Rosetta mission: behind the scenes of a historic landing
Philae landing controllers made no secret that, for all their expertise and preparation, luck had its part to play
A sharp turn and the European Space Agency’s $1.5bn Rosetta probe swooped down over comet 67P and from an altitude of 20km released its robotic lander, Philae, on a slow, silent descent to the surface. It had taken mission controllers 10 years and 6bn km to get to this point of cosmic theatre playing, 510m km from Earth, beyond the orbit of Mars. Mission control – watched by the world – held its breath.
The landing craft separated from its mothership at 0835GMT on Wednesday morning. The night before, a fault on the lander had nearly scuppered the plan. An upwards-facing nitrogen thruster, intended to fire on touchdown to prevent the lander from bouncing off the surface failed to respond to commands. Managers decided to gamble. They gave the green light for go.
Fred Jansen, Rosetta mission manager, said: “Twenty years ago we said we wanted to go to a comet that we knew nothing about. There are risks but I think our chances of success are 75 percent.”
The mood at European Space Operations Centre in Darmstadt, Germany, was one of high excitement and no little anxiety. At such a distance, signals take 28 minutes and 20 seconds to travel from Earth to Rosetta and vice versa.
At 0903 the message from Rosetta arrived. The lander had separated. From here on, mission controllers could only watch and wait. “Philae is gone. It is on its path down to the comet,” said Andrea Accomazzo, Rosetta flight operations director. “Philae is on its own now,” said Stephan Ulamec, the lander’s manager.
Shortly after separation, Matt Taylor, Rosetta project scientist, said: “Exploration is about going to the limits. Everyone is nervous, everyone is on tenterhooks. But we know the risk is worth it.”
Minutes after the separation signal arrived Rosetta lost contact with Philae. The interruption in communications, caused by Rosetta’s defensive manoeuvre to a safe distance from the comet, was expected. But even scheduled events can take their toll on fingernails. The team faced a two-hour wait before re-establishing contact with the falling probe. Two hours came and went. Then, contact. At Darmstadt, cheers filled the room. “It’s still alive,” said one of the staffers.
Mission controllers made no secret that, for all their expertise and preparation, luck had its part to play. They had chosen the landing site because it had the best chance of being flat, well lit, to charge Philae’s batteries through its solar arrays, and with a good view of the whole comet. But landing on an active comet had never been done before. The probe could land anywhere in a 1 square kilometre area. “We’ll need some luck not to land on a boulder or a steep slope,” said Ulamec, as Philae continued its descent.
The discoverers of comet 67P, Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, were mingling with the other scientists and media at the ESA’s Darmstadt centre. Pondering the comet, Gerasimenko said: “It reminds me of a boot.”
With contact re-established, the first instrument data and images from Philae’s cameras began to stream back to Rosetta which relayed them on to Earth. The lander’s suite of microcameras had taken a parting shot of Rosetta moments after separation.
Looking back towards the mothership, one of Rosetta’s 14m-long solar arrays was clearly visible in an image bearing a bright flare from the sun. Another image, taken from Rosetta’s Osiris camera, showed a gleaming white Philae, with landing legs unfurled, falling into the darkness.
ESA managers expected to hear if Philae touched down safely during the hour after 1530GMT. This was the moment: the culmination of years of work for engineers and scientists on the lander. Many had had far too little sleep. “I’m knackered but I’m also very, very excited. We’re close now,” said Taylor.
The historic moment in the Darmstadt control centre was captured by ESA’s TV cameras. At 1603 they received a signal from Philae. A touchdown signal. But the jubilation was not immediate. Brows furrowed, whispers were exchanged. Then came the announcement. “We are on the comet!” said Ulamec. “We see the lander sitting on the rock,” said Accomazzo.
Then, confusion. At first, ESA reported that Philae had fired its twin harpoons to secure it to the comet surface, and rewound the tethers attached to them. Then it became clear the harpoons had not fired. With nothing to secure the lander to the comet, its chances of remaining there, let alone doing useful science, were unknown.
As celebrations broke out in Darmstadt, ESA staff at the Lander Control Centre in Cologne had also spotted that something was wrong. Data beamed back from Philae showed the lander was still moving. Specifically, it was rotating. Impossible on the ground, it meant the spacecraft had touched down and bounced back off again. Finally, the craft seemed to come to a standstill. Thirty minutes later, Rosetta lost contact, as the comet spun and Philae disappeared over the horizon.
On Thursday morning, scientists re-established contact with Philae around 0600GMT. “The emotions all came back again when I heard we had telemetry,” said Jansen. “I went into the room where the instrument principal investigators are and I saw the images on Jean-Pierre’s screen and everyone was just: ‘yes!’.”
Data collected from the lander overnight began to arrive. Poring over the numbers, a picture of what happened emerged. Philae touched down within 50 metres of its target on a site that would have proved perfect for its future operations. The landing gear flexed, the signal to fire the harpoons was given, but for some unknown reason the anchors did not work. The gravity of the comet was so weak that Philae bounced off. Having touched down at 1533 it soared 1km off the surface and came back down nearly two hours later, at 1726. It then bounced again, more modestly, and came to rest at 1733.
That Philae had come to a rest was the good news. But there was bad news too. In the time it took Philae to land for good, the comet had rotated. Instead of settling on flat, exposed terrain with plenty of sunlight, the lander came to rest on more hostile ground. At around 1640, ESA released a panoramic image from Philae that revealed the lander’s precarious position. Closed in by rock and with one leg in the sky, most of the craft is in darkness.
But science is being done on the comet. Data is coming back. And nothing has dented the elation of the team that placed the lander on the comet. “What’s really impressive here is not the degree of failure, but the degree of success,” said Jean Pierre Bibring, Rosetta’s lead scientist. It is amazing where we are. We are at the limit of what humankind could do.”
