On 12 November, history was made when ESA’s Rosetta mission soft-landed its Philae probe on Comet 67P/Churyumov–Gerasimenko (Comet 67P/C-G). This is the first time ever that man has landed a spacecraft on a comet.
After a tense wait during the seven-hour descent to the surface of Comet 67P/C-G, the signal confirming the successful touchdown arrived on Earth at 16:03 GMT (17:03 CET).
Rosetta was launched on 2 March 2004 and travelled 6,4 billion km through the Solar System before arriving at the comet on 6 August 2014.
The landing site, named Agilkia and located on the head of the bizarre double-lobed object, was chosen just six weeks after arrival based on images and data collected at distances of 30–100 km from the comet. Those first images soon revealed the comet as a world littered with boulders, towering cliffs and daunting precipices and pits, with jets of gas and dust streaming from the surface.
Following a period spent at 10 km to allow further close-up study of the chosen landing site, Rosetta moved onto a more distant trajectory to prepare for Philae’s deployment.
Deployment was confirmed at 09:03 GMT (10:03 CET) at a distance of 22,5 km from the centre of the comet. During the descent, which was made without propulsion or guidance, Philae took images and recorded information about the comet’s environment.
Touchdown was planned to take place at a speed of around 1 m/s, with the three-legged landing gear absorbing the impact to prevent rebound, and an ice screw in each foot driving into the surface.
But during the final health checks of the lander before separation, a problem was detected with the small thruster on top that was designed to counteract the recoil of the harpoons to push the lander down onto the surface. The conditions of landing – including whether or not the thruster performed – along with the exact location of Philae on the comet are being analysed.
Over the next 2,5 days, the lander will conduct its primary science mission, assuming that its main battery remains in good health. An extended science phase using the rechargeable secondary battery may be possible, assuming Sun illumination conditions allow and dust settling on the solar panels does not prevent it. This extended phase could last until March 2015, after which conditions inside the lander are expected to be too hot for it to continue operating.
Science highlights from the primary phase will include a full panoramic view of the landing site, including a section in 3D, high-resolution images of the surface immediately underneath the lander, on-the-spot analysis of the composition of the comet’s surface materials, and a drill that will take samples from a depth of 23 cm and feed them to an onboard laboratory for analysis.
The lander will also measure the electrical and mechanical characteristics of the surface. In addition, low-frequency radio signals will be beamed between Philae and the orbiter through the nucleus to probe the internal structure.
The detailed surface measurements that Philae makes at its landing site will complement and calibrate the extensive remote observations made by the orbiter covering the whole comet.
ESA’s animation below shows Philae’s landing on Comet 67P/C-G: