Two Rings Discovered Around the Centaur Chariklo

The tiny world of Chariklo and its two mysterious, icy rings.

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Last Wednesday was an interesting day for astronomy, as the discovery of not one, but two strange new worlds right here in the Solar System were announced. A new dwarf planet, a tiny world orbiting the Sun far beyond Pluto, seems to have got most of the headlines. But I'm more interested in the other discovery, published here in Nature.

Most of the asteroids and other smaller worlds in the Solar System tend to be found in two areas: The Asteroid Belt  in between Mars and Jupiter, and the icy Kuiper Belt (of which Pluto is a member) out beyond the orbit of Neptune. The space in between is far from empty however.

In the late 1970s astronomers started noticing small objects orbiting in amongst the giant outer planets. Called Centaurs, these small rocks raised a lot of questions. Their orbits are inherently unstable, crossing over the orbits of  the huge gas giants. It's only a matter of time before they run smack into Jupiter, Saturn, Uranus or Neptune, colliding with them or being thrown out of the Solar System. This means they can't have been there long, so where did they come from?   

We also don't know what they really are. Are they comets, thrown in from the Oort Cloud in the depths of space? Maybe they're more like asteroids, scattered out of the Kuiper Belt by Neptune's gravity? Perhaps a mix of both? Whatever they are, they don't fit a neat explanation, and that makes them interesting.

In June 2013 several telescopes in South America were trained on the largest Centaur, a flattened sphere of rock 127 kilometers across called Chariklo. They were watching for a event called a stellar occultation, waiting for Chariklo to pass in between Earth and a background star. By looking at the way the Centaur blocked out the light coming from the star, the astronomers behind the telescope were hoping to find out things like what shape Chariklo is and how it rotates.

Right on time, the star disappeared as Chariklo moved across it, reappearing six seconds later as the Centaur moved on.

But just before it disappeared , the star dimmed very slightly for a split second, and then nearly vanished completely for a second more. As is came out from behind Chariklo it did it again, but this time in the opposite order: Flashing off, then dimming slightly.

The occultation of a background star by Chariklo, reveling the rings.

This image is one of my new favorite pictures. It shows the actual data from one of the telescopes. Time is along the bottom, going from left to right. Going up the graph shows how bright the background star was, going from 0 to 1, where zero means no light and one is the normal amount of light coming from the star. 

Going across you can clearly see the light dim a bit, dim a bit more, then go away completely as the star passes behind Chariklo. Then it comes back again and does the opposite.

This means that there must be something else blocking out the star for a moment as it gets close to Chariklo, causing two perfectly symmetrical dips in the star's light, either side of the Centaur. Only one thing would do that: Chariklo must have a pair of rings. 

Up until now we've only found four objects with rings: Jupiter, Uranus, Neptune and the stunning band of ice and dust around Saturn. These planets have one main thing in common: They're huge, many times the size of the Earth. That the fifth known ring system would belong to a rock smaller than Wales (to use a standard unit of measurement) was completely unexpected. 

The reason I like the plot so much is that is so clearly shows what's actually there: A big thing in the middle with two rings around it. Most plots need a long time and lots of extra information to work out what they show, but here it's nice and clear. 

The rings around such a small object raise a lot of questions. For a start, look at how much of the star's light the inner ring blocks. Over three-quarters doesn't get through. These rings aren't wispy clouds of dust, they're bits of ice and rock just as thick as Saturn's rings. How did rings like that form?

Chariklo's gravity is so small that is shouldn't be able to hold on to the rings for very long at all. As the Sun warms up the particles they radiate the heat away in all directions. But the Sun's heat is only coming from one direction, an imbalance that over time will give them a little kick, enough to escape Chariklo's tiny gravity. Either the rings are very young and won't be there much longer, or Chariklo has moons (that we haven't spotted) holding the rings in place. 

The discoverers suggest a few ideas about how the rings may have formed. Maybe something bumped into Chariklo, throwing up material off its surface but not so fast as to give it enough force to fly off entirely. Alternatively Chariklo may be slightly active, blowing stuff off itself in weak volcanic eruptions. Perhaps it could once have had two moons, which collided to form the ring.

We may never know the answers.  But even if the rest of Chariklo's secrets remain hidden, its intriguing and mysterious rings show us just how much we have to learn about space, even in our own back yard.

Follow me on Twitter for more blogs and science news. In other astronomy-related news, please don't buy or read the Mail. Ever.

A more traditional set of rings around Saturn. You're in this picture: Small dot, bottom right.

     

The Curious Core of Mercury

After a three decade long gap without a visiting spacecraft, the Messenger spacecraft closes in on Mercury. This first image revealed a whole new side to the closest planet to the Sun (Literally: no one had ever seen this side of the planet before).

A Random Tour of the Solar System, Part One.

Hello! After realising that this blog was beginning to turn into "exciting space news of the week", I've decided to mix it up a bit and embark on an irregular tour of the Solar System. My vague aim is to get through all eight planets by the end of my PhD, but we'll see...

Basic facts and figures about the planets are available across the internet, so instead of a general overview I'm going to concentrate on one or two things I find interesting about each planet. First up is the both the smallest planet and the closet to the Sun, Mercury!

Space can be a dangerous place.

On Friday two astronomers from the University of Bristol came to talk to us about their work in trying to figure out how solar systems are formed. This is a very active area, spurred on by the huge numbers of exoplanets, planets in orbit around stars other than the Sun, that have been discovered in the past twenty years.

Although we have the rough series of events that cause planets to form figured out, the details are proving tricky. The models and simulations are very inefficient, only producing planets in a few cases. This is a problem, because planets are everywhere. Just last week, the team working on the Kepler space telescope, a dedicated planet hunter, announced the discovery of 715 new planets (although opinion amongst the exoplanet PhD students at Warwick is divided as to how much we can believe that result).

Teams around the world are working on how to explain how all of these planets came to be. The group from Bristol is focusing on how the newly forming planets interact with each other, and in particular what happens when they collide.
 
The evidence for those collisions is all around us. The leading theory as to how Earth gained a Moon so similar in size to it involves a massive collision between the young Earth and another object, roughly as big as Mars. From the huge cloud of debris left after that cataclysmic event came the Earth and Moon we know today.

Further out in the Solar System, the ice giant Uranus orbits on its side, again most likely the result of a massive collision. And a collision may have paid an even bigger part in the history of Mercury.

In 1974 the Mariner 10 spacecraft became the first man-made object to visit Mercury, swinging past in a brief flyby. As Mariner 10 curved around the innermost planet, a rocky ball with a diameter only a third of the size of the Earth's, the tiny spacecraft's orbit was subtly shaped by Mercury's gravity. Because of this, astronomers were able to accurately measure the mass of the Solar System's smallest planet for the first time.

What they found was surprising: Mercury appeared to be far too heavy, a much denser object than they had expected. The density of a planet should increase as its mass goes up, with the material squeezed tighter and tighter by the increased gravity. The results from Mariner 10 showed that Mercury has a much higher density than Mars, the second smallest planet, despite having only half of the mass.

This means that Mercury's iron core, which only makes up a small part of the volume of the other rocky planets, must be huge. The core takes up 85% of Mercury's volume, most of the planet. In contrast, Earth's core makes up only 15% of its volume.

Comparison of the interior structure of Earth and Mercury. The core, which makes up only a small part of the Earth, takes up the bulk of Mercury's interior.

According to our current knowledge of how to make a planet, Mercury could never have formed with a core that big. The structure of a planet is determined firstly by how large they are, but also what they're made of.

Once they get big enough that their interior melts, the heaver elements such as iron sink to the middle and form the core, leaving behind the lighter rocks that make up the mantle and crust above it. This process, known as differentiation, has occurred on all of the rocky planets, as well as some of the larger asteroids.

If all of the planets formed out of roughly the same mix of stuff, which we've no reason to believe that they didn't, then a planet the size of Mercury simply wouldn't have enough iron to form such a large core. A core that big should have formed a much larger planet around it. Something strange happened to Mercury, leaving it with a lot less material that it should have had.

There are a number of ideas as to what that strange thing was. When the solar system was first formed the Sun was a lot more variable than it was now, going through massive temperature swings as it settled down into being a star. If Mercury formed close in to the Sun when it was relativity cool, the next time the Sun heated up it could have melted the new planet's outer layers, blowing them away in a intense solar wind.

Another suggestion also points to Mercury's closeness to the new star. Mercury is only just over a third of the distance away from the Sun than the Earth is. That close, the protoplanetery disk of gas and dust, out of which the planets formed, was very dense. Many of the small clumps of material know as planetesimals, the first stages in building a world, would have been caught by the friction of the disk, spiralling down into the Sun before they could begin to form planets. This would have affected the less dense material more, leaving only the heavier stuff to be built into Mercury.

The third explanation is a lot more dramatic. There is one very easy way to get rid of a large chunk of a planet: hit it with something big. Very big.

Mercury's huge core may have been the result of a massive collision, a glancing blow blasting away its outer layers.

The collision hypothesis suggests that another roughly Mercury-sized planet, long since gone, smashed into it soon after it formed. Mercury was dealt a glancing blow, blasting away its outer layers.

The debris would have been quickly scattered away, pushed by the gravity  of nearby Venus and accelerated away by the effects of the Sun's heat, before it had a chance to collapse back onto Mercury. The molten ball of iron and rock left by the collision would then have slowly cooled into the Mercury we see today.

Mariner 10 flew past Mercury twice more, flying too fast to go into orbit, before heading off into space, never to encounter the planet again. Without enough information, which of the three ideas about how Mercury gained its massive core was correct would remain a mystery for over thirty years.

Shielded from the intense heat from the Sun, Messenger closes in on Mercury in this artist's impression

On the 3rd of August 2004 the Mercury Surface, Space Environment, Geochemistry and Ranging, or Messenger, spacecraft blasted off from Cape Canaveral in Florida. Four years later it flew past Mercury.

Like Mariner 10 it would fly past twice more. Unlike Mariner 10, Messenger would be back again, and this time it would be there to stay. In March 2011 Messenger became the first man-made object to enter into orbit around Mercury.

The environment Messenger found itself in is, to say the least, harsh. To protect it from the heat of the Sun, one side of the spacecraft is hidden under a thick shield. But the surface of Mercury can reach temperatures of over 400 degrees Celsius during the day, heating up space around it and threatening Messenger's unprotected underside. Messenger therefore was put into a very elliptical orbit, spending most of its time away from the planet before quickly swooping down to study the baking hot surface below.

There Messenger used its array of instruments to try and shed light on the mystery of Mercury's massive core. Far more advanced than Mariner 10, Messenger was able to study the chemistry of the rocks in Mercury's crust in detail.

In particular, Messenger was looking for potassium, along with uranium and thorium. Potassium is part of a group of elements known as volatiles, with a much lower boiling temperature than the other two. If Messenger found that Mercury had a lot less potassium than uranium of thorium, it would suggest that the surface had once been molten, supporting either the collision hypothesis, or the idea that Mercury had been melted by a hot young Sun.

However, Messenger found just the opposite. There was more potassium than uranium or thorium, showing that the surface couldn't have melted. It seems like that dramatic collision never happened.

That puts us back to square one in many respects, as the third theory, the one which didn't require melting, is far lest developed than those that did. The leading theory now suggest that Mercury formed out of iron-rich meteorites and comets, but planetary scientists are still a long way from a complete explanation for how this odd little planet came to exist.

Messenger is still in orbit, continuing to study the solar system's smallest planet. In 2023 it will be joined by the massive (and over budget) Bepi Columbo spacecraft. Maybe then Mercury will finally give up its secrets.

I seem to have got a bit carried away taking about the core, so I'll have to leave my second Mercury mystery for another post. For now, I'll just tell you what it is. Mercury, which as I mentioned has daytime temperatures of upwards of 400 degrees, appears to have water ice at its south pole. Ice! There's a lot more to Mercury that just a big core.

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The areas of Mercury that receive little or no sunlight, shown in orange . Protected from the roasting heat, Messenger has detected water ice in the deep craters at Mercury's south pole

What the Billion-Star Mapper Gaia Will Tell Us...

Artist's impression of Gaia as it begins its mission to map out the Milky Way

On 19th December 2013 astronomers around the world watched with baited breath as a Soyuz rocket blasted off from the European spaceport in French Guiana. Nineteen days later the rocket's precious cargo reached its operational orbit, 1.5 million kilometres away from the Earth. Then, last Thursday, the European Space Agency's newest flagship, Gaia, send back its first clear image.

Image of a star cluster in the Large Magellanic Cloud, taken to check that Gaia's telescopes are properly focused. 

It isn't much to look at. A simple patch of stars, a cluster in the Large Magellanic cloud. But Gaia isn't designed to take pretty pictures. In fact, its not going to take many pictures at all: This image is simple for calibration, a test to make sure that Gaia's twin telescopes are focused and working.

What Gaia will do though is rather special. Over the next few years, Gaia will return to this patch of stars and observe it again. And again. And yet again, seventy times in all. And it will do the same thing for another billion stars.

Gaia's mission is to make a map of the galaxy. For those billion stars, it will return data telling us how bright they are, what their colour spectrum is like, what their position is. And by doing this seventy times for each star, it will allow us to see how each of those things changes over time.

Measuring distances in space is very difficult. You can't do it based on how bright things are, because how do you know if the star you're looking at is dim but close, or bright and far away? Knowing the true brightness of an object, known as the absolute magnitude, is one to the key things needed to start to say more about it than "that's pretty". But to know the absolute magnitude, we need to measure the distance...

In the last post I mentioned standard candles, objects that we know have the same absolute magnitude. Comparing the apparent brightness of these objects can be used to measure distance, but it's only a relative scale: It just tells us how far away one standard candle is compared with another. If we want to know how far away an object is in actual units, like light-years,or the more commonly used parsec (3.26 light-years), we need a different technique.

How to measure distances with Stellar Parallax : By comparing the position of a star relative to background stars at different points in its orbit Gaia will be able to accurately measure the distance to one billion stars, 1% of the stars in the Milky Way.

The technique Gaia will use, stellar parallax, is simple in principle. Hold one finger out in front of you, then close one eye. Open that eye, and close the other. Your finger will appear to move against the background, a consequence of the slightly different angles that your eyes are seeing it from. If you measure how far it moved and how far apart your eyes are, a simple bit of maths will give you the distance between your face and your finger.

Instead of two eyes, Gaia will use the orbit of the Earth around the Sun to create the parallax it needs. If it observes a star twice, six months apart, it will have travelled to the other side of the Earth's orbit around the Sun in between the observations. This will allow it to pinpoint the distance to the stars that it measures, building up a map of the Galaxy.

Simple in principle, but not so in practice. Because of the vast distances between the stars, the difference in their position caused by parallax is tiny. Gaia will have to measure changes in position of just 24 microseconds, a distance in the sky 75 million times smaller than that taken up by the Moon. Gaia could see both sides of a human hair one thousand kilometres away. What's more, most of the stars it will observe are thousands of times fainter than those we can see with our eyes.

To achieve this amazing precision has required building one of the most advanced spacecraft ever flow. Inside its body, a squat 3.5 metre cylinder, Gaia carries two rectangular telescopes, with ten mirrors focusing light down onto a billion pixel camera, the largest ever put into space.

Beneath the telescope is a complex system of mircothrusters, tiny rockets that keep Gaia in a perfectly balanced, slow spin, scanning across the sky. At the base of the spacecraft is a huge sunshield, a ten metre wide circle of insulating material that keeps Gaia at a stable temperature. The sunshield had to be folded up to fit into the rocket. Had it failed to open, the mission would have been over before it began.

But unfold it did, allowing Gaia to fly to L2. a point in space 1.5 million kilometres away where the gravity of the Earth and the Sun are balanced. Here Gaia will travel around the Sun at the same pace as the Earth, allowing us to easily keep in contact, but is far enough away that the heat and light reflected off Earth will not affect its measurements.

Gaia in the clean room before launch, testing the deployment of its massive sunshield. The telescopes are housed in the main cylindrical structure, with power provided by solar panels underneath the shield.

Those measurements promise to be spectacular. As well as measuring the position of a billion stars, it will also tell how those positions change over time. Our map of the galaxy will be a moving one, allowing us to explore the dynamics of the Milky Way like never before. We'll be able to test our ideas of how it was formed and how it evolved into what we see now, as well as what will happen to it in the future. Gaia may even shed light on that most elusive of substances, Dark Matter.

Not all the movement of stars is caused by parallax. Gaia will be able to detect the tiny tugs on stars caused by the gravity of orbiting planets, many of which are completely invisible to our current methods of planet-hunting. Gaia is predicted to detect seven thousand new exoplanets, double the current number of candidates.

In addition to this, Gaia will detect hundreds of thousands of asteroids in our own solar system, thousands of previously undetected brown dwarf stars, supernova in distant galaxies, and explore the cores of active galaxies, the quasars.

Beyond its predicted results, the most significant result from Gaia will be the huge catalouge of measurements it will create, seventy billion results that will keep astronomers busy for decaeds to come. Gaia's most exciting discoveries will be things that we haven't even thought of yet.

Almost every astronomer has something they want Gaia to do. I've lost count of the number of times I've heard phrases along the lines of "Gaia will tell us..." since starting my PhD, whether its the answer to a great mystery of science or simply the exact distance to a favourite star. Personally, I'm hoping to use Gaia to find the first planet around a White Dwarf star, but it will be a few years before that is possible and I certainly wont be the only one looking!

Gaia's first batch of data will be released to the public in October next year. Expect some very interesting astronomy news soon after...

Ice cascades off Gaia's Soyuz launcher as it blasts off into a new age of astronomy

For new blogs and other science news, follow me on Twitter.

Talking of new blogs, does anyone have any requests for space subjects they'd like me to talk about? I realised the other day that my plans for the next three posts involved two spacecraft and a rocket, which isn't a particularly wide area! I'm going to try and cover a wide variety of areas within astronomy, but if anyone asks for one subject in particular I can make that a priority. (If the answer for anyone is exoplanets, then I recommend Lost in Transits, a blog by another PhD student here at Warwick.)
 

The M82 Supernova SN2014J

Something went bang in the night...

I thought I was doing well last week. Tuesday evening I had my Rosetta blog finished, ready to publish the next morning. I was sure people would still be interested in it two days after the event. I mean, it's not as if the closest supernova since 1987 was going to go off during the night?

Ahem.

At 7.20pm last Tuesday evening a group of students together with their Professor, Steve Fossey, were in the UCL observatory for a practical astronomy class. It was a cloudy night, and the only bright thing left to look at in the sky was the Cigar Galaxy, officially known as M82. Turning their telescope on it, they noticed something odd: A point of light near the centre of the image so bright , it was outshining the rest of the galaxy. Quickly turning another telescope on it, they saw the same thing. A star had exploded in one of the most violent and energetic event in the Universe. A supernova.

The discovery of the supernova (bottom image) compared with an older image of M82 above it.

Fossey contacted the Palomar Transient Factory (PTF), a collection of linked telescopes in the USA that specialise in studying supernova and other unique events. They confirmed that it was a supernova and, at just twelve million light years away, the closest one to Earth since 1987. As the tenth supernova discovered so far this year, it was christened SN2014J.

They then took a spectra, splitting the light from the explosion into its separate wavelengths, or colours. Different materials absorb or release light at different wavelengths, so by looking for dark gaps or bright spikes in the spectra the PTF team could see what the exploding star was made of.

Although they are all exploding stars, the type of star which causes the supernova can vary. Many are the deaths of huge stars, eight times the mass of the Sun or more, collapsing in on themselves then blowing to pieces as they run out of the hydrogen fuel they need to hold themselves up. It was a supernova of this kind, known as core-collapse supernovae, that produced the millisecond pulsar I talked about in the first post. SN1987A, the closest recent supernova, was one of these.

The closest recent Supernova, which went off in 1897 in the Large Magellanic cloud, a dwarf galaxy orbiting our own Milky Way. This picture  was taken by the Hubble Space Telescope in 2006, showing the still-expanding cloud of debris from the massive explosion.

The spectra of the M82 supernova showed that it was something different. SN2014J is a Type 1A supernova, the explosion of a White Dwarf star.

White dwarfs are the leftover cores of stars like our Sun. Stars are held in a delicate balance, stopped from collapsing under their own weight by the energy generated by hydrogen fusion in their cores. When this fuel runs out the core starts to collapse, whilst the rest of the star expands into a red giant. The outer layers are gradually blown away, leaving behind a hot, dense core of carbon and oxygen.

White dwarfs are strange objects. They are so dense that they have around the same mass as the Sun squeezed in to an object no bigger than the Earth. They are held up only by something called electron degeneracy pressure, the fundamental rule that two things can't be in the same place at once.

This pressure can only hold up so much. If the mass of the white dwarf goes over 1.44 times that of the Sun then it collapses, exploding so violently that it releases more energy than the entirety of the galaxy that contains it. These are known as supernova progenitors.

How to get a white dwarf above that 1.44 Solar mass limit is a subject of debate. For decades the accepted model was that the white dwarfs are in binary systems with another star. When that star turns into a red giant, its outer layers get close enough to the white dwarf that the gravity of the white dwarf beings to pull the material of the red giant onto itself. If this carries on for too long the white dwarf slips over the mass limit and it's all over.

Artists impression of the traditional model for the cause, or progenitor, of a Type 1A supernova. Material from a Red Giant flows onto a White Dwarf, causing it to grow above 1.44 Solar masses and explode. 

Although that model makes sense, actually getting the physics to work has been tricky. Few computer simulations can get the white dwarf to explode. In many of them the material being dumped onto the white dwarf is simply blown away before it can build up to the limit.

This has lead to suggestions that instead of a white dwarf taking matter from a red giant, Type 1A supernovae may be caused by the collision of two white dwarfs. This model works better in simulations, as well as providing a better explanation for some observations.

Working out which of these options cause Type1A supernovae is important because of one crucial difference. In the white dwarf-red giant model, every white dwarf explodes at exactly 1.44 Solar masses. With two white dwarfs, the double degenerate model, that's not necessarily true. And that has implications for the spookiest ghost in physics: Dark Energy.

If white dwarfs always explode at 1.44 solar masses, then the explosion will always have roughly the same brightness. This means that if we see two Type 1A supernova, one of which is brighter than the other, then the dimmer supernova must be further way. Objects with that behaviour are known as standard candles, and are our main way of measuring distances in space. By observing Type 1A supernova in far away galaxies, we can measure the size of the Universe.

In the late 1990s Type1A supernovae were used to investigate how quickly the universe was expanding. The expectation was that as time went on the expansion of the Universe will slow down, as gravity begins to overcome the initial outwards explosion of the Big Bang. The Type1A measurements showed the opposite. The expansion of the Universe is accelerating, and we don't know why. The cause of the acceleration is known as Dark Energy, although the only things we know about it are that it isn't dark and it probably isn't energy.

Should Type1A supernovae turn out not to be standard candles then this whole measurement and much of modern cosmology would be thrown into doubt. What makes SN2014J so interesting then is that it is close enough to investigate what the progenitors were. 

Already the Hubble Space Telescope has swung round to take a look, joining a host of other telescopes in space and around the world to study the new supernova. Meanwhile astronomers are scouring older pictures of M82, trying to spot what stars were at the exact spot where the supernova went off.

Hubble image of the new supernova. Astronomers everywhere have been shocked by the appearance of a giant arrow over M82...

My own experience with the supernova has been quite interesting. The news was breaking as I woke up last Wednesday. I tweeted about it, and a few hours later was emailed by a Spanish science news website asking for more details! The article is in Spanish, but Google Translate does a fair job on it.

Because this supernova is so close, anyone can take a look. It's not quite bright enough to see with the naked eye, but a small telescope should be able to spot it. Over the next week or so it it should brighten so much that it will be visible through binoculars. M82 is just above the Plough, near the North Star.

I hope you get a chance to go out and see it. Space science is often a long process, with missions and observations taking years to come to fruition and gives results. Events like this, coming out of nowhere, are relatively rare, but have the potential to reshape our knowledge of the Universe.

For new blogs and various other science news (including any more nearby supernovae), follow me on Twitter. 

UPDATE: On 27th February, I finally managed to get the telescope out and have a look. I only have a small telescope so M82 wasn't much more than a fuzzy blob, but there was definitely a small point of light just off centre. Success!

Esa's Comet Hunting Rosetta Probe Wakes Up

Rosetta deploys Philae for the first ever landing on a comet in this artist's impression

In space, no one can hear your alarm clock.

At exactly 10am GMT on Monday morning, 807 million kilometres from Earth, an alarm clock went off. For the first time in two and a half years, a spacecraft stirred into life. Heaters switched on, warming up star trackers so that the spacecraft could find out where it was. Thrusters fired, stopping a slow spin that had kept its solar panels facing the Sun during it's long sleep. Six hours after the first alarm, enough systems were active for Rosetta to point at it's distant home and tell its creators that it was awake.

At 6.18 that evening, 48 minutes into the one hour window that Rosetta had to contact Earth, the message was received. A ten year voyage is about to come to a dramatic conclusion.

Launched in 2004, Rosetta's mission looks fairly straightforwards on paper. Fly out to Comet Churyumov-Gerasimenko, a four kilometre wide ball of ice and dust, go into orbit around it, and deploy a small lander called Philae to touchdown on the surface. Sounds simple enough.

(Comet Churyumov-Gerasimenko also rejoices in the the much more pronounceable but less impressive name of 67P)

This mission would however require one on the most complex journeys ever taken by a spacecraft, a six billion kilometre quest around the inner solar system in order to catch up with Churyumov-Gerasimenko.

Comets travel around the Sun in wildly different orbits to planets like the one we live on. Churyumov-Gerasimenko orbits the Sun once every six and a half years, sweeping out a huge ellipse between the mostly circular orbits of Earth and Jupiter. At closest approach it is three times closer to the Sun than when it reaches the edge of it's orbit. 

This means that, at any given point in its orbit, Churyumov-Gerasimenko is travelling at a very different speed to the planets around it, as well as any spacecraft that launch off those planets. Previous missions to comets, such as the rather spectacular Deep Impact, haven't had to worry about that, simply flying past the comets at tremendous speed in one brief encounter, desperately snapping pictures as they went.

However, for Rosetta to go into orbit around Churyumov-Gerasimenko it will have to be able to match speeds with the comet, slowing down enough to be captured by the comet's weak gravity. No rocket on Earth was (or is) capable of providing the change in velocity, or delta-V, needed to accomplish this feat. 

Instead Rosetta has spent the last ten years looping backwards and forwards between the inner solar system. Flying past first Earth, then Mars, then twice back around Earth, Rosetta has used the planets' gravity to sling it out into the orbit it needs to catch Churyumov-Gerasimenko at just the right moment. 

As Rosetta swung by Earth for the last time in 2009, it took this rather spectacular picture.

Speeding away from the inner solar system, flying past and imaging two asteroids on the way, Rosetta's orbit carried it out beyond the orbit of Jupiter. 800 million kilometres from the Sun, the spacecraft's two 14 meter long solar panels could no longer provide enough power to keep Rosetta alive. Shutting down everything except for the main computer and a few heaters, Rosetta went to sleep for two and a half years.  

Rosetta's awakening on Monday was both a technological and media triumph.Every major new website was running with the story. It made the front page of the Guardian and the Independent, #WakeUpRosetta trended on Twitter and Newsnight gave Jeremy Paxman an excuse to show off his scientific ignorance.

It's easy to see why. Esa could have talked about a preprogrammed reactivation sequence and establishing contact. Instead they described it as an alarm clock going off, as Rosetta waking up and calling home.

By talking about it in these very human terms, people have connected with the space mission far more than if it had just been seen as a technological marvel. Rosetta waking up has reminded people that our spacecraft aren't just computers and wires. They're us, an extension of our desire to explore, to investigate our home around us. Robots that we cast out into space to go where we can't go (yet).

Rosetta may be awake, but its real mission hasn't even started. The next few months will be spent testing the eleven scientific instruments on board, ready for when it finally catches up with Comet Churyumov-Gerasimenko in May.

Once there Rosetta will study the comet as it plunges towards the sun, watching it change from a dormant ball of gas and dust to one of the most active objects in the Solar System. By August it will have closed into within twenty five kilometres: close enough to enter orbit. Throughout the approach it will have to dodge eruptions of gas and rock as the comet heats up and begins to develop a tail. Any impact to Rosetta's delicate instruments or solar panels could prove fatal.  

Rosetta will map the comet in detail, providing a new understanding of the processes that take place on these tiny objects. It will sample the gas and dust streaming off the comet, studying it's chemistry and behaviour.

Churyumov-Gerasimenko represents a unique laboratory, a sample of the primordial building blocks of the planets untouched since the dawn of the solar system. By understanding this tiny world we will be able to shed light on how our world and everything around it came into being.

Perhaps the most interesting results will come when Rosetta samples the water that will start to pour off the comet, eventually creating a massive tail behind it. Our current ideas about how Earth was formed suggest that almost all of the water that life relies on was delivered by impacting comets (A theory backed up by some work in the area I'm studying for my PhD). Rosetta's discoveries at Churyumov-Gerasimenko could help see if that really was the case or not.

Rosetta's images of the comets will provide more than just pretty pictures. They will also be scouting the comet out for a suitable landing site. Sometime in November a small spacecraft called Philae will detach from Rosetta to attempt the most difficult part of the mission: The first spacecraft to land on a comet.

If all goes to plan, Philae will drift down from a height of around one kilometre, using a harpoon to drag itself onto the comet's surface. Churyumov-Gerasimenko's gravity is too small to hold it in place, so once down Philae will drill into the surface, clinging onto the side of the increasing volatile comet. There it will deploy nine experiments of it's own, capturing hopefully superb close up images and taking samples from under the surface. Philae is expected to last around a week, but could return data for many months.

If everything goes to plan, by the time the mission finishes in 2015 Rosetta and Philae will have revolutionised our knowledge of comets and answered some of the biggest gaps in our knowledge of how our solar system, our planet and everything alive on it came to exist. Not only that, but the images of a comet exploding into life as it reaches the heat of the inner solar system should be spectacular.

Comet ISON grows a spectacular tail. Soon Rosetta will show all of us how this happens from the inside.

Rosetta is now awake, hurtling towards it's distant target. The Rosetta stone revealed the secrets of ancient Egypt, but the spacecraft that it gives its name to is ready to unlock even greater mysteries.

New blogs will be posted on Twitter, which is also the best place to keep up to date with Rosetta's progress.


   

Two White Dwarfs and a Pulsar: The Mysterious Triple Stellar System PSR J0337+1715

Artist's impression of the triple stellar system

They're stars, but not as we know them

Last Sunday evening astronomers announced the discovery of a star system so strange that it made the BBC news website. Three technically dead, but very much still kicking, stars that could allow us to explore one of the most important gaps in our knowledge of the Universe.

The paper is surprisingly readable, although I realise I may not be the best judge! If you don't want to slog through it, read on...

The system, with the exciting name of , is made of three stars all orbiting each other over 4000 light years from the Earth. These stars aren't like our Sun: instead they show two of the different things that can happen to a star when it runs out of the hydrogen fuel that it needs to produce energy.

At the centre of the system is a pulsar, the aftermath of the explosive death of a truly massive star. It's a neutron star, an object so dense that it has one and a half times as much mass as our Sun packed into a ball just a dozen or so kilometres across.

Stranger still, this object is spinning at a rate of 366 times every second, sending out a beam of radio light from its poles that can be picked up by telescopes here on Earth. These are know as millisecond pulsars, or MSPs. The telescopes are so good that this beam can be measured to within an accuracy of one ten-thousandth of a second every half an hour.

This precision allowed the observers to spot tiny variations in the arrival time of the beams, suggesting that there was another object close by that was affecting the path of the beam. Looking through old telescope data allowed the culprit to be spotted. It was a faint, blue dot. Another dead star.

This time it was a white dwarf, formed from a star very similar to our Sun. These are the end-points of the evolution of smaller stars, a slowly cooling hot ball about the size of the Earth, with a fifth of the Sun's mass packed in. The white dwarf is orbiting so close to the pulsar that they spin around each other in just a day and a half.

(My PhD involves studying what's left of any planets that were around white dwarf stars. But that's a subject for another day!)

However, the white dwarf couldn't explain all of the timing variations, meaning that there must be a third object, invisible to our telescopes, orbiting the pulsar-white dwarf pair. Only one object fits the bill: A second white dwarf, larger than the first but older and dimmer, orbiting the inner pair at a distance slightly less than that of the Earth from the Sun.

With three dead stars, this system must have had a turbulent history. It would have started as three normal, main sequence stars like the Sun. Two would have been just a bit bigger than the Sun, but they would have orbited around a monster ten times the mass of our star.

The more massive a star is, the shorter it lives. As the central star quickly burned through its fuel it would have swelled up, encasing the other two stars in a common envelope of gas and plasma, before exploding in a supernova to leave behind the spinning pulsar.

About a billion years later the outer star would also run out of fuel. Smaller stars don't blow up. Instead they swell up to form red giants, before blowing away their outer layers and shrinking down into white dwarf stars. As it grew in size material from the outer star fell onto the inner pair, changing the dynamics of the system so that now the stars all orbit each other in almost perfect, flat circles.

After another billion years the inner star would also have died. This time its outer layers flowed onto the pulsar, speeding it up until it was spinning around once every 2.73 milliseconds. The inner star then finished evolving into a white dwarf, forming the system as we see it today.     

The history of the triple star system . Starting as three normal stars, the central star exploded to form a pulsar. The two other stars then became white dwarfs, to leave the system as it is today.

This history is fascinating in itself, but what makes the triple star system even more interesting is its potential to test our understanding of how the entire universe works.

Since 1917 our knowledge of the night sky has been underpinned by Albert Einstein's Theory of General Relativity, an explanation of how gravity holds all of the stars, planets and galaxies in the universe together. General Relativity has passed every test that we've thrown at it and explained everything that we can see.

It's also wrong.

We know that it is wrong because it doesn't fit together with the other great theory of modern physics, Quantum Theory. This explains the physics of the smallest building blocks of the universe, the particles and atoms that make up us and everything around us. Like General Relativity, it has been tremendously successful at explaining the world around us.

But put the two together and it all falls apart. General Relativity is hopeless at explaining the very small, Quantum Theory doesn't show any way to build a galaxy. The search for a theory that would explain both of them, the semi-mythical quantum gravity, is one of the biggest challenges of modern physics.

One of the problems when searching for quantum gravity is that it's hard to find places where the effects of both Quantum Theory and General Relativity apply, a laboratory where we could test them both together. PSR J0337+1715 could provide such a laboratory.

A key feature of General Relativity is the Strong Equivalence Principle. This says that all objects behave the same in another objects gravity, regardless of their own gravity. Showing this is easy: Find two objects that are the same shape, but with different weights. Drop them from the same height, and they will hit the ground at exactly the same time, every time.

This means that two objects orbiting another should orbit in exactly the same way, regardless of how much gravity they have. Planet Earth orbits the Sun in the same way as a tiny satellite. 

But Quantum Theory disagrees. It suggests that there should be very small changes between how objects with different gravity react to something else's gravity.     

We can't test this in our solar system, or in any others that we've seen. The differences in gravity between the planets, moons and asteroids are only spread over a factor of a thousand or so, too small to see any difference that might provide clues to quantum gravity.

The newly discovered triple star system is different. There, the gravity of the pulsar in the centre is roughly one hundred thousand times bigger than that of the white dwarf next to it. By seeing how they react to the gravity of the third white dwarf, we just might be able to see the tiny cracks in General Relativity that would help us to build a new theory of the universe.    

There are some strange thing out there among the stars. But the stranger the things we find become, the closer we get to understanding it all.
 
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P.S. Some of you may be wondering what happened to my last blog, Realising We're in the Future. It's still there, but I just haven't had the time to research topics that aren't related to my work in enough detail. Who knows, it may be back if something particularly interesting happens!