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Telescopes and space missions

Rendezvous with a comet

02 Oct 2014
Taken from the October 2014 issue of Physics World

After making a successful rendezvous with Comet 67P/Churyumov–Gerasimenko earlier this year, Europe’s Rosetta craft is now riding alongside this celestial body and next month is set to land a probe on its surface. Matt Taylor describes the excitement of this unique project and the scientific insights that it hopes to achieve

Getting up close

“I’ll not lie to you. It’s tense here at @esa #WakeUpRosetta,” Tweeted the astronomer and author Stuart Clark at 6.48 p.m. Central European Time.

“Tense? The signal’s late, so it’s past tense,” responded Joel Parker, deputy principal investigator of the ALICE instrument on-board the Rosetta craft.

As a project scientist on the mission, I couldn’t resist adding: “It ain’t past tense till the fat lady sings.”

Clark, Parker and I were among 300 or so scientists, engineers, journalists and other dignitaries to have congregated in the H-building of the European Space Operations Centre (ESOC) at the European Space Agency (ESA) in Darmstadt on 20 January this year. We had gathered to witness the Rosetta “wake-up event”, when the spacecraft was supposed to emerge from a planned 31-month-long hibernation. But what had begun as a relaxed group of colleagues at 6.30 p.m. – the start of the one-hour window when we expected the first signal from the craft to arrive – had gradually evolved into something akin to a room full of parents waiting for their children to return from their first long trip away on their own.

Having worked at ESA since 2005 with project scientist duties on the Cluster and Double Star missions, and as Rosetta project scientist since 2013, I was, of course, very confident that the spacecraft would come out of hibernation at the planned time, as my colleagues were too. Eventually, as expected, the signal duly arrived – but it was to be a nail-biting wait. As Andrea Accomazzo, Rosetta’s flight director, later admitted: “That was the longest hour of my life.”

Audacious and exciting

The idea for Rosetta was born in the mid-1980s during the exciting days of the Giotto mission and its visit to Comet Halley and Comet Grigg–Skjellerup. Giotto provided us with a massive leap in our understanding of comets, measuring for the first time the “nucleus” of a comet – the lumpy main body of the object. The mission showed that comets are some of the darkest objects in the solar system, reflecting barely 2–4% of incoming light. It also revealed that jets of dust and gas spring from the nucleus, feeding the fuzzy outer atmosphere – or “coma” – around it. But although Giotto got quite near – approaching to within 600 km of its quarry – it became clear what we needed to do next, which was to get up close and personal with a comet.

The armada of spacecraft that visited Halley around the time of Giotto was later joined by a number of missions to other comets. But all flew by relatively fast (at speeds of no less than several tens to hundreds of kilometres per second) and remained far away, typically approaching to within no more than several hundred kilometres of their target. Rosetta is unique in that it is the first craft to have made a rendezvous with a comet and is currently riding alongside its prey and will continue to do so through 2015, including perihelion, the comet’s closest approach to the Sun, next summer. If that isn’t enough, we will also deploy a lander – Philae – onto the comet’s surface next month.

But Rosetta is no ordinary project in other respects too. Most planetary-science and exploration missions usually start with astronomers observing their destination remotely – either with the naked eye, or by using ground-based or near-Earth space-based facilities. Robotic craft are then sent to orbit and map the body of interest – and only afterwards do we look to land a craft. The Rosetta mission will instead condense two space-mission steps into one. As Alan Stern, principal investigator of Rosetta’s ALICE instrument, once noted, “We are going to orbit and land on a comet for the first time with the same mission. That’s what makes Rosetta so audacious and exciting!”

Space scientists and astronomers find asteroids and comets intriguing because these small objects are the “leftovers” from when the solar system formed, some 4.5 billion years ago. What makes comets extra interesting is that they were flung out into the frozen outskirts of the solar system in regions known as the Kuiper belt and the Oort cloud. Being in such a chilly environment – about 50 K in the case of the Kuiper belt – the molecular composition of these bodies has hardly changed over time, meaning that they can give us clues as to what the early solar system was like.

We now know that comets are a mixture of dust, rock and frozen gases, containing a number of different molecules, notably water ice. What is more, there are indications that the isotopic make-up of water in comets – in particular those from the Kuiper belt with “short” orbits of less than 200 years – is very similar to that found on Earth, meaning that comets could have brought water to our planet. They are also laden with organic molecules, which are the building blocks of amino acids, and the ingredient for life. So by studying the physical and chemical make-up of these bodies, we can obtain information about where in the early solar system it was made, the journey it subsequently took and its connection to the evolution of the planets themselves. The elemental make-up of comets even gives us clues about the creation of the Sun itself.

From dream to reality

The Rosetta mission was approved way back in November 1993 as a “cornerstone” mission of ESA’s Horizons 2000 Science programme – the other three craft getting the nod being the XMM orbiting X-ray observatory, the SOHO solar-observation craft and the Cluster mission to study the Earth’s magnetosphere. The original idea for Rosetta was for the spacecraft to take a sample of a comet and return it to laboratories here on Earth. But as this plan was deemed far too expensive, the next best option was pursued – to send a laboratory to the comet instead.

The mission is named after the famous Rosetta stone, which – along with an obelisk found in a temple on an island in the Nile called Philae – led to the deciphering of Egyptian hieroglyphics almost 200 years ago. The Rosetta mission, by providing an unprecedented characterization of a comet, will take us on a similarly exciting journey back in time, but even further into the past – to the beginning of our solar system in fact. In doing so, we hope to unlock the mysteries of how our solar system formed and how it developed into what we see today.

Rosetta will characterize the comet’s nucleus in full, measuring its mass and gravitational field, and obtaining data on its physical, chemical, mineralogical and isotopic make-up. The mission will also probe the morphology of the comet’s surface. Rosetta will in particular be able to examine the development of cometary activity and the processes in the surface layer of the nucleus and coma, looking at how the stream of particles from the Sun, known as the solar wind, interacts with that region. With a mass of almost three tonnes at launch, the spacecraft and lander have specific instruments to address these scientific objectives, with an impressive payload of remote-sensing and in situ experiments.

Originally scheduled for take-off in January 2003, Rosetta was initially delayed due to a failure of the Ariane launch vehicle in 2002 before eventually blasting off on 2 March 2004 on an Ariane 5 G+ rocket from Europe’s spaceport in Kourou, French Guiana. As a result of the delay, the mission’s original destination – Comet 46P/Wirtanen – had to be abandoned, with Comet 67P/Churyumov–Gerasimenko being selected instead as the Rosetta target. This particular comet, which is larger than the original target and equally as interesting from a scientific point of view, was discovered in 1969 by researchers Klim Churyumov and Svetlana Gerasimenko, with the “67” indicating that it was the 67th short-period (P) comet to have been discovered – Comet Halley/1P having been the first. Rosetta’s new target 67P takes about six and a half years to orbit the Sun, getting to within 180 million kilometres of the Sun at its closest approach and being 840 million kilometres away at its furthest point. Space-based measurements of the comet suggest that it is about 4.5 km long, roughly 3–4 km across and rotates once every 12 hours.

The long and winding road

After Rosetta took off in 2004, it exploited the gravity of the Earth and Mars to get out onto a perfect orbit to chase down Comet 67P (see figure 1 above). Over the following 10 years, it has flown past the Earth three times and once past Mars, with scientists using each of these fly-bys to test the craft and its instruments as well as carry out a variety of observations. For example, in one of the Earth fly-bys, Rosetta’s plasma instruments saw strange modes of waves of unknown origin near the Earth. When these observations were combined with those from ESA’s Cluster mission, which was located in the solar wind at the same time, space scientists concluded that the waves were driven by specific magnetic- and electric-field configurations in the solar wind interacting with near-Earth space. Meanwhile, in a subsequent Earth fly-by, measurements taken by Rosetta’s Visual, Infrared and Thermal Imaging Spectrometer were compared with data from Earth-observing satellites, including ESA’s Envisat craft, to get important information about how to optimize Rosetta operations near the comet. (In effect, we were checking the response of Rosetta’s instruments, calibrating them against the many measurements made with existing Earth-observation satellites to see how well Rosetta’s kit was doing.)

During Rosetta’s decade-long journey to Comet 67P, its trajectory has also brought the craft near to two other small bodies in the solar system – Asteroid 2867 Steins in September 2008 and Asteroid 21 Lutetia in July 2010. These planned encounters addressed one of the prime goals of the mission, which was to obtain accurate measurements of the size of these small bodies and characterize their surface features, such as the number of impact craters, which can indicate how old the asteroid is. As a result of these fly-bys, the Steins asteroid was found to be diamond in shape, with dimensions of 6.67 × 5.81 × 4.47 km. The unusual shape is thought to be caused by the so-called Yarkovsky–O’Keefe–Radzievskii–Paddack (or “YORP”) effect, whereby visible photons from the Sun get absorbed by an asteroid and then re-radiated at infrared wavelengths, taking momentum from the body and altering its rotation rate. In this case, the change in rate made some material move towards the equator of Steins, resulting in the diamond shape. In fact, this was the first time the YORP effect had been seen in an object in the “main” asteroid belt lying between the orbits of Mars and Jupiter.

As for Lutetia, it was found to have dimensions of 132 × 101 × 76 km and to have a density of 3400 kg m–3 – one of the highest of any known asteroid – implying that it must contain a lot of iron. In spite of this, we do not think that the asteroid has a solid, dense iron core as is the case for the terrestrial, or “rocky”, planets Mercury, Earth, Venus and Mars. The observation can only be explained if the asteroid was subjected to some internal heating early in its history, but did not melt completely and so did not end up with a well-defined iron core. That would be why the surface is relatively “primordial” (in other words, it is young and has not changed much over time), which would not be possible if a full molten phase had existed.

In June 2011, nearly a year after the Lutetia fly-by, Rosetta was out to about 668,000 km from the Sun. But not having enough power to operate the spacecraft safely, we put it into a slow spin to keep it stable. A command to enter “hibernation mode” was then released. Rosetta was moving further and further away from the Sun, which meant that it was approaching a point where there was simply not enough power to supply the platform, even with its massive solar arrays. With such low power, we could only maintain a few essential components, such as heaters and the computer, which would be used to wake the craft back up in January 2014. It is worth noting, in passing, that Rosetta has travelled further from the Sun than any other solar-powered spacecraft in history.

The last radio-frequency pulse from the spacecraft was detected at about 14:12:00 Coordinated Universal Time (UTC) on 8 June 2011 and when no further signal after that was identified, Rosetta’s successful entry into hibernation was confirmed. Fred Jansen – Rosetta’s mission manager and the person responsible for the entire project – has likened hibernation to putting your TV on standby, although waking up the craft is not exactly the same as switching your TV back on. “Instead of us pressing the remote control to take it out of standby, we are relying on an internal alarm clock to trigger the spacecraft to come out of hibernation,” he says.

Wake up and then some

And so on 20 January 2014 – after 957 days in hibernation and having passed through the leg of its orbit that took it furthest from the Sun – Rosetta was finally on its return journey to the inner solar system. Once it had got to within 807,224,610 km of the Earth, we knew that the craft would have enough solar power available to wake up and start the main phase of operation. Many people around the world were waiting in anticipation for the next step in Rosetta’s voyage, not least the members of the mission’s instrument and operations teams, who had worked for decades to get to this stage. The craft’s internal alarm clock was due to begin the wake-up process at 10:00 UTC, taking Rosetta out of its standby mode and triggering a number of on-board activities, including the craft warming itself up, switching on various critical components and orienting itself towards Earth. Once all this had happened, the signal – taking into account the 45-minute journey to travel to Earth – was expected to reach all those gathered in the H-building at the ESOC in Darmstadt between 6.30 p.m. and 7.30 p.m. local time.

As 6.30 p.m. came and went, the mood was jovial. Gerhard Schwehm – the previous Rosetta mission manager and a current project scientist – was of the strong belief that the signal would come at 6.45 p.m. and I even queried if he actually had an app on his phone to simulate a signal. But as the minutes ticked by, the relaxed attitude of many of us began to dissolve into apprehension. Our levels of adrenaline and worry began to rise. Having spent the day surrounded by journalists constantly asking us how we were feeling, this was the hour where emotions would finally show.

The room went silent, interspersed with only an occasional murmur, all faces fixed on a noisy, fuzzy line on our computer screens.

Then, shortly after 7.10 p.m. I overheard a colleague comment on signal strength and some of us started to lean forward to inspect the line more closely.

“Was that…?” asked Markus Bauer, ESA’s science and robotic exploration communications officer, as he and I looked at each other. “Did you see that?”

Holger Sierks – a principal investigator of Rosetta’s OSIRIS camera – seemed to nod, as did Parker and Schwehm. Then the clear spike continued for more than a couple of frames and an ecstatic Andrea Accomazzo punched the air in delight.

“Hello, world!” came the Tweet from the official @ESA_Rosetta account. We were back in business.

To anyone considering that this episode was staged, it honestly was not. And I can assure you that I have never hugged so many scientific colleagues in such a short period of time before.

Rosetta had done it, but that was only the start. Since that January day, we have started mapping and characterizing the comet to consolidate our knowledge of how to orbit it and to find out how best (and, crucially, where) to deploy the Philae lander – the very first time anyone has tried to carry out such an outlandish feat. The landing is set to take place on 11 November and there is sure to be more drama and excitement in store. Compared with the wake up, the landing will be even more nerve-wracking – it will not be a gut-wrenching few minutes, but a torturous few hours.

The main phase of the mission is now in full swing, and it is down to us to do the science we have promised. ESA staff and instrument scientists have been busy since May finalizing operations so that we can get the most out of Rosetta before its mission ends in December 2015. By then, we will have witnessed one of the most unique and amazing rides of all time – escorting Comet 67P/Churyumov–Gerasimenko through its closest approach to the Sun, seeing how it changes from a rather inert frozen object to something interacting fully with the power of the Sun, with ice subliming (changing directly from ice to gas) and lifting the dusty material from the surface of the nucleus with it. This is a process that generates the dust and gas tails, and that is at its strongest when a comet is at its nearest point to the Sun. We are set to get a ringside seat to watch this process grow, peak and then start to wane. In doing so, we will have become intimate – as never before – with an object we barely know, and understand even less about.

Quite a whirlwind romance lies in store!

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