Comet-mapping Rosetta spacecraft prepares for duty

 作者:袁黍识     |      日期:2019-03-05 01:19:02
By Jacob Aron IT WILL be the biggest highlight of the trip so far. As New Scientist went to press, the European Space Agency’s Rosetta spacecraft was preparing to rendezvous with the comet it has been chasing for the past 10 years. Mapping and landing on comet 67P/Churyumov-Gerasimenko will be a challenge unlike anything space explorers have ever faced. “When you go to Mars nowadays, you know everything,” says Rosetta flight director Andrea Accomazzo. Not so for Rosetta’s target. “We don’t know the mass, we don’t know the gravity field, we don’t know how to fly around this object.” Other spacecraft have flown by comets at high speed or orbited asteroids. But orbiting a comet is a different matter. Because a comet’s gravity is too weak to slow a visiting probe, Rosetta will initially manoeuvre in a series of controlled triangles, starting at a distance of 100 kilometres and moving closer in over the next few weeks. Eventually it will enter a proper orbit determined only by gravity, getting as close as 10 kilometres (see diagram). Rosetta may be in for a bumpy ride. Gases frozen beneath Churyumov-Gerasimenko’s surface could escape explosively as it nears the sun and warms up. As they spurt out, they will push against Rosetta’s solar panels, possibly as strongly as the comet’s gravity, but in the opposite direction. Weirder still, the comet is made of two blobs fused together in a shape likened to a rubber duck. That makes its gravitational pull tricky to predict, so creating a detailed 3D model of the comet is a top priority. Cartographers know the difficulties involved in wrapping a flat map around a globe, but at least they can lay down lines of latitude and longitude with ease when mapping our planet. Doing that with a rubber ducky is more difficult, says team member Holger Sierks. “The whole coordinate system on this body is tricky.” There will be no time for ESA researchers to catch their breath. They must ensure that Philae, a smaller probe riding along with Rosetta, touches down on Churyumov-Gerasimenko by 11 November. Any delays will make landing much trickier as the comet becomes increasingly active on its approach to the sun. “We have to do this at warp speed. We have very little time between the discovery of a new world and landing,” says Sierks. ESA will announce a shortlist of landing sites for Philae in September. Data from all of Rosetta’s scientific instruments and measurements of how the comet affects the probe’s trajectory will influence the eventual choice. Measurements released last week indicate that the comet’s average surface temperature is -70 °C, around 20 to 30 °C warmer than predicted. That makes it likely that the surface is dark and dusty rather than coated in clean ice. Understanding how the surface temperature changes as the comet nears the sun will be a factor in where to land, as certain sites may become unstable. One spot that might already be off limits is the “neck” of the duck. Despite being of great scientific interest, it may prove too difficult to land on, as gravity may not be acting at a right angle to the surface. For now, assuming they’re released swiftly (see “Let the space fans in“), we can enjoy the first high-resolution images of the comet, which Rosetta was due to send on 6 August. “It’s the end of a 10-year trip in the solar system and the start of the exploration of a new world,