Spin Up To Crash Course

Simulated view of Cassini releasing Huygens probe against the backdrop of Titan, the Earth-like moon. Click image for larger view
Credit: NASA/JPL

On Christmas Day 2004, the Cassini spacecraft flawlessly released ESA’s Huygens probe, passing another challenging milestone for Cassini-Huygens mission. But, with no telemetry data from Huygens, how do we know the separation went well?

At 3:00 CET on 25 December, the critical sequence loaded into the software on board Cassini was executed and, within a few seconds, Huygens was sent on its 20-day trip towards Titan. As data from Cassini confirm, the pyrotechnic devices were fired to release a set of three loaded springs, which gently pushed Huygens away from the mother spacecraft. The probe was expected to be released at a relative velocity of about 0.35 meters per second with a spin rate of about 7.5 revolutions per minute.

Telemetry data from Cassini confirming the separation were collected by NASA’s Deep Space Network stations in Madrid, Spain, and Goldstone, California, when the telemetry playback signal from Cassini eventually reached the Earth.

However, these data showed only that the Cassini systems had worked, and that the Cassini ‘attitude perturbation’ (how Cassini moved in reaction to the probe’s release) were as expected. Within hours, the preliminary analysis of this data confirmed that Huygens was on the expected trajectory and spinning within the expected range. The spin imparted to Huygens is vitally important to ensure that the probe remains in a stable attitude and on course when it enters Titan’s atmosphere.

The Huygens’ probe will enter Titan’s thick atmosphere and may record alien thunder on its microphone.
Credit: ESA

How do we know that it is on the right course and how accurately can we tell?

Since Huygens has no propulsion system of its own, it had to be put on course for its descent before it was released. As planned, a fine tuning of the Cassini trajectory took place on 22 December to place Huygens on its nominal entry trajectory.

While Huygens will remain on this trajectory until it plunges into Titan’s atmosphere on 14 January, the Cassini orbiter performed a deflection maneuver on 28 December to avoid crashing onto the moon.

Huygens is scheduled to reach Titan’s upper atmosphere at about 10:06 CET (4:06 AM EST) on 14 January, entering the atmosphere at a relatively steep angle of 65° and a velocity of about six kilometers per second (>10,000 miles per hour).

The fine-tuning maneuver, called ‘Targeting Clean-up’, was critical: if the entry angle is too steep, the probe could overheat and burn up in the atmosphere; if the angle is too shallow, the probe might skim like a pebble on the surface of a lake and miss its target.

After the probe’s separation from Cassini, telemetry data were collected by NASA’s Deep Space Network stations in Madrid, Spain, and Goldstone, California. From these data confirming the release, we know the speed after release, and that the probe is spinning as planned to keep stable. Images from Cassini’s cameras showing the probe drifting away were taken on 25-27 October.

Simulation of Cassini pointing away from Titan. Click image for larger view
Credit: NASA/JPL

Although only a few pixels across, these images taken at different distances between the probe and the orbiter helped navigators to reconstruct the probe’s trajectory. Using the backdrop of known stars, and pinpointing Huygens’s position relative to Cassini, the probe’s trajectory was reconstructed using radio and optical navigation techniques.

This information is important to help establish the required geometry between the probe and the orbiter for radio communications during the probe descent on 14 January. It also shows that the probe and Cassini are well within the predicted trajectory accuracy.

So how could we check the spin rate was correct?

When the Huygens probe was being designed more than 10 years ago, it was required that the probe had to be magnetically ‘clean’ when switched off, meaning that any residual permanent magnetic fields must not interfere with the sensitive Cassini magnetometers. Later, when the probe was built, it was found that there was still a weak magnetic field produced, but within acceptable limits for Cassini’s magnetometer sensors.

However, because magnetic fields have a ‘direction’ as well as a strength, and this weak field was slightly off-center, it effectively gave the probe a ‘left’ and a ‘right’ side (it behaves like a small magnet with a north and south pole). With the implication being that if you can detect this magnetic field, then you can also detect how it is rotating.

Moon eclipsing Saturn as seen by Cassini. Click image for larger view
Credit: NASA/JPL

Following an initial suggestion by Jean-Pierre Lebreton, the Huygens Project Scientist, scientists on the Cassini Dual Technique Magnetometer (MAG) team, from Imperial College, London, and Braunschweig, confirmed that their instrument should be able to detect this small rotating magnetic field and plans were put in place to measure this during the probe release period.

Magnetometers are direct-sensing instruments that detect and measure both the strength and direction of magnetic fields in the vicinity of the instrument. The Cassini MAG is measuring these fields while Cassini is in orbit around Saturn as well as during the close Titan encounters. But, just after separation on 25 December, the MAG scientists detected fluctuations in the magnetic field around Cassini that could only have come from Huygens rotating and moving away.

Professor Michele Dougherty, Principal Investigator for MAG, said, "What was observed by MAG just after the probe separation on 25 December 2004, were weak but clear fluctuations in both magnetic sensors which reside on the 11-meter magnetometer boom. These fluctuations were a clear indication of the Huygens probe moving away from the Cassini orbiter. This signature confirmed the spin rate of the probe at 7.5 revolutions per minute, the ideal rate which was predicted, and that Huygens is well on its way to Titan."

Former MAG Principal Investigator David Southwood, who is now the Director of Science at ESA, said, "Detecting the spin was immensely reassuring – not only did it show Huygens was rotating correctly, but also because the spin is directly related to the departure velocity, that Huygens was headed off at the right speed. It was really great to do it with an instrument I knew so well."

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
5.44Timer 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.
10.06Huygens 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.
10.10Pilot 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.

10.11Huygens 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.

10.25Main 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.
10.42Surface 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.
11.50Gas 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.
12.23Descent 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.
12.27Surface 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.
14.37Cassini 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.
15.07First 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?