The Huygens venture literally hangs on the three different parachutes that must control the Probe's speed and attitude at successive phases of the descent on to Titan. A pilot parachute about 2.5 m in diameter, adapted to supersonic speeds, will check the headlong rush into the atmosphere and release the main parachute, about eight meters in diameter. This will operate for about 15 minutes in the thin upper layers of Titan's stratosphere. It will then be replaced by the stabilizer parachute, which is about three meters in diameter. This size of parachute for the main phase of the descent is selected to ensure that the Probe will reach the surface before the batteries go flat.

In the drop test in Sweden the Huygens model reached Mach 0.8 before the pilot chute was deployed. This could not match the full Mach 1.5 velocity at which it will operate on Titan. Wind-tunnel tests have validated the performance of the pilot chute at supersonic speed.

Much engineering effort has gone into developing the parachutes and the system that deploys and jettisons them. This starts with an explosive piston that ejects the pilot chute. The same system is responsible for extracting the descent module from its protective coverings. The Earth's air substituted for Titan's atmosphere in a high-altitude drop test that verified the complex sequence.

In May 1995 a 100 m helium balloon lifted a full-scale model of Huygens 37.5 km above the Esrange upper-atmosphere facility at Kiruna, Sweden. The model Probe was released from the gondola hanging underneath the balloon. After gathering speed, it went into the whole automatic procedure to release the parachutes and shed the back cover and front shield. Instruments and cameras in the model Probe recorded the performances of the separation mechanisms and of the parachutes.

The stabilizer chute on which Huygens will make its landing on the surface of Titan was known to be insufficient to slow the test Probe for a soft impact on the Earth. A fourth and larger parachute was therefore added for the test, to achieve the intended impact speed of 28 km h^-1.

"The drop test was very exciting," says Hamid Hassan of ESA. "It was almost like a project within a project, but the effort was well rewarded. The complete success of the test gives us confidence that what Huygens did at Kiruna it can do again more than a billion kilometres away, on Titan."

Huygens descent module after landing at Kiruna.