Finding Philae: How it Was Done, and Why it Might Not Matter [IN DEPTH]

*UPDATE: Philae makes contact (June 13)*

It's been eight months since the European Space Agency's (ESA) Philae lander fell away from the Rosetta Spacecraft, on its way toward making history. Officially, it was the first piece of human technology ever to successfully land on a comet as it hurtled through space. However, soon after touchdown, the historic lander went missing. Now, investigators think they have found where Philae is hiding.

"Landed?" More Like "Bounced"

When the touchdown signal from Philae was sent and confirmed on November 12, the lander's instruments reportedly began to "think" that the craft had settled, booting up and starting a phase of passive analytic experiments.

However, expert interpretation of that data quickly revealed that Philae was still moving after it first hit the surface of comet 67P/Churyumov-Gerasimenko. Its landing harpoons, designed to anchor the lander, had not deployed - technical difficulties that could have ended with Philae tearing away from the comet's weak gravity to float aimlessly in space.

Thankfully, that's not what happened. It was initially revealed that the lander likely skated over the cold and dusty surface of 67P for about two hours, bouncing three times and traveling an estimated 3,340 feet (~1 km) in an unknown direction.

Telemetry and audio data (you can hear Philae touchdown here) confirmed that the bouncing lander did not end up where it was supposed to, but due to the convoluted shape of the "rubber-ducky" comet, the Philae team had very little hope of quickly finding their robot. They lost contact with it two days later. (Scroll to read on...)

Philae, Shouting in the Dark

Then, on November 15, Philae called out via radio signaling to the orbiting Rosetta craft that, while lost, it was still very much intact. This gave the team some hope, as they were able to determine that while the lander's solar panels - essential for keeping its battery charged - were largely in the shade of 67P, some sunlight still reached it.

Even hidden in shadow, Philae reportedly gets 1.5 hours of sunlight each 12-hour day on the comet. The intended landing zone, for comparison, offered nearly seven hours of sunlight.

Still, that information alone allowed the Philae team to narrow down where their lander might be.

Finding Philae

Additional images taken by its ROLIS and CIVA cameras, along with telemetry and data returned by its instruments during its nearly 60 hours of surface operations, built up a picture of Philae's final landing site.

However, the team couldn't work from this alone. Help was recruited from Philae's parent-craft, Rosetta, and its high-resolution OSIRIS camera. At a distance of over 18 kilometers (~11mi), and taking into account the size, reflectivity, and orientation of Philae, investigators started looking for bright spots no more than a few pixels wide that could be the lost lander.

This was no small task. Stephan Ulamec, lander manager with the DLR German Aerospace Agency, memorably complained during April's European Geosciences Union General Assembly in Vienna that Philae's glow "is not very distinct."

"And, yes, the later in the evening and the more wine you drink - the more landers you're able to 'identify' in this terrain," he joked with his peers.

Still, after many sleepless nights (and plenty of wine?) five key candidates were highlighted for further investigation. (Scroll to read on...)

These were then narrowed down to only one or two promising locations after an in-depth analysis of Philae's radio calls in the dark - called the CONSERT experiment - was concluded.

"Combining data on the signal travel time between the two spacecraft with the known trajectory of Rosetta and the current best shape model for the comet, the CONSERT team have been able to establish the location of Philae to within an ellipse roughly 16 x 160 metres in size," the ESA recently announced.

"We have identified several possible lander candidates in OSIRIS images, both inside the CONSERT region of interest and nearby," Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research (MPS) in Germany, added proudly. (Scroll to read on...)

[Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA] Sweeping OSIRIS passes spotted a new and promising bright spot located just outside the CONSERT error ellipse (marked). Zooming in, experts have argued that this is a prime candidate for the spacecraft, based on shape and light reflection.

The Best and Brightest

Lastly, the team turned to images of these promising locations before and after Philae's touchdown. They sought to compare and contrast the images, hoping that little had changed on 67P's surface save for Philae's unannounced presence, despite the fact that we are now well aware that the comet's surface is constantly changing as it draws closer to the Sun.

"Although the pre- and post-landing images were taken at different spatial resolutions, local topographic details match well, except for one bright spot present on post-landing images," Philippe Lamy, member of the OSIRIS team at the Laboratoire d'Astrophysique de Marseille, said in a statement. "We suggest [this] is a good candidate for the lander." (Scroll to read on...)

A good candidate, but not definitely the craft. In fact, if you were to ask any of these experts if we will ever know for sure where the historic lander is, they will all tell you the same thing: we'll have to wait and see.

The thing is, even knowing where it is, there is little the Philae and Rosetta teams can do about the craft's current situation. Without enough sunlight, and its preliminary battery dried up, the lander cannot get to work on its intended mission.

Our best bet, Ulmac has argued on several occasions, is to hope that conditions on the comet will change where Philae will start to see more sun. Knowing the lander's location then, would simply help experts be prepared for when (if ever) that happens.

"The conditions for Philae's wake-up are becoming more and more favourable as the comet approaches the Sun," Ulmac said. "The team at DLR's Lander Control Center has continued to prepare long term operations for Philae and its instruments in the hope that it does wake up soon."

It's a display of perseverance that shouldn't exactly come as a surprise anymore. If this past year of space exploration has taught us anything, it's that scientists and dreamers (often one-and-the-same) never give up; and that's the way it should be.

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