Like the Duracell Bunny, the four Cluster spacecraft that ESA sent into space 15 years ago have continued working long after anyone expected them to. And in its extended lifespan, the mission has paid dividends. Over the last decade, transmissions of data from Cluster enabled a scientist at KTH Royal Institute of Technology to make a major contribution toward the understanding of Earth’s aurora.
By any measure, Cluster has been successful. But it’s an especially rewarding outcome for a mission that was set back by a rocket crash on its first launch attempt. The original four Cluster satellites never made it into orbit. The four replacement satellites, which are still in use today, were launched aboard Soyuz-Fregat rockets in 2000.
Space missions are typically guaranteed to work for a few years, during which time all the primary scientific objectives are supposed to be met. Per-Arne Lindqvist, a space and plasma researcher at KTH, says that unmanned craft often continue working longer. Yet the fact that Cluster consists of four vehicles and has been in operation a full 13 years beyond its original expiration date has been somewhat of a pleasant surprise.
Even stranger is that by surviving so long, the four spacecraft have actually been able to get a better view of Earth’s aurora, which made possible the recent breakthrough in the modeling of the inner workings of the so-called “black auroras”.
KTH contributed to Cluster’s detectors for measuring electric fields and waves (EFW); and the university hosts the Scandinavian data centre for analysis and distribution of EFW data. (More than 2,000 publications of Cluster results have been released, of which 250 come from the EFW team). Cluster’s mission was to observe Earth’s magnetosphere—the invisible bubble of highly-charged electrical and magnetic fields surrounding the planet. But, as a result of its long lifespan, it wound up doing a little bonus work:
“The main thing about Cluster and the aurora is that the original orbit was such that it was not best suited for studies on the magnetic field lines where the auroral acceleration takes place,” Lindqvist says. “Over the years, due to natural disturbances, the orbit has evolved in both shape, size and orientation, so coverage of the auroral field lines became much better after the original mission duration. And of course, data over several years enable statistical studies.”
Beginning about two years after the original Cluster mission expiration date, Tomas Karlsson, a space and plasma physics researcher at KTH Royal Institute of Technology, started to notice something strange in the data being sent down from the project’s four satellites.
“I noticed a weird combination of electrical and magnetic field measurements that were different from normal, and I wanted to understand the physics behind the data,” Karlsson says. “On each occasion, the Cluster spacecraft were flying over the night-time auroral region.
“These events are very rare, and if I had only seen it once, I might not have attached any importance to it,” he says. “But when I found a number if similar events, over the years of data, this made me think that they were relevant to look closer at.”
And then there is all of the follow-up study of the results that continued at KTH and other universities around the world after the first two years of the mission.
ESA has extended the project in two-year increments ever since 2002, and while there is a probability that the project could extend beyond 2016, that probability is decreasing all the time. “The main problems on the spacecraft side are power to operate everything, and fuel to keep the spacecraft orbit constellation and attitude,” Lindqvist says.
And the instruments are not going to run forever. Karlsson says they are degrading at different rates on each of the four spacecraft. “Some of our electric field probes have failed at different times on different spacecraft,” he says. “We can still do good measurements even if we lose two out of four probes on a satellite, but with poorer time resolution.
“If one spacecraft would fail completely, I guess there would be serious discussion about a continuation. But one must also consider the added value to other missions, such as the upcoming MMS mission (which is in orbit and being commissioned now), to have the Cluster spacecraft to correlate measurements with.”
Lindqvist adds that the only thing keeping the power going in the Cluster spacecraft now is the Sun. “All batteries have been dead for a few years, so the spacecraft operate only in sunlight. But, the solar panels have actually degraded much less than anticipated. The fuel is running out but there is still a small amount left.