This series of images of Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 12 August 2015, just a few hours before the comet reached the closest point to the Sun along its 6.5-year orbit, or perihelion.

The images were taken from a distance of about 330 km from the comet. The comet’s activity, at its peak intensity around perihelion and in the weeks that follow, is clearly visible in these spectacular images. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Ode to Philae: A Brief Mission That Made a Big Impact
published during a waxing gibbous moon.
02/16/2016
Philae

Philae. Credit: ESA

The world fell in love with animated versions of Rosetta and Philae through a clever marketing campaign that included videos, photos and even communication between the two probes via Twitter. The anthropomorphized robotic duo won our hearts as we watched Rosetta’s “little brother” Philae ride on her back while the pair raced for their target: a duck-shaped ball of ice and rock known as comet 67P/Churymouv-Gerasimenko (comet 67P for short).

We watched Rosetta and Philae grow up to become intrepid space explorers through a series of videos and family photo albums. They come from a long line of space-faring robots as their “grandfather” Giotto explored Halley’s comet during ESA’s first deep space mission, and their cousin “Stardust” was the first to return a sample to Earth.

The pair launched in 2004, well before the era of social media dominance, embarking on a journey that would go down in history. Nearly a decade later as the ambitious mission kicked into high gear, scientists are confident it will help unlock the secrets of comets and help us understand their role in the development of our Solar System.

On November 12, 2014, the plucky little lander detached from Rosetta and embarked on a perilous 7-hour journey to be the first to successfully land on a comet. ESA enthralled us with an animated video showing Philae packing a lunch, and putting on boots and waving goodbye to Rosetta before his upcoming free fall:

People all over the world huddled around their laptops, waiting on news of Philae’s fate. Many even changed their profile pictures on social media to show support for the lander. When the signal arrived, the world breathed a unanimous sigh of relief. However, not long after scientists knew Philae survived the descent, they noticed something wasn’t quite right.

Philae

Series of 19 images captured by Rosetta’s OSIRIS camera as the Philae lander descended to the surface of Comet 67P/Churyumov–Gerasimenko on 12 November 2014. The timestamp marked on the images are in GMT (onboard spacecraft time). Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The signal coming in from the lander was wonky, telling the team that Philae touched down on the surface, but it’s anchoring system failed. Without its harpoons to hold it in place, the washing machine-sized lander was sent bouncing across the comet’s surface. Subsequent analysis showed that Philae bounced three times before landing in what would be its final resting spot.

Equipped with two batteries, Philae could power itself for less than three days before it would need recharging from a set of solar panels covering its exterior. Philae’s targeted landing spot, dubbed Agilkia, was targeted for its exposure to sunlight. However, the science team quickly determined that Philae had missed its targeted landing spot and ended up in the shadows. Researchers knew Philae’s days were numbered and were determined to make the most of what little time they had. For nearly 60 hours after touching down on comet 67P, Philae’s suite of 10 instruments were turned on to collect as much data as possible.

As soon as the data was downloaded, Philae’s ground team decided to rotate the lander, in hopes of positioning its largest solar panel towards the Sun. This maneuver would not prolong Philae’s life, but would give it the best chance to power back up in the future. Philae then entered into hibernation mode until its solar panels could collect enough energy for the lander to power back up. Researchers thought they had heard from the little lander for the last time on November 14.

ESA released a video showing Philae growing very tired and settling in for a long nap:

Now the world would wait and hope that one day, the beloved little lander would turn back on. Researchers planned to start listening for Philae in March of 2015, hoping that as comet 67P steadily approached the Sun, Philae’s solar panel would be hard at work collecting much needed energy. Philae’s interior needed to be at least -45 degrees Celsius (-49 degrees Fahrenheit) before it could power back up, and in the frigid blackness of space, that is no simple task.

Philae

This series of images of Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 12 August 2015, just a few hours before the comet reached the closest point to the Sun along its 6.5-year orbit, or perihelion. The images were taken from a distance of about 330 km from the comet. The comet’s activity, at its peak intensity around perihelion and in the weeks that follow, is clearly visible in these spectacular images. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Months went by with no word from Philae, when suddenly on June 13th, the world rejoiced as ESA announced it received a signal from the plucky little lander. “Philae is doing very well: It has an operating temperature of -35ºC and has 24 Watts available,” explains DLR Philae Project Manager Dr. Stephan Ulamec. “The lander is ready for operations.”

Further analysis showed Philae had awoken prior to June 13, but was unable to contact Rosetta or Earth. Philae “spoke” with the team for 85 seconds as scientists were feverishly downloading as much data as possible during the brief encounter. Philae remained in spotty communication with the team through July 9th. It’s possible that comet outgassing could have shifted Philae’s position, interrupting its signal.

“When you see what’s happening on the surface of the comet as it gets closer to perihelion — jets going off, and God knows what else — it wouldn’t take a lot of that going off close to the lander to move it around a bit,” said Ian Wright of the U.K.’s The Open University and principal investigator of Philae’s Ptolemy instrument.

“Philae is obviously still functional, because it sends us data, even if it does so at irregular intervals and at surprising times,” Philae project manager Stephan Ulamec, of the German Aerospace Center (DLR), said in a statement. “Several times, we were afraid that the lander would remain off — but it has repeatedly taught us otherwise.”

The team hoped that if a jet did knock Philae’s antenna out of alignment or moved the lander, that a subsequent jet could put it back in a more favorable condition. Scientists had hoped that Philae would provide a front row view of what happens to a comet as it reaches perihelion — the point in its orbit closest to the Sun — but have not heard from the lander since last summer. At this point in the mission, comet 67P is on its way back out into the Solar System and will not receive the heat needed to power Philae back up.

So, we must say farewell to one of our favorite robotic explorers. Philae will rest on its comet perch for as long as comet 67P remains in tact. Farewell Philae, your mission was brief, but you will always hold a place in our hearts.