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Last Updated on Sunday, 02 May 2010 12:24
Published on Friday, 25 February 2005 00:00

22 months ago, NASA's Cassini spacecraft left Earth to begin its seven year odyssey to the beautiful ringed planet, Saturn. Early tomorrow morning, after swinging around the inner Solar System and completing two flybys of the planet Venus, Cassini will briefly return home for a third boost in speed which will kick it away from the Sun and towards Jupiter and Saturn.

The spacecraft, launched on its mission to Saturn in October 1997, will make its closest approach to Earth at an altitude of 1,166 km (725 miles, or about five times higher than the Space Shuttle's orbit) over the eastern South Pacific at -23.5 degrees latitude and 231.5 degrees longitude. It may be visible from small islands in that area, such as Pitcairn Island or Easter Island.

Although the primary purpose of this Earth flyby is to increase the spacecraft's speed by borrowing some energy from our planet - a manoeuvre known as a gravity assist - the encounter will also be of considerable interest to scientists from the UK. Nine of Cassini's 12 scientific instruments will make observations of the Earth-Moon system during the spacecraft's passage, including studies of Earth's magnetic environment and images of the Moon.

UK scientists are involved with a number of the science instruments which will be switched on during this brief encounter.

One of the most significant events prior to the flyby was yesterday's successful deployment of the 11 metre long boom to which the two magnetometer sensors of Cassini's dual technique magnetometer instrument (MAG) are attached. Principal scientific investigator for MAG is Professor David Southwood of Imperial College, London and one of the co-investigators is ICL's Dr.Michele Dougherty.

The MAG instrument was turned on 44 hours before the closest approach to the Earth and scientists at Imperial College are already very excited about the data they are receiving from the instrument whilst the spacecraft is making its way towards the Earth. The flyby is extremely important to them since it can be used as an instrument calibration exercise, since the magnetic field of the Earth is very well known. Of prime scientific interest will be data from the interaction between the solar wind and Earth's magnetic field and plasma environment. In particular, studies of the enormous tadpole-shaped magnetotail, which stretches millions of kilometres 'downwind' on the planet's leeward side, will be carried out.

The Cassini Plasma Spectrometer (CAPS) instrument, with its Ion Beam Spectrometer and Electron Spectrometer, was turned on today about 16 hours before closest approach to Earth. 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 our magnetic planet. "We know a lot about Earth's magnetic field, so this will be an ideal opportunity to calibrate our instrument in preparation for arrival at Saturn," said Dr. Coates.

The Earth flyby will also provide an opportunity to check out some of the other instruments. The Radio and Plasma Wave Science (RPWS) instrument will be tested during studies of low frequency waves in the sea of charged particles surrounding the Earth. Dr Hugo Alleyne and the late Dr. Les Woolliscroft of the University of Sheffield provided data compression software for the RPWS experiment.

Also undergoing calibration during the flyby will be the wide and narrow angle cameras of Cassini's Imaging Science Subsystem (ISS). Although no images of the Earth will be taken, about a dozen views of the half Moon will be snapped in visible and infrared light using various configurations of filters. As the Moon comes into Cassini's view, the cameras will obtain shots of both the illuminated and dark hemispheres. Professor Carl Murray of Queen Mary and Westfield College, London is a member of the ISS team. He and his colleagues at QMW produced the ISS calibration software used during the flyby.

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 25 March 1999. It is expected to continue sending back data for nearly another decade. The instrument has already detected a number of dust impacts, some of which seem to have originated beyond our Solar System. 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.

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.

The Earth swingby will bend Cassini's flight path so that it heads towards Jupiter. Passing about 9.7 million km (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 km (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). Principal scientific investigator for the Surface Science Package on Huygens is Dr. John Zarnecki of the University of Kent (Canterbury).

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).


Dr. John Zarnecki, University of Kent, Canterbury, CT2 7NR.
Tel: +44 (0)1227-823237.
Fax: +44 (0)1227-827558.
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Professor Carl Murray, Astronomy Unit, Queen Mary & Westfield College, University of London, London E1 4NS.
Tel: +44 (0)171-975-5456.
Fax: +44 (0)181-981-9587.
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Dr. Michele Dougherty, Physics Dept., Imperial College, Prince Consort Road, London SW7 2BZ. Tel: +44 (0)171-594-7757.
Fax: +44 (0)171-594-7772.
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Professor Fred Taylor, Dept. of Atmospheric Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU.
Tel: +44 (0)1865-272903.
Fax: +44 (0)1865-272924.
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Dr Andrew Coates, Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT.
Tel: +44 (0)1483 204145.
Fax: +44 (0)1483-278312.
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Dr. Hugo Alleyne, Dept. of Automated Control and Systems Engineering, Sheffield University, Mappin Street, Sheffield S1 3JD.
Tel: +44 (0)114-282-5235.
Fax: +44 (0)114-273-1729.
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Professor Tony McDonnell, University of Kent, Canterbury, CT2 7NR.
Tel: +44 (0)1227-459616.
Fax: +44 (0)1227-762616.
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Further information on the Cassini Earth flyby 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:

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Peter Bond, RAS Press Officer (Space Science).
10 Harrier Close, Cranleigh,
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