麻豆传媒

The 7.5-year-old Cassini spacecraft in orbit around Saturn is beginning to show wear, with three of its 12 main instruments experiencing difficulties. But mission officials believe the glitches will cause minimal losses to science.

鈥淭he three that are acting up a bit are disappointments, but not a major setback by any means,鈥� says Robert Mitchell, Cassini鈥檚 programme manager at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US.

The problems in two instruments can be traced to the motors that point their detectors. In late 2004, mission managers lost the ability to control a motor that steers one of three detectors on Cassini鈥檚 plasma spectrometer, which studies charged particles.

鈥淚t moves but we can鈥檛 control it,鈥� Mitchell told 麻豆传媒. He says engineers may try to lock it into a fixed position or program new software to regain control of the motor. 鈥淲e haven鈥檛 given up on getting it back up and going again,鈥� he says.

Another motor controlling one of three detectors on the Magnetospheric Imaging Instrument (MIMI), which measures magnetic fields, may be impossible to fix. A mechanical failure may have caused the motor to stop turning in response to commands in late January 2005, and mission managers turned the motor off in mid-February.

鈥淚t鈥檚 like you have a telescope that works just fine but if you want to point it you can鈥檛,鈥� says Mitchell. The instrument鈥檚 other two detectors 鈥� like those on the plasma spectrometer 鈥� are fully functional, though. So the scientific losses from this problem are estimated to be less than 10% of the total data it could have taken, says Mitchell.

Bad vibrations

But MIMI鈥檚 motor glitch did add to problems with the third instrument, called the Composite Infrared Spectrometer (CIRS). CIRS acts as a thermometer, studying the heat from objects such as Saturn鈥檚 rings, and it can also map the molecules in the atmospheres of Saturn and one of its moons, Titan.

It does this by taking the spectrum of infrared light from its targets 鈥� a sensitive process that involves taking the light in along two different paths and studying the patterns it produces when it meets up again. A mirror sets the lengths of the paths by moving at a constant rate through a distance of about 1 centimetre.

But vibrations on the spacecraft have interfered with the mirror鈥檚 motion, in some cases causing it to reverse direction. CIRS is a 鈥渧ery delicate instrument鈥�, says Michael Flasar, the instrument鈥檚 principal investigator at NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US. 鈥淎nytime there鈥檚 a vibration on the craft, we seem to be very sensitive to it.鈥�

MIMI鈥檚 motor malfunction in January caused vibrations that were 鈥減articularly onerous鈥� for CIRS, says Flasar. The problem was magnified because 鈥� just weeks beforehand, on 24 December 2004 鈥� the 349-kilogram Huygens space probe had been jettisoned from Cassini to land on Titan. 鈥淗uygens鈥檚 mass was apparently acting as a damper on the vibrations,鈥� says Flasar.

The vibrations have lessened since MIMI鈥檚 motor was shut down, 鈥渂ut we鈥檙e not out of the woods yet鈥�, says Flasar.

His team will have to process the spectra carefully to remove the 鈥渘oise鈥� from the vibrations. But Flasar adds: 鈥淓ven during the worst of the problems, we鈥檝e been able to rescue some good data.鈥�