Titan Surface Landing

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

Ground UTC	Time wrt Entry	Event
6 January 2005
11:53	-07d 21h	Spacecraft configured for probe relay. All instruments except MAG are turned off
7 January 2005
09:00	-07d 00h	Probe relay critical sequence begins. 8 day quiet period of minimal spacecraft activity before relay begins; Orbiter on thruster control
14 January 2005
06:26	- 02h 40m	Set Solid State Recorder pointers for probe recording
06:38	- 02h 28m	Transition to thruster control for relay
06:48	- 02h 18m	Perform final recorder configuration for relay
06:50	- 02h 16m	Turn on probe receivers
07:02	- 02h 04m	Turn Orbiter to point to Titan
07:14	- 01h 52m	Turn to Titan complete
07:17	- 01h 49m	Disable X band downlink
08:44	- 00h 22m	Probe turns transmitters on; Low power mode
09:06	+ 00h 00m	Probe reaches interface altitude. Entry altitude = 1270 km
09:08	+ 00h 02m	Probe feels maximum deceleration
09:09	+ 00h 03m	Pilot chute deployed at 170-190 km altitude Speed = Mach 1.5 Pilot chute is 2.6m in diameter
09:09	+ 00h 03m	Aft cover released, main parachute deployed Altitude 160-180 km Speed = Mach 1.5 2.5 seconds after pilot chute deployed main chute is 8.3m in diameter
09:10	+ 00h 04m	Probe begins transmission to Orbiter
09:10	+ 00h 04m	Release front shield Transmitters to high power Istruments configured for descent Measurements begin 152-175 km Speed < Mach 0.6
09:25	+ 00h 19m	Main parachute separation Deploy stabilizing drogue chute 110-140 km altitude Drogue is 3m in diameter
09:42	+ 00h 36m	Surface proximity sensor activated at 60 km altitude
09:49	+ 00h 43m	Possible icing effects to Probe at 50 km altitude
11:12	+ 02h 06m	Titan-C orbiter closest approach. Inbound 60 000 km flyby at 5.4 kms-1, 93 deg phase
11:23	+ 02h 17m	Descent imager lamp on
11:27	+ 02h 21m	Surface impact, end descent phase. May vary ± 15 min depending on descent time
13:37	+ 04h 31m	Orbiter stops collecting probe data. Maximum of 4 hours and 36 minutes of data collection
13:39	+ 04h 33m	Write protect probe data partitions. Partitions A5 and B5 of Solid State Recorder are protected from further data writing.
13:47	+ 04h 41m	Turn Orbiter to point to Earth
13:50	+ 04h 44m	Turn to Earth complete
13:59	+ 04h 53m	Critical sequence ends; S07 background sequence B begins
14:00	+ 04h 54m	Post-Probe tracking begins. Canberra 70m station receiving; 10 minutes for DSN lockup allocated
14:07	+ 05h 01m	First telemetry data sent to Earth
14:10	+ 05h 04m	Playback of probe data begins at Canberra at 66,360 bps
16:50	+ 07h 44m	End playback of first partition First copy of probe data received at Earth
16:57	+ 07h 51m	Ascending ring-plane crossing Distance = 18.4 Saturn radii
17:53	+ 08h 47m	Start tracking at Madrid 70m. Data rate upgraded to 142,200 bps
21:00	+ 11h 54m	End first full playback of all probe data. Complete set of all copies probe data received at Earth
15 January 2005
00:22	+ 15h 16m	First complete set of probe data reaches Huygens Operations Center. No later than time listed; likely up to ~3 hours earlier
02:28	+ 17h 22m	Start tracking at Goldstone 70m
11:00	+ 01d 02h	Power on of orbiter instruments, if Orbiter is healthy and playback proceeding per plan
12:23	+ 01d 03h	End nominal playback of Probe data

Pre-mission predicted times.

Last Update: 23 August 2012