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Supplied by the European Space Agency (ESA), the Huygens probe is attached to NASA's Cassini spacecraft that will study Saturn and its rings and moons. The Huygens probe is a craft designed to land on the surface of Titan, one of Saturn's more intriguing moons. The probe is the first of ESA's Horizon 2000 long-term scientific programme, which resembles NASA's ambitious Discovery Program. Cassini will orbit Saturn four times before releasing Huygens on its kamikaze mission of discovery.

Intrigue of Titan

Ever since Voyager I flew by Titan in 1980, scientists have been preparing for an eventual mission there. A joint NASA-ESA mission was actually proposed in 1982 by Daniel Gautier of the ESA. Titan is the only moon in the solar system to possess an atmosphere. Its atmosphere is mainly nitrogen, but also contains methane and other hydrocarbons. The presence of methane is a mystery to scientists since light from the sun tends to rapidly convert methane into ethane; there must be some mechanism on Titan to replenish the methane in the atmosphere. Competing theories suggest either methane oceans shielded from the sun by the thick atmosphere or underground reservoirs of methane. Because of its hydrocarbon-rich nitrogen atmosphere, many scientists believe Titan closely resembles a primitive Earth, before organic life on the surface began to transform the atmosphere. Data from within the atmosphere of Titan could provide many hints to the formation of the early Earth.

Entry, Descent, and Landing

After being released by the Cassini probe, the 318 kg Huygens probe will drift towards Titan and enter the atmosphere. Now at the mercy of Titan's 250 km/hr winds, the probe is expected to plummet in a highly askew manner. Soon after entry, the Entry Assembly (ENA) that protected the probe during entry will eject itself and reveal the more vulnerable Descent Module (DM).  The ENA includes the heat protection equipment and deceleration mechanism, while the DM has only the exposed scientific instruments, a simple spin control device, and a parachute. Most expect the probe to be destroyed on landing or soon after, not by the impact but by harsh methane surface.

Onboard Instrumentation and their Scientific Objectives

Soon after atmospheric entry, the Huygens probe will measure wind speeds as well as its own speed using Doppler Wind Experiment (DWE) which will also measure the swing of the probe on its parachute. The DWE's ultra-stable oscillator will help to stabilize the probe to maximize effective communications with the orbiter. A radar altimeter will then make measurements of the surface and the altitude of the probe along its descent. The Huygens Atmospheric Structure Instrument (HASI) will make detailed measurements of wind gusts in the atmosphere, as well as being able to compute the density of the atmosphere given the known properties of the probe itself. HASI will also measure the electromagnetic properties of Titan's atmosphere. The Descent Imager/Spectral Radiometer (DISR) will analyze radiation in the atmosphere. It will also focus on aerosols scattered by sunlight and calculate the size and amount of the particles. To measure the actual chemical composition of Titan's atmosphere, the Huygens probe will use a Gas Chromatograph Mass Spectrometer (GCMS). Working in tandem with the GCMS, the Aerosol Collector and Pyrolyser (ACP) will collect an atmospheric sample during descent, separate the aerosols, and flush the particles to the GCMS to be analyzed. In the event that the probe successfully lands in tact on the surface and is still able to operate, a special Surface-Science Package (SSP) will continue to collect data about the surface of the moon until it runs out of power or is destroyed. If it does reach the surface undamaged, the other onboard instrumentation will continue to collect data as well.

Probe Support Equipment (PSE)

The Huygens probe will communicate with 30 kg of ESA hardware left on the the Cassini orbiter. This Probe Support Equipment will receive all data collected by the Huygens probe during its mission of descent into Titan. The PSE consists of a device to receive information from the Huygens probe while it is away, as well as an umbilical cord that provides power to the probe during its long trip to the Saturnian system. A Spin Eject Device (SED) will launch the probe from Cassini, and will remain functionless onboard the host craft.

Christiaan Huygens

While the Cassini mission is named for the French/Italian astronomer Jacques-Dominique Cassini, discoverer of many of Saturn's moons, the Huygens probe is named for Dutch astronomer and inventor Christiaan Huygens, who personally discovered Titan in 1655. In addition to his achievements in astronomy, Huygens invented a technique for polishing telescope lenses as well as the pendulum clock. His groundbreaking work with light led to a greater understanding of reflection and refraction; he was the first to document the phenomenon of double refraction.

Critical Mission Dates

  • October 15, 1997 - Launch
  • April 27, 1999 - First Venus flyby
  • June 24, 1999 - Second Venus flyby
  • August 18, 1999 - Earth flyby
  • December 30, 2000 - Jupiter flyby
  • July 1, 2004 - Cassini/Huygens reaches Saturn
  • December 25, 2004 - Probe deployment
  • January 14, 2005 - Probe reaches Titan


Other ESA Missions
·Double Star·
·Mars Express·

A New World

On January 14, 2005, mankind added another celestial body to the roster of those it had reached out and touched. The Huygens probe, launched from the Cassini mothership 21 days earlier, landed on the surface of Titan and invited it to the very exclusive club which at the time only included the Moon, Mars, and Venus (the comet Tempel 1 was inducted on July 4, 2005).

Titan had long fascinated astronomers with its several unique characteristics. Not only is it one of the largest moons in the solar system, but it is the only satellite with a real atmosphere (not counting the constant haze of volcanic output that surrounds Io). The atmosphere is primarily composed of methane, and murky enough to hide any details of the surface from Earthbound observation.

Since it was difficult to predict what Huygens would encounter when it landed on the planet, it was designed to cope with as many situations as possible. The probe had a tough shell to withstand a touchdown on rocky terrain, and would float if it landed in liquid. The probe was too small to contain a transmitter capable of sending useful amounts of data directly to Earth, so it would relay transmissions through its parent craft. This would also help preserve its 1800 watt-hour power source, which was expected to keep the probe going for 2.5 hours of descent and half an hour of surface activity. Huygens was equipped with a battery of instruments already noded above, ready to examine the inner atmosphere and surface of a truly brand-new world.


The separation, glide, and descent phases of the mission were uneventful- at least in terms of deviation from expectations. The probe was quiescent during most of its lone voyage, while Cassini performed its own remote examination of Titan and moved into position for its role as relay. Upon encountering the outer reaches of Titan's 600km-deep atmosphere, Huygens awoke and established its radio link to Cassini. During the descent, the probe sent back a stream of images of the approaching surface; the probe was rotating as it fell and measured wind behaviors as well as capturing images of rolling clouds, a barren and rocky landscape, and even what appeared to be river channels full of liquid methane. The probe eventually came to rest on gravelly terrain covered with pebbles and dust, on the edge of a large, dark region it had seen on the way down, and returned close-up images of the surface and measurements of the properties of the low-level atmosphere.


The knowledge gained from the Huygens mission was enhanced by two extremely fortunate events. Firstly, the probe's power source, predicted to last for half an hour after touchdown, was able to hold out for over twice that long- one hour and 12 minutes. Secondly, an error in communications between Cassini and Huygens resulted in the loss of data from the probe's Doppler Wind Experiment- but a team on Earth, following the probe through telescopes, were able to make equivalent measurements from the observed movement of the probe and salvage the DWE team's project.

After Cassini-Huygens, the future looks bright for exploration of the outer solar system. NASA is moving ahead on its Project Prometheus mission, a return to the gas giants and a testbed for future nuclear-powered spacecraft. They are also ready to collaborate with the ESA again on a mission to the surface of Europa (go ahead and say it; I'll wait), now that it's been proven that moons can be just as interesting, and withhold just as many secrets, as the planets themselves.

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