On March 25, 1655, Dutch astronomer Christiaan Huygens discovered Saturn’s moon Titan with a homemade telescope. Nearly 340 years later on January 14, 2005, a spacecraft named in his honour landed on its surface. The Huygens probe was the European Space Agency’s half of a joint mission with NASA, the Cassini-Huygens mission, and it remains the furthest landed payload in the solar system.
The seeds for the Cassini-Huygens mission were first sown by NASA in the late 1970s. At the time the agency was developing the Galileo mission to Jupiter (that finally launched in 1989) and determined that a similar mission exploring the Saturnian system was a logical next step. When the European Space Agency’s (ESA) Directorate of Scientific Programmes put out a call for mission proposals, a team of European and US scientists came forward in November of 1982 with the idea of a joint program to explore the ringed planet and its system of moons.
Eight years and a number of preliminary studies later, the plan was finalized by November of 1990. NASA would build an orbiter spacecraft called Cassini in honour of the French-Italian astronomer Jean-Dominique Cassini who discovered a number of Saturn’s moons and ring features (like the Cassini division) between 1671 and 1685. Cassini would carry a probe, the ESA-built Huygens, which would actually land on the surface of Titan. From the start time was tight. The best launch window for the mission was in October 1997; it would allow Cassini-Huygens to make a flyby of Jupiter and use the giant planet’s gravity to shave two years off the trip to Saturn. But it meant the two space agencies had just seven years to build their spacecraft.
Incredibly, both NASA and the ESA stayed on track and Cassini-Huygens launched during the primary window on October 15, 1997. For six and a half years the spacecraft flew through largely empty space en route to Saturn, waking up at least every six month for health check. Then, on May 18, 2004, the spacecraft reached the Saturnian system. They stayed together before the lander prepared to take centre stage. On December 25th, it separated from the orbiter and struck out on its own.
The Huygens Lander
The ESA designed 705-pound Huygens lander was a robotic laboratory, the first (and so far only) probe designed to take in situ measurements of Titan’s surface and atmosphere. Huygens had six main science goals: to determine the moon’s atmospheric composition; to investigate the energy sources that drive atmospheric chemistry; to study aerosol properties and cloud physics in Titan’s dense atmosphere; to measure winds and global temperatures; to determine the moon’s surface properties and infer a little about its internal structure; and to investigate the upper atmosphere and ionosphere.
To meet these science goals, Huygens was loaded with a suite of science instruments. It carried a Gas Chromatograph and Mass Spectrometer, instruments designed to identify and quantify the the various gases in the atmosphere. Gaseous samples were collected by an Aerosol Collector and Pyrolyser, which was cleverly designed to take a sample high in the atmosphere and store it until the spacecraft had time to really study it.
Huygens also carried a Descent Imager/Spectral Radiometer. This was a multi-sensor optical instrument designed to take pictures and make spectral measurements throughout Huygens’ descent to Titan’s in wavelengths ranging from the ultraviolet to the infrared. The central feature of this instrument was its two cameras that looked downwards and sideways. They were programed to capture images throughout Huygens’ descent, images that could reconstructed into a panorama of Titan.
The Huygens lander also carried an Atmosphere Structure Instrument. Another multi-sensor instrument, this was designed to measure the atmosphere's physical properties and electrical properties. There was also a Doppler Wind Experiment was on board. Once it hit the surface, Huygens would use its Surface Science Package, the simple suite of designed to take the in situ measurements on the surface.
Falling to Titan
A little more than two weeks after it separated from Cassini, Huygens began its descent through Titan’s atmosphere January 14, 2005. It started its fall through the moon’s hazy atmosphere about 790 miles from the surface where friction with the thick clouds was enough to slow the lander slowed from its 11,185 mile per hour entry speed to a relatively gentle 870 mph. At an altitude of 100 miles, the probe's scientific instruments were uncovered and exposed to Titan's atmosphere. This was followed almost immediately by a parachute; the first in a series deployed automatically from the top of the lander at about 95 miles, slowing the lander to 220 miles per hour. About 75 miles from the surface, the larger main parachute was jettisoned and replaced by a smaller stabilizing chute.
All this happened in just three minutes. Hanging under its terminal smaller parachute, it took Huygens an additional two and a quarter hours to finish its descent from 75 miles.
What Huygens Saw
The dense clouds that envelop Titan surprised scientists. Not only were they more conducive than expected – Huygens found evidence of possible lightning during its descent – the clouds extended almost all the way to the moon’s surface. Not until Huygens was 25 miles from landing did the probe start to see surface details emerging through the cloud. And these were surprisingly Earth-like details. Pictures returned from the lander showed evidence of erosion from possible flowing methane. Huygens passed over some lighter areas that turned out to be elevated terrain before drifting over a darker region that turned out to be a smooth area perfect for a landing.
Huygens landed in this dark spot, a site scattered with water-ice pebbles as big as a few inches in diameter. The area had a consistency similar to soft, wet sand with a fragile crust on top. When it landed on this yielding surface, Huygens bounced, slid, and wobbled for 10 seconds before finally coming to a stop.
The probe lasted about 90 minutes on Titan’s surface, taking its measurements and photographs of the moon that it sent back to Cassini that would in turn relay the information back to Earth. Then it fell silent.
Huygens is still there, though by now likely weathered and warped into a version of itself by Titan’s alien atmosphere.