INFO 06-1996: Encouragement from Jupiter for Europe's Titan Probe
5 April 1996Engineers in charge of the European Space Agency's Huygens mission have reviewed the success of the recent Jupiter Probe with their counterparts from NASA, at a meeting in Ottobrunn, Germany. Huygens is designed to parachute into the atmosphere of Titan, the largest moon of the planet Saturn. It shares important features with the Probe, released from NASA's Galileo spacecraft, which plunged into the atmosphere of Jupiter on 7 December 1995.
Huygens will transmit scientific information for 150 minutes, from the outer reaches of Titan's cold atmosphere and all the way down to its enigmatic surface. For comparison, the Jupiter Probe radioed scientific data for 58 minutes as it descended about 200 kilometres into the outer part of the atmosphere of the giant planet.
The parachutes controlling various stages of Huygens' descent will rely upon a system for deployment designed and developed in Europe that is nevertheless similar to that used by the Jupiter Probe. The elaborate sequence of operations in Huygens worked perfectly during a dramatic drop test from a stratospheric balloon over Sweden in May 1995, which approximated as closely as possible to events on Titan. The performance of the American Probe at Jupiter renews the European engineers' confidence in their own descent control system, and also in the lithium sulphur-dioxide batteries which were chosen to power both Probes.
"The systems work after long storage in space," comments Hamid Hassan, ESA's Project Manager for Huygens. "Huygens will spend seven years travelling to Saturn's vicinity aboard the Cassini Orbiter. The Jupiter Probe was a passenger in Galileo for six years before its release, so there is no reason to doubt that Huygens will work just as well."
Huygens will enter the outer atmosphere of Titan at 20 000 kilometres per hour. A heat shield 2.7 metres in diameter will withstand the friction and slow the Probe to a speed at which parachutes can be deployed. The size of the parachute for the main phase of the descent is chosen to allow Huygens to reach the surface in about 2 hours. The batteries powering Huygens will last for about 2.5 hours.
Prepared for surprises
A different perspective on the Jupiter Probe comes from Jean-Pierre Lebreton, ESA's Project Scientist for Huygens. The results contradicted many preconceptions of the Galileo scientists, particularly about the abundance of water and the structure of cloud layers. Arguments continue about whether the Probe hit by chance a patch of unusual weather, whether instruments were misreading, or whether ideas about the giant planet need a thorough shake-up.
"The Jupiter experience teaches us to be more modest in our predictions about what a new world will be like," says Dr Lebreton, "It shows the limitations of theories made from telescope studies and flybys, and confirms the need for on-the-spot observations. We know far less about Titan than about Jupiter. So a real understanding of Titan must await the arrival of Cassini-Huygens in eight years' time." Hazy orange clouds obscure Titan and leave scientists guessing about what Huygens will find. As speculation and debate continue, the biggest uncertainty concerns the nature of Titan's surface. Some experts expect to find large lakes of liquid hydrocarbons, while others suspect that the surface is dry. The hypothesis that a global ocean might cover Titan is out of fashion at present, because of radar results and reasoning about the effects of tides in a global ocean.
The multinational teams of scientists who have developed the instruments on Huygens are prepared for surprises. For example, the Surface Science Package is designed for a wet or a dry landing, and will give appropriate results in either case.
Further tests planned
With just eighteen months to go until the launch of the joint Cassini-Huygens mission in October 1997, the spring of 1996 is a busy time for the Huygens teams. The first European flight hardware reached NASA's Jet Propulsion Laboratory (JPL) for incorporation in the Flight Model of the Cassini Orbiter. This is the Probe Support Avionics, which receives and processes the signals from the Probe at Titan. The Engineering Model of Huygens has also gone to JPL, for comprehensive electrical tests of the Cassini spacecraft.
ESA is planning to carry out further tests on the Flight Model of Huygens, which is due for delivery in less than a year's time. The aim is to settle questions remaining after searching reviews of the Probe's design and readiness. Shock tests will check that Huygens is not harmed by the firing of pyrotechnic devices used to release the protective shell and the parachutes, after the Probe's incandescent entry into Titan's atmosphere.
In addition, the Titan Test will be repeated. This subjects the Probe to a simulation of the very cold atmosphere of the target moon. A previous test showed some components in Huygens approaching the lower limit of acceptable temperatures. The repeated test will verify that subsequent minor modifications have succeeded in reducing effect of the chilling.
Background facts about the Cassini-Huygens mission
Huygens is a medium-sized mission of ESA's Horizon 2000 programme for space science, and a contribution to the joint NASA-ESA Cassini mission.
Christiaan Huygens discovered Saturn's moon Titan in 1655, and the mission named after him aims to deliver a 343-kilogram Probe to Titan and carry a package of scientific instruments through the atmosphere. Six sets of instruments will analyse the chemical composition of the atmosphere, observe the weather and topography of Titan, and examine the nature of its surface. Titan is larger than the planet Mercury, and its unique atmosphere rich in nitrogen and hydrocarbons may resemble the atmosphere of the primitive Earth, before life began.
Nominal dates for the Huygens mission are as follows:
- Launch - 6 October 1997
- Arrival at Saturn - 26 June 2004
- Release of Huygens - 6 November 2004
- Entry into Titan's atmosphere - 27 November 2004
The Saturn Orbiter, the other element in the Cassini mission, will relay the signals from Huygens to the Earth, before settling down to prolonged observations of Saturn and its rings and moons. European and American scientists are partners in all the experiments, both in the Orbiter and in the Huygens Probe.
Farthest out for Europe
Huygens will travel to a greater distance from the Sun than any previous ESA mission, out to the orbit of Saturn at 1400 million kilometres, or nearly ten times the Sun-Earth distance. For comparison, the farthest-ranging mission at present is Ulysses. It orbits over the poles of the Sun and out to the orbit of Jupiter, 800 million kilometres from the Sun. As no other mission planned or contemplated by ESA at present will go as far as Saturn, Huygens is likely to hold the European record for many years.