Cassini probe shows signs of wear

12:45 08 April 2005, NewScientist.com news service by Maggie McKee

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.

“The three that are acting up a bit are disappointments, but not a major setback by any means,” says Robert Mitchell, Cassini’s programme manager at NASA’s 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’s plasma spectrometer, which studies charged particles.

“It moves but we can’t control it,” Mitchell told New Scientist. He says engineers may try to lock it into a fixed position or program new software to regain control of the motor. “We haven’t given up on getting it back up and going again,” 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.

“It’s like you have a telescope that works just fine but if you want to point it you can’t,” says Mitchell. The instrument’s other two detectors – like those on the plasma spectrometer – 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’s 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’s 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 – 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’s motion, in some cases causing it to reverse direction. CIRS is a “very delicate instrument”, says Michael Flasar, the instrument’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US. “Anytime there’s a vibration on the craft, we seem to be very sensitive to it.”

MIMI’s motor malfunction in January caused vibrations that were “particularly onerous” for CIRS, says Flasar. The problem was magnified because – just weeks beforehand, on 24 December 2004 – the 349-kilogram Huygens space probe had been jettisoned from Cassini to land on Saturn. “Huygens’s mass was apparently acting as a damper on the vibrations,” says Flasar.

The vibrations have lessened since MIMI’s motor was shut down, “but we’re not out of the woods yet”, says Flasar.

His team will have to process the spectra carefully to remove the “noise” from the vibrations. But Flasar adds: “Even during the worst of the problems, we’ve been able to rescue some good data.”