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Catching Up On Mars
Paris - Aug 26, 2003 On 27 August 2003, Mars is less than 56 million kilometres away -- approaching closer to our planet than it has done in over 60,000 years. About the same time as this closest approach, Mars Express passes the halfway mark of its journey, in terms of distance along its trajectory. On 1 September 2003, as it hurtles through space at 10,800 kilometres per hour, the spacecraft will have covered over 242 million kilometres, half of the total of 485 million kilometres needed to arrive at Mars. Note that the distance travelled is not the same as the distance between the Earth and Mars. At first glance, you might think that this time would be better than three months ago to launch a spacecraft to Mars, given the advantage of a shorter route. However, planning interplanetary journeys is more complicated than just taking account of the distances.
Playing catch up Imagine the Solar System as an athletics racetrack. If you were watching a 400-metres race from the centre of the track and wanted to intercept one of the runners taking part, you could simply chase the runner you want to catch. If you are fast enough, you might eventually catch up but only after using a lot of energy and travelling a long way. A much better way to catch your athlete is simply to walk across the centre to the other side of the circular track. It is a much shorter distance and you use a lot less energy and time getting there. You calculate your walk so that you arrive at the other side of the track at the same time as they do. Too early and you are waiting for them. Too late and you have missed them completely -- you would have to wait one lap until they came around again.
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