Friday, 21 November 2014

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.
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.  
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

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