Venus Assists Cassini on its Space Odyssey to Saturn
Venus Assists Cassini on its Space Odyssey to Saturn
UK astronomers are about to receive the first substantial set of scientific data to be returned during the Cassini spacecraft's six-year odyssey to the ringed planet Saturn.
At 21.30 BST on June 24, Cassini and its European-built Huygens probe flew about 620 kilometers (388 miles) above the cloud tops of Venus in order to obtain a boost in speed from the planet's gravity. A number of science instruments in which UK scientists are involved were switched on during this brief encounter. However, they will have to wait for a while to study the new information. Data obtained during the flyby will trickle back to Earth at a fairly slow rate via the low gain antenna since Cassini's more powerful high gain antenna is acting as a sunshield and pointing away from the Earth.
The Cassini Plasma Spectrometer (CAPS) instrument, with its Ion Beam Spectrometer and Electron Spectrometer, was turned on about 7 hours before closest approach to Venus. Dr. Andrew Coates of Mullard Space Science Laboratory is the team leader for the Electron Spectrometer part of this instrument, while Rutherford Appleton Laboratory is also involved. CAPS is investigating how the electrically charged particles of the solar wind interact with the non-magnetic planet. "This will be a useful practice for the Cassini investigation of Saturn's giant moon Titan, which we think also has no magnetic field," said Dr. Coates.
At the same time, the Radio and Plasma Wave Science (RPWS) instrument was used to study radio emissions from the planet and waves in the sea of charged particles surrounding Venus. RPWS was activated just over eleven hours before Venus closest approach. Although a search for radio 'whistlers' caused by lightning proved fruitless during Cassini's first flyby of Venus in 1998, many scientists are convinced that such storms are common on the cloud-shrouded planet. Dr Hugo Alleyne and the late Dr. Les Woolliscroft of the University of Sheffield provided data compression software for the RPWS experiment.
Although Venus has no internal dynamo to generate a magnetic field, it does act as a barrier, slowing the incoming solar wind to subsonic speeds on its sunward side. This sudden deceleration causes a shock wave as the solar particles approach and flow around the planet. Cassini's Dual Technique Magnetometer (MAG) should have observed this Venusian bowshock as the spacecraft ploughed through the boundary into a region known as the ionosheath. Principal scientific investigator for MAG is Professor David Southwood of Imperial College, London.
Although Cassini's Imaging Science Subsystem (ISS) was activated to obtain about 10 images of the cloud tops during the flyby, the pictures themselves will be of little scientific value. The main purpose of the images was to calibrate the wide and narrow angle cameras in preparation for more intensive use on arrival at Saturn. Imaging opportunities were limited during the flyby since the spacecraft and its cameras were not to be manoeuvred and remained pointing in one fixed direction. Professor Carl Murray of Queen Mary and Westfield College, London is a member of the ISS team.
While most of the spacecraft's instruments have been switched off since launch in October 1997, the Cosmic Dust Analyser (CDA) instrument has been operating continuously since March 25. It is expected to continue sending back data for nearly another decade. During its first 41 days of measurements, the instrument detected at least 7 dust impacts, but the source of these particles is not yet determined. However, it seems likely that CDA's chemical analyser, which has been provided by the University of Kent, has returned the first data ever obtained on the composition of an interplanetary dust particle. Professor Tony McDonnell from Kent is one of the CDA investigators. Rutherford Appleton Laboratory also contributed to the design and manufacture of this instrument.
Next Stop Earth
Saturn is so far away that it will take Cassini six years and nine months to get there. Since there are no launch vehicles powerful enough to send such a huge spacecraft directly to Saturn, mission planners turned to the tried and tested technique of gravity assists. This enables an energy exchange in which the spacecraft gains speed and alters its flight direction while reducing the orbital velocity of the planet by a minute amount.
In order to benefit from this technique and reach the ringed planet, the 5650 kilogram Cassini had to be launched inward towards Venus. Only after completing two Venus flybys, a flyby of Earth and one of Jupiter, will the bus-sized spacecraft have accelerated sufficiently to reach Saturn.
On August 18, Cassini will fly past Earth at an altitude of 1,166 kilometers (725 miles), about five times higher than the Space Shuttle's orbit. This Earth swingby will bend Cassini's flight path so that it heads towards Jupiter. Passing about 9.7 million kilometers (6 million miles) from the gas giant on 30 December 2000, it will use Jupiter's gravity to change course and speed for its final destination of Saturn.
Saturn is ten times further from the Sun than the Earth - about 1,430 million kilometers (900 million miles). Cassini's arrival is scheduled for 1 July 2004. Over the following four years, it will conduct 27 different scientific investigations of the giant planet's atmosphere and magnetosphere, its magnificent rings, and sixteen of the known moons. The largest of these, Titan, is particularly fascinating since it has a thick, cloudy atmosphere which is mostly made of nitrogen but also contains hydrocarbons such as methane - similar to the atmosphere of the early Earth but much colder.
Cassini will complete more than 60 orbits of Saturn, including about 45 close flybys of Titan and about 20 flybys of some of the smaller, icy moons. This tour is made possible by using planet-sized Titan's gravity to alter Cassini's orbit each time the craft swoops to within a few thousand kilometres of the moon's orange cloud tops.
The spacecraft is named after the 17th century Italian-French astronomer, Giovanni Domenico (or Jean-Dominique) Cassini who made a number of important discoveries about the Saturn system.
The mission is an international venture involving the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Italian Space Agency (ASI), as well as several European academic and industrial partners. The United States is responsible for the main Cassini spacecraft which will be inserted into orbit around Saturn in July 2004. Attached to the mother craft will be the European Space Agency's Huygens probe, whose task is to parachute onto the unexplored surface of Titan, the largest of Saturn's moons. (see RAS press notice 97/36)
More than 616,400 signatures sent to NASA from citizens in 81 countries have been recorded on a high-tech data disk installed on the Cassini spacecraft.
UK investment in the Cassini-Huygens mission amounts to 7.4 million pounds, of which 4.48 million pounds is being spent on the orbiter experiments and 2.92 million pounds on the probe investigations. Most of this is provided by the Particle Physics and Astronomy Research Council (PPARC).
PPARC is the UK Government-funded body providing support for basic research in elementary particles and the forces of Nature; planetary and solar research, including space physics; astronomy, astrophysics and cosmology.
Further information on the Cassini-Venus and Earth flybys can be found at:
Further information on the Cassini mission can be found on these Web pages:
Artist's impressions of the mission are available from:
Peter Bond, RAS Press Officer (Space Science).
Phone: +44 (0)1483-268672
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