Huygens descends.

On Christmas eve, the Huygens probe was released from the Cassini spacecraft, and is now hurtling toward its January 14th hot date with Titan. Cassini snapped a picture of it about 12 hours after the release, but it just looks like a pixelated white splotch. The probe is in sleep mode, set to awaken about 45 minutes before it enters the giant moon's upper atmosphere. Three sets of parachutes will then open to slow the descent and provide a stable platform for the scientific instrumentation to start measuring. Surface impact is expected about 2.5 hours later at an estimated speed of 15 miles per hour. The doodads onboard are:

1. Huygens Atmospheric Structure Instrument - measures the density, temperature, pressure, and electrical properties of the atmosphere, and of the ocean should it land in liquid.

2. Doppler Wind Experiment - measures wind gusts and their effects on the probe during descent and will help stabilize communications.

3. Descent Imager/Spectral Radiometer - measures radiation flow and light intensity, and includes top, bottom, and side cameras plus a lamp.

4. Gas Chromatograph Mass Spectrometer - identifies and measures the chemicals in Titan's atmosphere during descent and, if it lands safely, the surface material.

5. Aerosol Collector and Pyrolyser - collects and heats aerosol particles to vaporize volatiles and decompose the complex organic materials, then analyzes them.

And then the big finale...

6. Surface-Science Package - "An acoustic sounder, activated during the last 100 meters of the descent, will continuously determine the distance to the surface, measuring the rate of descent and the surface roughness (e.g., due to waves). If the surface is liquid, the sounder will measure the speed of sound in the "ocean" and possibly also the subsurface structure (depth). During descent, measurements of the speed of sound will give information on atmospheric composition and temperature, and an accelerometer will accurately record the deceleration profile at impact, indicating the hardness and structure of the surface. A tilt sensor will measure any pendulum motion during the descent and will indicate the probe attitude after landing and show any motion due to waves. If the surface is, indeed, liquid, other sensors will measure its density, temperature and light reflecting properties, thermal conductivity, heat capacity, and electrical permittivity."

If you're into this sort of thing, you can read all about the probe's operational subsystems, too. The biggest disappointment? The five batteries will only live about three hours (at most) before the craft goes dark for good. Keep your fingers crossed for pictures from the surface.