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·