.
"When everything goes as planned, flying a mission can be routine," says ESA's Manfred Warhaut, head of mission operations. "But when unexpected trouble occurs, and there's nothing in the manuals, you really want to have an experienced and talented team on hand to solve the problem."