Our SpaceFlight Heritage: Huygens lands on Titan a decade ago

Joe Latrell

Descent Imager/Spectral Radiometer (DISR) image of Titan taken at 2km altitude during the descent. (Click for full-size image.) Image Credit: ESA/NASA/JPL/University of Arizona

Ten years ago the clouds of an alien world were pierced by a small space probe hurled from the planet Earth. On Jan. 14, 2005, NASA and the European Space Agency (ESA) successfully deployed the Huygens probe to the cloud-covered Saturnian moon Titan.

The small probe was a first. Never before had humanity landed anything on a moon of the outer Solar System. Titan, shrouded in mystery for decades, was about to reveal some of its secrets.

The 703 lbs (319 kg) Huygens probe was carried out to Saturn as a piggyback payload on the Cassini spacecraft, a joint operation of NASA, ESA, and the Italian Space Agency. The Cassini-Huygens package was launched from Earth on October 15, 1997. After a trip lasting seven years, the Huygens probe detached from Cassini on December 25, 2004. It landed safely on Titan on January 14, 2005.

The spacecraft used a heat shield to reduce velocity before deploying a parachute system in the upper atmosphere of Titan. The Huygens probe relied on a timer, set before separation with Cassini, to control its wake-up and preparation for landing. The probe had batteries that would last 153 minutes. The parachute system would float the probe to the surface of Titan over the course of two hours with a brief bit of time planned for operations on the ground. While the probe floated on its parachutes, Cassini listened and recorded the transmissions for 3 hours using its high gain antenna. When the time was up, Cassini then aimed the antenna at Earth to relay Huygens’ findings.

In addition to Cassini listening in, very large radio telescopes on Earth were doing the same thing. The strength of the signal from the Huygens probe was comparable to that of the Galileo probe that orbited Jupiter from 1995 to 2003. Using the detected carrier signal, scientists on Earth were able to detect wind speed and, by using the Doppler effect, pinpoint the landing site of the probe to within 0.6 miles (1 km) – all from a distance of over 745 million miles (1.2 billion km).

Scientists did not know if the Huygens probe would land on a solid surface. Many thought there was a high probability it would land in a sea of liquid methane. To counter this possibility, the Huygens probe was designed to float for about three minutes. Fortunately, the Huygens probe landed on solid ground.

The surface of titan was not as scientists had expected. The landing site was covered in small rocks of water ice. The ground was a sandy texture. The rocks appeared to have been worn smooth through some liquid erosion process, perhaps rain of methane or ethane. The atmosphere, captured in photos from the probe, appeared thin and hazy. The colors seemed saturated in oranges and yellows. The temperature recorded at the lander’s final location was -290.83 °F (-179.35 °C).

The Huygens probe has six instrument packages on board. The Huygens Atmospheric Structure Instrument (HASI), which was designed to collect information on the atmosphere during the decent phase. The Doppler Wind Experiment (DWE) was used to measure wind speed. The Descent Imager/Spectral Radiometer (DISR) was used to study the radiation balance of the atmosphere. The Gas Chromatograph Mass Spectrometer (GC/MS), and the Aerosol Collector and Pyrolyser (ACP) were on board to measure particulates and gasses in the Titan atmosphere. The final package, the Surface Science Package (SSP), was built to study the landing site of the Huygens probe, measuring the physical properties of the surrounding area.

Some science data was lost when the probe failed to collect some wind data due to a software design error. That same error caused the loss of 350 of the planned 700 photos taken while the probe was operational.

While the Huygens probe has fallen silent, the Cassini orbiter continues to return scientific data from Saturn.

Video Courtesy of European Space Agency