Back in business. Artist's impression of Rosetta, which started sending new signals to Earth today, and its comet lander.


Rosetta Awakes and Prepares to Chase Comet

The European comet-chasing probe Rosetta is up and running again today after it successfully roused itself from a 2.5-year sleep and signaled anxious controllers on the ground. The spacecraft had been put into hibernation during the most distant part of its 10-year journey in pursuit of comet 67 P/Churyumov-Gerasimenko because sunlight was too dim to keep its solar-powered systems running. Dozing in a slow stabilizing spin, Rosetta could not receive signals from the ground, so there was a risk that some problem might prevent it from responding to its preset alarm call at 10:00 GMT this morning. Even then, there were many processes to go through before news reached Earth: The spacecraft’s heaters would need to warm up its systems, its startrackers get a fix, boosters halt the spin, solar arrays turn towards the sun, and, finally, its communications antenna would need to point at Earth.

It was not till 18.18 GMT today that the signal was picked up by NASA’s ground stations at Goldstone, California, and Canberra in Australia, and transmitted to the European Space Agency’s (ESA’s) control center at Darmstadt in Germany. “This was one alarm clock not to hit snooze on, and after a tense day we are absolutely delighted to have our spacecraft awake and back online,” Fred Jansen, ESA’s Rosetta mission manager, said in a statement.

Rosetta is still some 9 million kilometers from the comet. As it closes with it over the coming months, all its system and instruments will be checked and validated. To get into the right orbit to chase the comet, the spacecraft has travelled 6.2 billion kilometers around the solar system, got a boost by swinging past planets four times, had close encounters with two asteroids, and slept for the past 957 days.

Scientific observations will begin in earnest in May, following a maneuver to synchronize Rosetta’s orbit with that of comet 67 P. The spacecraft is attempting to do something never done before: to catch up with a comet as it speeds toward the sun and to go into orbit around it, taking all sorts of measurements as the sun’s rays and solar wind start to heat it up and blow material off into a comet’s characteristic tail.

Previous comet missions, such as ESA’s Giotto and NASA’s Stardust, were brief flybys, catching a few hours in the life of a comet as it zipped past. Rosetta will ride along with its comet for more than 18 months, right through its close approach to the sun, or perihelion. Previous missions were snapshots, says Gerhard Schwehm, Rosetta’s former project scientist and mission manager; Rosetta is “a full blown movie.”

As the spacecraft edges closer over the summer, different instruments will start to work: cameras and spectrometers at various wavelengths, mass spectrometers, chemical analyzers, and sounding instruments. Chris Carr of Imperial College London, the principal investigator for a suite of instruments called the Rosetta Plasma Consortium, says he hopes theirs may be the first on the spacecraft to actually detect the comet as solar wind begins to blow gas off its surface and the sun’s ultraviolet light ionizes it, leaving a trail of plasma.

As the comet gets closer to the sun and is heated more and more, it will produce a complex and dynamic atmosphere of gas, dust, and plasma called a coma, full of outbursts and jets. “We’ll get a full history of the time evolution of [67 P’s] plasma environment. A comet’s plasma is completely different to that of a planet,” Carr says.

In November, potentially the most difficult part of the mission will take place when Rosetta dispatches a lander called Philae onto the comet’s surface. Once it touches down, the lander will fire a harpoon into the comet to hold itself in position; the comet itself doesn’t provide enough gravity. The lander has the ability to extract a sample of material from several centimeters below the comet’s surface. This has huge interest for researchers because comets, which form and live most of their lives in the outer reaches of the solar system, are pristine relics of the stuff planets were originally made of. Getting hold of some of that material will help scientists understand the nature of the planets we see today.

It will also help answer questions about how comets might have influenced the evolution of our own planet. Earth seems to have more water than models would suggest, and one theory is that early in its life, comets rained down on its surface, depositing water. Rosetta’s lander has the ability to measure the relative abundances of hydrogen and its isotope deuterium in the water on the comet. If that isotope ratio is similar to that in the water on Earth, it will support the idea that comets put the water there. Some researchers also believe comets seeded Earth with organic compounds that later produced life. The lander will look for such complex organic molecules.

Carr says that many have been working on this project for decades. At the start, “it seemed so far off,” he says. “I never thought this day would come.”

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