That Time We Tried to Harpoon a Comet: Remembering Rosetta and Philae

November 12^th marks the anniversary of the 2014 attempt by the European Space Agency’s Philae lander to land on the surface of the comet 67P/Churyumov-Gerasimenko as part of the ongoing mission of the Rosetta spacecraft, which was in orbit around the comet. The landing…didn’t go as planned. Philae was supposed to fire landing harpoons to try and bring it to a gentle stop on the surface of the comet, but they didn’t fire.

So on the surface what I’m about to write about is a failed aspect (Philae) of an otherwise very successful space science mission (Rosetta). But I want you to think about what I just said: we humans once not only put a spacecraft into close orbit around an active comet but, not content with that feat, we attempted to land on the surface of the comet by harpooning it.

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I never felt like the Rosetta/Philae mission got the respect it deserved for being such an absolutely bonkers idea that we actually made happen. And it was even mostly successful! Rosetta is long dead and Philae even longer dead (more on that later) but I feel like this anniversary is as good a reason as any to write an appreciation post for the sheer chutzpah this mission required.

Not to mention I get to use some of the most desolately lovely images ever taken by any spacecraft ever. That alone is worth the trip down memory lane.

The Rosetta Stone of Space

Even the mission’s name showed the incredible idea that lay behind it. The spacecraft Rosetta was named for the Rosetta Stone, a large hunk of rock with an inscription carved into it in three languages, including ancient Egyptian hieroglyphs. In the 1800s the Stone became the key piece of the puzzle when it came to first deciphering that ancient script. The Philae lander was named for an obelisk that similarly has the same inscription on it in ancient Egyptian and Greek.

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Just as these namesakes were key to unlocking the language of ancient Egypt, the Rosetta/Philae space mission was designed to unlock the ancient secrets of the solar system through an up-close study of an ancient stony fragment, in this case the comet 67P/Churyumov-Gerasimenko. In the Planetarium we always nicknamed this comet ChuGer just for ease, so I’m just gonna call it that from here on.

ChuGer was found in 1969 by Soviet astronomers Klim Churyumov and Svetlana Gerasimenko. It has an elliptical orbit around the Sun that carries it from a little inside the orbit of Mars at its closest to just barely outside the orbit of Jupiter at its farthest. It only takes about 6.4 years to complete each orbit making it a very short period comet.

So if you wanted to catch a comet by the tail, this one would be a good one to grab. Not much else was known about it. In fact, if you look at the best model we had of this comet prior to Rosetta’s launch, it looks like an amorphous gray blob.

Rosetta

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Rosetta launched on March 2, 2004 with Philae tucked into a compartment inside of it. Looks are not generally a factor when designing a spacecraft, but I’ve always found Rosetta’s design to be an elegant one. The central spacecraft was fairly small (less than 10 ft/3 m across) and its solar arrays were very long, each one stretching 46 feet/ 14 m out from the central bus. That made the spacecraft resemble a particularly delicate bird with long slender wings. This loveliness may have been accidental but there’s no harm in appreciating it when it’s there!

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Since the most efficient way to get to the outer solar system is rarely the most direct, Philae took over ten years to reach its target, slipping into orbit around ChuGer in August of 2014. And up close the comet looked like a completely different world than the models suggested. 

It wouldn’t be any exaggeration, I think, to say that ChuGer looks very much like what its, a cold fragment left over from the early solar system, its eons of existence visible in the pockmarked surface slowly weathering away as it continues to drift through the void. It’s also not much of an exaggeration to say it looks like a demented duck with a particularly vicious skin condition, depending on what kind of mood you’re in.

Point is we didn’t know ANY of that until Rosetta got close to it. Our pre-launch gray-blob models of this icy chunk of space debris didn’t account for the demented duck shape AT ALL. I remember how fun it was watching the comet come into sharper and sharper focus as Rosetta approached, a feeling that would only be equaled when New Horizons did the same thing to Pluto a  year later. It was a good time for space nerds. 

Philae

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While Rosetta resembled an elegant bird, Philae looked like an 8-bit animation of a virus, but a virus with an ambitious task before it: become the first spacecraft to touch down on a comet. Just over three months after entering obit around ChuGer, Rosetta released Philae into space, and after a seven hour descent the European Space Agency jubilantly announced to the world that they had, in fact, achieved the first soft touchdown on a comet.

And technically that was correct. Philae was transmitting a signal from the surface of ChuGer. But the pictures it was transmitting were…odd, and seemed to indicate that the spacecraft was resting at an awkward sideways angle. It turns out that the landing had gone quite awry and the landing harpoons designed to pull Philae to a firm anchorage on the surface failed to fire.

As a result Philae wound up touching down not once but three times, experiencing two giant bounces back towards space as a result of ChuGer’s small gravitational pull before coming to rest against a cliff, stuck in shadow where its solar panels could not get the light needed to keep the little bug going. Nevertheless, Philae continued operating for 57 hours before it went silent for good, and while it never actually got to harpoon a comet, it does hold the record for being the first thing to achieve a soft touchdown on one.

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Coming Alive

After Philae’s demise, Rosetta continued to orbit ChuGer. All told, it spent over two years circling the comet. That two years coincided with the comet’s closest approach to the Sun, which happened in August 2015. That meant Rosetta had a front row seat to watching ChuGer come to life, shooting off jets and suffering the sorts of outbursts from its surface that is just part of being a comet getting closer to the Sun.

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While this resulted in some truly brilliant images, it also meant Rosetta was taking unusual risks for a spacecraft, spending all its time in close proximity (about 18 miles above the comet’s surface) to something that was constantly exploding a little bit. Debris was flying everywhere.

Fortunately Rosetta got through the riskiest part of ChuGer’s orbit around the Sun with nary a scratch and with an absolute tanker-load of data, much of which is still being combed through today. Then, of course, ChuGer began to move outwards from the Sun, getting farther from Rosetta’s power source every day. And that meant mission engineers had a decision to make.

Going For It

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While it is possible for solar-powered spacecraft to survive with the power levels they receive out by Jupiter’s orbit (look at Juno, after all), they have to be specifically designed for it (read: absolutely covered in solar panels) and Rosetta wasn’t. It didn’t have the power to continue operations through aphelion, ChuGer’s farthest distance from the Sun.

It was, engineers concluded, potentially possible that Rosetta could be put into hibernation through this phase of the orbit, potentially possible that it could eke out enough power to keep its heaters running, making it potentially possible that Rosetta could wake back up when the comet started moving back towards the Sun. Potentially. Possibly.

ESA engineers looked at the odds, and decided Rosetta would get a blaze of glory instead. In September 2016, with power supplies lessening every day, Rosetta began to descend. It spent 14.5 hours moving closer and closer to ChuGer’s surface, taking and transmitting data at every moment right up until the instant it made contact. The last image it transmitted was taken from less than 70 feet (21 m) above the surface.

Dreaming Big

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When you think about it, putting a spacecraft into orbit around an active comet was an incredibly high-risk idea. There’s a reason no one had done it before. But Rosetta did it. The concept of landing on a spitting comet was insane. It’s all spiky parts and low gravity, after all. But Philae did it. The safe move when the sunlight levels got too low was absolutely to let the spacecraft go gently to sleep, with the extra benefit of hoping it might (potentially, possibly) wake up later. The idea of kamikaze diving the comet was nuts. But the ESA did it.

This mission was the very definition of dreaming big, the kind of beautiful interweaving of science and engineering to do something that nobody had ever dared to try before. And it paid off: Rosetta was able to find evidence that gases in Earth’s atmosphere may have originally been delivered by comets like ChuGer, to discover that it’s possible for comets to form their own kind of faint aurora, and to watch the comet’s surface change as it became active.

And Rosetta’s descendants continue to multiply. The Hera, JUICE, and BepiColombo missions, all currently in flight to their destinations, all were built using lessons learned from Rosetta and Philae. A more direct descendant, Comet Interceptor, is being built right now. Because that’s the beauty of these science missions—even when the spacecraft are gone, they leave their data and lessons behind. It’s possible to keep learning from them forever—as long as you have the ambition to send them out in the first place.