How to Watch the Comet Landing!

On the 12th of November the European Space Agency will make the first ever attempt to land a space probe on a comet. If all goes to plan, the Rosetta orbiter will deploy the Philae lander into a seven-hour drift onto the surface of Comet 67P  Churyumov–Gerasimenko.  I'll do a full blog after the event, but here's some hopefully useful bits and pieces to follow the high point of the Rosetta mission:

The first port of call is the ESA Livestream, where all the major events will be shown and new data announced. If I've made this work right, it should be playing above this paragraph. It can also be found (along with lots of other stuff) on the Rosetta homepage.

Timeline of Philae's seven-hour descent onto Comet 67P. The signal from a successful touchdown should arrive at or after 4pm GMT. Click on the image to enlarge. Image Credit: ESA 

Above are the key points in the landing sequence. A much more detailed version is available here.

Image of Philae's targeted landing site, known as Agilkia. Image Credit: ESA

A bit of a wider view: The image above shows the target landing site on the "head" of Comet 67P. It's been named  Agilka, after an island in the River Nile where the temple from the island of Philae was moved to avoid flooding caused by the building of a dam. Philae was the place where the Rosetta Stone was found.

Apart from the ESA Livestream, the best place to stay up to date with the landing is proably Twitter. I will be tweeting updates and my feed should hopefully be appearing below this paragraph. You should also have a look at #CometLanding.

Good luck Philae!


Tweets by @AstroDave2

ALMA Spots Planets Forming Around a Young Star

High-resolution image of the protoplanetary disc around HL Tau, a young star roughly 450 light-years from Earth. The image, which was taken by the ALMA telescope, shows gaps and rings in the disc carved out by new-born planets. Image Credit: ALMA (ESO/NAOJ/NRAO) 

This morning the image at the top of this page was doing the rounds on Twitter. I, like several others, glanced at it and initially moved on. I've seen plenty of artist's impressions like it before. It took me a while to realise that this isn't a painting. This is a real image from the ALMA telescope, showing the birth of a solar system.

The image shows a star surrounded by a protoplanetary disc, a huge ring of gas and dust around twice the diameter of Neptune's orbit. Invisible at the wavelengths of light that ALMA sees, the central star is a young object called HL Tau, which is  just a million years old. That might seem old, but our own Sun, which is otherwise quite similar to HL Tau, is 4.6 billion years old. HL Tau is a star at the very beginning of its life.

This makes the disc partly expected, but partly mysterious. For the past few decades most models of how planets form have been based on discs like these, the leftover debris from the cloud that collapsed to form the star.  HL Tau is making planets.

Although the entire process is till not fully understood, the theory suggests that slight irregularities in the disc can cause some areas to become more dense. This makes them clump together, growing from dust into small rocks. As they get larger their gravity gets stronger, pulling in more and more material until they begin to look like small planets or asteroids. These planetesimals begin to collide, combining to eventually form planets.  

The ALMA image is a resounding confirmation of this theory, showing this process in action. The disc has huge gaps in it, gaps which are carved out by newly-forming planets. This is the mysterious part, as the presence of these very well defined gaps, at such a young star, show that the planets must be growing much quicker than many simulations suggest.

Not all of the gaps will have planets in them. Some of them will be formed by resonances. This means that, for example, an area of the disc could be going round the star a certain, precise  number of times in the time it takes a further out planet to go round once.

For example, a dust particle in the disc could be going round the star four times for every time planet, which is further away form the star, goes round once. The planet and the dust will then be lined up at exactly the same place each time the planet goes around the star. The gravity of the planet will give the dust an identical tug or kick each time, moving it out of it's orbit.

As this will happen to all of the dust in the same resonant orbit, eventually a gap is cleared. We see the same behaviour in this Solar System- the many rings of Saturn are shaped and sculpted by moons in resonant orbits. Which of the rings in HL Tau are formed by planets, and which are cleared out orbital resonances, will take more observations to find. The full research paper on this observation is yet to be published, so maybe we'll find out then.

Hubble Space Telescope image of the clouds of star-forming gas and dust around HL Tau. Image Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA

HL Tau is in the constellation Taurus, currently visible in the late evening in the Eastern sky, near the Moon. But you wont spot the disc, it's far too small.

This leads to the second incredible part of this image: The resolution. The image was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), a huge array of 43 (and counting) telescopes designed observing in the submilliter wavelength range, between infrared and radio waves. The telescopes are spread out by up to fifteen kilometers, allowing them to take extremely sharp images.

ALMA can distinguish between objects separated on the sky by just 35 milliarcseconds. For comparison, your eye has a resolution of around one arcminute- nearly two thousand times worse. Resolution like ALMA would allow you to see both sides of a penny placed over one hundred kilometers away.

It's this high resolution that has allowed ALMA to see the protoplanetary disc in such exquisite detail, picking out the tracks formed by the new, growing planets. And this is in many ways just a proof of concept, a test of ALMA's capabilities. Hopefully we'll be seeing many more amazing things with this telescope in the future.

New blogs will be posted, as ever, on Twitter.

COMING SOON: Next Wednesday (12th) The Rosetta spacecraft will deploy the Philae lander to make the first attempt to land on a comet. I'll be tweeting and blogging along, and I highly recommend keeping track of, in my opinion, the most exciting space event of the year.

Hubble spots Comet Siding Spring flying past Mars

Comet Siding Spring makes near miss of Mars in this image form the Hubble Space Telescope. Click to enlarge! Image Credit: NASA, ESA, PSI, JHU/APL, STScI/AURA 

Last Sunday the comet C/2013 A1  Siding Spring flew past Mars at a distance of just 140 thousand kilometers, or one third of the distance between the Earth and the Moon. It's the closest we've ever seen a comet get to a rocky planet- so close, in fact, that initial observations suggested it might even hit.

Data and observations are pouring in from the flotilla of orbiters around the Red Planet, and I'll certainly write more on this story as more results become available. For this blog post though I just want to show this amazing image, taken by the Hubble Space Telescope, showing Siding Spring at its closest point to Mars. Click on the image to zoom in.

This photo, which is defiantly now on my list of favourite space images, is actually a composite of two observations, one of Siding Spring and another of Mars. Although both objects would have fitted in the field of view of Wide Field Camera 3, the instrument used to obtain the image, Mars is around ten thousand times brighter than the comet. An exposure long enough to see any detail in Siding Spring would have captured Mars as just a shining white blob! A second problem that Siding Spring was moving across the sky much faster than Mars. Hubble had to track across the sky in time with it's motion, so a picture of Mars taken at the same time would have been a blur.

Photographic trickery aside, the result is incredible. I especially like the amount of detail on Mars, as well as the structure visible in Siding Spring's tail.

Much more on Siding Spring to come! Followed by the the main comet-related event of the year on 12th November, when the Rosetta spacecraft will send down a lander to make the first attempt at landing on a comet.

As always, follow me on Twitter for more stuff about space.

Two new spacecraft join the Mars flotilla

The best view of Mars form Earth, taken with the Hubble Space Telescope. A growing number of spacecraft have been sent to study the Red Planet from close-up, including MAVEN and MOM ,which arrived this week. Image Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA

On the 14th of July 1965, Mariner 4 became the first spacecraft to successfully flyby Mars, providing the first close-up images of the fourth planet from the Sun. Since then a host of spacecraft from several nations and space agencys  have flown past, orbited or even landed on Mars (along with many, many failures).

Growing interest in an eventual human mission to Mars has seen a surge in such missions over the past few years, most of them successful. Last week two new spacecraft joined the international flotilla of orbiters and rovers, including India's first interplanetary mission. So here, in order of arrival, are all of the active missions and what they're teaching us about Mars

Mars Odyssey

Artist's impression of Mars Odyssey, the oldest active spacecraft at Mars. Image Credit: NASA/JPL-Caltech

The first spacecraft to arrive at Mars in the twenty-first century was Mars Odyssey. Named after the book (and film) 2001: A Space Odyssey, this NASA orbiter reached Mars in, appropriately enough, 2001. Designed to study the chemistry of the Martian surface, its key discovery was the detection in 2002 of vast amounts of water ice lying just below the ground (click that link to be amazed by 12 year old internet...)

Whilst it has continued to make scientific observations, Odyssey has in more recent years fulfilled a vital role as a communications relay, transmitting information from the various landers and rovers on the surface to Earth and relaying commands back. I quite like this fact- we're beginning to build a space-based communications infrastructure at another planet!

Mars Express

Mars Express, the first European Mars orbiter. The long booms form the MARSIS sub-surface sounding radar, used to map the geology of the top few kilometres of the Martian crust. Image Credit: NASA  

Arriving in December 2003, Mars Express was the first European Space Agency mission to another planet. Based on the design of my current favourite mission, Rosetta, and sister craft to the near-identical Venus Express, Mars Express carries instruments to measure the chemical composition of the Martian atmosphere, surface and even subsurface. It also has a nifty spectroscopic camera allowing it to take high resolution, 3D images of Mars and, thanks to its unusually elliptical orbit, Mars' largest moon, Phobos.

In 2004 the spectrometers on Mars Express made an intriguing observation: signs of what could have been methane in the atmosphere. Methane should only last a few hundred years in an atmosphere before it reacts with the other chemicals around it, so for it to be present in detectable amounts means that something must be producing it. We know of several geological processes that could achieve this, but most methane production on Earth is biological. Could Mars Express have seen signs of life?

Mars Express also carried a lander, the British-built Beagle 2. Sadly however the landing was a failure, and contact was lost with Beagle 2 shortly after it entered the atmosphere on Christmas Day 2003. The reason for its loss is still unknown.

Opportunity

Panorama of Endurance Crater taken by the Opportunity Rover in 2004. One of the rover's solar panels can be seen in the bottom right (click to make bigger). Image Credit: NASA/JPL/Cornell 

The Mars Exploration Rover Opportunity is currently over ten years into a 90 day mission. Yep, you read that right.

January 2004 saw the arrival of two identical, six wheeled rovers on Mars. Following on from the highly successful Pathfinder mission, Spirit and Opportunity parachuted through the thin atmosphere and landed via an innovative airbag system. Original planned to last just three months and drive around a kilometre across the surface, both rovers far exceeded their targets. Spirit became stuck in sand in 2009 and didn't survive the winter (the xkcd on the topic is essential reading), but Opportunity is still going strong, having covered a distance of over 40 kilometres.

The full list of discoveries made by this stupendously successful mission would be several posts on its own, so in the interests of word count I'll talk about just one. Early in its mission Opportunity was sent to investigate the wreckage of the heat shield that had protected it during its entry into the Martian atmosphere. Near the heat shield was a strange, dark-coloured rock, out of place with the geology around it. Opportunity had discovered the first meteorite on anther planet.

The meteorite, dubbed Heat Shield Rock, was a lump of iron and nickel leas than half a metre across. Its existence was a mystery: Mars' thin atmosphere couldn't have slowed it down enough to stop it vaporising when it hit the ground. So perhaps at some point in the past Mars had a much thicker atmosphere, an atmosphere that it has since lost?

Mars Reconnaissance Orbiter
   

Over 250km above the Martian surface, the Mars Reconnaissance Orbiter easily spots the 1.6 metre long Opportunity. Image Credit: NASA

Since the before the start of the Space Age, landing people on Mars been high up the wish-list of things to do in space. A key requirement for that, as well as for larger robotic landers, is high-resolution mapping of the Martian surface. In 2006 the Mars Reconnaissance Orbiter (MRO) arrived to do just that.

Significantly larger than its predecessors, MRO's main instrument is 0.5 metre downwards-pointing telescope. The High Resolution Imaging Science Experiment, or HiRISE, is the largest telescope ever sent to another planet and can image the Martian surface at resolutions down almost 30sm/pixel.

HiRISE, together with several other instruments, has allowed us to explore huge swaths of the Martian surface in great detail. Among its many achievements has been providing evidence for brief flows of running water, as well as spotting parachuting landers heading down to the surface.

Curiosity 

The largest lander ever sent to another planet, the Mars Science Laboratory Curiosity landed via a highly complex skycrane system in 2012. Here, the nuclear-powered rover takes a selfie, next to a rock that it has drilled into  (middle left) to obtain a sample for it's onboard laboratory. Image Credit: NASA 

A common complaint about a perceived lack of technological progress is "where's my jetpack?" Whilst a person using a jetpack would actually be a really silly idea, that question does now have an answer. It's on a nuclear powered, laser equipped mobile science lab on Mars.

At 900kg, the Mars Science Laboratory, better known as Curiosity, was far too large for it to land using air bags like Spirit and Opportunity. Instead they used a skycrane, a rocket powered aircraft that slowed the rover down from 200 mph to zero before lowering it down on cables. This video has the full details of an operation that surely ranks among  the most difficult and technologically impressive achievements of humankind.

Curiosity's primary mission on Mars was to determine if the conditions on Mars could at some point in its past have been suitable for life. By the end of its first (Earth) year on the Red Planet Curiosity had met its scientific objectives, showing that the rocks around it had once formed part of a lake bed, with water and all of the chemical ingredients needed for life.

The Mars of several billion years ago was evidently very different to the barren planet we see today. However, Curiosity found no trace of the methane in the atmosphere that had been detected years earlier by Mars Express.      

Completing its primary mission in August, it has not all been smooth driving for Curiosity. NASA's recent Senior Review of its planetary exploration missions found that the rover was not being used to its full scientific potential, and that a better balance between  driving and taking data needs to be found. Whatever its troubles, Curiosity will certainly make more exciting discoveries over the next few years, as it begins to climb a 6km high mountain.

MAVEN

Arriving at Mars last week, MAVEN has been sent to find out what happened to Mars' atmosphere. Image Credit: NASA

Finally we come to last week's new arrivals. First to arrive was the  Mars Atmosphere and Volatile Evolution (MAVEN), a NASA mission. As a wide range of general scientific capabilities is already present at Mars, MAVEN's mission is somewhat more specialized than previous spacecraft. Its primary objective is to find out what happened to Mars atmosphere.

To support, for example, the prehistoric running water and intact meteorites found by previous missions, Mars must have had a thick atmosphere similar to the Earth's. Yet all that remains now is thin shell of carbon dioxide. Where did the atmosphere go?

In an attempt to answer these questions MAVEN will be sent on a daring mission into the upper reaches of Mars' atmosphere. The bent shape of its solar panels, seen in the artists impression above, will help with this, allowing it to remain stable as it becomes in effect our first interplanetary aircraft.

There, its advanced suite of spectrometers along with a magnetometer, will measure in detail the composition of the atmosphere and, crucially, its interaction with the solar wind. As Mars has no global magnetic field to protect it, the force of the solar wind has become the prime suspect in the case of the missing atmosphere.

MOM

MOM is the only Mars orbiter capable of taking images of the whole of Mars in one go. Compare with the HST image at the start! Image Credit: ISRO

Arriving into Martian orbit on Friday 24th September, The Indian Space Research Organisation's Mars Orbiter Mission (MOM) has one notable difference with all the spacecraft to have come before it: The price tag.

Although the quote value of $74 million probably doesn't take all of its costs into account, MOM still cost many times less than MAVEN. Despite this, it has a small yet advanced suite of instruments, These include the first camera capable of taking full-disc images of Mars, as well as a dedicated methane detector which will try and finally nail down the story of this elusive gas in Mars' atmosphere. Although MOM only has a six-month mission planned, plans made this week to share science data with NASA suggest that it many well keep going for some time.

India's success in pacing a spacecraft into Martin orbit may be the start of a new stage in space exploration, showing that exploring the solar system isn't limited to a few select countries and can be done without spending billions. India, and the countries that follow it, will reap the technological benefits from these missions just as the "traditional" spacefarers have done before.

And that's it! With seven working spacecraft now on or orbiting Mars, as well as more missions launching soon, we are learning more about the fourth planet from the Sun than ever before. Within our lifetimes we may finally find out whether Mars once supported life, or even if it has any now. And the more we study the Red Planet and how to get there, the closer humankind gets to finally voyaging from Earth to join our robotic explorers.

Follow me on Twitter for new blogs and other sciencey things.

Blink and You'll Miss it: The Brief Life of a Gasous Disc around a White Dwarf

Artist's impression of a debris disc in orbit around a white dwarf. Over the past eight years we have seen such a disc suddenly form, then rapidly disappear around the white dwarf SDSS J1617+1620. Image Credit: NASA, ESA, STScI, and G. Bacon. 

 Today I published my first research paper, graced with the snappy name of "Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf". The tone of the title is a hint to the tone of the paper, so I'll attempt to provide a much more readable post about what we've done here.

(Note: Where I use "we " below I'm generally referring to all of the authors of the paper. I didn't actually start working on this project until January, so other people have done a lot of work on this before me.)

History of a dying star

Around four hundred million years ago, SDSS J1670+1620 was a star similar to our Sun (although with 2-3 times the mass). As it reached the end of the hydrogen fuel supply in its core, it swelled up into a red giant, then blew away its outer layers to become the tiny white dwarf that we see today.

Just like the Sun, SDSS J1670+1620 hosted a system of planets, asteroids and other objects. As the star underwent the turbulent transformation into a white dwarf its inner planets were destroyed, tumbling into the red giant.

The rest of the system may have survived relatively unscathed. The remaining planets would have started moving out into wider orbits as SDSS J1617+1620 shrank down to a tiny ball of carbon and oxygen the size of the Earth, with only around a third of its original mass.

Systems like SDSS J1617+1620 are referred to as evolved planetary systems. Over the past couple of decades astronomers have discovered dozens of systems like this, and we now know that they exist at around at least half of all white dwarfs.

Around 33 (and counting) white dwarfs we've detected rings of dust. These structures, as wide across as the Sun but only a few millimetres thick, are formed by asteroids thrown in towards the white dwarf by distant planets. When they get too close the gravity of the dead star overcomes the forces bonding the rock of the asteroids together, ripping them apart into a dusty disc.

Such a disc was observed around SDSS J1617+1620 in 2012. But as well as a dust disc, we also saw signs of a much rarer phenomenon: Another disc, this one made of gas.

How to find a gas disc

The way we find these gaseous discs is, in my opinion, rather cool- another one of those places where a seemingly complicated plot clearly shows what's actually there.

"Diagram" showing how we observe gas discs. Instead of a single colour in the spectrum, the light emitted from calcium is spread out into a double peak. The red- and blue-shifted light is from the parts of the disc moving away from and towards us respectively. Image Credit: Me.

The diagram above, as well as demonstrating my amazing Paint skills, shows how we find the gas discs. First the light from the white dwarf is split into a spectrum separating it out into all the different colours coming from the Star. A rainbow is the result of this happening to the Sun; in effect we are making a star rainbow.

Each element in the object emitting the light leaves its mark on the spectrum, emitting or absorbing light at very specific wavelengths. When we look at the emission from calcium, we see something odd. Instead of a sharp line, the emission is smeared out into as distinctive double-peak, with one peak on each side of where the line should be.

This is a result of the Doppler Effect. If an object emitting light is moving towards us, the light waves get bunched up, resulting in a lower wavelength. The light appears to be slightly more blue than expected. As it moves away, the light is stretched out and the wavelength increases, becoming more red. The Doppler Effect has many applications, most famously being used by Edwin Hubble to show that the universe is expanding.  

With this in mind we can see that the double-peak emission must be coming from a disc around the white dwarf. As the disc rotates, the side of it moving away from us emits red-shifted light, and the side moving towards us emits blue-shifted light.

Not only can we tell that there's a disc, but we can also measure its size and location. The amount that the light is shifted by depends on the speed of the material in the disc, which depends in turn on how far away the gas is from the white dwarf. Measuring the wavelengths of the inner and outer extent of the peaks can therefore tell us the exact position of the disc.

Many mysteries

Doing this measurement has had a surprising result. Including this one, we've seen gaseous discs around just seven white dwarfs. They should have been easy to explain: As the dust gets closer into the white dwarf, the heat should sublimate it into gas.

But when the positions of the gas discs were measured, they were all found to be too far away from the white dwarf for this to happen. The question is then: if it's not the dust sublimating, how did they form? And why do we see them at only a small number of white dwarfs?

A key missing piece of evidence is variability. As the discs are formed by asteroids, which are regularly scattered in towards the white dwarf, we know that these are highly active systems. But despite being observed over several years or even decades, none of the gas or dust discs have shown any changes with time.

Until now.

Slide show of some of the spectra of SDSS J1617+1620. In the space of two years double-peaked emission lines revealing a gaseous disc appeared out of nowhere. A couple of years later the emission had decreased dramatically, and by last year the disc had completely disappeared.

The disappearing disc

After discovering the first gaseous disc around a white dwarf in 2006, a search was made in the Sloan Digital Sky Survey for more white dwarfs with double-peaked calcium emission lines.

Two observations had been made of  SDSS J1617+1620 one in 2006 and later in 2008. The later spectrum showed clear evidence for a gaseous disc, a huge structure as wide across as the Sun. However, in the earlier spectrum the disc was nowhere to be seen.

Intrigued by this clear, unprecedented evidence for variability, we started observing the white dwarf with ever more powerful telescopes. To our surprise, the gaseous disc dispersed almost as quickly as it has formed. Just a year after the observations first showing the disc the emission from the disc had fallen by more than half. The gas disc continued to dissipate, until by 2013 there was nothing left.

How did this happen? The video above shows one possibility. We know that SDSS J1617+1620 is orbited by a dust debris disc, formed form the shattered remains of an asteroid. A second asteroid, thrown in from the outer reaches of the system by an orbiting planet, could have impacted in this disc, creating a short-lived burst of gas.

A more intriguing possibility is that we're actually seeing the formation of the dust disc itself. Depending on how close it gets to the white dwarf, an asteroid being pulled apart by gravity might not turn entirely to dust in one go. It might take several orbits, passing close to the white dwarf and loosing a little more mass to the forming disc each time before moving away again.

Each time the debris of the doomed asteroid returned it would interact with the growing dust disc, forming a gaseous layer like the one we're seen. The intriguing possibility here is that the asteroid may return again in a few years time, perhaps creating another gas disc. This could provide a key insight into the dynamics of evolved planetary systems, and we plan to keep observing this star to find out if the gas disc returns.

However it formed, we also need to try and explain how the gas disc disappeared so quickly. Here we have a helping hand, as there has been plenty of theory work done studying the behaviour of gaseous and dusty discs.

Each particle of dust orbits the white dwarf independently, interacting very rarely with the rest of the dust in the disc. The gas, however, behaves more like one big object, with each particle mixing and colliding with those around it. This has the effect of slowing the gas (and dust) down, causing it to spiral inwards and fall onto the white dwarf. The more gas, the faster the accretion.

This accretion is easily visible as metal-pollution in the otherwise pure hydrogen atmosphere of SDSS J1617+1620. We should be able to test this theory, seeing if the accretion rate changes in time with the variability of the gas disc

However when we measured the accretion rates, they stayed the same. This could mean that our ideas about the behaviour of the gas discs are wrong, but its more likely that there was too little gas to cause a noticeable effect- this would also explain the short lifetime of the disc. Alternatively there could be a delay between the formation of the disc and the change in accretion rate as the disturbed material moves inward. More reason the keep watching!

The future

 As we were finishing off this paper, another paper was published showing variability at another white dwarf- but this time it was the dusty part of the disc changing, not the gas. With clear evidence that both types of disc at white dwarfs can change, we may be able to begin to answer some of the outstanding questions about these intriguing systems.

Studying these systems also provides an insight into the future of our own Solar System. Five billion years from now, the Sun will begin the transformation into a white dwarf. The more we learn about evolved planetary systems, the better we'll understand what happens next.

The full paper is available here. I haven't gone into all of the details about what we've discovered at SDSS J1617+1620 as that would be a really long post, but if you want to find out more feel free to ask me any questions in the comments or on Twitter.

Rosetta Enters orbit around Comet 67P/Churyumov-Gerasimenko

A strange new world: Comet 67P/Churyumov-Gerasimenko as seen by the Rosetta spacecraft, which became the first spacecraft to orbit a comet on 6th August. Image Credit: ESA.

After a 10 year voyage through the the solar system, the European Space Agency's robotic explorer Rosetta has become the first spacecraft to enter orbit around a comet. Since waking up in January from a two-and-a-half year hibernation, Rosetta has been steadily gaining on on comet Churyumov-Gerasimenko, also known as 67P. On Wednesday morning it finally arrived, burning its main engine for 6 minutes and 26 seconds to reach a relative speed with Comet 67P of just one meter per second.

The surface of Comet 67P/Churyumov-Gerasimenko taken from a distance of 100km with Rosetta's OSIRIS science camera. Image Credit: ESA

A few hours later Rosetta returned the first close up images from the comet, our first good look at a completely new world since the Dawn spacecraft orbited Vesta in 2011. The image above, with a scale of around 2.5 meters per pixel, reveals a varied topography strewn with boulders.

Previous missions and telescope observations have revealed that comets like Churyumov-Gerasimenko are "dirty snowballs", irregular mixes of ice and dust. Working out how that chemical composition and the varying geological activity of the comet has produced such a landscape is one of the questions that Rosetta will try and answer.       


Video: ESA

Unlike most space missions, Rosetta's initial orbit around Comet 67P doesn't follow the standard circle or ellipse. Until the mass of the comet can be measured by observing its gravitational pull on Rosetta, the ground controllers at ESA don't know exactly what manoeuvres will be needed to reach a stable orbit.

Instead, as the video shows, Rosetta will fly around the comet in a strange triangular orbit, flying in hyperbolic arcs with thruster burns at each corner. From there the orbit will be slowly lowered, until the spacecraft is in an ellipse just 10km above the surface of Comet 67P.

An overexposed image of Comet 67P taken on 2nd August, revealing jets of material streaming from the surface. Image Credit: ESA 

The seeming tranquillity of the first close-up images is deceptive. I've already written about how Rosetta has seen the activity of Churyumov-Gerasimenko increase as it gets closer to the Sun, and this overexposed image shows two distinct plumes of material streaming out of the surface of the comet.

This activity will continue to increase during Rosetta's time at the comet. By the time Churyumov-Gerasimenko reaches perihelion, the closest point in its orbit to the Sun in a year's time,  the plumes will have grown into a characteristic tail, or coma.


Video: DLR

Arguably the most exciting phase of the mission is still to come. In November Rosetta will deploy the Philae lander, a fridge-sized box that will attempt to become the first man-made object to land on a comet. Rosetta has already made an initial search of Churyumov-Gerasimenko for possible landing sites, shown as green cricles in the video. Over the next few months this will be narrowed down  to one area for Philae to target, guiding itself in with a pair of harpoons.

With a successful orbital insertion, the Rosetta mission is shaping up to be one of the most exciting space missions ever carried out. The pictures and data that it is returning are already fantastic, and I'm sure I'll write about it again as the mission continues.

P.S. Last time a wrote about Rosetta, I was contacted by a group working on a website where you can see a visualisation of the whole mission, charting the entire ten-year voyage up until now. I recommend a look.

Follow me on Twitter for new blogs.     

Planets Across the HR Diagram Part 1

I've just got back from my first conference in Cambridge, "Characterizing Planetary Systems Across the HR Diagram". The conference attempted to provide an opportunity for people working on planets in different ways, like trying to find out how they formed, how they behave, what they look like and a host of other questions, to try and join up some of the gaps between their areas.

Over the next couple of posts I'll try and record some of the bits of the many talks and presentations I found the most interesting, but I should probably start by explaining the name of the conference.

A simplified Hertzsprung-Russel (HR) Diagram, a plot of all of the stars in the sky. The horizontal axis shows a star's surface temperature. The vertical axis shows how bright the star is.  Image Credit: ESO.

The Hertzprung-Russel, or HR, diagram is a plot of all of the stars on the sky according to how bright they are and what temperature they are. A star's position on the diagram can tell us a huge amount about what it is like and at what stage in its life it is.

Along the horizontal axis is the colour that the light from the star appears as, going from blue through yellow to red. Stars give of light in a particular way known as a blackbody spectrum, a consequence of which is that the colour that a star appears as is directly linked (with a few other things taken into account) to its surface temperature. This means that the horizontal axis is also a measure of a stars temperature. As shown in the picture, the temperature (confusingly) increases from right to left.

The vertical axis shows how bright a star is, the stars luminosity or magnitude. Note that this isn't a measurement of how bright the star appears to be from Earth (the apparent magnitude). This doesn't tell us what the star is like, as a dim, nearby star can have the same apparent magnitude as a bright star further away. The HR diagram measures how bright a star actually is, defined either by its magnitude at a set distance (the absolute magnitude) or by how much energy it gives out (its luminosity). The HR diagram shows here measures the luminosity of the stars as compared to the luminosity of the Sun.

The position of a star on the HR diagram is related to what stage it is in its life-cycle. Stars spend most of their lives on the wavy line going from bottom right to top left across the diagram. This is known as the Main-Sequence (MS), and it's where the Sun is now. Less massive, redder stars like red dwarfs are towards the bottom right of the MS, whilst massive, hot blue stars are near the top left.

At the end of their lives, most stars swell up into red giant stars many times bigger than the Sun. Whilst the temperature of the star doesn't change that much, the surface area and hence the luminosity of the star will increase dramatically. They therefore move up the HR diagram into the top right.      

From here around 5% of stars explode as supernovae. The rest blow off their outer layers, leaving behind a tiny, very dim but very hot white dwarf. This moves them to the bottom of the HR diagram.

All of this means that by plotting a star on the HR diagram we can immediately tell what kind of star it is, and at what stage it is in its life. This has many applications. For example we can use it to tell how old star clusters are, looking to see if the massive, shorter-lived stars in the top left of the HR diagram are missing.

So that's the HR diagram part of the conference name explained. Now for the "Characterizing Planetary Systems..." part.

In his talk on the first day, Kevin Schlaufman showed us a different version of the HR diagram:

Kevin Schlaufmann's image of the known exoplanet-hosting stars on the HR diagram. Image Credit: Schlaufmann et al 2013. 

This HR diagram shows only those stars that have been confirmed to host exoplanets. The most notable feature is how empty it is compared to the full HR diagram, showing the huge gaps in our knowledge about planets around giants, white dwarfs and the top end of the main sequence.

This is partly due to the techniques we use to search for exoplanets, which tend to be biased towards finding planets at smaller stars. But it also revels how little we know about planets in some of these areas.

The aim of the conference was to bring astronomers who worked on planets in some forms over all of the HR diagram, be that studying the formation of planets at the very beginning of a stars life, observations of the debris discs around giant stars, or the remnants of planetary systems at white dwarfs. By trying to bring all of those disparate areas together, we can hopefully begin to fill in some of the gaps in the planetary HR diagram.

Over the next couple of posts I'll try and do a whistle stop tour of some of the talks at the conference, highlighting those areas I found interesting (/understood). And then I'm off to another conference... until then, new blogs will be posted on Twitter.