Asleep in the dark near Jupiter, a spacecraft is almost ready to make the first ever landing on a comet – if we can wake it. Stuart Clark joins mission control
YOU know that anxious feeling the night before an exam or a job interview, when the alarm clock absolutely, positively has to work. Take that feeling and double it. Multiply it by a large factor, add the number you first thought of… you get the picture. That’s how restless mission controllers at the are feeling right now.
For if there is one alarm clock in the whole solar system that has to work, it is the one on their . Set for 1000 GMT on 20 January, it will, if all goes to plan, wake Rosetta from a three-year slumber travelling through the deepest reaches of space. Bleary-eyed and disoriented, the craft will keep mission controllers on tenterhooks for a few hours more until they hear a faint beep – just enough to tell them that the €1 billion, 3-tonne spacecraft is alive and well.
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Launched in 2004, Rosetta’s mission is to give us our first up close and personal look at a comet. By August 2014, it will have caught up with comet and will hang on to its coat-tails all the way from the frigid outer reaches of the solar system, near Jupiter, to the cauldron of the inner solar system. Rosetta’s orbit around “Chury†will take it to within a few tens of metres above the surface – a vantage point that will allow it to map the comet’s exterior.
(Image: Scott Garrett)
Video: How the Rosetta spacecraft will orbit a comet
By November, scientists and engineers will have chosen a site to drop Philae, Rosetta’s washing-machine-sized lander. It will drift down from the mother ship and latch on to the surface of the comet. Once settled, Philae will begin to reveal secrets about the solar system and maybe even give us clues about the origin of life (see “Secrets of the solar system“).
First, though, Rosetta must wake up. For most of the past three years, it has been in hibernation mode, deep asleep except for the tick of an oscillating crystal and a dozing onboard computer with one eye on the clock. Everything else is dark: the cameras, the instruments, even the communications equipment. Nothing has been heard from the comet chaser for 32 months.
That’s because to reach its rendezvous with Chury, Rosetta has travelled deeper into space than any solar-powered spacecraft before it. Near Jupiter’s orbit, where sunlight levels drop to around 4 per cent of those on Earth, its giant solar panels – the largest ever flown in space – can only generate enough power to keep some internal heaters going. Almost everything else on Rosetta had to be turned off.
For spacecraft operators who are used to daily communication, the silence has been a wrench. “It is very weird to be out of contact for such a long time,†says Roberto Porto who works at the ) in Darmstadt, Germany.
I first met Porto in 2010 when Rosetta flew past asteroid Lutetia. He called me into the control room to witness a manoeuvre. He showed me four graphs on a computer screen and held up a clenched fist with his little finger and thumb extended, as if imitating a telephone. His fingers stood for the solar panels and his fist the bulk of the spacecraft, which is about the size of a small car.
As the lines snaked across the screen, he gently rolled his fist. “The spacecraft is doing this,†he told me. His mental connection to the craft was extraordinary. At the time, he realised that something had happened inside him. “That fly-by was the first time I felt really attached to the spacecraft,†he recalls. Turning Rosetta off a year later was far from easy. “It was like saying goodbye to a friend and knowing you weren’t going to see them for almost three years. I guess it was worse for the people who had worked on the mission for longer than me.â€
One such person is . He joined the Rosetta mission in 1996 and is now the spacecraft’s operation manager. He knew early on that hibernation would be a challenge. “In the development phase of the mission, I tried to convince my manager many, many times not to design a hibernation mode. We had no other choice but to create this,†he says.
No one on Accomazzo’s team wanted to be the one to send Rosetta to sleep, just in case it never woke up. So when the time came, they turned to , who designed the hibernation mode for Astrium, the company that built the spacecraft. “We said to him, ‘OK, you have created this monster, so you can be the one to send the signal’,†says Accomazzo.
Eibner was unperturbed. “Everything had worked up until that point, so I didn’t see anything to be worried about,†he says. Preparing the spacecraft was an arduous procedure that lasted a few days. To remain stable after the thrusters were turned off, Rosetta had to be gradually shut down and then made to gently spin. Finally, when the only remaining link was a beep sounding every few seconds in the control room, Eibner sent the last command. The spacecraft’s faint heartbeat stopped. Rosetta was as deeply asleep as any spacecraft could be and still be alive.
Fast-forward to now, and a team is waiting like anxious parents for Rosetta to call home. Before the spacecraft does that on 20 January, it has a lot to do on its own. When the onboard computer realises it is time to wake up, it will switch on two heaters. These will gradually warm up Rosetta’s star trackers, the sensors that allow the spacecraft to determine its orientation.
After six hours of heating, thrusters will slow down its rotation to a leisurely crawl. The deceleration will take less than 1 minute. Next, the spacecraft must find Earth. It does this by turning on its star trackers, which will scan the skies for familiar patterns of stars. Once it has worked out where it is and where it is pointing, it will consult the onboard map to find Earth’s position. Then the spacecraft will start moving to point its antenna towards home. Next, it will switch on the transmitter and the signal will begin its 800 million kilometre journey to Earth. At this distance, the radio waves will take about 45 minutes to reach us.
The quickest all of this can happen is in about 7½ hours, with most of that time spent waiting for the star trackers to warm up. Even if the signal does not arrive on time, panic would be premature. The timings are inherently uncertain. For example, the computer only looks at the clock every 15 minutes to see if 1000 GMT has passed, so there could be a delay of at least a quarter of an hour. If the spacecraft is in any doubt about the state of the star trackers or the rotation manoeuvre, then software is designed to stop and start the particular process again. Altogether, such shenanigans could delay the receipt of the first transmission by around 2 hours.
As nail-biting as those 8 to 10 hours will be, the spacecraft has been up and about operating independently before. Rosetta is a master at doing things for itself. In 2007, it skimmed past the night-time side of Mars, and had to rely on battery power to keep itself alive. Even then, it managed to take a of its solar panels above the Syrtis region of the Red Planet.
“Rosetta has a lot to do before it can call home: it will be a nail-biting 8 to 10 hours between the wake-up call and the spacecraft’s first sign of lifeâ€
Three years later, the spacecraft had to turn away from Earth to concentrate on gathering data as it flew by asteroid Lutetia. It worked on its own for 40 minutes before turning around to beam its results back to Earth. Shortly afterwards, as a test, the team placed Rosetta into temporary hibernation for a week.
Busy doing nothing
The team has also learned a thing or two from Rosetta’s sister spacecraft, Mars Express and Venus Express. Both of these are more-or-less identical to Rosetta. All told, the flight team now has 25 flight-years of experience operating these three probes and if there is one lesson they have learned, it is this, says Accomazzo: “Many of the problems we have met were because we were in control of the spacecraft. We either made a mistake or discovered a feature of the spacecraft that didn’t work as expected. Every time we left the spacecraft alone, we didn’t have a problem.â€
Nevertheless, they are taking Rosetta’s awakening very seriously. “We haven’t seen the spacecraft for nearly three years. The tension is more like you experience at a launch,†he says.
With the whole mission resting on the outcome of Rosetta’s wake-up call, the team has prepared for every eventuality. They have been running simulations using software models and a full-size replica of the spacecraft that is housed at ESOC. And they have been working out what to do if the signal doesn’t arrive – nothing, at least initially. “The risk is that we send a command to the spacecraft but the spacecraft is already doing something, so we don’t want to interfere,†says Accomazzo.
Once awake, some niggles may appear. Like many of us on waking up, Rosetta will take a while to really get going. Only by May will all the subsystems and instruments have been switched on and checked. By this time, the spacecraft should be back close enough to the sun to generate juice to power all its systems (see “The comet chaserâ€).
In particular, Porto is waiting to see what happens with the reaction wheels that help point the spacecraft in the right direction. The failure of reaction wheels is what crippled NASA’s planet-hunting Kepler spacecraft and Porto has found that two of Rosetta’s four wheels experience unexpected friction when turning at high speed. During the hibernation, the team has used the replica of Rosetta in the lab to understand the behaviour and design workarounds.
They are also prepared for some damage to the solar panels caused by micrometeoroid strikes during the hibernation. While this will inevitably degrade Rosetta’s ability to generate electricity, it is an accepted part of spacecraft ageing. Armelle Hubault, who joined the team in 2004, is sanguine about what they will find. You can hear her shrug as she says, “there will be ageing of the spacecraft to take into account but we know this spacecraft very well nowâ€.
Accomazzo also sounds philosophical: “I spent many years being really scared of this hibernation mode, but after years of flight my confidence has grown dramatically.â€
Hubault sums the team’s feeling up, simply and directly. “We are ready,†she says.
Wake up Rosetta, it’s time to go to work.
Secrets of the solar system
Comets date back to when the planets formed 4.6 billion years ago. They preserve a record of the more volatile material that was available for planet creation, including water and other simple compounds such as ammonia, methane and carbon dioxide. They also contain organic molecules that would have been available as ingredients during the still mysterious event that sparked the origin of life on Earth. The thinking is that comets will be the Rosetta stones that will allow us to decipher the mysteries of the early solar system and our origins – hence the name of the mission. Rosetta will be the first spacecraft to orbit a comet and map its surface as the pair of them travel closer to the sun. It will also launch a robotic lander to analyse the comet’s surface (see main story).
Until now, space scientists have only ever looked at comets from a distance. The few space missions that have got close have briefly sampled the tail material, taken pictures of the mountain-sized nucleus that boils off to produce the tail, and departed again. These snapshots have shown that comets are as individual as people, diverse in appearance and behaviour.
That behaviour can change dramatically as the comet approaches the sun. The increased illumination heats the icy landscape, driving more of it to turn to gas and escape. Certain spots are more vigorous than others, causing jets to form. The Rosetta spacecraft will observe this process unfolding for the very first time on comet Churyumov-Gerasimenko.
Events will peak when “Chury†makes its closest approach to the sun on 13 August 2015, with the Philae lander anchored to its surface and Rosetta still in orbit. No one expects there to be the kind of drama that surrounded the recent comet ISON, which disintegrated shortly after its closest approach. Chury never gets that close to the sun, staying outside Earth’s orbit.
Nevertheless, the flight team can never totally relax. There will be a marked increase in activity on the comet that will inevitably put Philae and Rosetta itself in danger. When it comes to comets, no one can truly predict what will happen.
This article will appear in print under the headline “Wakey, wakey, Rosettaâ€
