On 14 January 2005 the Huygens probe successfully landed on the surface of Titan, Saturn's largest moon. It made history as the first probe to land on an object in the outer Solar System.

Titan is one of the most interesting bodies in the Solar System. It is the Solar System's second largest moon, and the only moon known to have an appreciably dense atmosphere. Many have speculated the presence of liquid on Titan, as many of its surface features suggest.

The fully-automated Huygens probe was developed by the European Space Agency (ESA), with the objective of analysing the clouds, atmosphere, and surface of Titan. It is named after Christiaan huygens, the Dutch astronomer who discovered Titan in 1655.

The probe detached from the Cassini spacecraft in December 2004 and began a 20-day descent towards Titan. As it entered Titan's atmosphere, the probe was slowed down by the front heat shield from an entry speed of 6 kms-1 to 400 ms-1 (Mach 1.5) by about 160 km altitude. Huygens then deployed a series of parachutes to reduce its speed further, and ejected the heatshield. Then throughout the two and a half hour decent, the instruments on board the probe measured the temperature, pressure, density and energy balance in the atmosphere. The Cassini orbiter remained in orbit and the probe support equipment (PSE) on the orbiter tracked Huygens and recovered data sent back from the probe.

Huygens' surface landing was successful; the probe remained operational and in contact with the Cassini orbiter for 1h 10 minutes after it had landed. This was much longer than anticipated, since the landing was soft and all 5 batteries survived. Measurements taken by a penetrometer as the probe landed, along with a view of the surface (above-right), indicate the landing site is solid and covered with ice pebbles.

Image taken as Huygens descended towards Titan The image on the left was taken as Huygens descended towards Titan.  It shows hills lined with surface channels meeting a low-lying, flat, dark region. It was initially suggested this dark region could be a lake of fluid, but it has since been established that the dark region in this photograph is where the probe landed, which is known to be solid. However the channels observed in the hills, as well the apparent weathering of rocks observed near the probe, increase the likelihood that liquid at least did once exist on Titan.

Analysing the Huygens probe release with MAG
When Huygens was released from Cassini to begin its descent towards Titan, engineers wanted a way to "see" what was happening, as quickly as possible, to make sure that everything went smoothly. Since there was no camera pointing at Huygens, it was impossible to see visually what went on initially during the release, until much later when a distant photograph of the probe was captured as it sped off towards Titan.

However, MAG was able to successfully detect the probe release by analysing the change in magnetic signature of the Cassini spacecraft, since Huygens was adding its own contribution to the intrinsic field while it was attached. Not only were we able to see the probe release clearly, but we also accurately determined the spin rate of Huygens, since it was detached with a "twist" to set it spinning for stability. Had things not gone according to plan, Huygens could have been lost or Cassini damaged or crippled, with disastrous consequences for the mission. By measuring the spin rate, which was 7.5 revolutions per minute as predicted, we confirmed without any doubt that the probe release had indeed gone well.

The plot below shows the predicted change in spacecraft magnetic field overlaid with the actual data recorded by MAG. The results were strikingly accurate. You can also download an animation that illustrates the motion of Huygens as it was released from the main spacecraft (NOTE: file size 1.41MB).

The plot below (click to enlarge) shows the predicted change in spacecraft magnetic field overlaid with the actual data recorded by MAG. The results were strikingly accurate. You can also download an animation that illustrates the motion of Huygens as it was released from the main spacecraft (NOTE: file size 1.41MB)