The First Landing on a Comet

Released from the Rosetta orbiter, the fridge-sized Philae lander drifts down to become the first spacecraft to land on a comet. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA 

And the second, and the third...

At 8.35 GMT last Wednesday morning, five hundred million kilometres from the Earth, a tiny lander called Philae detached from the side of the Rosetta spacecraft. 28 minutes later the signal confirming the separation arrived at ESA’s Space Operation Centre (ESOC) in Darmstadt, Germany. The first ever attempt to land a spacecraft on a comet had begun.

Unlike most spacecraft landings, Philae would not land using rocket engines or parachutes. Rosetta had pushed it away in (it was hoped) just the right direction, at just the right speed to fall gently down onto its target.

The target was Comet 67P/Churyumov–Gerasimenko, an irregular lump of dust and ice less than five kilometres across at its widest point. Separating from Rosetta 22.5 kilometres from the surface, the low gravity of Comet 67P pulled Philae into a leisurely, seven-hour descent. 

As it fell towards Comet 67P, Philae had time to spin round and take a picture of Rosetta...
Image Credit: ESA/Rosetta/Philae/CIVA 

...whilst Rosetta watched Philae disappear into the darkness.
Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA 

Imaged during its descent by Rosetta's OSIRIS camera in the sequence above, Philae is a 100kg box filled with ten scientific instruments, including cameras, spectrometers, a drill and two labs for analysing surface samples. And, crucially, two harpoons.

These harpoons were to fire as Philae touched down onto the surface of the comet, anchoring itself securely to 67P. The plan had been for a small thruster on the top of the lander to ignite at the same time, holding Philae down onto the surface. But that morning, the team at mission control had discovered that the thruster had stopped working. Only the harpoons could stop Philae from rebounding off the surface of Churyumov–Gerasimenko and back into space.

A picture of the first landing site from 40 meters above the surface. Image Credit: ESA/Rosetta/Philae/ROLIS/DLR

At this point I had to go to a seminar, and spent the next tow hours failing to pay attention to the speaker whilst surreptitiously checking Twitter for news. If Tom Shanks is reading this, then sorry! But I got out in time to celebrate with the rest of the world as, at 16.03 GMT, the signal arrived at ESOC: Philae had landed, the first spacecraft to touch down on a comet. There was much rejoicing.

But the celebrations were short lived. As the mission controllers studied the data relayed back by the orbiting Rosetta, they realised that the crucial harpoons had failed to deploy. Worse still, the signal from the lander was fading in and out, and the power being generated by its solar panels was varying wildly. By the evening, a tentative explanation had been found: Philae had bounced straight off the comet and gone into a spin.

In a series of incredibly detailed images, the orbiting Rosetta spacecraft tracks Philae's wild flight across the surface of Comet 67P. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

By the next morning the full tale of the landing had been put together. Philae had landed at 15:34 GMT, thudding down in exactly the right place. But without the thruster or harpoons to hold it down, the tiny spacecraft had bounced back up again, heading off the comet at a leisurely 38cm/s. Thanks to the extremely low gravity of 67P, Philae flew over the surface for nearly two hours, flying almost a kilometre high. During that time the comet turned underneath it, the targeted landing site slipping away.

When Philae hit the ground again it made a second bounce, this time for only seven minutes. When it finally came to a halt, the lander was over a kilometre from the spot where it had first touched down. But exactly where Philae had ended up was a mystery.  

Panoramic view of Philae's landing site, with the spacecraft superimposed. It wasn't meant to be this dark... Image Credit: ESA/Rosetta/Philae/CIVA

The first images from the landing site showed a very different place to the flat, sunny target. Philae appeared to be at a tilt, with one leg sicking into space. Worse still, the bulk of the lander's solar panels were in the shadow of a large cliff. If Philae wasn't able to move, then it would only get around 1.5 hours of sunlight each day- nowhere near enough to recharge it's batteries.

But Philae was designed with this scenario in mind. Although the solar panels would have allowed it to carry on working for several months, it had been built with enough battery power to complete all of its initial science observations. While the mission controllers pondered a way to move away from the cliff, Philae's ten instruments swung into action.

The ten scientific instruments Philae used to study the surface of Churyumov–Gerasimenko.  Image Credit: ESA/ATG medialab

The full results from the measurements made by Philae have yet to be released, but a few preliminary discoveries have been announced. Particularly intriguing was the data collected by the Multi-Purpose Sensors for Surface and Subsurface Science, or MUPUS. This instrument deployed a small hammer, deigned to dig into the surface of Churyumov–Gerasimenko and measure the temperature at different depths.

Surprisingly, even at it's most powerful setting, the hammer couldn't make a dent in the surface of 67P. The ground beneath Philae, long expected to be a porous, loosely bound mix of dust and ice, was actually rock-solid. Although this conflicted with accepted knowledge (always a good kind measurement to make), a solid ice crust would explain why Philae bounced so high after its first touchdown. The low density of the comet suggests that, beneath this icy crust, the material of Comet 67P is much less tightly packed.

Another instrument, the Cometary Sampling and Composition Experiment or COSAC, detected signs of organic chemical compounds on the surface of Comet 67P. These carbon-rich compounds, which give the comet its deep black colour, are one of the key reasons we are interested in these icy worlds. It is thought that many of the ingredients needed for life on Earth, such as water and some amino acids, were originally delivered here by impacting comets.

With battery power running low, Philae ran through all of it's remaining scientific instruments, drilling into the surface to collect material for its onboard laboratories, receiving and transmitting radar data from Rosetta to map the insides of the comet, and taking yet more images.

By the time its batteries finally gave out, Philae had achieved all of its planned science operations. Despite the bumpy landing, the mission had been a complete success.  

So much hard work.. getting tired... my battery voltage is approaching the limit soon now pic.twitter.com/GHl4B8NPzm
— Philae Lander (@Philae2014) November 14, 2014

At 36 minutes past midnight on Saturday morning, mission control at ESOC lost contact with Philae. But there's still hope for the little lander. Just before its batteries gave out, Philae had managed to turn itself, bringing it's largest solar panel out of the shade into the faint sunlight.

As Churyumov–Gerasimenko flies ever closer to the Sun, there's a small chance that Philae's batteries will recharge. We may yet be hearing more from the tiny lander. Even if this is the end of Philae's epic adventure, Rosetta is still in orbit of the comet, continuing to revolutionize our knowledge of these tiny, mysterious worlds.



Note: you my have noticed that I haven't commented on #shirtstorm- it's outside the scope of what I wanted (and feel qualified) to talk about, but I recommend and broadly agree with articles like these on the issue.

Another Note: I've stared writing for Astrobites! These are daily summaries  of recent scientific papers, written by astronomy postgraduate students. The style is a bit more technical than this blog, but it's worth a look if you want to to keep up to date with astronomy research. I'll be writing there once a month, and my first post is here. 

Follow me on Twitter for new blogs.

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

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.

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.     

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. 

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.