The Strangest Star: A Neutron Star Inside a Red Giant

Artist's impression of a neutron star, which may sit at the heart of a red supergiant in a newly discovered Thorne-Zytkow object.  Image Credit: NASA

Some of the stuff in space is truly weird. Last month a group of astronomers announced the discovery of what might be one of the strangest: A star inside another star.

The star, which has the exciting name HV 2112, is thought to be a Thorne-Zytkow Object (TZO). This type of star, the existence of which was first suggested in 1977, is made up of a red supergiant, a vast dying star many times the size of the Sun, with a neutron star sitting at its core.

Neutron stars are some of the most extreme objects in the universe. The remains of the explosive deaths of massive stars, neutron stars are the densest objects we know of (black holes excepted, because they're just odd).

Most matter, including you, me and everything we see around us, is made of atoms, comprised of a tiny nucleus surrounded by a cloud of electrons. While the common statement that atoms are mostly empty space isn't quite true (there's all sorts of fields and virtual particles flying around in there), the vast majority of the mass of an atom is concentrated into a tiny area relative to it's overall size.

Not so for neutron stars. They, as the name suggests, are made almost entirely out of neutrons, one of the particles found in atomic nuclei. The neutrons are packed in literally as tightly as it is possible to be, making the star ridiculously dense.

Though they have a mass of between one to two times the mass of the Sun, neutron stars never get bigger than about twenty kilometres across. This means that a sugar cube-sized lump of neutron star weighs around a billion tons.
 
(To use the accepted measurement standards of this blog, a 10km radius gives it a surface area of roughly 0.06 Wales)

The TZO would have started off as a binary, two massive stars closely orbiting each other. When the larger of the two stars ran out of hydrogen fuel in its core, it blew up in a supernova explosion, its core collapsing to form the neutron star.

From that point there are two ways by which the neutron star could have fallen into the core of its companion. The physics of supernovae  are still not fully understood, but it is thought that in many cases the explosion may not be symmetric. The explosion could have given the neutron star a "kick", slamming it into its nearby companion.

Alternatively the neutron star may have remained as it was until its companion also reached the end of its life and swelled up into a red giant. The forming red giant would have engulfed the neutron star, slowing it down until it spiralled down into the core of the red giant.

However once the neutron star reaches the core of the companion it begins to look just like every other red supergiant. Without being able to see the buried neutron star, how can we tell that HV2112 is a TZO and not just a (far more common) red supergiant?

There are two main ways to move heat around inside a star. Heat transfer in the Sun is dominated by radiation, where material that gets hot shines out light, heating up material around it and so on. In the Sun energy is transferred by radiation for about 70% of the distance from the core to the surface.

For the last 30% this changes, and energy is transferred by convection currents, where the material itself moves. Hot material expands and becomes less dense, so rises up through the cooler materiel around it until it reaches the surface. It then radiates its heat in to space, cooling down and sinking back into the Sun to be heated up again, and so on. This mix of rising hot material and sinking cooler stuff creates a granulated pattern of convection cells on the Sun's surface (a structure I studied as part of my Masters degree).

Granulation on the surface of the Sun, caused by convection cells formed of hot (light) and cool (dark) material. This phenomena can reveal the presence of a neutron star at the heart of a red supergiant. Image Credit: NASA

It's these convection currents that allow us to distinguish between a normal red supergiant and a neutron star-cored TZO. Unlike the Sun, in a red supergiant the convection zone extends all of the way to the core. This means that material is being regularly transferred from the core to the surface: material that in a TZO has recently been in contact with the neutron star core.

The interaction between the surface of the neutron star and the mass of hydrogen around it ignites a unique nuclear reaction, the rapid-proton process. This creates large amounts of otherwise uncommon elements, such as rubidium, strontium, yttrium, zirconium and molybdenum. The convection currents then carry these material up to the surface, where we can see them with out telescopes.     

By looking at the the spectrum of HV 2112 the discoverers were able to observe all of these elements and compare them with the material they expected to see in the atmosphere of the neutron star. They found much higher levels of rubidium, lithium and molybdenum than they expected, leading them to suggest that HV 2112 had a neutron star hidden at its core. The 37 year search for a TZO may well be over.

The team behind the discovery were careful to point out that, although the signs of HV 2112 being a TZO are all there, there were other signs in its atmosphere that didn't quite fit. Some of the ratios of different metals were different from what we'd expect, and the way the light from the star varied over time was unusual. However none of these anomalies are showstoppers, although they show us just how much more we have to lean about these giant stars.

If it is a TZO, HV 2112 is certainly one of the strangest, most interesting objects around. We'll just have to see what weird and wonderful things we'll find next...

The discovery paper is here. Follow me on Twitter for new blogs.

SpaceX reveal the Dragon V2 Manned Spacecraft.

SpaceX's new Dragon V2 Spacecraft, a seven person capsule that may soon be taking astronauts to the International Space Station. It was revealed to the public on the 29th of June. Photo Credit: SpaceX

On Thursday evening (well, Friday morning here)  SpaceX founder Elon Musk took to the stage to reveal the Dragon Version 2, SpaceX's entry to the race to build the next manned spacecraft for Nasa.

In my not so humble opinion, it looks amazing.

The design is very different from the current, cargo carrying version of the Dragon. Instead of the blunt-nosed capsule based on traditional spacecraft designs, the Dragon V2 is a sleek cone, flanked by pairs of SuperDraco rocket engines.

It's these (3D printed!) engines that are the Dragon V2's key innovation. They are powerful enough to act as a launch escape system, blasting the capsule away from an exploding rocket in the event of an accident. But their primary purpose is to allow the Dragon V2 to do what no space craft has done before: Land propulsively, coming down onto a landing pad using rockets, without the use of a parachute.

 This will hopefully allow it, along with its Falcon 9 launch rocket, to be rapidly reused, massively reducing the prohibitive cost of spaceflight. SpaceX have released an animation showing just how this would work:

The video shows my favourite bit of the new spacecraft: The trunk, the disposable rear section holding cargo and the Dragon's solar panels, has rocket fins! Combined with the propulsive landing, this makes the spacecraft remind me of old-school, 1930s Flash Gorden-esque sci-fi.

The interior of the Dragon V2, with seats for seven astronauts, is incredibly futuristic.Photo credit: SpaceX

The interior, however, is anything but old fashioned. With it's spacious, seven seat design and touch screen controls, the Dragon's insides are radically different from anything thats gone before. My first reaction to it was "that can't be real"- it's just nothing like the cluttered, button heavy Soyuz or Space Shuttle cockpits. But this is the first time we've seen a new manned spacecraft for decades, so we really should expect not to recognize it. It will be interesting to see what the interiors of spacecraft built by SpaceX's competetors look like. Indeed, the glimpses Boeing have released into their CST 100 craft look similarly futuristic.

The Dragon V2 certainly looks beautiful, but will it work? Only time will tell. Pending Nasa funding, SpaceX plan to start flying it into orbit next year, with its first manned flight penciled in for 2016. If all goes well, we could soon finally have a true twenty-first century spacecraft.

Rosetta's Destination Comet Grows a Tail

Comet 67P/Churyumov–Gerasimenko (centre), target for ESA's Rosetta space probe, begins to develop a tail.

Image credit: ESA

Just a short post today, as I'm writing a lot of other stuff at the moment (more on that to follow...).

Back in January I wrote about the European Space Agency's exciting Rosetta mission as it reactivated from a three year hibernation. Last week the mission reached another major milestone, although not one it had any control over.

Rosetta's target, the comet 67P/Churyumov–Gerasimenko, has begun to wake up.

The comet, seen as a fast moving white dot in this series of images taken with Rosetta's main camera, has suddenly exploded into life, beginning to form the huge tail that comets are famous for. As Rosetta closes into orbit with 67P/Churyumov–Gerasimenko in August, we will be able to see this fascinating process unfold up-close for the first time.

Comets actually spend most of their time without tails, orbiting for millions if not billions of years in the far reaches of the Solar System. Occasionally a chance encounter with another object will give them just enough of a gravitational nudge to send them tumbling into towards the Sun.

Once there, the increasing heat from the Sun begins to melt the ice that makes up most of the comet. The ice is embedded and mixed in with rock and dust, so as it sublimates (turns directly from solid to gas) it beings to blast that dust out in volcano-like eruptions. The escaping gas and dust flows out into a huge cloud, or coma, out of which forms the tail. And it really is big: Whilst the nucleus of a comet may only be few kilometers across, the tail can grow to lengths of hundreds of millions of kilometers. That's significantly larger than the Sun, although the coma is much less dense.

Comets actually have two tails, seen clearly in the image of comet Hale-Bopp below (One of my earliest memories is of being taken outside at night to watch this comet as it flew past Earth in 1997). The blue tail is formed of charged particles, or ions, individual charged particles released from the comet. These interact with the magnetic field carried in the solar wind, so stream out from the comet in a straight line directly away from the Sun.

The white tail is formed out of the dust liberated by the escaping water, pushed away from the comet by sunlight. As they fall back from the comet each dust particle moves into a separate orbit, slightly slower than the orbit of the comet thanks to the increased distance from the Sun. This causes the tail stretch out into a curve, as each successive particle out from the comet gets slightly further behind on its orbit.

Thanks to this behavior comet tails are a key tool in our attempts to understand the solar wind, which has a big affect on the satellites and other space technology that modern civilization depends on. One of Rosetta's targets as it closes in on 67P/Churyumov–Gerasimenko is to investigate this relationship between the comet and the solar wind from the inside.

Comet Hale-Bopp, which flew past Earth in 1997. The separate ion (blue) and dust (white) tails can be clearly seen.
Image credit: Nasa. 

Rosetta is closing in on its target, aiming to reach orbit in August. As I hope I've shown you, the show has only just begun...

New blogs and other stuff on Twitter. 

Spaceflight via Russian Trampolines

You can't get off Earth using a trampoline, but you might just be able to escape Mars biggest moon, Phobos. Image: NASA

Since the Space Shuttle was retired in 2011, NASA has had to rely on Russian Soyuz rockets to get its astronauts into space. This hasn't come cheap, costing $70million a seat. But they've had to grin and bear it, as the next American spacecraft capable of carrying humans (probably the SpaceX Dragon) wont be ready until at least 2017.

As you may have noticed in the news, US-Russian relationships are rather strained at the moment, and the Ukraine sanctions are beginning to affect the Russian space industry. Yesterday Dmitry Rogozin, the Russian deputy prime minister, tweeted that:

“After analysing the sanctions against our space industry, I suggest to the USA to bring their astronauts to the International Space Station using a trampoline,”

Now this could have lead to a very interesting long blog post about the politics of space exploration, but there's a far more important question the ask: Could you actually get into space using a trampoline?

The key factor in getting off a planet is the escape velocity, how fast you need to go to escape a given object's gravity. For Earth this value is about 11.2 kilometres per second, which is fairly fast.

How fast can you go using a trampoline?

The Guinness Book of World Records lists the highest height achieved by a team on a trampoline as 6.73 meters, using two people to provide the bounce for a third. With a bit of maths, that means that the speed they were going as they left the trampoline was about 11.5 meters per second.

Not fast enough for Earth, but the Solar System is full of smaller objects. Which is the largest with an escape velocity of less than 11.5 meters per second?

It turns out to be Phobos, one of the moons of Mars, which comes in with an escape velocity of 11.4 metres per second. Its a small world, a lumpy rock with an average diameter of eleven kilometres and a mass nearly two billion times smaller than that of the Earth.

This might be a practical question, as Phobos has been mentioned as a possible destination for Mars-bound astronauts. It takes much less fuel to get there than to land on Mars, and could be used as a base to remotely control Mars rovers without the annoying time delay caused by the signals having to go from Mars to Earth and back.  

So if in 20 years time you're an adventurer stranded on a desolate Martian moon, make sure you've got a trampoline.

Note: Discussions over lunch resulted in the decision that the speed you could jump on the trampoline wouldn't be affected by the lower gravity, as the key factor is the energy you're  producing in your legs. You'd jump higher, but accelerate less slowly down towards the trampoline, affects which would cancel out. If you can do some maths showing this is wrong, please let me know. 

New blogs will be posted on Twitter, as always.

Falcon 9, Skylon and the Future of Spaceflight

It costs over £1000 to send a single kilogram of anything into space. In that one figure lies the ruin of many predictions of the future.

In 1968 Arthur C Clarke and Stanley Kubrick presented the world with their vision of the future in 2001: A Space Odyssey. They imagined a future of mankind in space, complete with bases on the Moon, space planes, voyages to the gas giants and a vast circular space station.

A PanAm space plane approaches the huge Space Station V in 2001: A Space Odyssey...

As far as we've got: The International Space Station is much smaller than Space Station V. Although its still just about the most incredible thing we've ever built.

None of this has happened. Neither have countless other predictions of the future. Wernher Von Braun imagined a complex space infrastructure; Nasa planned to have humans on Mars in the 1980s; The Space Shuttle promised easy access to space for all.

Many factors contributed to this, not least the changing requirements of politics at the end of the Space Race, but the major one has been cost. No one predicted just how ridiculously expensive space flight would be.

One of the main reasons behind this expense is that every time a rocket is launched, at a cost of around $100 million, it is thrown away. These high launch costs lead to even more expensive spacecraft. Without the chance to launch repair missions or backups, satellites have to be built to never fail, with multiple redundant systems. Communications satellites cost hundreds of millions, and the price tags of science missions are often measured in billions.   

It can be argued then that the key to the future of space flight lies in reusability. And true reusability, not like that of the Space Shuttle which ended up costing over a billion dollars per launch.

However getting a rocket back to use again turns out to be rather tricky. Rockets are built in stages to save on fuel. Each part burns through its own stock of propellant before dropping off, leaving the next stage(s) to get the now much lighter rocket into space.

When the first stage detaches it is going very fast, travelling through very thin air. With no thrust remaining, the stage follows a parabolic arc downwards, at which point it smashes into the thicker air near the ground and turns into tiny pieces.

Even if it survives this, the next stop is the ocean or ground, both of which are rather bad for you at high speed. Amazon founder Jeff Bezos recently recovered parts of the Saturn V moon rockets from the Atlantic. There wasn't much left. The space shuttle boosters, which parachuted down into the ocean from relatively low heights, required months of refurbishment after being soaked in salty water.

That's just the first stage. Recovering the later parts of the rocket from space means surviving the extreme temperatures of reentry, impossible without a heavy heat shield. And every kilogram you use on reusability is one kilogram less put into space. 

With these challenges, it isn't much of a surprise that it's taken fifty years for someone to have a go.

With landing legs folded up around its base, the latest SpaceX Falcon 9 rocket blasts off from Cape Canaveral. The first stage would later be successfully landed  into the ocean. Photo Credit: SpaceX 

Last Friday a SpaceX Falcon 9 rocket blasted off on an otherwise routine mission to the International Space Station, (accidentally drenching itself in dirty water as it did so). What made this rocket unique was its choice of optional extras: Landing legs.

The Falcon 9 version 1.1 is deliberately more powerful than it needs to be. So when the first stage separated it had more than enough fuel left to fire up three of its nine engines, slowing it down enough to safely fall back down to Earth. As it reached the ocean with landing legs unfolded, one engine fired again to lower it gently down into the Atlantic. The first stage of a rocket has, for the first time, successfully landed in one piece. 

Stormy seas have meant that SpaceX have been unable (at time of writing) to recover the stage, but that doesn't matter too much. Just getting the rocket down has provided them with the knowledge they need for the next step: Flying the stage all the way back to the pad, ready to be used again.

Being able to reuse a rocket has huge advantages. Currently a Falcon 9, the most cost affective rocket available in the world at the moment, has a price of $56.6million. (I find it amusing that SpaceX list the prices of their rockets online- do they take Paypal?)

However, the fuel for a launch comes to about $200 thousand. Being able to reuse the first, largest stage of a rocket, turning it around with no more maintenance than an airliner gets between flights, could dramatically slash the cost of flying into space. SpaceX already has long term plans to bring back not just the first stage, but the whole rocket.

Using some of the rocket's fuel to retun it to Earth comes with a price, of course: A 30% hit to the amount of payload the Falcon 9 can carry to space. This is where, I think, reusable rockets will benefit all of us. If SpaceX start saying rockets capable of lifting 4 ton satellites cost half a million dollars each, but lifting 5 tons will cost fifty million, then there's going to be a huge pressure to miniaturise technology, much of which will probably end up in your pocket.

Concept art of the Reaction Engines Skylon, a huge space plane scheduled to being flying at the end of this decade

While SpaceX are trying make rockets work like aeroplanes,  across the Atlantic a plan is coming together to turn planes into rockets. The British company Reaction Engines is working on bringing to reality one of the staples of science fiction: The space plane

Known as Skylon, the 85 metre long behemoth will take what is both an old fashioned and radically different way into space. Taking off from a runway, its unique hybrid SABREs (Synergetic Air-Breathing Rocket Engine) will run like jet engines at first, taking in air from the atmosphere and burning it with on board hydrogen fuel.

Then, when the plane reaches five and a half times the speed of sound, the SABREs will switch to rocket mode to take the Skylon and fifteen tons of payload into low Earth orbit. Cargo delivered to space, the Skylon will then fly back to Earth, landing on a runway and ready to fly again in a couple of days.

Whilst space planes have been imagined for years (see the first image), Reaction Engines have only just managed to develop the key piece of technology needed to actually make one fly. At Mach 5.5 the air hitting the engines will be hugely and quickly compressed, causing it to heat up to temperatures of thousands of degrees. To avoid melting the engine, the air must be cooled to well below zero degrees Celsius in a fraction of a second.

The solution is to use a precooler, sending the air between a series of tiny pipes through which the plane's cold hydrogen fuel is being pumped, taking away the tremendous heat. Last year Reaction Engines successfully demonstrated this technology, securing funding to work towards a full scale prototype. If all goes to plan, the first Skylons could begin flying at the end of the decade.

Funding is Skylon's main challenge. Reaction Engines have raised only a small amount of the roughly $10 billion they need to bring the plane to life, mainly from private industries. This is a deliberate policy, as it will only be worth having space planes that can fly every day if there will be something to launch every day. If the space industry thinks not, they simply won't fund it. 

SpaceX's reusable Dragon spacecraft becomes the first privately-operated vehicle to dock with the International Space Station in 2012. Photo Credit: SpaceX

If SpaceX and Reaction Engines succeed in their aims to create reusable spacecraft, and it's a big if, then it could completely change the paradigm of space flight. SpaceX are talking about sending people to Mars for half a million each, whilst Reaction Engines plan to launch dozens of people ta a time in Skylon. Asteroid mining, space tourism, huge space telescopes and more would all become more affordable.

The challenges ahead are huge. SpaceX has yet to recover, let alone re-fly, a single rocket stage. Skylon's mighty SABREs are at this point no more than small prototypes in an Oxfordshire backyard. Both companies may yet fail in ther dreams of reusability, a subject so complex that I've only been able to touch on the basics here.

But if they succed, then we may just get 2001's dream of a future in space. Just a little bit later than pannned.

New blogs and science news on Twitter, as always


   

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