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Is Philae's Time Coming to an End? - UPDATE

Marcia S. Smith
Posted: 14-Nov-2014
Updated: 14-Nov-2014 05:36 PM

UPDATE, November 14, 5:33 pm EST: The European Space Operations Center (ESOC) just tweeted (@esaoperations) that it has reacquired Philae.  The first contact was brief, but the signal was then reacquired and communications now are "stable" and "telemetry and science data are flowing."  We'll post another update later this evening.

The European Space Agency's (ESA's) Philae lander is on the surface of Comet 67P and its science instruments are working, but its ability to function and communicate with Earth could be quickly coming to an end -- perhaps today.   Philae landed in a place surrounded by rocks that are blocking sunlight from reaching its solar panels to recharge the batteries.  ESA scientists hope they can reestablish communications at least one more time, but will not know until about 6:00 pm Eastern Standard Time (23:00 GMT, midnight CET) today.

Philae (pronounced fee-LAY) has been traveling in space for 10 years aboard its mothership Rosetta.  They reached Comet 67P/Churyumov-Gerasimenko, which is currently about 510 million kilometers (315 million miles) from Earth, in August.  The two separated on Wednesday (November 12) and Philae landed on the comet while Rosetta remains in orbit. 

Both spacecraft are equipped with many scientific instruments and Rosetta also serves as a communications relay between Philae and Earth.   Whenever Rosetta moves below the horizon at the comet, communications between Philae and its Earth-based operators cease.  Thus there are only certain windows of opportunity when Earth and Philae can talk to each other and instructions uploaded or data downloaded.

The next communications pass will be at about 9:00 pm UTC tonight, Philae project manager Stephan Ulamec said during a Google Hangout update this morning.  Times are approximate and he added that the passes have been occurring about one hour later than expected.  Adding in the 28 minutes 20 seconds needed for a signal to travel from the comet to Earth, it will be "about midnight" when they know if contact was restored.  (At that point, he apparently was referring to midnight local time in Germany, which is Central European Time.  UTC is one hour earlier; EST is 6 hours earlier, making it about 6:00 pm EST.)

Comets have almost no gravity so Philae was equipped with harpoons that were supposed to fire into the comet's surface to hold it in place.  Philae also has a small engine that was supposed to provide a downward force as it reached the surface.  Finally, the three landing legs had ice screws to help hold it in place.   Operators knew the engine was not working even before Philae separated from Rosetta, but decided to proceed anyway confident of the other systems.  Unfortunately, the harpoons did not fire when Philae first touched down on the comet at 10:34 am EST on November 12.   It bounced about a kilometer (about half a mile) high and floated above the comet for almost two hours before landing for a second time, bouncing again, and then landing for a third and final time about seven minutes later.

Where it landed still is not known.  Cameras aboard Rosetta are searching for it, but so far without success.  Philae is quite small -- about one meter (three feet) on each side --  and it landed somewhere amidst rocks.  Not only do the rocks complicate finding it from orbit, but they are preventing sunlight from recharging Philae's batteries. 

Valentina Lommatsch from the German space agency's (DLR's) lander control center clarified that all three of Philae's legs are touching the surface despite earlier reports that one was sticking up in the air. The problem is that we are "really unlucky" to be "in a corner surrounded by rocks."

Philae's primary battery had 64 hours of life when it landed, enough to complete its primary scientific mission.  The hope was for an extended mission of many months, however, allowing many more investigations using Philae's 10 scientific instruments, but those chances are looking slim.

Ulamec said that even regaining contact with the spacecraft tonight "is not secure, maybe the battery will be empty before we get contact again."   Of the three solar panels on Philae, one is getting a low level of illumination for about 1 hour 20 minutes each "day" and the others about 20 minutes each, versus the 6-7 hours planned. 

Last night when they uploaded commands to Philae, they calculated that it had about 100 watt-hours left and 80 were needed to execute those instructions (to activate scientific experiments), Lommatsch explained.  Unfortunately one of the last commands, which would have put it into low power mode to extend the battery life, did not reach the lander and "it will be really, really close" as to whether communications can be restored, she cautioned.  Later she said the lander needs 5.1 watts to boot.  Before a battery can be recharged, its temperature must be raised to 0 degrees Celsius and that requires 50-60 watt hours per day.  Then there still must be enough daylight left to actually recharge it.  "Looks a bit bad, but we can always hope," Lommatsch said.

ESA experts are assessing what they might be able to do with Philae if communications are restored to increase the chances of recharging, perhaps by moving the lander -- either rotating it using its flywheel, or trying to get it to essentially hop out of its current location to a better spot.  If nothing else, there is a chance -- although it sounded rather small -- that communications could be reestablished as the comet moves closer to the Sun and illumination increases. 

Ulamec was asked why ESA did not use a nuclear power source for the lander.  Some spacecraft, like NASA's Mars Curiosity rover, use Radioisotope Thermal Generators (RTGs) to power the spacecraft and its instruments.  RTGs use heat produced by the decay of plutonium-238.  Ulamec replied that launching nuclear power sources carries safety and political implications and, in any case, Europe does not have that technology.

Despite everything, the Philae scientists, engineers and operators appear delighted with what they have been able to accomplish already, even if there is no more.  Rosetta project scientist Matt Taylor said that a great deal of science data has been obtained from Philae and Rosetta and will be presented at the upcoming fall meeting of the American Geophysical Union (AGU 2014) next month in San Francisco, CA.

Flight director Andrea Accomazzo urged everyone to focus on what has been done, not what could have been done.  "This is already fantastic," he enthused, while adding that he really wants to know exactly where Philae is and to hear from it tonight.


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