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SYFY WIRE Bad Astronomy

Goodnight, Rosetta

By Phil Plait

On Friday, the Rosetta mission came to a close. At 11:19 UTC, the radio signal received at Earth from the spacecraft was cut off when the orbiter became a lander, slowly impacting and coming to rest on the surface of a comet.

At that moment, it became more than it once was; it became a part of the comet it had been chasing since it was launched on March 2, 2004.

It was August 2014 when Rosetta first approached and made rendezvous with 67P/Churyumov-Gerasimenko, a chunk of ice and rock and gravel and dust orbiting the Sun every 6½. The wonders of the comet had already been streaming in; even from a distance Rosetta found it not to be a single monolithic object, but two distinct lobes connected by a narrow neck.

It was a rubber duckie comet.

But the discoveries came crashing in. The comet had no impact sites, none. The surface must be incredibly young, or else it would have accumulated impact craters over the eons. While expected—comets were known to have volatile surfaces that change over short time periods, even by human standards—the surface was still odd and magnificent, like sculpted half-frozen whipped cream.

Active pits were seen where water ice, heated by the Sun, turned into a gas and blew away. Aeolian features were also photographed, shapes carved and sculpted by the whisper of gas escaping from the comet’s interior. Smooth regions were found where finer grains of material flowed down in the comet’s gravity, despite its pull being so weak you could launch yourself into interplanetary space with a single good jump.

One of my favorite mysteries—why so many objects like asteroids and comets are not single objects but instead double-lobed—may have finally been solved using Rosetta’s observations. The pieces collided slowly, sticking together. Over time, sunlight causes the comet to rotate more rapidly until the pieces fly apart, but their gravity draws them together again, starting the process all over once more. It’s a poetic and lovely dance that may be common in small bodies.

And then there was Philae, the plucky, anthropomorphized lander that failed in its primary mission to land on a comet safely, but still managed to squeak out some science even after bouncing several times on the surface and landing sideways wedged up against a towering rock face. It managed to send back some data despite its predicament, and we learned yet even more about the comet. Where it landed exactly was unknown until just a few weeks ago, when high-resolution images finally were able to show its last resting place.

And now Rosetta has come to join it. The decision to land the orbiter on the comet is not an easy one, but it makes sense. The funding used to run it can be used for other missions, and its success had already led to it being given a lengthy mission extension. Moreover, the comet orbits the Sun on an ellipse, and it’s well away from us now, the signal weaker. Also, from its view the Sun and Earth would be close together in the sky, making communicating with it difficult for many weeks at a time.

There comes a time in every mission when it must be ended, and we move on to what’s next. It can be bittersweet, and this is no exception. Rosetta was a triumph, blazing new ground and overcoming adversity to succeed. It’s a monument to what we can do when we work together and really try.

As a tribute to that, the folks at ESA made this lovely animation to celebrate the last moments of the mission:

Make sure you watch it to the very end; it doesn’t stop when you first think it does.

… and if you’re wondering what the reference there is at the very end, it’s referring to this phenomenal, wondrous video: “Ambition.”

This is the true legacy of Rosetta, the first mission to dare land on a comet and follow it around the Sun. The science it returned was why we sent it. But the inspiration it instills in us will stir generations of explorers to come.

Per inspiratione, ad astra.

For more about Rosetta, Philae, sand 67P, here are some articles I wrote over the years:

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