Philae on comet. (Credit: ESA)

Philae on comet.
(Credit: ESA)

After arriving and checking out landing sites at Comet 67P/Churyumov-Gerasimenko the Rosetta spacecraft released the Philae probe, which landed on the comet on November 12, 2014.

After touching down in the Agilkia region as planned, Philae did not secure itself to the comet and bounced to a new location in Abydos. Its flight across the surface is depicted in a new animation, using data collected by Rosetta and Philae to reconstruct the lander’s rotation and attitude.

In the year since landing, a thorough analysis has also now been performed to determine why Philae bounced.

The probe had three methods to secure itself to the comet after landing – ice screws, harpoons and a small thruster. The ice screws are relatively soft, but Agilkia turned out to be very hard and the screws did not penetrate the surface.

The harpoons were capable of working in both softer and harder materials. They were supposed to fire on contact and lock Philae to the surface, while a thruster on top of the lander was meant to push it down to counteract the recoil from the harpoon. Attempts to arm the thruster the night before failed and it is believed that a seal did not open or a sensor failed.

So when Philae landed, the harpoons did not fire. If scientists can regain communications with Philae, they will try to re-fire the harpoons to assure that the probe remains secure on the surface and to give the sensors the opportunity to measure the comet’s temperature below the surface.

Despite the bouncing, Philae did complete 80 percent of its planned first science sequence before falling into hibernation in the early hours of November 15, 2014 when the primary battery was exhausted. The location in which Philae finally landed did not have enough sunlight to charge the secondary batteries so that the science measurements could not be performed and communicated.

The science team hoped that as the comet moved nearer to the sun, there would be enough energy generated by the solar panels to reactivate Philae. Things worked as hoped and there were contacts with the lander on June 13, 2015 and eight periodic contacts up to July 9.

However, due to increased activity on the comet caused by the increase in sunlight Rosetta had to retreat several hundred kilometers for safety reasons and that put it out of range with Philae.

With the comet’s activity subsiding, Rosetta has started to approach again and has reached within 170 km of the surface.

Meanwhile, the lander teams have continued their analysis of the data received during the contacts with Philae in June and July and are trying to determine the status of the lander when it first woke up.

During the analysis the teams concluded that two receivers and one of the two transmitters were likely no longer working and that now the other transmitter is also suffering problems switching off to on affecting possible contacts.

The teams are using the information to determine if a strategy can be created to regain regular contact. However, it is a race against time because the comet is heading out beyond the orbit of Mars and temperatures on the surface are falling.

The teams believe that they have until the end of January before the lander’s internal temperature gets too cold to operate.

In the meantime, Rosetta continues to return unique data and has analyzed changes to the comet’s surface, atmosphere and plasma environment.

Next year it is planned that Rosetta will travel through and beyond the comet’s tail 2000 km. Some very close flybys are also planned as the teams prepare to land the orbiter on the comet. The plan is to end the mission with a controlled impact of Rosetta on the surface. This idea emerged around six months ago, when an extension of operations from December 2015 to September 2016 was announced.

The solar-powered Rosetta will no longer receive enough sunlight to operate as the comet recedes from the sun, out beyond the orbit of Jupiter on its 6.5-year circuit. It will travel even further out than during the previous 31 months of deep-space hibernation that ended in January 2014.

In addition, Rosetta and the comet will travel very close to the sun, making the relay of data and commands very difficult.

For now, the Rosetta teams are investigating maneuvers needed for operating close to the comet in the weeks leading up to the dramatic mission finale. According to Sylvain Lodiot, ESA’s Rosetta spacecraft operations manager, the teams are discussing exactly what the final end of the mission will involve.

“The schedule we’re looking at would first involve a move into highly elliptical orbits –- perhaps as low as 1 km – in August, before moving out to a more distant point for a final approach that will set Rosetta on a slow collision course with the comet at the end of September,” said Lodiot.

It is expected that science observations would continue throughout and up to almost the end of the mission, permitting Rosetta’s instruments to gather unique data at unprecedentedly close distances.


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