The 'Huygens' probe has sent back some spectacular pictures of the strange world of Saturn's moon, Titan, writes Dick Ahlstrom
Not splash, not bump but splat is the best description of the moment the Huygens satellite touched down on Saturn's moon, Titan. The probe came to rest in a puddle of mud after its seven year, 2bn-kilometre journey, according to scientists overseeing the mission.
The various scientific teams have been pouring over the data beamed back to Earth last Friday by Huygens' sister satellite, Cassini, which continues to orbit Saturn. It is still early days in efforts to interpret the data and another major scientific briefing is scheduled for tomorrow at the European Space Operations Centre in Darmstadt, Germany.
The teams responsible for each of Huygens's six on-board instruments are preparing a preliminary view of what the satellite has found. A more detailed understanding will come only after months and perhaps years, according to mission controllers in Germany.
One thing that has surprised scientists is how "easy" Huygens's landing was. Planners had assumed Titan was awash with liquid methane, implying the probe would splash down. Instead the data suggests Huygens plopped into muck of an undetermined variety. Once there it beamed back information for much longer than the three minutes hoped for by mission controllers.
"I think the biggest surprise is that we survived landing and that we lasted so long," states Charles See, a member of the University of Arizona team overseeing the DISR (Descent Imager and Spectral Radiometer) instrument on Huygens. "There wasn't even a glitch at impact. That landing was a lot friendlier than we anticipated."
Huygens sent back data and pictures from the surface collected by its Surface Science Package (SSP). Overseen by a team from the UK, the SSP had nine independent sensor systems meant to capture data relating to Titan's physical surface and its major constituents.
The SSP was also designed to measure wave height, density and electrical properties of any liquid in which it landed but as we now know, it encountered no ocean and saw no waves.
A team of 30 in the Descent Trajectory Working Group are attempting to recreate where the probe travelled as it parachuted down onto Titan's surface. Huygens slammed into Titan's thin upper atmosphere at about four times the speed of sound, according to the scientific team. Its heat shields began to slow it down until its main parachute pulled its speed back to about 180 kilometres per hour, a bit faster than our new motorway speed limit.
Once into the lower atmosphere the probe slowed its progress to about 5.4 metres per second and began drifting sideways at about 1.5 metres per second. Huygens rocked more than expected during its descent through the high altitude haze that envelops Titan. Data analysis of on-board instruments showed it tilted at between 10 and 20 degrees as it passed through the haze.
Below this, Huygens settled into a more gentle descent, tilting no more than three degrees. "The ride was bumpier than we thought it would be," says Martin Tomasko, principal investigator for the DISR instrument. The team thinks this was caused by a change in wind profile at about 25 kilometres above the surface.
The DISR team also has information about Huygens's orientation after landing. "The probe's parachute disappeared from sight on landing, so the probe probably isn't pointing east or we would have seen the parachute," according to team member Mike Bushroe.
They also believe the satellite may have sunk down into the surface a bit after landing. Its downward-looking high-resolution camera lens picked up some of the material into which it landed. "Either that or we steamed hydrocarbons off the surface and they collected onto the lens," suggests Charles See.
There is much exciting science to come as the Huygens dataset is dissected. Other instruments on board included the French led Aerosol Collector and Pyrolyser experiment that twice sampled the atmosphere at 40 kilometres and 20 kilometres above Titan as the probe descended.
The US-built Gas Chromatograph and Mass Spectrometer on board Huygens measured the chemical composition of the atmosphere from 170 kilometres down the the surface. It also analysed samples trapped by the French collector experiment.
A German team put together the Doppler Wind Experiment, designed to measure wind strength at accuracies better than one metre per second. The Huygens Atmospheric Structure Instrument measured the physical and electrical properties of the moon's atmosphere. It took temperature and pressure readings and also carried Huygens's "ear" on Titan, a microphone that captured sounds during the descent and while on the surface.
The DISR experiment included a 20-watt landing light, meant to illuminate the surface as Huygens made its final, fateful descent onto the surface. It was designed to switch on at 700 metres above the surface and remain illuminated for at least 15 minutes after touchdown.
"In fact not only did the landing lamp turn on at exactly 700 metres, but also it was still shining more than an hour later, when Cassini moved beyond Titan's horizon for its ongoing exploratory tour of the giant moon and the Saturnian system," adds Tomasko.
Huygens was still beaming up its signal to Cassini as the orbiter set slowly below Titan's horizon. Huygens sent back well over an hour's data from the surface and continued to do so as Cassini went out of reach.
Huygens now sits silent, a part of Titan's frozen, minus-180 degrees Celsius landscape having completed its mission to open up a strange new world to human investigation.