Descent Preparation and Communication

The Huygens mission officially started at around 09:06 UTC when the Huygens probe reached the predetermined interface altitude of 1270 km above the surface of Titan. Prior to this mark two crucial events took place that marked a transition from the cruise phase to operations phase:

• 04:44 UTC
  Mission Timer Unit (MTU) activated the Huygens probe.
• 07:02 UTC
  Cassini turned to point the HGA at the landing site in order to communicate with the probe. Communications would last for a maximum of 4h 36 minutes after interface at which point Huygens would pass over the horizon relative to Cassini.

Trajectory of the Huygens Probe and the Cassini Orbiter from Entry - 7 hours until Entry + 4 hours, in the Titan reference frame. (Image courtesy NASA)

Descent To Surface

The mission is thought to have followed the pre-mission schedule (listed in detail below) reasonably accurately. The only real variation is the surface descent phase took 2h27m50s as opposed to the pre-mission estimate of 2h21m.

It will be some time before scientists full assess the descent profile taken by Huygens - although there is lots of information to work with. Alongside the spacecraft instrument readings the descent of Huygens was tracked by radio telescopes around the world, which monitored the carrier signal sent by a transmitter activated when the first parachute deployed. The confirmation of the detection of this signal by the Green Bank 110m dish at around 10:30 UTC was the first indication that the mission had gone to plan.

From interface altitude at 1270 km the probe desecened through haze until an altitude of about 30 km above the surface. This was around half the altitude suggested by pre-mission estimates of between 70 to 50 km above the surface.

At an altitude of 700m above the surface the descent lamp was activated. The purpose of this lamp was to not to illuminate the landing site, the light levels on the surface of Titan are roughly 1000 times less than sunlight and 1000 times stronger than a full moon, but to provide a monochromatic light source and enable scientists to accurately determine the reflectivity of the surface.

Surface Phase

The surface phase of the mission lasted 1h 10 minutes - considerably longer than had been anticipated. There were three main reasons for this:

• The initial mission scenario allowed Huygens and Cassini to communicate for around 2h15m, the revised scenario allowed for 4h36m
• There was no failure on the batteries so all five could be used to power the probe. System redundancy meant Huygens only required four batteries to complete a nominal mission
• The landing on the surface was soft meaning no damage was done to the probe.

Early indications are that the surface where Titan landed consists of a thin frozen crust that is around 10 cm thick with a less dense layer benath.

Timeline of Events

Pre-mission predicted times.