The real mission of the DISR instrument occurs when the Huygens probe detaches from Cassini and performs a parachute-assisted descent through the atmosphere of Titan, Saturn's largest moon.
Titan is interesting to scientists because it contains a thick atmosphere with substantial amounts of methane. Only two other bodies in our solar system beside the earth and gas giants have atmospheres (that is, clouds, possible precipitation...some sort of "barrier" between the surface and space): Venus, and Titan. The surface of Titan has never been seen clearly by telescopes or other orbiters.
Some scientists believe that the surface of Titan is covered with large methane or ethane oceans, which may help to explain the "replenishing" cycle of the clouds, similar to the water cycle on earth. Others believe that there is no liquid water, that instead Titan is a barren, frigid, desert-looking wasteland. No one knows for sure.
DISR is a very sophisticated optical instrument, containing 14 separate ways which light can enter the instrument and be measured. It will take pictures of the surface as the probe descends and measure the types and sizes of aerosols in the atmosphere. To have the opportunity to make these measurements, however, a few key things will need to happen:
30 minutes prior to entering Titan's atmosphere, Huygens' instruments will be turned on to prepare for descent. At this point, the probe will be traveling at 13,424 miles per hour (supersonic speeds). To keep Huygens from burning up like a meteorite, a 27-foot parachute is deployed to slow the spacecraft down after entering the atmosphere. Meanwhile the probe's heat shield helps to keep the delicate instruments from being destroyed. Another parachute is deployed when the heat shield is released, as the probe slows further. When the heat shield is a sufficient distance away from the probe to avoid instrument contamination, the instruments begin to turn on and collect data. Two minutes after the heat shield is discarded, DISR turns on.
A smaller 10-foot parachute is deployed for the remainder of the descent, although when the probe hits the surface, even though it has slowed considerably,, it will still be "the equivalent of riding you bicycle into a brick wall," says Marty Tomasko.
DISR will collect approximately 700 images as the probe spirals toward Titan's surface, producing mosaics of the ground and horizon in various resolutions. (For more information on the DISR instruments, please see the 'Instruments' section.) The descent is projected to last from two to two and a half hours, although a half hour of battery power is reserved if Huygens survives the descent and lands on a solid piece of Titan's surface for ground data collection.