Landing a spacecraft in outer solar system

On January 14, 2005, the Huygens probe parachuted to the surface of Titan, Saturn’s largest moon. With that, Huygens not only became the first spacecraft ever to land in the outer solar system, but had also carried out the most distant landing from Earth. A.S.Ganesh takes a look at the success of the Huygens probe…

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.

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.

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.

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.

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.