Rosetta is a first-class mission, because it is a mission of firsts. First to fly-by asteroid Steins, a rare metal-poor asteroid. First to travel past asteroid Lutetia, which might be related to a class of rare, metal-rich meteorites. First to attempt to detect an atmosphere on an asteroid. First to catch up to a comet and travel alongside it. First to listen to the plasma “wind” from a comet. First to orbit a comet. First to capture high resolution images of the surface of a comet. First to see the boulders, canyons and cliffs that cover the surface of the oddest-shaped comet yet observed.
These are what the mission has achieved so far. Still to come, we hope: the first mission to travel with a comet as it develops its tails. The first mission to travel around the sun with a comet. The first mission to observe a comet returning to dormancy. And of course, the Rosetta mission is the first in which there will be an attempt to land on the nucleus of a comet. Not just land there, but stay there, sample the comet and analyse its water and dust, checking for the building blocks of life.
In the video below, space scientist Monica Grady, a member of the Rosetta mission’s science team, talks to science editor Akshat Rathi about humanity’s fascination with these dirty iceballs and explains how Rosetta hopes to answer some of the most fundamental questions about life on Earth.
This is a transcript of the video:
So, Monica, humans have been fascinated by comets for a very long time. Can you tell me more about it?
Ever since the Chinese astronomers from 2,000 years ago observed the night sky, comets have been harbingers of doom and disaster. There is a picture of a comet in the Bayeux apestry which commemorates the Battle of Hastings from 1066. There are pictures of comets in one of Giotto’s famous pictures. There’s connections, or connections have been made, between comets and the Star of Bethlehem; between comets and battles, disasters, the birth of kings, the death of kings. All this is in sorts of history, mythology and religion going on for thousands and thousands of years.
Edmund Halley predicted that comets came into and out of the solar system and in regular orbits and there is a comet which takes his name, Halley’s Comet, which comes round every 76 years.
One of the most famous of cometary scientists was somebody called Fred Whipple, who described them as “dirty snowballs”, the idea that they were a mixture of rock and snow. Now we call them more like “icy dirtballs”. It’s actually more dust there than there is ice.
What about scientific explorations of comets?
There have been several space missions to comets. The first one was the Giotto mission of 1985. That was a European Space Agency mission and it went to Halley’s Comet and it flew past the cometary nucleus and took the first picture that we have of a comet’s nucleus. But it just went straight past. But that mission sowed the seeds of the Rosetta Mission.
There have been two other missions to comets: there’s been the Stardust Mission, which was a NASA mission which went through the tail of a comet and collected material from the comet’s tail and brought it back to the Earth.
And then, there was Deep Impact which was a NASA mission where a comet ran into a copper projectile. So, NASA let go this great, big copper thing and the comet ran into it. What Deep Impact did was it excavated a big crater in the side of the comet and we could observe the interior of a comet for the first time. So we could see what actually happened when the impact evaporated the ice and release a lot of the dust and we could see what was deeper down.
The Stardust Mission brought back a bit of Comet Wild 2. But it only brought back solid material. It didn’t bring back any gas and it didn’t bring back any ice. And of the solid material that came back, it’s a bit difficult to disentangle some of the data for the organic compounds because of the way the material was collected.
Why was 67P – and I can never pronounce its full name – chosen as the target comet and not the many billions out there?
Comet 67P is also known as Churyumov–Gerasimenko, which is a bit of a tongue-twister. It’s named after two astronomers Churyumov and Gerasimenko, who observed it first. It’s a comet which is part of the inner solar system so it travels round partly under the influence of Jupiter but, of course, mainly under the influence of the sun. The reason it was chosen is because its orbit is in the plane of the solar system. So all the planets, the Earth, orbit the sun, right? Now some comets have really inclined orbits and that makes it very difficult to track them. So, Comet C-G has got this orbit, which is more or less what we call the plane of the ecliptic so we could track it relatively easily.
People assume that comets come from the outer edges of the solar system. But 67P is different. What about it is different?
There are two big groups or classes of comets: there are the long-period comets and the short-period comets.
The long-period comets are ones which live at the outermost fringes of the solar system. They inhabit a region called the Oort Cloud, which nobody’s ever seen or observed but we believe it’s there because when you observe the paths, the orbits, of some comets, especially the ones that are really steeply inclined, they always go out to this region, which is about 50,000 times as far away from the sun as the Earth is. So, these are the really long-period comets. They only might come in and out of the solar system every three or five thousand years.
The short-period comets are ones that used to be long-period comets but they’ve been grabbed by Jupiter or they’ve been scattered inwards by some events, perhaps the passage of a nearby star or something like that. That is why Halley’s Comet comes round every 76 years. The difference between the two types of comets is the length of their orbits and the length of time they stay within the solar system.
From when the planning began, to the actual comet landing, it has taken 20 years. Why did it take so long?
The Rosetta mission has been a long time in the planning. The roots of its mission was in the success of the Giotto mission from 1985 to 1986. Once the results had come back, it was, hey, we’ve learnt a lot about comets but there’s even more we need to know. So, planning started more or less then.
But there aren’t that many space missions. You’ve got to wait your turn in the queue for one thing. You’ve got to have an excellent science case so that the European Space Agency will accept that there is really interesting work to be done. You’ve got to get the science team together, you’ve got to persuade the politicians of the different countries. So all that took from say 1986 to 1996.
Then you’ve got to build the instruments and when you have built your instruments you have to deliver it to the rocket for it to be launched. Now, usually an instrument has to be delivered, maybe one, maximum of two years, prior to launch, where they’re all integrated because you know, the Brits have built this one and the French have built that one and the Germans have built that one, and the Italians have built that one and you’ve got to make sure that all the plugs and sockets all fit together and that they all communicate with each other and that, you know, if you switch this one on, it doesn’t fuse that one … That sort of thing, so integration takes a lot of time. So, you’re allowed two years for integration and so Rosetta was integrated, it was delivered almost to the rocket.
Unfortunately what then happened was that the launch previous to the one Rosetta was going to go on exploded on takeoff. That was the mission which was going to launch the Cluster mission. And that put everything back another two years because there had to an investigation into the explosion of Cluster so it’s actually taken two years longer than it should have done.
It took all that time to actually get into the launchpad but didn’t mean that we were really to actually start taking any data. The mission had to be launched and it doesn’t just go from Earth to comet, it had to go twice around the Earth and once around Mars. But to build up enough speed and thrust so that it start to get on the right trajectory so that it would catch up with the comet. So that’s what it did, it took nearly ten years to actually catch up with the comet.
For the first eight years, it was doing tests, it passed a couple of asteroids and took some measurements. For the last two of those years, it was asleep. It rested, saved its batteries and it woke up in January of this year. In August of this year, it actually caught up with the comet and between August and November, it has been doing a whole load of manoeuvres to bring it closer and closer into the comet so now it’s only a few kilometres away from the comet’s surface.
The mission’s objectives are to understand more about the origin and the evolution of our solar system. That is a big goal. What have you achieved so far?
The aims of the Rosetta mission are very grand: it’s to understand the origin and evolution of the solar system by looking at primitive material that was formed when the solar system was formed. It’s going to look at the water and the carbon and the organics to see how they relate to the water and the organics on Earth, and to life on Earth.
Now, the main part of that scientific investigation will take place when the Philae lander actually arrives on the comet’s surface. But till now, we’ve been doing a lot of science. Obviously, we’ve had the fantastic images of the comet’s nucleus, we know its temperature, we know its density, we know its angular momentum, we know its spin speed, we’ve got some idea of the differences in composition of the surface of the comet. And so, there’s been a lot of information so far about the comet itself taken remotely from the instruments actually on the Rosetta spacecraft.
If the comet landing succeeds, what are the realistic expectations?
Assuming that Philae lands successfully on the surface of the comet, what the instruments on board will do is drill a small amount of material from the surface and the sub-surface regions of the comet and this will be brought up and placed in the ovens on the comet, where they will be heated up and melted and then eventually burned. The melting will melt the ice, so that we get an idea of what gases are trapped in the ice, we’ll get an idea of the composition of the ice – its hydrogen and its oxygen – and then we’ll get an idea of the composition of the more solid material. How much carbon there is there? How much sulphur?
Can Rosetta give us a definitive answer to whether water of Earth came from the water on comets?
One of the aims of the Rosetta mission is to see what the relationship is between the water in the comet and water on the Earth. Now assuming they all formed in the same place at the same time, they should be the same. But when the Earth formed, it got very, very hot. Its surface was completely molten and it is assumed that it lost practically all of its water. And so when the Earth cooled down and after the moon had formed, the Earth would have been quite a dry planet. What we want to see is how much of its water it managed to retain in the early times when it was molten and how much has been added subsequently by bodies like comets.
The mission is to end in late 2015 when the comet comes close to the sun. Do you think Philae will survive? And what about Rosetta?
The comet keeps going. It’s going to get close to the sun and then it’s going to go round the sun and then it’s going to come back out again. As it goes on that journey, it will develop a huge big tail as more and more of the ice from the surface of the cometary nucleus sublimes.
Now, it’s very unlikely that Philae will actually survive the growth of those tails because these are very strong jets of gas. These will be coming up as geysers and if one comes up just underneath where Philae is then it won’t survive for very long. However the Rosetta mother-craft will be flying alongside the comet, it will watch as the tail develops, it will go with the comet as it swings round the sun and then continue to track Comet C-G as the tail then decays away. So, Rosetta is planned to actually survive the tail formation but sadly Philae isn’t. But it might do, you never know.
What about applications on Earth for the technologies that have been developed to build Rosetta?
One of the really great things about being involved with a space mission is that you do lots of experiments first in the laboratory using huge, great, big pieces of equipment. And then the real challenge is to shrink down those bits of equipment into instruments that don’t weigh very much, and don’t need much power. Colleagues at the Open University have done that successfully.
They’ve taken a piece of equipment, which is the size of a room, they’ve shrunk it down to something which is about the size of a shoe box – and that’s the Ptolemy instrument which is on board of the Philae lander. This is great, it’s going to go and sniff the comet, it’s going to measure the gas and the dust in the comet and we’ll learn about the formation of the early solar system, which is fantastic for planetary scientists.
What the taxpayer does benefit from though is that we can take the know-how that we’ve got from taking something very big and making it very small – we’ve made it portable. And it’s something which can test for volatile compounds and so we can use that it all sorts of applications: you can use it on submarines to test their quality; you can use it out in field hospitals to test for disease; you can put it on an orbiting satellite going round the Earth to look at carbon dioxide emissions from forests and so on. As soon as you’ve built something small and portable then you can do all sorts of things with it. And that is where a lot of the value of space research comes in to.
For the scientist, it’s valuable to do research. But for the taxpayer, we also get that additional spin-off as well.
What next for cometary exploration after Rosetta?
The Rosetta mission has been very ambitious. But it’s limited. It’s sending something to the comet – and making very sophisticated measurements using very sophisticated instruments – but it’s a sort of one-shot. What we really need to do is to be able to repeat these things to much more sophisticated experiments that we can do, that we couldn’t do ten, 15 years ago when the mission was being planned. What we really need to do, what we want to do, is bring some of that comet nucleus back to the Earth. That, I think, would be the next step in the age of cometary exploration: planning a comet nucleus sample return mission.
That would be great, and I hope that happens in my lifetime. Thank you so much for your time today, Monica.
Monica Grady receives funding from the STFC and the AHRC