Rosetta: The whole story
It was one of the most audacious space missions ever. How did it feel to be part of the Rosetta team as it touched down on a comet? As the spacecraft nears its closest approach to the Sun, Richard Hollingham recounts the drama, the disappointments and the jubilation of a 30-year quest.
14 November 2014: 23.49 GMT
…Philae control room, Cologne, Germany, 56 hours and 15 minutes after first touchdown on Comet 67P/Churyumov–Gerasimenko…
All eyes are on the main screen where a graph displays the lander’s battery power trickling down towards zero. It is the last flurry of activity from the first human object to have landed on a comet. It is a mission that has taken nearly 30 years.
“It was a very special moment,” says Philae’s technical manager Koen Geurts, who was sitting in front of the power console. “One science team after another cheered as their results came in.”
…With energy almost spent, Philae sends back the last of its data…
"We were doing science up to the very last second," says Geurts.
…Philae shuts down…
The European Space Agency (Esa) Rosetta mission to orbit and land on a comet has been celebrated around the world as a remarkable achievement.
But the journey to reach 67P/Churyumov–Gerasimenko has taken 30 years and faced tremendous technical, scientific and emotional challenges.
It is now six months after the Philae landing. With the comet getting warmer and more active as it heads ever closer to the Sun, it is possible that the robot lander may reactivate and shed more light on the comet’s internal workings.
Rosetta and Philae’s far-reaching story includes an exploding rocket and hasty change of destination; asteroid encounters, the perils of a three-year hibernation and media pressures. They have suffered computer glitches and faulty thrusters. A failure that almost caused the landing to be postponed may even have saved it.
“The Rosetta mission is never going to be nominal,” says project scientist Matt Taylor. “Apparently it’s quite difficult to fly around a comet.”
It starts with a comet. The most famous comet of all: Halley’s Comet due to make its second appearance of the 20th Century.
This time a space probe would be sent 150 million kilometres to study it: Esa’s first deep space mission, Giotto.
But as Giotto was being prepared for launch, a small group of space scientists gathered to discuss an even more ambitious mission.
“The 23rd of May 1985 was Rosetta’s date of birth,” says Gerhard Schwehm, who was at the meeting.
Named after the Rosetta stone, the famous Ancient Egyptian relic that enabled scholars to first decipher hieroglyphics, the original mission concept was very different to the orbiter and lander that was eventually launched.
A massive (and massively expensive) retro-looking spacecraft, the original Rosetta was a joint Nasa and Esa project designed to land on a comet and return a 10kg sample back to Earth.
Then, on 28 January 1986, on the day of the first joint Esa-Nasa meeting to discuss the comet mission, the Space Shuttle Challenger exploded, killing seven astronauts.
“I believe that this was one of the events that changed Nasa’s whole strategy and they had to rethink their whole programme,” Schwehm says.
The sample return mission was designed as a smash-and-grab raid – getting on and off a comet as quickly as possible – but Schwehm had another idea.
“What if we could leave a monitoring station on the comet?” he suggested at a planning meeting.
“That is the idea we went with – if you can’t bring the sample back to our laboratories, then bring our laboratories to the comet.”
And so the idea of the Philae lander was born. By the end of the 1980s it had also become clear that a sample return mission was neither feasible nor affordable.
The mission started in earnest in 1992.
Even in its revised form, with a lander and orbiter packed with scientific instruments, it was going to be a complex and challenging undertaking.
“If you’d asked the scientists back in the 1990s what were the chances of a successful Rosetta mission, you would have got some pretty negative answers,” says Trevor Morley, a specialist in interplanetary navigation with Esa’s flight dynamics team.
“The flight to the comet was complex,” admits spacecraft operations manager Andrea Accomazzo, who has worked on the project since 1996.
With a comet shortlist drawn up, Rosetta’s flight was going to take almost a decade and involve travelling some 800 million kilometres from the Sun to the orbit of Jupiter.
The space probe had to meet the comet at the right place, right time and right speed.
This posed serious problems for the spacecraft design. Previous deep space missions, including Voyager and Cassini-Huygens, had relied on American “nuclear battery” technology for power. But with Nasa suddenly out of the picture, this was no longer an option. It was back to the drawing board.
Instead, Esa, its national space agency partners and specialist contractors across Europe had to develop powerful new solar arrays that would work in the shadowy depths of our Solar System. They also had to ensure that the spacecraft got neither too cold out near Jupiter or too hot as it approached the Sun.
In 2002, with construction and testing complete and Nasa back on board contributing three instruments, the three-tonne Rosetta with its fridge-sized Philae lander was shipped to the European spaceport at Kourou in French Guiana and the target plotted: Comet 46P/Wirtanen.
The two-week launch window opened on 13 January 2003.
Esa had even commissioned the famous Dutch pottery maker Delft to produce souvenir blue-and-white plates featuring the destination comet.
11 December 2002
Expectation, however, quickly turned to despair. “I believe I was one of the first to realise that something had gone wrong,” says Schwehm.
In the run-up to the Christmas holidays, Schwehm was at the European Space Research and Technology Centre (Estec) in the Netherlands, watching the launch coverage of a new version of the Ariane 5 rocket.
Rosetta was due to launch on the next Ariane 5 just a few weeks later.
Three minutes after lift-off, the rocket failed, taking two communications satellites with it.
“I don’t want to repeat the words I used when I saw it,” says Schwehm.
Christmas 2002, Schwehm recalls, was frantic.
“Everybody was on holiday, scattered across Europe,” he says. “I was like a telephone switchboard for the mission as we discussed what to do.”
At first the team investigated trying to switch to an alternative launcher. But there was only one other heavy rocket capable of carrying Rosetta: the Russian Proton.
Although the Russians could make a Proton available, Rosetta was filled with highly corrosive propellant and fears of a leak meant flying it half way across the world in a cargo plane was considered far too dangerous.
The only option was to wait until Ariane 5 was cleared to fly. This meant having to go back to square one, choosing a different target comet and plotting a new course to reach it.
“It was a big disappointment,” says Accomazzo. “Our colleagues in mission analysis had to go back to studies from the end of the 1990s and the best candidate was 67P/Churyumov–Gerasimenko.”
“However, within a short time we were ready with a new mission,” he says. “This was fantastic.”
Launch and journey
Rosetta launched on 2 March 2004, powering into the night sky over French Guiana.
“Launch is like birth,” says Schwehm. “Every launch is risky, it’s the most important point for a spacecraft – it was a big relief when it went beautifully into orbit and then onto its interplanetary path.”
It was the start of a long and perilous journey that would span 6.4 billion kilometres.
To save fuel, the spacecraft would use the gravitational attraction of Earth and Mars to increase or decrease velocity relative to the Sun. As well as these gravity assist or “sling-shot” manoeuvres, the journey would also take in two asteroids and three years of deep-space hibernation.
“Rosetta started off with a huge amount of propellant,” says Morley.
Rosetta’s first trajectory took it accelerating out of Earth’s orbit towards Mars.
“At that time, Mars was 300 million kilometres from the Earth and we swung by the planet with an error of less than a kilometre,” says Morley.
Later in the year, Rosetta returned to the Earth picking up speed to take it onto the main asteroid belt between Mars and Jupiter.
This region was a valuable scientific destination in its own right and enabled flight controllers to put the spacecraft through its paces by rendezvousing with asteroid Steins, a 6.5km-long space boulder pockmarked with craters.
“The asteroid fly-bys were triggered by science but they were very interesting to do from a technical point of view,” says Accomazzo.
Another pass by Earth in 2009 and asteroid Lutetia in 2010, sent the spacecraft off towards Jupiter. Because the solar arrays would not be able to generate enough power at this distance, it was time to put the spacecraft to sleep.
“The hibernation was the toughest part,” says Accomazzo. “The risk was significant, much higher than any other mission. But we had no other option.”
Rosetta then spent the next 31 months in hibernation…
“There was nervousness and apprehension and that was increasing in the hour when we were expecting to get the signal,” says Rosetta project scientist Matt Taylor, who joined the mission in summer 2013 and was watching Rosetta wake up at mission control in the European Space Operations Centre (Esoc) at Darmstadt in Germany.
“We began the hour joking and laughing, but there’d been a glitch," says Taylor. “One of the people in the room who was nervous was the guy who wrote the software for the spacecraft to come out of hibernation.”
“Then you can hear murmurs from certain people,” says Taylor. “But ultimately we got the spike on the screen to say it was okay.”
From mid-March onwards, with instruments activated, Rosetta began to get its first glimpses of Comet 67P/Churyumov–Gerasimenko.
It became clear that something was strange. As the images resolved, the comet turned out to look nothing like the potato-shaped souvenir model comets on the scientists’ desks.
Accomazzo suggested that it looked more like his son’s plastic bath duck.
As rendezvous got closer and closer, the team at Esoc took centre stage.
“Most people believe the landing was the most difficult but first we had to approach the comet,” says Accomazzo.
“We didn’t know anything about the body – its mass, shape, all those variables – the real masterpiece was to measure the comet’s properties to a level where we could learn to fly round it.”
On 6 August, Rosetta was within 100 kilometres of the comet and flying in formation at 55,000km/h (34,000 mph).
For a successful landing the mission called for a smooth landing site some 500 metres across, considerably larger than a football field. There was no such place.
The discussion to select a landing site involved dozens of meetings and hundreds of people, although the ultimate decision came down to Jansen.
“It was ‘why can’t we do this’, ‘why can’t we do that’…,” says Taylor. “There was not a fantastic landing site, we went for the least worst we had.”
“Once we decided, it was clear that this was the best landing site,” says Accomazzo.
“Then the lander decided to go somewhere else.”
11 November 2014
Engineers in the control room at the German space agency, DLR, in Cologne had switched Philae on 78 times since launch, to check systems, update software and monitor performance.
Then they came to power the lander up for the final time.
“We had a problem at boot,” says Geurts, the mission’s technical manager. “We still don’t know why but it’s not something you want when you’re starting the most important operational procedure in 12 years.
“This was causing a headache immediately,” he says.
They switched it off. Then they switched it back on again.
Philae sparked into life.
But there were more problems.
As the lander powered up, they discovered that a thruster on the top of the box-like robotic explorer had failed. This small propulsion unit was designed to press Philae down into the surface when it landed, holding it in place while harpoon anchors fired.
Back at Esoc in Darmstadt, journalists from around the world had gathered. The surrounding streets were jammed with TV satellite trucks for live coverage of the landing. One German network had even cleared the schedules to follow events.
Esa had promoted the landing with a series of animated films in the style of a children’s story and even commissioned a short movie, Ambition.
The media pressure was intense.
It was Jansen’s call whether to go ahead.
“I wouldn’t be good at my job if I couldn’t make difficult calls,” he says. “But on the evening before, there were a few curses.”
“Fred had the weight of the world on him,” says Taylor. “We had a meeting at around 10 that evening, all the team leaders, what are the criteria for a go/no-go scenario? We’ll try this, this and this and if they’re all go, we’ll go ahead.”
With commands uploaded to the spacecraft, all they could do was wait until it reported back some four hours later.
“We went into one of the hotel bars, but that was full of journalists, so we had to go to the empty smoking lounge to have a drink and chat about what had happened,” Taylor says.
At 3 o’clock in the morning, Taylor was woken by a text.
The landing was on.
At 08:35 GMT Philae was released from Rosetta to descend towards the comet.
“During the descent the lander was very quiet,” says Accomazzo, who was sitting at the main mission control console in Darmstadt. “I was under a lot of pressure to announce the landing.”
“When it came to the point where we thought we had landed – because we saw the readings change at exactly the expected time – I said [with a smile] ‘I believe we have landed’.”
Although Philae’s scientific instruments had activated on cue, the telemetry coming back from the spacecraft was not stable. In the smaller Philae control room at DLR in Cologne, there was less jubilation.
“We had the anchor experts sitting next to us – making concerned noises – and saying it was not exactly what we would expect,” says Geurts.
“Of more concern, we were still seeing a rotation of the spacecraft – this meant we were still somewhere in free space.”
No-one was entirely sure what was going on. Careful analysis over the next two hours confirmed that the harpoons designed to anchor the lander had failed to fire and Philae had bounced a kilometre back out into space.
But Geurts believes the earlier thruster failure, the one that everyone was so worried about, may have prevented it travelling too far.
“We’ve done some assessments and believe the thruster would have loaded the landing mechanism like a spring, pushing it down to jump even higher… to never land again.”
At 17.32 GMT the lander’s rotation suddenly stopped.
“Nobody knew how to feel,” says Geurts. “You were happy on one hand but so many questions on the other.”
“The big question was power,” says Accomazzo. “We had limited energy stored in the battery, which is not rechargeable.”
Mission controllers at Esoc and Cologne set to work reorganising the sequence of scientific experiments. They postponed using the drill, for fear it could tip the lander over.
“It became pretty obvious that the lander was somewhere in a cave, hole or crevasse,” says Accomazzo.
“Only a couple of the solar arrays were seeing the Sun and only for a very short time, this immediately put a lot of time pressure to complete as much as possible before the batteries ran out.”
In Cologne, the Philae science and engineering teams were working around the clock. They had around 54 hours of power left.
“The scientists were pushing us to the limit and we managed to operate every instrument – even drilling,” says Geurts. “We decided to take more risks.”
“By this time, all the media had gone, just the engineers and scientists here in Cologne witnessing how the battery was dying.”
“When every team got their data back we had beer and champagne,” he says.
“That was really the moment we were happy.”
“I didn’t realise it was going to be this intense,” says Taylor.
He looks more relaxed than I’ve seen him in months, sat at his desk at Estec surrounded by books and scientific papers. On his windowsill is a row of plastic bath ducks, alongside a model of the comet. The wall is covered with diagrams of the spacecraft and drawings from his children.
“Family life, personal life, health is destroyed in some ways but my family knows it’s not going to last forever,” he says. “My kids have become engaged in what I do,” Taylor says. “For once they’ve started to say, ‘Dad’s quite cool!’”
In the office across the corridor, Jansen has stepped down from his role as mission manager.
“It has been too intense,” says Jansen. “There was no break, there were no weekends, you were working all the time.
“The last thing I did late at night was answer emails and you start again at six in the morning doing the same thing,” he adds.
Nevertheless both agree, without hesitation, that the mission has been worth the effort.
Over the past few months the first science results have started to come in, including studies of the comet’s water, magnetic field, dust and chemical composition. Comets are believed to be the oldest objects in the Solar System and 67P/Churyumov–Gerasimenko provides scientists with a time machine into the past – before the Sun and planets were fully formed.
But it will take years to analyse the vast amount of data returned by Philae and still being generated by Rosetta.
As it approached the Sun, the comet’s surface began to warm and an increasing amount of gas and dust was vented into space. This caused problems for Rosetta and its support team on Earth. The orbiter recently went into a "safe mode" when its navigation system became confused by the dust.
On 14 June 2015, it was announced that Philae, which had lain dormant for months, sparked back into life. The mission’s control centre received its first message from the lander since its batteries ran dry last November. Philae is expected to send large amounts of data it had collected before going silent last year, giving the scientists back on Earth more insight into the comet.
“People ask me what the cost is of Rosetta and I say one billion euros,” says Accomazzo, “and they say ‘wow!’ but Ronaldo cost 100 million euros, Real Madrid costs much more than Rosetta every season – it’s definitely worth flying something like this.”
The final word goes to the man who started it all, Gerhard Schwehm. Now retired, the former head of the mission spends much of his time compiling a history of the project.
Find out more
As it happened: Rosetta comet landing
Philae: The tense final moments around comet touchdown
Rosetta: The wild wild world of comet chasing
Are these the five craziest space missions?
Writer: Richard Hollingham
Editor: Stephen Dowling
Art Editor: Olivia Howitt
Production Editor: Howard Timberlake
Video Editor: Paul Ireson
Rosetta journey illustrations: Gavin Cammell and Rob Turvey
Rosetta journey animation: Pierangelo Pirak
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