The Cassini-Huygens mission is one of the most ambitious and successful missions that we have achieved in the history of planetary exploration. A cooperative project between NASA (National Aeronautics and Space Administration), ESA (European Space Agency) and the Italian Space Agency (ASI), the Cassini-Huygens was envisioned to better understand Saturn and its system.
Built like a shellfish
While the orbiter Cassini – the main spacecraft that was named after Italian astronomer Giovanni Cassini – was provided by NASA, ESA supplied the lander, the Huygens probe, named after Dutch physicist Christiaan Huygens. Crucial to the overall exploration, Huygens was a unique spacecraft. At about 2.7 m wide and 318 kg in weight, Huygens was built like a shellfish – a hard shell to protect its delicate interior from high temperatures.
In this March 1997 file photo, a technician checks the heatshield of the space probe Huygens in a cleanroom in Germany. | Photo Credit: UWE LEIN
The Huygens probe had two parts: the Entry Assembly Module and the Descent Module. Equipment to control Huygens after its separation from Cassini and a heat shield were contained in the Entry Assembly Module. The Descent Module, meanwhile, contained scientific instruments and parachutes to help its descent to the surface of Titan, Saturn’s largest moon.
Four gravity assists
Launched on October 15, 1997, Cassini’s journey to Saturn was made possible by four gravity assists – two Venus flybys (1998 and 1999), and one each with Earth (1999) and Jupiter (2000). It entered Saturn’s system in May 2004, meaning that the gravitational pull of Saturn was now stronger than the tug from the sun.
It was on Christmas day in 2004 that the Huygens probe separated from the Cassini spacecraft. Having been in hibernation for over seven years, Huygens – designed to investigate Titan’s atmosphere, including chemical properties, temperature, wind, and pressure profiles – began its 22-day trip to Titan.
On January 14, 2005, Huygens first entered Titan’s atmosphere and within four minutes deployed its 8.5 m diameter main parachute. Huygens began transmitting important information back to Cassini just a minute later. This continued for another two hours, until its impact on the surface of Titan – about 7 km from its target point at a velocity of 4.54 m/s. A communications programme problem, however, meant that the number of images Huygens transmitted to Cassini was nearly halved – 376 as opposed to the expected 700.
Survives the landing
Even though Huygens wasn’t designed to survive the landing, the scientists hadn’t completely ruled it out. The scientists on Earth were therefore ecstatic when Huygens, now the first spacecraft to land in the outer solar system, continued transmissions for another three hours and 10 minutes, sending 224 images of the same view of its surroundings.
Picture taken January 14, 2005 shows one of the first images returned by the Huygens probe. This is the coloured view, following processing to add reflection spectra data, and gives a better indication of the actual colour of the surface. | Photo Credit: HO
The most distant landing from Earth likely placed Huygens on a surface that resembled sand made of ice grains. The pictures that were beamed back not only showed a flat plain with pebbles, but also evidence of liquid acting on the surface in the recent past. The existence of liquid hydrocarbon lakes in the polar regions of Titan was confirmed in the following years with additional data.
Expectations outdone
A couple of mission extensions meant that the Cassini orbiter functioned for another 12 years, aggregating unprecedented amounts of data about Saturn and its entire system. The mission finally came to an end on September 15, 2017, when Cassini, on its 293rd orbit of Saturn, intentionally plunged into Saturn’s atmosphere – a move devised as a means to protect potential places in Saturn’s system where microbial life could have emerged. Most estimates suggest that Cassini was completely destroyed after being burnt up in Saturn’s atmosphere less than a minute after its final transmission.
