Philae sleeps, hope rests in the sun

"Rosetta’s lander has completed its primary science mission after nearly 57 hours on Comet 67P/Churyumov–Gerasimenko" the European Space Agency (ESA) announced in the early hours of Saturday morning.

Philae’s last moments of operation and contact with its mothership Rosetta were dramatic. Rosetta had lost contact with its lander at 09:58 GMT (10:58 CET) on Friday, but was able to regain contact with it later at 22:19 GMT (23:19 CET). By the time contact was permanently lost at 00:36 GMT (01:36 CET) on Saturday morning, Philae was able to return all the data it gathered to Rosetta. ESA scientists have said Philae had completed the vast bulk of experiments and measurements planned during the first phase of its operation based on the power of its primary battery.

In these last two hours of contact with Philae, ESA staff at the DLR German Aerospace Agency, who monitored Philae’s progress from ESA’s Space Operations Centre in Darmstadt, Germany, had attempted to to rotate Philae’s main body in the hope of exposing one of the larger solar panels to the meager sunlight that was falling on one of the smaller panels. Such move would have allowed its secondary batteries to recharge and the probe remain operational for longer.

Thus, "the lander’s body was lifted by about 4 cm and rotated about 35°, but as the last science data fed back to Earth, Philae’s power rapidly depleted" ESA said.

Hope lies in the sun

“We still hope that at a later stage of the mission, perhaps when we are nearer to the Sun, that we might have enough solar illumination to wake up the lander and re-establish communication” said Stephan Ulamec, lander manager at the DLR German Aerospace Agency.

The Rosetta orbiter will continue listening for a signal from Philae each time its orbit brings it into line-of-sight visibility with Philae. However ESA has downplayed hopes adding that "given the low recharge current coming from the solar panels at this time, it is unlikely that contact will be re-established with the lander in the near future."

Rosetta is now moving back into a 30 kilometer orbit around the comet and over the coming months it will start to fly in more distant ‘unbound’ orbits, while performing a series of daring flybys past the comet, some within just 8 km of its center. Rosetta will continue studying the comet in great detail as the later becomes more active, en route to its closest encounter with the Sun on 13 August next year.

In fact, this is primarily the time ESA scientists are now looking at in hoping to get Philae awoken from its hibernation. 67P comet will next reach its closest to the Sun on 13 August 2015, at 186 million kilometers from the Sun (67P orbits the Sun once every 6.6 years).

Some space enthusiasts following the mission closely via Social Media have been asking ESA if reflecting sunlight from Rosetta’s solar panels onto Philae could provide a solution. Sadly, this cannot be the case. Firstly, engineers would need to navigate Rosetta to point directly at Philae, and this is impossible give the distance between the two bodies. Secondly, the sunbeam would be so broad that it would be useless.

Rosetta mission’s amazing success powered by solar PV

A few observers have focused on Philae’s last two days of operation on the unknown grounds of the 67P comet to launch a criticism on the solar energy’s pitfalls and incompetence. The critique is at least disproportionate. To reach the comet, the Rosetta spaceship needed to travel 6.4 billion of kilometers over the last 10 years based on power from its two 14-meter-long solar panels. In mid 2012 Rosetta recorded its maximum distance from the Sun, at about 800 million kilometers. And during its trip to the comet it had also to be put in hibernation to help it save power and minimize mission costs.

Philae too in its 57 hours of operation on the comet collected unprecedented data. As pv magazine wrote on Thursday, Philae’s mission on the comet was initially planed by ESA to last for only two days and a half, powered by its primary battery. ESA had warned from an early time that the risks of landing and operating Philae on the comet were plenty and not fully known. Risks varied covering among others the power challenge and the roughness of the comet’s surface. The failure of Philae’s two harpoons to anchor it successfully onto the comet’s surface was always a risk. The lander bounced twice on landing, resulting in it coming to rest at a shady area about 1 kilometer away of the targeted landing zone. The reported failure of Philae’s solar panels were incorrect, since its PV system worked perfectly and the problem was solely the lack of irradiation.

As reported by Ulamec, one of Philae’s panels was receiving sunlight for about one hour and twenty minutes and two more panels for about twenty-thirty minutes every comet day (which lasts 12.4 hours).

The decision to use solar panels for power Philae was ESA’s alone. All previous deep space probes have used Radio-isotope Thermoelectric Generators (RTGs) instead. However, "ESA has not developed RTG technology, so the agency decided to develop solar cells that could fill the same function," ESA said.

But ESA does not sound in the least bit disappointed about Philae’s job on the comet. Philae’s mission was primarily designed to last around 60 hours on its initial battery charge. Philae’s solar panels were added in an effort to extend the mission for months based on the sun powering the secondary batteries. And even though this wasn’t finally possible, Ulamec said, by Friday Philae had gathered about 80% of the date ESA was hoping it to do so.

“The data collected by Philae and Rosetta is set to make this mission a game-changer in cometary science,” says Matt Taylor, ESA’s Rosetta project scientist.

Meanwhile, Rosetta will continue performing its tasks, observing the comet from a distance and while it is powered by its solar panels.

Philae’s last tweet from the comet was: "My #lifeonacomet has just begun @ESA_Rosetta. I’ll tell you more about my new home, comet #67P soon… zzzzz".