The European Space Agency’s small Philae lander, which had been carried by the Rosetta spacecraft to the Comet 67P/Churyumov-Gerasimenko, has successfully landed. Separation from its mother craft was confirmed at ESA’s Space Operation Centre, ESOC, in Darmstadt, Germany, at 0903 GMT. The small craft then took several hours to free fall, eventually landing on the Comet after a series of bounces on 12 November 2014.
The initial contact was detected as being made at 1533 GMT with the confirmation signal received on Earth 28 minutes later. However, the spacecraft had actually bounced off the surface immediately after touchdown and then recontacted and bounced again at 1726 GMT.There had been prior concern that the craft might bounce off the comet into oblivion during the “harpooning” procedure after the rearward firing cold gas thruster failed to pressurise. As it was, the harpoon system is thought to have failed anyway and the initial bounce off the surface in the very low gravity environment was simply due to the direct collision. The first bounce was up to 1km high but the second bounce was much lower.
After its second bounce a more permanent touch down was made. The screw down feet on the legs apparently managed to maintain a purchase on the surface, with two out of three grounded. This happened at 1732 GMT with the signal received on Earth about 28 minutes later. The mark left by the bounces and the final landing sight was subsequently photographed by Rosetta.
Philae relayed its surface images (including those of its feet) back to Earth via its Rosetta mother ship. After being out of communication visibility with the lander since 0958 GMT on 14 November 2014, Rosetta regained contact with Philae at 2219 GMT.
The battery powered Philae craft has solar cell recharging and it was hoped that this would last long enough to allow scientific observations of the sublimation mechanism and gas outflows as the comet approached the sun. However, on its final landing, Philae found itself in the shadow of a cliff, which meant that it did not get enough sunlight to recharge its battery properly. The lander’s body was able to be lifted by about 4 cm and was rotated about 35 degrees in an attempt to receive more solar energy but the solar illumination was still not enough.
Noting the worsening power situation, ground engineers triggered drilling operations to start so that this could be analysed by the on-board laboratory, and its results transmitted back to Earth via Rosetta, before the power ran out. Without a strong attachment to the surface, this subsurface drilling operation was hazardous in that any residual drilling “torque” risked flinging the craft back off into space (according to Newton’s third law). In the event this did not happen.
The sampling itself apparently went well. It is hoped the scientific measurements of the constituents of the sampled water ice will provide evidence to prove whether or not comets provided the vehicle for water molecules to arrive on Earth. A German thermometry experiment ended in failure however, as it broke as it was being hammered into the harder than expected ice beneath the dust.
The Philae craft did eventually run out of power as the battery was drained. ESA still hopes that this will be a temporary situation and that Philae will be able to come out of hibernation as the comet gets closer to the sun. In this instance the sunlight should become more constant and this should be enough to recharge the battery to allow communications to be restarted via the Rosetta spacecraft. While less than ideal for solar array illumination, Philae’s current shadowed position may actually help protect the spacecraft from the worst of the solar heating.
Having previously maintained as best a stable telecommunications relay station position as it could relative to the Philae, Rosetta itself has moved back into an observational orbit around the comet.
Comment by David Todd: Whatever else is achieved, this fantastic mission has, at least, shown that the manned landings on comets depicted in science fiction films such as ‘Armageddon’ (1998), are now a very real possibility. Others foresaw humans riding on comets. In his 1877 book ‘Hector Servadac’ (aka ‘Off on a Comet’), the 19th Century French writer, Jules Verne even foresaw men residing on the surface of a comet – even if they were there unintentionally after the Comet Gallia’s “graze” with the Earth pulled off some land masses. Part one of the cartoon version of the story is available here: http://www.youtube.com/watch?v=wuMCEpnlgZY
