Rosetta has quietly become one of the coolest vessels currently out exploring our solar system. The European Space Agency mission is doing something that's never been done: It's going to harpoon and land a small spacecraft on a comet's surface.
This week, mission scientists narrowed down and selected that landing site, meaning we're a couple of months away from one small step for comet-chasing spacecraft kind.
Rosetta launched on March 2, 2004. It spent the first ten years of its life bouncing around between the Earth, Mars, and the asteroid belt to gain speed and perfect its trajectory so it would intercept its target, the somewhat awkwardly shaped, 2.45-mile wide Comet 67P/Churyumov–Gerasimenko. The spacecraft arrived and entered into an triangular around the comet this past August. And since then, one of its main tasks has been to find a suitable landing site for its Philae lander.
The Philae lander is part of what makes this mission so awesome. It's a 220-pound, box-shaped spacecraft currently hitching a ride aboard Rosetta. When it goes in for a landing attempt in November, it will self-eject from its host spacecraft, unfold its three legs, and touch down on the surface. Its legs are designed to make touchdown as smooth as possible by absorbing the shock of landing to prevent bouncing and are ready to rotate, lift, and tilt to stop the lander from falling over.
But before the legs can do their job, Philae needs as safe a site as possible to make that landing. Since going into orbit around the comet, Rosetta has been taking some stunning images of Comet P67 in the name of finding that site.
"As we have seen from recent close-up images, the comet is a beautiful but dramatic world—it is scientifically exciting, but its shape makes it operationally challenging," said Stephan Ulamec, Philae lander manager at the DLR German Aerospace Center.
On this dynamic, irregular little world, the team was looking at segments with few boulders, flat terrain, and ample sunlight. Unfortunately, no candidate landing sites met all the criteria the team was looking for.
But one site stood out as the best option, an area known as Site J. The site sits on the comet's head. Slopes in the area are less than 30º relative to local vertical, which will hopefully stop Philae from falling over when it touches down. The site also has few boulders or large rocks, giving Philae a clear landing area. It also gets a decent amount of sunlight, a key factor in recharging Philae's batteries after the landing phase.
Site J is a scientifically interesting spot as well. From here, Philae will be able to analyze untouched material on the comet's surface, delve into the properties of its nucleus, and study the processes that drive its activity. A backup site, landing site C, is on the comet's body.
Philae's landing attempt will come on November 11. The craft will use harpoons and ice screws to anchor itself into the comet, then take a 360° panoramic image to give scientists a clear picture of what they have at their disposal. Then we're going to see some really neat science—just to reiterate, this is a spacecraft landing on a comet, after all.
"We will make the first ever in situ analysis of a comet at this site, giving us an unparalleled insight into the composition, structure and evolution of a comet," said Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.
Philae will study the comet's plasma and magnetic environment, measure surface and subsurface temperature, and collect subsurface samples for on site analysis. ESA hopes to confirm Philae's landing date by September 26. Pending a comprehensive readiness review on October 14, we should be good to go to land on a comet!