At a press conference in London hosted by the Particle Physics and Astronomy Research Council, Chris Carr of Imperial College London described how the magnetometer instrument onboard the Venus Express spacecraft will measure the magnetic field around the planet.

Scientists hope their results will confirm why Venus is so inhospitable in comparison to Earth and has almost no water, in spite of the similarities between the two planets. Earth and Venus formed at the same time from the same basic materials and they are very similar in size and mass.

Scientists believe that Venus is inhospitable because its atmosphere is being eroded by the solar wind, a magnetised, electrically charged gas that streams off the Sun at a million miles per hour. This 'plasma' from the Sun slams into an electrically charged part of Venus's atmosphere known as the ionosphere, which is ionised by solar radiation.

The ionosphere provides a magnetic barrier against the solar wind but scientists believe that this barrier has much less protective power than Earth's internal magnetic field. This internal magnetic field creates a 'bubble' around Earth that protects it from the solar wind.

Chris Carr, who helped to build the instrument alongside colleagues at the Space Research Institute in Austria and the Institute for Extraterrestrial Physics in Braunschweig, Germany, explained: "We are going to make a 'map' of the plasma around Venus. By measuring the magnetic field, we can analyse the complex physical processes that result when the solar wind and Venus's ionosphere collide."

The Venus Express magnetometer is similar to the one that the Imperial team has been involved in building and controlling onboard the Cassini spacecraft. Cassini's magnetometer recently revealed an unexpected magnetic signature from the Saturnian moon Enceladus, a surprise which led to the discovery of an atmosphere on this tiny moon.

Chris Carr added: "We are going to be able to get a substantially enhanced picture of the space environment around Venus using the new high-resolution magnetometer, coupled with a new high-resolution plasma analyzer. We have a huge number of questions that we hope these instruments can help answer."

Even though the solar wind is travelling at such immense speeds, the Venus ionosphere still presents a significant magnetic barrier. How do these plasmas mix? How much energy is transferred from the Sun into the Venus atmosphere?" he said.

The Magnetometer for the Venus Express mission consists of two small sensors about 5cm by 5cm and weighing about 200g. One is mounted on the end of a metre-long deployable boom and the other sits directly on the spacecraft's body. The use of two sensors means that the stray magnetic fields produced by the spacecraft can be taken into account when the team is measuring Venuss magnetic field.

Venus Express sets off on 26 October 2005 and is due to reach Venus in April 2006.

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