In 2014, ESA's Rosetta spacecraft arrived at comet 67p/Churyumov-Gerasimenko. Over the next two years, it examined every aspect of the comet revolutionising our understanding of this cosmic wanderers. Now, 6.5 years later, 67P returns to perihelion - it's closest approach to the Sun - and so we take a look back at the mission – what we've learned from it already, and what we still have yet to study.
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Ezzy: Greetings, listeners. It's time for the November episode of Radio Astronomy. I'm News Editor Ezzy Pearson and I'm joined on the podcast today by production editor Neil McKim.
Ezzy: Coming up later, we'll be telling you how you can see the Pleiades in the night sky in our stargazing tip of the month. But first, we'll take a look at a topic that we've learnt about whilst putting together this month's issue. On November 3rd, the comet 67P Churyumov-Gerasimenko returns to the inner solar system and passes through perihelion, its closest approach to the Sun for the first time in six and a half years. The comets are fairly typical comet, but it has become one of the best known comets in the world thanks to a mission called Rosetta.
Neil: The mission itself was approved back in November 1993 by the European Space Agency, and finally in 2004 ESA launched the billion euro costing Rosetta spacecraft on an Ariane 5 rocket with an aim to chase, go into orbit around and land on a comet. In fact, it was undertaking the most detailed exploration of a comet ever taken. The original target was the comet Wirtanen, but the target was changed to 67P before launch. The mission Rosetta was named after the famous Rosetta Stone, which led to the deciphering of Egyptian hieroglyphics because it was hoped that it would unlock clues about other comets. It arrived at 67P on the 6th August 2014 after a 10 year journey through the Solar System, which took it to Mars on the way. And then it spent two years circling the comet as it grew closer to the Sun. The spacecraft gathered data and observed the comet's surface, turning from ice to gas, creating its tail. Its spectrometers analyse gas coming off the comet to work out chemical composition. And as Rosetta drew close, it discovered that 67P is a double lobed comet, with further investigation revealing that its volume was largely made up of empty space. The comet's hard surface, meanwhile, proved tricky for landing the mission's Philae landing craft, as we'll talk about in a minute. As the comet drew closer to the Sun, the amount of gas given off was dramatically increasing, which made it very difficult for Rosetta to stay in orbit. The dust pushed against the solar panels, but Rosetta survived and completed its mission on the 30th September 2016. Rosetta began a descent to smaller spot on the comet's smaller lobe. The mission ended with a controlled landing on the surface.
Ezzy: So the Rosetta mission started in. Well, it arrived at the comet back in 2014, know having been travelling through space for I think it was about 10 years, almost 10 years and it was it was one of the first big missions when I was just becoming a science journalist, so I've been following along with it for quite some time. But it is certainly an interesting one and I think one of the most interesting things that happened during that mission. Certainly, the one that was probably the best publicised was the landing of the Philae lander. So this was a 100 kilogram lander. It was about the size of a washing machine is always the example everybody uses, and it was kind of a box with three legs on it. And the idea was that it was going to go to the surface of the comet, latch on and take some samples and so on. Unfortunately, Ten years is a long time for a spacecraft to be travelling through space, and during that time, some of the mechanisms on board Philae that were supposed to make it attach to the surface broke down. So it had a a couple of screws that were supposed to screw... On the feet that were supposed to screw into the surface when it landed. It was also supposed to have some thrusters that just kind of slowed it down slightly, so it didn't come into hard and bounce away. It also had a harpoon that was supposed to fire into the surface of the comet and really sort of hold on onto that surface and latch on, and it was those lost to the thrusters that were were had unfortunately failed during flight. And also, the harpoon wasn't looking too great either. There was, you know, with two of the kind of failsafes not working people were worried like, well, should we even bother with the landing at all? At which point they took the decision of there's no point having it attached to Rosetta doing nothing. You might as well try. You never know. It might work. And so on the 12th November 2014, Philae began its long seven hour descent towards the surface of [67P]. It was heading for an area called Agilika, which was a very nice, sunny looking area so that Philae would be able to charge its solar panels and continue operating after its batteries run out. It would look like it was a nice soft surface, nice and dusty, so that could help, you know it wouldn't have such a hard landing and make it more likely to rebound. Unfortunately, that didn't work. With the latching mechanisms broken, it came down on the surface too hard. It turned out that dusty surface was actually just a thin covering over some rock hard ice. And Philae Bounced off and travelled about a kilometre away, ending up in the shadow of a cliff in an area known as Abydos. The lander, unfortunately, also came down on its side, which meant that its drill that was supposed to come out of the bottom and take some samples and put it onto the onboard ovens and so on couldn't deploy properly. It was just pushing the lander away, and there were a bit worried that they were actually going to end up tipping it onto its back entirely so they couldn't use the drill on board, either. However, despite this, Philae managed to operate for 60 hours on the surface of 67P, taking lots of data readings about what the surface was like in that area managing to actually get a glimpse of not just one place on the surface, but two because it bounced. Unfortunately, because it was in the shadow of a cliff, its solar panels couldn't charge, and it couldn't continue after that initial 60 hours of its battery life ran out. It did make a brief return. There was a couple of chirps that were called from Philae between the 30th June and the 9th July in 2015, and there was hopes that was when the comet was passing closest to the Sun. It was getting the longest time, and there was hopes that maybe, you know, some some gas escaping from the comet would shift the lander, and it might be able to charge properly. But unfortunately, that didn't happen, and we never heard from again after that time. So it was it wasn't exactly the full mission that they'd been hoping to get from Philae, but it still managed to do a lot on the surface of the comet, and we still got quite a lot from it about comets.
Neil: And wasn't they rediscovered by Rosetta later on as well? An image of it?
Ezzy: Yes, it was. Yes. So that's one of the things when you're you're operating something on the surface of a of a planet... Or in this case, a comet. It's always really useful to know exactly where it is. And so they were they had Rosetta as it was doing its observations. It was taking lots and lots of pictures of the surface. And one of the things that they were looking for was to see if they could find the lander in its shadow of a cliff. And they did eventually find it. You could see it sort of two legs sticking up in the air as it was coming around. And of course, one of the problems of trying to find something that's in shadow is like, we need light to be able to see it. So it took a long time for them to find that one. They needed to wait until the angle of the light was just right to be able to show off its legs.
Neil: I've been looking a little bit about comets in general and why we're so interested in discovering comets. So basically, comets are the frozen leftovers from the formation of the Solar System, and they're composed of dust, rocks and ices. They range from a few kilometres to tens of kilometres in width. But as they orbit closer to the sun, they heat up and emit gases and dust into glowing head that can be larger than a planet. This material forms a tail that can stretch for millions of kilometres. There are billions of comets in the Solar System, which are typically located in one of two regions. The most distant portion is in the Oort Cloud at the edge of the Solar System, about 100,000 times more distant from the Sun than Earth, and which is estimated to contain billions of comets. The so-called long period comets are thought to originate there. And these have an orbital period of more than 200 years. Closer in just beyond the orbit of Neptune is the Kuiper Belt, which also contains billions of comets and extends for 30 to 50 times the Earth-Sun distance. Some comets, however, escape from these regions and journey into the inner Solar System. And every year many new comets are discovered in this region, including Sun-grazers. As the name suggests, Sun-grazers travel close to the Sun are sometimes partially or completely destroyed by their encounter. Comet ISON is a well known example of a sun grazing comet that was destroyed by an encounter with the Sun in 2013. 67P itself is named after its discoverers Klim Churyumov and Svetlana Gerasimenko, astronomers from Kiev who spotted the comet for the first time in 1969 on a photographic plate, the P identifies short period comets of a well established orbit around the Sun, taking less than 200 years to complete a solar revolution. While the number 67 refers to this particular comet's position in the list of satellites catalogued periodic comets. The most famous comet, Halley, is designated 1P.
Ezzy: So Halley is a great example of one of the reasons why comets are so captivating. You know, it first visited, was first seen streaking across the sky. Well, probably has been since antiquity, but it was kind of first noted down by Halley in 18th century, 17th century. And then he was the first person to notice that it was coming back again and again and again every 75 years or so. And it was that sort of the beginning of people realising that maybe these were things that were worth studying. They weren't just, you know, omens of bad luck and so forth that people could see constantly coming across the sky. And began, you know, the sort of real research into them. And now they are incredibly useful tools for learning about our own Solar System. Because as you said, Neil, they are the leftovers from the origin of the Solar System. They can tell us what was around at the beginning of the Solar System and basically the raw ingredients that were there before the formation of the planets and so on, and where these things might have been in the Solar System. And that's hugely important. And one of the things that 67P and the Rosetta mission really taught us and really hammered down was about the history of water in our Solar System. That was kind of one of the big discoveries because there's been a long debate about where exactly the water on Earth comes from, because the water, the way that we understand the formation of the planets happened there can't have been... The planet would have been too hot for water to condense onto the surface when it was first forming, it must have been brought in afterwards. And one of the ways that people think that might have happened was comets. However, there's something called the D/H ratio, which is the ratio of something called heavy water to regular water. And that doesn't change over time, you know, it stays stable over billions of billions of years. And so if water from a comet has the same ratio as we find of that ratio on Earth, then that means comets probably brought that water to Earth. Except that's not what we find. Rosetta really hammered home the fact that this had double the amount of heavy water as we find on Earth. That's just it's not possible that most of our water came from comets, at least comets not-unlike 67P. So that really was one of the big changes in the way that we understand how our Solar System formed is comets. They might have been important in formation, but they weren't important for moving walked around the Solar System.
Ezzy: The other thing that Rosetta discovered was that 67P was throwing off a huge amount of water. So When it was out in deep space, when Rosetta first got there, it was only sort of putting out about a mug full of water every second. By the time it got, the comet got to perihelion, its closest approach to the Sun, it was throwing out two bathtubs full a second. That is a lot of water. And in case anybody at home is wondering how on earth something can stand to lose two bath tubs are full of water a second for billions of years and be safe... Still have anything left. It's because actually the comet hasn't been coming through perihelion for billions of years. It's existed for billions of years since the beginning of the Solar System. But it's only started coming in towards the Sun enough to become what's known as an active comet, and that's where it forms its lovely coma and gives off lots of water. And it's only come that close in since 1959, back then it had a it used to orbit much further out, and then it had an interaction with Jupiter and got swung in close into the Solar System because that confused me. At first I was like, I just don't understand how something could be giving off this much water. I know space deals were big numbers, but that one seems a little bit too big. And it's also why comets can regularly disintegrate, and we still have so many left. It's because most of them are still all the way out there. But water wasn't the only thing that they were looking for around 67P. And there's lots of other things they weren't expecting to find that they discovered. They found atomic oxygen, which is very strange to find somewhere as cold as a comet. You would expect if there was oxygen around, it would have been incorporated into water or something like that. The fact that it was still, you know, in some an atomic form means that it must have been trapped inside the comet and emerged later. So that taught us a lot about how these comets are formed. And also, we found out that the comet was pretty much empty space, so the inside of the comet was probably about 75 to 85 percent empty space. Voids. It was this sort of loosely held together pile of stones covered in a hard crust of ice. And that's actually what Philae bounced off of. That was this hard crust of ice that we think now was probably formed because of this constant thawing freeze creating, you know, the the the water vapour creates this coma melts and creates this coma. And then as it goes back into deep space, it freezes back onto the surface, forming this very hard crust around the edge of the comet. We also found glycerine around the comet, which is what's known as an amino acid, and those are the building blocks of life. It's one of the most simple building blocks of life. But the fact that we found it in a comet, meaning it was in one of these like primordial pieces left over from the formation of the Solar System means it was around at the beginning of the Solar System. We didn't form these things entirely on Earth. They were around in the solar nebula before the planets formed, which has big consequences for, you know, how we think life grew and evolved on our planet. But most of those discoveries that we've made so far that they made initially were based on the data of just one instrument. In order to compare and contrast -because Rosetta had 11 instruments on it, plus it had Philae which had a whole bunch more - But in order to compare and contrast those, you need to do various, frankly, quite boring technical things about balancing and calibrating and making sure everything you understand how each of them relates to each other. And so they spent three years after the mission doing that releasing the data in 2019. And so that means now people have had two years to go through that consolidated data and we are now beginning to see a whole new slew of discoveries coming out of Rosetta, which is really cool. There is one that really caught my eye the other day, which was the fact that we've now discovered Aurora around comets and the one that they found was in the ultraviolet. So it's not visible to human eyes, but apparently it was all the way around the comet. It's not like on Earth where it's just around the poles. It's this kind of like halo around the comet, which I think is just an incredibly evocative image. And there was signs of the oxygen line, which is the red in the aurora. So the aurora is mostly known for the green, but there is also quite a lot of other colours in it. And one of the prominent ones is it's the red line from oxygen, and they saw that around the comet. Unfortunately, the instruments weren't quite calibrated enough to really sort of look into that. But fortunately, 67P is offering us a chance to get a second look because it is coming into the inner Solar System. Again, we get to have another look at it. Is it just remotely this time. You know, Rosetta isn't going to somehow pop back up again? Well, at least we don't think it will. It's been on a comet for a very long time. Probably hasn't done very well. Comet's quite cold, but people will be able to look at it remotely. And in fact, if any of our listeners at home are interested in comet photography or astronomy is actually visible with with most home telescopes. If you want to have a look, we have tips on that in our skyguide, but professional telescopes will also be looking at it. And so all of those things that they wanted to follow up on, they will now be able to do. And that was the real magic of the Rosetta mission. It learnt about a comet, a single comet in incredible detail and told us what a comet look like up close, so that now when we look at astronomers, look at it from the ground or from space based observatories like Hubble, we can understand what we're seeing. So it's learning about one thing in great detail to be able to better understand them entirely. And so that, I think, is going to be the real legacy of the Rosetta mission. And there's still a lot of discoveries to come. People still like people are still going through the Giotto mission, which was in the 1990s, and papers are still being released on that fairly regularly and that was just a flyby for a few minutes. This Rosetta mission was there for two years with a whole bunch more instruments and a whole bunch of science being done. So we haven't definitely haven't heard the last of Rosetta, and I'm looking forward to coming back in another six and a half years when it's gone through perihelion again.
Neil: The sky starts to get quite dark in November. Looking towards the eastern part of the sky in the early evening, You should be able to spot the small but distinctive open cluster known as the Pleiades. In the constellation of Taurus the Bull. It's roughly a third of the way up the sky when due east, a position achieved at 8:30 p.m. on the 1st November 7.30pm mid-month and 6:30 p.m. by the month's close. The name of the Pleiades comes from a Greek legend, though the seven daughters of the God Atlas and the ocean nymph Pleianae an alternative name for the Pleiades is the Seven Sisters, supposedly because that's how many stars are obvious to the naked eye. But you'll see many more by looking through binoculars or a telescope.
Ezzy: So that's it from us this month. You can read more about the Rosetta mission in the November issue of BBC Sky at night magazine. But we also take a deeper look at Venus, tour through the sparkling stars for Bonfire Night and get to know the lunar rays that streak across the Moon's surface. And that's not forgetting all the regular sections that will help you unlock the wonders of the night sky, Find the right equipment to observe and discover the best things to see after dark this month from all of us here at BBC Sky at night Magazine. Goodbye.