Goodbye Rosetta, the Comet Orbiter Sacrificing Itself for Science

It's not au revoir, it's goodbye. On Friday, the European Space Agency's Rosetta orbiter will end its mission—by smashing into the comet it's been chasing since it woke up over two and a half years ago.

Rosetta has just hours left before it makes a "controlled descent" to the surface of Comet 67P/Churyumov-Gerasimenko around 1:20PM Central European Summer Time. The descent may be controlled, but the spacecraft was never built to land; it will not be heard from again after the collision.

"As soon as we lose signal, that's it; that's the end of the mission. Probably before that, the tears will start rolling," Rosetta Project Scientist Matt Taylor told me in a phone call.

Rosetta made its final manoeuvre into a collision path on Thursday evening and will hit the comet at a speed around one metre per second, which Taylor characterised as "relative walking pace." As soon as it hits the comet, the antenna won't be pointing at the Earth any more; in the mission control room in Darmstadt, Germany, confirmation will come when communications end.

"I find that quite appropriate; we don't want to have it rolling around in the dust trying to find the Earth," said Taylor.

While it will take years for researchers to study all the data that has come from Rosetta and its comet lander Philae, Taylor said that Friday will be especially tough for those who have dedicated a large part of their life's work to the spacecraft's operations.

"You feel like you shouldn't turn up, you shouldn't go to the end of the mission, you don't really want to go, because it's a sad time," he said.

Before Rosetta's last dive, the spacecraft will take pictures of Comet 67P from right up close and gather observations from a unique vantage point as it glides right through the comet's coma, "tasting" the cometary environment.

"That is really unique, in particular when we get below two kilometres, because we really haven't got that bit before with Rosetta," said Taylor.

ESA is bringing Rosetta down in a region known as Ma'at. This is on the "head" lobe of the rubber ducky-shaped comet. They chose this area because it contains pits that Taylor said could give a kind of "inside view" into the comet. The walls of the pits are also covered in what the team calls "goosebumps" or "dinosaur eggs"—lumpy structures one to three metres in size.

"These features we think are the kind of primordial building blocks of the comet," said Taylor. "So we really want to get some nice high-resolution images of those on the side walls of the pits."

In order to get as much data as possible before Rosetta bids its silent goodbye, the researchers won't be downloading to the spacecraft's solid state memory as they usually would, but livestreaming as much data as possible directly through the spacecraft's antenna.

While the Philae lander has languished on the comet surface for a while now, it won't be reunited with its mothership. Rosetta should end up on the ducky's "temple," about a kilometre away from Philae on its forehead.

Despite not always plain sailing, the Rosetta mission has already provided a wealth of observations for cometary scientists to work with in an attempt to understand more about how our Solar System was formed, and we can expect to see the plucky orbiter's legacy sealed in science papers to come over the next years.

So here's to a good send-off for Rosetta.

"Ultimately, even if something goes wrong to be honest with you, we've done so much with this mission," said Taylor. "I think it will just allow people to get closure."

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