Some space missions go out with a bang, others with a victorious return to Earth, but Rosetta’s final moment was marked simply by radio silence.  Shortly after noon (BST) on Friday, the European Space Agency (ESA) spacecraft finally collided with the duck-shaped comet that it has been pursuing through space for twelve-and-a-half years, switched off its transmitters and hung up the phone to its controllers on Earth.  “This is the culmination of tremendous scientific and technical success for this mission,” said Patrick Martin, Rosetta’s mission manager, after confirming touchdown. “It was historic, it was pioneering and it is revolutionising how we see comets. Farewell Rosetta, you’ve done the job: that was space science at its best.”  As the $1bn craft met the surface, indicated by a green wiggle flatlining on a computer screen at mission headquarters, scientists were left to wonder how the final moments played out.  Rosetta may have come gently to rest in the snow-drift texture of the comet’s surface or bounced back into space, as the Philae lander did when it was dispatched in 2014. Rosetta’s precise fate will never be known, because it was pre-programmed to shut down all communication on contact and there are no telescopes on Earth powerful enough to see it.  “This is space; anything can happen out there,” Professor Mark McCaughrean, a senior science advisor at ESA. “It’s a bit like: does a tree make a sound in the forest if no one is there to hear it?”  The mission’s end met with muted applause and hugs at ESA’s Operations Centre in Darmstadt, where some of the assembled scientists had spent more than 20 years on the project.  Professor Ian Wright, principal investigator on the Ptolemy instrument on the Philae lander, said: “It’s a bit like watching the boxed set of a TV series and knowing you’ve only got one episode to go. Sure, you are going to find out ‘who done it’ but even so, you’ve been enjoying it so much you don’t want it to stop.”  Alan Stern, of the Southwest Research Institute, Texas, who was principal investigator on the Alice instrument, compared the mission to a glorious holiday: “You are sad it is going to end but you are still happy you went on the journey.”  Rosetta touched down on the “head” of the comet at a speed of about 90 centimetres per second - around a walking pace. As it made the descent, the craft captured a series of final images, the last one taken about 10 metres from the surface.  The images show spectacular details of the Ma’at region of the comet, marked by large pits and “goose bump” structures. In the very last images, Rosetta’s cameras are too close to focus, and the approaching grey, gravelly-looking surface can just be made out.  Despite looking like dust-covered rocks, previous measurements show that the comet is extremely porous, with about 70% of its interior being empty space.  “When you see these beautiful images of the comet you should not think of it as rock,” said Björn Davidsson, a Rosetta scientist based at Nasa’s jet propulsion laboratory. “It’s something like spun sugar or cotton candy – something very, very fluffy.”  Rosetta arrived at 67P/Churyumov-Gerasimenko in August 2014, after spending a decade chasing down the comet.  In the two years it spent in orbit, it sent back spectacular photographs of the comet’s dramatic landscapes as well as reams of scientific data measuring its composition, density and magnetic field. Scientists discovered that the comet contains organic compounds that may have seeded life on Earth as well as water and molecular oxygen.  Understanding the composition of comets gives scientists a snapshot of the materials that were blowing around in the clouds of dust and gas from which the planets in our solar system formed around 4.6bn years ago. Comets have essentially been in a deep freeze ever since, making them valuable time capsules.  Monica Grady, a space scientist at the Open University who works on Ptolemy, said: “It was a very difficult and complex mission and the engineers pulled it off almost flawlessly. Lots of kudos to ESA for pulling it off.”  Scientists chose to send Rosetta into a controlled descent because as Comet 67P heads towards the orbit of Jupiter, away from the sun, its solar power supply was destined to run out. Rather than risk putting the probe into a lengthy hibernation until it came closer to the sun again, ESA decided to acquire some last images and data.  “I’ve seen certain rock bands with certain singers that can’t sing anymore. They should have stopped when they were fully functioning,” said project scientist Matt Taylor earlier this week. “And that is what we are doing here with Rosetta. It is maximising what we can do with the spacecraft at this time. This plunge is the only way to get this science.”  After the mission’s end, ESA scientist Laurence O’Rouke, who found the lost Philae lander a few weeks ago, said: “I’m surprised how sad I feel. We were all prepared for this but it is still a bittersweet moment. It does allow us to look back and see who much Rosetta and Philae achieved – and that is overwhelming.”  Taylor said: “Rosetta was rock and roll. It turned everything up to eleven.”
ESA’s Rosetta spacecraft has finally collided with the comet it has been orbiting for the past two years, ending contact with its controllers back on Earth