Huygens, Phone Home

Darmstadt, Germany
January 14, 2005

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Huygens parachutes onto Titan. ESA’s Huygens probe descends through Titan’s mysterious atmosphere to unveil the hidden surface (artist’s impression) Credit: ESA

Huygens appears to be on track for its historic descent through Titan’s atmosphere later today. Engineers who manage the giant radio telescope at Green Bank, West Virginia, reported at about 11:30 am GMT (5:30 am EST) that they had received a faint signal from the probe. The Green Bank radio telescope is one of a network of 18 radio telescopes that ring the Earth. The network was set up to ensure that any signals sent by the probe, or by Cassini after Huygens has transmitted its data to the orbiter, are detected by at least one of the tracking stations.

The signal received at Green Bank was quite faint. The transmitter onboard Huygens that sent it has about the same strength as the antenna on a mobile phone. Detecting it was an impressive feat, considering that was sent from more about 1.2 billion kilometers (about 750 million miles) away.

What Green Bank actually detected was the signal sent by Huygens to Cassini. Huygens’ transmitter is too weak to send information back to Earth directly, so it relays its scientific data through Cassini. Although the Huygens signal contained actual scientific data it was too faint to resolve the data stream. Rather, the giant radio dish detected a carrier signal, much like the dial tone you hear when you pick up a telephone. Its receipt was a critical indicator that the Huygens missions is proceeding as planned, that the probe had completed its high-speed entry into Titan’s atmosphere and that at least one of its three parachutes had deployed properly.

Data from Huygens will be relayed to Earth by Cassini later today. Cassini’s orbit around Saturn has been adjusted so that it is positioned to receive Huygens’ signals during the probe’s descent through Titan’s atmosphere and for a short time after it reaches the surface. The first of Huygens’ images is expected to arrive on Earth shortly after 5:00 pm GMT (11:00 am EST).

Timeline of expected events during the Huygens descent to the surface of Titan on 14 January 2005. (CET is Central European Time or six hours ahead of EST. Therefore the 10:10 CET pilot parachute deploy occurs at 4:10 AM EST).


Time (CET) Event
5.44 Timer triggers power-up of onboard electronics
Triggered by a pre-set timer, Huygens’s onboard electronics power up and the transmitter is set into low-power mode, awaiting the start of transmission.
10.06 Huygens reaches ‘interface altitude’
The ‘interface altitude’ is defined as 1270 kilometres above the surface of the moon where entry into Titan’s atmosphere takes place.
10.10 Pilot parachute deploys
The parachute deploys when Huygens detects that it has slowed to 400 metres per second, at about 180 kilometres above Titan’s surface. The pilot parachute is the probe’s smallest, only 2.6 metres in diameter. Its sole purpose is to pull off the probe’s rear cover, which protected Huygens from the frictional heat of entry.

2.5 seconds after the pilot parachute is deployed, the rear cover is released and the pilot parachute is pulled away. The main parachute, which is 8.3 metres in diameter, unfurls.

10.11 Huygens begins transmitting to Cassini and front shield released
At about 160 kilometres above the surface, the front shield is released.

42 seconds after the pilot parachute is deployed, inlet ports are opened up for the Gas Chromatograph Mass Spectrometer and Aerosol Collector Pyrolyser instruments, and booms are extended to expose the Huygens Atmospheric Structure Instruments.

The Descent Imager/Spectral Radiometer will capture its first panorama, and it will continue capturing images and spectral data throughout the descent. The Surface Science Package will also be switched on, measuring atmospheric properties.

10.25 Main parachute separates and drogue parachute deploys
The drogue parachute is 3 metres in diameter. At this level in the atmosphere, about 125 kilometres in altitude, the large main parachute would slow Huygens down so much that the batteries would not last for the entire descent to the surface. The drogue parachute will allow it to descend at the right pace to gather the maximum amount of data.
10.42 Surface proximity sensor activated
Until this point, all of Huygens’s actions have been based on clock timers. At a height of 60 kilometres, it will be able to detect its own altitude using a pair of radar altimeters, which will be able to measure the exact distance to the surface. The probe will constantly monitor its spin rate and altitude and feed this information to the science instruments. All times after this are approximate.
11.50 Gas Chromatograph Mass Spectrometer begins sampling atmosphere
This is the last of Huygens’s instruments to be activated fully. The descent is expected to take 137 minutes in total, plus or minus 15 minutes. Throughout its descent, the spacecraft will continue to spin at a rate of between 1 and 20 rotations per minute, allowing the camera and other instruments to see the entire panorama around the descending spacecraft.
12.23 Descent Imager/Spectral Radiometer lamp turned on
Close to the surface, Huygens’s camera instrument will turn on a light. The light is particularly important for the ‘Spectral Radiometer’ part of the instrument to determine the composition of Titan’s surface accurately.
12.27 Surface touchdown
This time may vary by plus or minus 15 minutes depending on how Titan’s atmosphere and winds affect Huygens’s parachuting descent. Huygens will hit the surface at a speed of 5-6 metres per second. Huygens could land on a hard surface of rock or ice or possibly land on an ethane sea. In either case, Huygens’s Surface Science Package is designed to capture every piece of information about the surface that can be determined in the three remaining minutes that Huygens is designed to survive after landing.
14.37 Cassini stops collecting data
Huygens’s landing site drops below Titan’s horizon as seen by Cassini and the orbiter stops collecting data. Cassini will listen for Huygens’s signal as long as there is the slightest possibility that it can be detected. Once Huygens’s landing site disappears below the horizon, there’s no more chance of signal, and Huygens’s work is finished.
15.07 First data sent to Earth
Cassini first turns its high-gain antenna to point towards Earth and then sends the first packet of data.

The time for the signal to travel from Titan to Earth then takes 67 minutes.

Getting data from Cassini to Earth is now routine, but for the Huygens mission, additional safeguards are put in place to make sure that none of Huygens’s data are lost. Giant radio antennas around the world will listen for Cassini as the orbiter relays repeated copies of Huygens data.

Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Prebiotic Laboratory
Planet Wannabe
Where is Cassini Now?