Where Is Philae, And When Will It Wake Up?

The hunt for Rosetta's misplaced lander on comet 67P/Churyumov-Gerasimenko is like staring out an airplane window trying to find a washing machine in a field of boulders. These gorgeous new images from the European Space Agency highlight the incredible challenge of finding a tiny robot on a huge comet.

The tumbling descent of Philae to its comet, as seen from the omnipresent Rosetta. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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The more details we get, the more impressed I am by the little Philae lander getting so much science done on comet 67P/Churyumov–Gerasimenko. Newly-released images of its spinning, tumbling descent from Rosetta in November last year make it look more like a snowflake than delicate, high-precision technology.

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Philae's view of its temporary first landing site, Agilkia, is about as informative as you'd expect from a rapidly spinning, bouncing probe. Image credit: ESA/Rosetta/Philae/CIVA

While we already determined Philae bounced its landing, now we're finally getting the images of where the science team suspects it landed. The photo below is Hatmehit, a large depression on the comets small lobe. That red cross on the depression rim marks what the OSIRIS team thinks is Philae. That makes the rim of Hatmehit not just the starting point to try to find the lander in subsequent images, but is also the basis for generating trajectory reconstructions of what happened to Philae once it hit the comet.

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The red crosshair identifies Philae on the rim of the Hatmehit depression. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Unfortunately, the little lander isn't giving away too many clues as to its final location: the view it sent home of its final resting place is of a rock wall now named Perihelion Cliff.

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Philae's view from its final landing location, Abydos, is of the towering Perihelion Cliff. Image credit: ESA/Rosetta/Philae/CIVA

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The search for the lander is painstakingly manual and low-technology. Philae is roughly the size of a washing machine, about 1 meter across. In a series of photographs from 20 to 30 kilometers from the comet's center (or approximately 18 to 28 kilometers above its surface), the little lander would be only a few pixels across. On the up side, the team could (and did) take photographs with filters specifically designed to highlight Philae's solar panels, which are a distinctly different material than the surrounding rocky fields.

The science team is going cross-eyed staring at the suspected landing site trying to find a cluster of pixels that could be the lander, but with so many boulders with the same optical characteristics, that's no easy task. To make it even worse, the hunt is best performed when the lander is illuminated, which only happens for 1.3 hours per comet revolution, and when Rosetta is in position to actually image the area. Alas, that particular combination only came together when Rosetta was in a terminator orbit with its cameras looking into long shadows, which is more ideal for hiding a lander than revealing it.

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Can you find Philae? If the lander is in this mosaic from December 13, 2014 as seen from an orbital distance of 20 kilometers, it's only about 3 pixels across. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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Rosetta has since moved on to a new orbit that is less optimized for lander-hunting, reducing the likelihood of finding misplaced Philae. The spacecraft will downright buzz the comet for Valentine's Day, swooping to within a mere 6 kilometers of the comet's surface on February 14th with the sun at its back to minimize shadows. However, the swoop is planned to maximize spectral imaging and directly sampling the comet's coma, and will be passing over the larger lobe with Philae no where nearby. After that, Rosetta will be orbiting from higher up for a grander view of comet 67P, eliminating the chances of continuing the hunt for Philae until later in the mission.

While this means the chances of pinpointing Philae's exact location are dwindling for now, that doesn't particularly matter in the greater scheme of things. Knowing Philae's location would be nice, but as ESA Rosetta's mission manager Fred Janson explains:

"[T]he location of Philae is not required to be able to operate it, and neither does it need to be awake for us to find it."

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This leads us to the truly iimportant question: will Philae wake up?

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Terrain model of Philae's probable predicament, wedged within a shadowy crevasse. Image credit: ESA/Rosetta/Philae/CNES/FD

At its original, intended landing location, Philae would've been basking in sunlight for 6.5 hours a day. Instead, tucked away in a crevasse under the shadow of cliffs, the lander is only being powered by sunlight for about 1.3 hours a day. But this is changing as the comet grows closer to the sun. If Philae is where we think it is, then by May the sun will be beating straight down on the landing site. Nothing can ever be easy for the lander: its unfortunate robotic yoga has left it at a strange tilt so its solar panels are unlikely to be in position to take full advantage of all this sun.

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If the lander did survive its long, cold sleep, and it does get enough sun to trickle-feed its batteries, Philae could theoretically wake up as early as March. Don't count on it offering a chipper "Hello!" right away, however: like any of us forced to stumble through a coffee-free awakening to confront the morning, the lander stays silent until it works up the power to deal with the rest of us. It will take until at least May or June to work up the necessary 17 watts to power its transmitter and call home to Rosetta. Even then, the orbiting spacecraft will need to be in position to hear the lander's call. Thankfully, that just requires being somewhere overhead — Rosetta is fine-tuned to the cries from its lander from up to 200 kilometers away.

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Comet 67P at a resolution of 2.4 m/pixel. Image credit: ESA/Rosetta/NAVCAM

The European Space Agency's various science teams all working with Rosetta, Philae, and their assorted instruments originally shared these photographs with the hordes of geoscientists at the annual American Geophysical Union meeting, but did not release them to us, the space-greedy public, until after their first round of research papers was published.

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To sooth our wounded curiosity on being a month behind in seeing the gorgeous photos, we're also getting a mosaic image hot off Rosetta's NavCam. Taken on January 21st, this is the comet as seen from an orbital distance of 27.9 kilometers from the comet's center, with the small lobe (and Philae!) in the foreground. Of particular interest in this photograph is a raised plateau the far horizon right of center. The crevices along the edge of this plateau may be indicators of a looming collapse, and a good place to keep an eye on for a dramatic pre/post landslide pair as the comet continues to grow more active with proximity to the sun.

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Read more about the hunt for Philae and the initial science findings about comet 67P on the ESA Rosetta blog.

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