The Huygens probe, riding aboard the Saturn-bound Cassini spacecraft, stepped flawlessly through a test run last week of the activities it will perform when it descends through the soupy atmosphere of Titan less than 28 months from now.

"All the probe subsystems and probe instruments did just what they are supposed to do," said European Space Agency systems engineer Shaun Standley, stationed at NASA's Jet Propulsion Laboratory, Pasadena, Calif. For the multinational Cassini-Huygens mission, NASA provided the large Cassini spacecraft, which will begin orbiting Saturn July 1, 2004, and the European Space Agency provided the Huygens probe, which will parachute into the atmosphere of Titan, Saturn's largest moon, on Jan. 14, 2005.

Last week's Huygens checkout was the 10th since launch on Oct. 15, 1997. The probe is sleeping for most of the seven-year journey. About every six months, though, engineers wake it up to check its health and exercise the moving parts in its valves and pumps.

"As nearly as possible, we put the probe through all the stages of the real descent sequence," Standley said. The sequence lasts about five hours. Since Huygens remains inside a protective shell, the simulation can't include every instrument activity nor, of course, one-time events such as parachute deployment. The checkout does turn on each instrument for the periods they will be used as the probe descends, take data from each, and send the data to Cassini for transmission to Earth. That allows evaluation of the subsystems, such as power, computers and transmitter, as well as each instrument.

Results of the checkout have been evaluated by engineers and scientists at the Huygens Probe Operations Center in Darmstadt, Germany, and at the home institutions for each of the probe's instruments in France, United Kingdom, Germany, and the United States.

The Huygens atmospheric structure instrument will analyze features such as temperature, pressure and lightning at different layers of Titan's atmosphere. Instruments named the gas chromatograph mass spectrometer and the aerosol collector and pyrolyser will work in tandem to collect, break down and identify particles and gases, including organic chemicals in the atmosphere. The descent imager/spectral radiometer will take pictures and spectra of the atmosphere and surface. The Doppler wind experiment will track how winds carry the probe. And the surface science package will investigate physical properties of Titan's surface.

The mission is a cooperative effort of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.