September 30. That’s the date the European Space Agency is going to deliberately crash the Rosetta spacecraft on Comet 67P/Churyumov-Gerasimenko. This won’t be a violent crash, though. Think of it more like a controlled descent.

The plan is for Rosetta to hit the comet’s surface at about 50 centimeters (20 inches) per second. About half the speed the Philae lander hit back in 2014 before bouncing a couple of times.

Sylvain Lodiot, the operations manager for the Rosetta spacecraft, says planning Rosetta’s slow crash is much more difficult than it was for Philae. With Philae, it was more of a detach and wait from the Rosetta spacecraft. Rosetta’s team needs to tweak the spacecraft’s orbit to get it in the perfect position for a slow descent.

That means the last six weeks before September 30th will be especially tricky. “The last six weeks will be particularly challenging as we fly eccentric orbits around the comet – in many ways this will be even riskier than the final descent itself,” Lodiot says.

The tricky part comes from Comet 67P’s shape. Its unique shape means the gravity around it isn’t uniform. Because of this, Rosetta mission controllers will need to be on their toes for trajectory corrections as they get Rosetta into position for its final day.

Rosetta’s final resting spot hasn’t been selected by the mission team just yet. They are looking at several regions of the comet to crash on and are weighing the trade-offs and going over the trajectories needed to get to each.

Why crash Rosetta at all?

Back in 2011, the spacecraft entered a 31-month hibernation period to conserve power. Why not do it again? It’s all about location. Right now, Comet 67P (and Rosetta) is soaring away from the sun.

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At its maximum distance, the comet will be over 850 million kilometers away from the sun. That’s farther than Rosetta has ever been before. And the vast distances means there’s not enough solar power to make sure Rosetta’s heaters could keep the spacecraft warm enough to survive the frigid temperatures of space.

Two years ago, Rosetta’s team made the decision to get what data they could from a crash landing versus hoping for the unlikely outcome the spacecraft would survive another deep hibernation.

Rosetta’s grand finale

12 hours before impact, the spacecraft will make one final course change. If everything goes as planned, Rosetta will strike the surface of Comet 67P at about 20 inches per second.

These final hours will yield “once-in-a-lifetime measurements” according to the ESA. We’ve seen incredible close-ups of Comet 67P before. But they will pale in comparison to the images Rosetta will capture in its final moments.

Like many missions before it, Rosetta’s won’t end on September 30.

“30 September will mark the end of spacecraft operations, but the beginning of the phase where the full focus of the teams will be on science. That is what the Rosetta mission was launched for and we have years of work ahead of us, thoroughly analyzing its data,” says ESA Rosetta project scientist Matt Taylor.

Rosetta’s watch of Comet 67P will soon be over, but the answers to the comet’s mysteries will be revealed in the months and years ahead.

I’ll leave you with three stunning images of the comet released over the past three weeks.

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