The 1990s were uncertain times in space exploration. The USSR / Russia essentially pulled out of the game. In the USA NASA’s mantra became ‘faster, better, cheaper’, and the two awful space shuttle disasters cast doubt on the reliability of that craft. Behind ‘faster, better, cheaper’ was a desire to reduce the lead time between mission conception and launch from eight years to five. Instead of putting tens of experiments on one huge expensive spacecraft the missions were broken up into smaller less expensive dedicated probes. More bang for the buck was the aim. There were some very notable successes. One was the low-cost Discovery mission NEAR-Shoemaker to the asteroid Eros. This orbited Eros for a year and finally landed on the surface on February 12 2001. And there was the Stardust mission to Comet Wild 2. This flew to within 236 km of the cometary nucleus on January 2 2004 and is returning to Earth with samples of cometary dust onboard, and will soft-land in the Utah dessert.

There are though, places that spacecraft cannot safely go. The rings of Saturn are full of ice-covered rocks and a spacecraft travelling through these rings would have too high a chance of hitting one of them. When the Giotto spacecraft visited Comet Halley in March 1986 it passed within about 600km of the nucleus, on the side facing away from the Sun. This miss-distance was picked because it gave the craft a 50/50 chance of survival. The minute cometary dust particles were approaching at a speed of 65 km s-1 and their kinetic energy was such that each impact could easily destroy the spacecraft. A special ‘bumper shield’ (devised by the famous American astronomer Fred Whipple) was used to break up the particles and spread their energy over a large area of thick ‘bullet-proof’ material. Small rocket jets were used to damp out the rolling effects caused by these impacts. The craft just survived, and subsequently went on to visit another comet Grigg-Skjellerup in 1992.

There are many things that spacecraft can do that are impossible from the surface of Earth. Travelling down through the clouds of Venus and Titan, and imaging the back of the Moon are perfect examples, as is the detection of the radiation belts and magnetic fields of distant planets.

And space exploration has been full of surprises. My favourites were the completely unexpected discoveries of volcanoes on the jovian satellite Io, the magnetic field of Uranus that is inclined at 60º to its spin axis and the huge Caloris crater on Mercury, the production of which nearly broke the planet apart. Even our nearby Moon was found to have strange enhancements of gravitational and magnetic fields that we called mascons and magcons.

Flybys and orbiters are fine for mapping surfaces and probing atmospheres and magnetospheres, but there is nothing quite like getting up-close and personal. For this you have to take the next step - land, rove about and start digging.

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Content last updated: 06/12/2004

Prof. David Hughes

David Hughes is Professor of Astronomy at the University of Sheffield and has worked there since 1965. Professor Hughes has published well over 200 research papers concentrating on the solar system and especially the minor bodies such as asteroids, comets, meteorites and meteoroids, and their origin, decay, size distribution and evolution. Hughes' ground-based and spacecraft observations of Halley's Comet led to an interest in the work, life and times of Edmond Halley and the history of astronomy in general. Hughes enjoys giving popular lectures, reviewing books and has had asteroid number 4205 named after him. At present he sits on the Council of the Royal Astronomical Society and the Swedish Space Research Advisory Committee.

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