UK Astronomers Ready For October Mission
On 6th October 1997, a Titan IV-Centaur rocket will blast off from Cape Canaveral carrying Cassini-Huygens, a spacecraft the size of a small bus, on a seven-year, 3.2 billion km trek to Saturn. Soon after arrival, the European-built Huygens probe will be released and parachute onto the unexplored surface of the giant moon, Titan. During the remainder of the mission, the Cassini orbiter will continue on a four-year tour of the Saturn system, sending back a stream of information on the planet, its eighteen known moons and its spectacular ring system.
Cassini-Huygens is a joint project between NASA, the European Space Agency (ESA) and ASI (the Italian space agency). NASA has primary responsibility for the Cassini orbiter while ESA is responsible for the Huygens probe. With funding from PPARC (the Particle Physics and Astronomy Research Council), the UK planetary science community has also been involved from the outset in the planning and construction of many of the instruments on board both spacecraft.
PPARC-funded scientists are involved in two instruments on the Huygens probe and seven instruments on the Cassini orbiter.
One of these is Dr Carl Murray of the Astronomy Unit, Queen Mary and Westfield College, London. In 1990, he was selected as a member of the Imaging Science Subsystem (ISS) for the Cassini orbiter. Over the four years of the mission the ISS instrument should be able to take as many as 300,000 images of the Saturn system, using the most sophisticated cameras ever sent to the outer Solar System. The instrument consists of wide- and narrow- angle cameras mounted on the side of the spacecraft. Although both cameras use modern 12-bit CCD technology to provide an image of 1,024 x 1,024 pixels, the optical assembly of the wide- angle camera is a spare built in the 1970s for the Voyager project.
As part of the preparation for Cassini, the Solar System Dynamics group at QMW has been involved in the calibration of laboratory test images obtained using the Cassini cameras and the development of software for the navigation and analysis of Cassini images. The QMW group has also investigated the effects of data compression on images of faint rings, and the possible hazards from dust in the Saturn system.
Recently QMW astronomers have been evaluating the possible orbital tours that Cassini might follow, seeing how each one meets the needs of ring scientists. As part of this effort they have collaborated with staff at the Royal Greenwich Observatory to provide ground-based astrometric measurements of the major satellites of Saturn, using the UK's jointly-owned 1-m Jacobus Kapteyn Telescope on the Canary island of La Palma. These data have now been used by the Jet Propulsion Laboratory in California to provide better orbits for the satellites, many of which will be targets for fly-bys during the Cassini mission.
The dynamics of narrow rings and any associated satellites are of particular interest to the astronomers at QMW. The Cassini images will help to solve ring puzzles that are common to all the major planets because Saturn has examples of the various types of rings found around Jupiter, Uranus and Neptune. Dr Murray is particularly keen to observe Saturn's "braided" F ring with the Cassini cameras. Images of the F ring's multiple strands and their gravitational interaction with each other and the small moons in the ring's vicinity will help to answer fundamental questions about the origin and evolution of rings. Such detailed knowledge helps astronomers understand the origin and evolution of planetary systems in general, since many of the dynamical processes occurring in narrow rings today also took place during the formation of planets 4.5 billion years ago. The Cassini cameras have other targets besides rings. These include the tracking of features in the atmosphere of Saturn and close-up images of moons such as Iapetus, an outer satellite that has one hemisphere covered in a mysterious dark substance. It is also hoped that the sensitivity of the Cassini cameras and the choice of filters will enable the ISS team to obtain images of the surface of Titan, despite the haze layer and thick atmosphere that envelopes the moon.
In order to reach Saturn, the spacecraft will have to complete a complicated tour of the inner Solar System. This will involve encounters with Venus (twice) and Earth, followed by a final gravity assist from Jupiter in December 2000. Arrival at Saturn is scheduled for July 2004. On its first orbit the Huygens probe will separate and plunge through the orange clouds into the atmosphere of Titan, Saturn's largest moon. It will continue to relay information back to the main spacecraft until it hits dry land or splashes down in an ocean of liquid hydrocarbons.
Over the next four years, the 7-m-tall Cassini orbiter will resemble a celestial billiard ball as it uses gravity assists from Titan to change its orbit. During its 1.7 billion km odyssey, it will return unique data on the huge gas planet, its retinue of moons and rings, and its local environment.
The planet Saturn has previously been visited by three spacecraft: Pioneer 11 in 1979, and the two Voyager spacecraft in 1980 and 1981. However, all these missions were "fly-bys", providing only a brief look at the Saturn system. In the next phase of the exploration of the Solar System, spacecraft are being put into orbit around the planets so that they can undertake detailed surveys. NASA's Galileo mission is currently in orbit around the giant planet Jupiter; and the Cassini-Huygens mission will undertake a similar survey of the Saturn system when it arrives in 2004.
The Imaging Science Subsystem (ISS) is one of three "facility instruments" on Cassini. The others are the Radio Science Subsystem and the Titan Radar Mapper. These important instruments have been built by the project, rather than individual institutions. They are designed and operated by a team of scientists, rather than a principal investigator and co-investigators. The ISS team consists of 14 scientists (11 from the United States and 3 from Europe) led by Professor Carolyn Porco of the University of Arizona.
Dr Murray speaks at the UK National Astronomy Meeting at the University of Southampton on Friday 11th April 1997 on the subject of Saturn's F-ring, which he calls 'The solar system's strangest ring'.
Images and further informationInformation about the Cassini mission can be found on the Internet at: http://www.jpl.nasa.gov/cassini/
Artist's impressions of various stages of the Cassini mission and views from Saturn's satellite are available from: http://www.maths.qmw.ac.uk/~kevin/SSD/art_cass
Further information about the Solar System Dynamics group at QMW and their involvement in Cassini can be found at: http://www.maths.qmw.ac.uk/~kevin/SSD/cass_qmw.html
Dr Jacqueline Mitton, RAS Public Relations Officer