Reflections on rosetta

Philae’s departure: The OSIRIS camera on Rosetta shows the Philae lander at 10:23 GMT on November 12th as it separated from the orbiter to head to the comet’s surface.

“It’s all down to Isaac Newton now. It’s down to the laws of physics.”

As the world waited to hear whether a refrigerator-sized probe had landed on a comet’s surface, European Space Agency senior science advisor Mark McCaughrean spoke to the live webfeed in Germany.

“If Isaac’s friendly to us, we’ll have a great landing,” he concluded.

A few hours later on 14 November, the tensely awaited confirmation arrived: the landing was successful and history has just been made.

Yet the story neither starts nor ends here. As researchers pour over the data from the lander’s primary science expedition, let’s take a brief tour of the mission, the goals and what it has achieved so far.

The International Rosetta Mission was launched by the European Space Agency in March 2004. The aim was to chase down Comet 67P and orbit its nucleus as it headed towards the inner Solar System. During this time, a small robotic lander would attempt to park directly on the comet’s surface. The orbiter would then continue to follow the path of the comet as it swept by the Sun.

It was an expedition that contained a whole lot of ‘firsts’. Previous missions have flown close to comets and returned samples from their coma (the nebulous envelope that engulfs the nucleus), but none has orbited the comet, nor followed it for an extended period of time. Moreover, there has never been a stable landing on a comet’s surface. Both Japan’s first Hayabusa spacecraft and NASA’s NEAR-Shoemaker have visited near-Earth asteroids, but these objects are substantially easier to reach and neither mission had a planned science program for the surface. The previous missions were outstanding achievements in their own right, but they were different from what Rosetta was attempting.

Apart from being a pretty exciting thing to do, the Rosetta mission has science goals to match its ambitious manoeuvres. Comets are snapshots of our early Solar System, left-over material from the planet-forming process that was scattered into the far outer reaches by gravitational footballers like Jupiter. As such, their composition and structure are examples of the building blocks from which our own planet was made.

Still more enticingly, comets may hold the answer to how life began on Earth’s rocky surface. Based on its toasty position relative to the Sun, ice could not have condensed out of the protoplanetary disc to become part of the early Earth’s core. So where did our oceans come from? The most popular theory is that water was delivered by comets, which formed in the frozen neighbourhood of the gas giants. As the massive planets’ gravitational pull scattered these smaller icy rocks, some were thrown inwards to deliver water to the newly formed Earth. By examining the molecules that form the comet, Rosetta can compare the result to terrestrial oceans and even search for the existence of complex organic molecules such as amino acids.

These goals gave the mission its name: Rosetta’s namesake is the stone in the British Museum inscribed with a message in three different languages. The comparison of this text helped decode the ancient script of hieroglyphs and thereby revealed a piece of the past. Rosetta is about to do the same on a scale measured in billions, rather than thousands, of years.

COMET-CHASING

Comet 67P was discovered in 1969 by Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko who lent their names to the comet’s official title: Comet 67P/Churyumov–Gerasimenko. The more commonly used part of the name, ‘67P’, refers to the comet being the 67th periodic comet to be discovered.

Rosetta caught up with the comet between Jupiter and Mars, at around 450 million km from the Sun, or three times the Earth-Sun distance. It was a route that took 10 years to complete, including three loops around Earth and one around Mars to give the spacecraft the required gravitational push. The resulting circuitous journey covered 6.4 billion km. An extremely nice interactive graphic has been created by the ESA to show Rosetta’s path and future destination.

To save power, Rosetta spent a large portion of the trip in hibernation, coming back on line in January this year. Its wakeup was cheerfully announced with a ‘Hello, World!’ on its twitter feed, although amusingly the social media platform did not exist when Rosetta left Earth. Upon arrival at the comet, several of Rosetta’s instruments also acquired twitter accounts and reported updates on what they were doing. They additionally offered words of encouragement to one another that was both adorably uplifting and informative.

With a gravitational field several hundred thousand times weaker than the Earth’s, Rosetta could not go into a true freefall orbit around the comet. Instead, it used thrusters to make a triangular motion about the nucleus that steadily brought it inwards to a minimum of 10 km from the comet’s surface. Loaded up with 11 scientific instruments, Rosetta was able to perform a thorough comet examination. This included transmitting spectacular close-ups of the comet’s nucleus, analysing the dust and gas in the coma and exploring the solid composition through scattered radio waves.

It also had to pick a touch-down site for the robotic lander – and this it had to do fast.

A LANDER CALLED PHILAE

The robotic lander was named Philae after the Egyptian island on which a discovered obelisk provided the missing clues for de-coding the hieroglyphs. Like the orbiter Rosetta, Philae was packed with instruments, including a drill and hammer, that could probe below the comet’s outer surface. But as with all machines, operations could only continue within a certain temperature range, and comet 67P was heating up.

Hurtling through the inner Solar System at 135,000 kilometres per hour, comet 67P is approaching the Sun. This is part of the Rosetta mission, which will observe the changes to the comet as it gets steadily warmer. However at around March next year, the surface will be too warm for Philae to function. Science on the comet surface needed to be over well before then.

After closely mapping the comet’s structure, five possible landing sites were selected. In a naming scheme that was a mental struggle for anyone with a moderate sense of order, the sites were labelled A-C, I and J. Comparisons between the amount of received solar radiation, the topology of the area, the ease of communicating with the orbiting Rosetta and how straight-forward Philae’s descent would be, resulted in site J being selected for touchdown. Since it was difficult to really stir up enthusiasm for a location bearing only a single letter designation, a public competition was held that renamed the location. The choice ‘Agilkia’ was selected for being the name of the island where Philae’s temples were relocated after flooding. It was time to move, and the ESA released a cartoon summary of the mission to date.

SHOULD I STAY OR SHOULD I GO?

As the time came for Philae to begin its descent, a series of go/no-go checks were performed by the ESA. While the lander was ultimately cleared for separation, not everything was perfect. Thrusters designed to push down and prevent Philae rebounding after arrival were not responding. This led to concerns that in comet 67P’s weak gravity, the small lander would bounce back into space. Yet, it was a risk worth taking: Philae was also equipped with two harpoons and ice screws that should hold it fast to the comet’s surface. The command for separation was sent.

So far from the Earth, communication between Rosetta and the ESA scientists takes almost half an hour. This produced the first tense wait to be broadcast through the live webfeed: would 10-year-old technology allow the lander to detach? When confirmation was received of its success, relieved scientists settled in for a longer wait: the seven hour descent of Philae from Rosetta to the comet’s surface. As McCaughrean announced, it was all down to Newton now.

The summary of this nail biting wait was superbly documented by the comic xkcd.

At 16:02 GMT, the awaited confirmation appeared: Philae was on the comet’s surface!

But it did not stay there.

Not only had the thrusters not worked, but the harpoons failed to fire. The result was what ESA engineers had dreaded and Philae lifted back off the comet’s surface. Miraculously, it came back down.

Later analysis of the incoming data would reveal that Philae had bounced twice. The first was a two-hour jump around a kilometre high and the same length across the comet. The second was a shorter seven-minute hop. When it came down again, the lander was approximately vertical, but one of its three feet was over empty space. This led scientists to be concerned about how steady Philae’s position would remain. Even more concerningly, the little lander seemed to be partially over-shadowed, suggesting it had stopped either in a hole or by the side of a cliff. This meant that while Philae was able to communicate with Rosetta, very little light was falling on its solar panels. Without a reasonable supply of sunlight, Philae would die when its battery ran out of juice in about two and half days’ time.

It was a dilemma for the ESA scientists: should they try re-firing the harpoons in the hope of shifting Philae to a better location? Or should they attempt to do as much of the science program as possible in this location, in case such a move would put the lander in an even worse position? With the battery providing a steady countdown, it was unlikely there would be enough juice left to fire the harpoons if they first attacked the science.

In the end, science won and Philae began loading its instruments with its new found environment. Due to its precarious positioning, the use of the hammer and drill were left until last amid fears that the recoil could move the probe. This was also a tricky ultimatum, since the instructions at the end of the program risked being cut as Philae’s battery went flat. Due to their ability to probe the substructure of the comet, these two instruments in particular were key to Philae’s science. Everyone held their breath yet again.

Data from Philae travel back to Earth via the orbiting Rosetta. As Rosetta dipped around the comet, the connection with Earth broke, providing two four-hour windows per day. When contact stopped after the first window on 14 November, scientists wondered if Philae would still be alive when the connection could be reestablished that evening.

It was. In fact, Philae completed 100% of its primary science program, including the drill and hammer whose data were successfully sent to Rosetta. With the main program finished, the ESA risked a controlled rotation, moving the lander around 35 degrees so its larger solar panel was out of the shadow. Sadly, this was not enough for a last minute revival. As the final window on Friday drew to a close, Philae’s battery ran down and the lander went into hibernation.

WHAT NOW?

While it was sad to say goodbye to Philae so soon, the extensive data it provided is now being analysed back on Earth. It will be a while before full results are released, but preliminary discussion from the Philae hammer twitter account suggests the comet surface was much harder than it had seemed from Rosetta’s photographs.

Rosetta meanwhile, will continue to follow comet 67P as it heads towards the Sun. It is anticipated to continue sending data back to Earth for the next twenty months, after which its own fuel supply will be depleted. There is some discussion as to what will happen to the orbiter at that time, with one suggestion being that it will be landed on the comet to reunite with Philae.

As for Philae, the story is not quite over. While its battery depletion has sent it into hibernation mode, it is possible it could reawaken as the comet approaches the Sun and more sunlight falls on its exposed solar panel. Speaking about this prospect, Valentina Lommatsch from the lander control centre, did warn the hope was not high, but it was not impossible. The key is the secondary battery needs to be recharged to boot the lander back into operation. This requires the battery to warm up to zero Celsius and then charge to at least 5.1 W; a feat that needs 50 – 60 Watt-hours per day to achieve. At present, there is only 90 minutes of sunlight reaching the lander, which provides between 1 and 4 watts or about 2.5 Watt-hours.

While these numbers did not look good, later consideration proposes that Philae’s unfortunate landing may yet reap some benefit. The shade that hides its solar panels may also protect the lander from overheating as the comet approaches the Sun. If it is able to operate beyond the original March deadline, there is a higher chance of regaining contact as the radiation incident on the exposed solar panel rises. On the German Aerospace Center’s website this week, lander project manager Stephen Ulamec states he believes next Spring could be the time to listen out for Philae.

“I’m very confident that Philae will resume contact with us and that we will be able to operate the instruments again,” says Ulamec.

Regardless of the prospects, Rosetta will remain listening for Philae for the foreseeable future. In the meantime, it has science to do.

A FINAL NOTE

One feature that has made this mission so exciting is the excellent material released by the ESA. Once Rosetta came out of hibernation, there have been a series of videos, blog posts and tweets updating everyone on the progress and problems faced. The result was a global audience for Philae’s landing and social media streams resembling a pop concerts’ screaming fan base. The website is definitely worth a visit. Be sure to check out the Rosetta blog and take a look at the videos released.

Congratulations ESA, on not only a historic mission, but on the best publicity I have ever seen and which I thoroughly enjoyed.

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