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Rosetta's scientists are finally learning how comets really work

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For the first time, we're getting up close, not just flying by

Rosetta hasn’t lived up to its name yet, but it’s starting to get closer.

Today, Science has published seven reports based on data from Rosetta, a spacecraft that has spent the past 10 years traveling to a comet nearly 280 million miles away. The European Space Agency launched the mission in 2004, but it wasn’t until last year that it reached its destination: a comet with the ungainly name of 67P/Churyumov-Gerasimenko. The ESA and NASA successfully put Rosetta into orbit around 67P in August, and they dropped a lander onto the surface in November — both historical firsts. And for the past five and a half months, scientists have been able to use Rosetta's bevy of instruments to gather information on everything from the water ions in 67P’s atmosphere to the composition of its barbell-shaped core.

Comets are like a time capsule from the birth of the solar system

Comets can give us a unique perspective on our own world, says University of Michigan professor Nilton Renno, a co-investigator in the 2007 Mars Phoenix mission. A comet is hypothetically formed from dust and gas present at the solar system’s formation — a time capsule from before our planet existed. "If you understand well the composition of the comet, you understand very well the composition of the cloud of dust that formed the entire solar system," Renno says.

The very first Rosetta findings, published in December, are an excellent example. Using a mass spectrometer, researchers examined the makeup of 67P’s ice, comparing its composition to our own water. The stark differences they found contradicted an earlier hypothesis about comets: that they had brought water to Earth by slamming into the newly formed planet. We don’t know enough to settle this or many other questions, but Rosetta could provide us with the most accurate information yet.

Rosetta

Smooth terrain on 67P's Imhotep region.

67P is far from the first comet to come under investigation. NASA’s Stardust mission flew by a comet in 2004 and returned to Earth carrying particles gathered from around a comet, and its Deep Impact mission launched a probe straight into one in 2005, transmitting images just before impact. But it’s the first time a craft has gotten so close for so long. Some researchers believe they could be correcting otherwise unavoidable biases with this approach. "This is really a big advantage," says Alessandra Rotundi of the Parthenope University of Naples, principal investigator of Rosetta’s Grain Impact Analyzer and Dust Accumulator (GIADA) sensor.

Using GIADA — a black box that measures particles’ speed and momentum using a laser screen and impact sensors — Rotundi and her team could measure the size and density of dust grains in the "coma," the fuzzy cloud that surrounds a comet’s core. The coma is formed as the comet heats up, parts of its solid nucleus turning directly to gas. Rosetta’s extended time near the comet will allow them to observe how 67P’s dust / gas ratio changes as it approaches the sun, and its close proximity will eliminate some sources of confusion. "A flyby is really short in time; it's also fast, and so it can cause issues in the measurements," says Rotundi. "It would have been much more difficult to know the velocity of each single grain." More accurate readings on the coma will give researchers information about what the comet itself is made of — and, by extension, when and where it was formed.

Comet flybys can gather some of the same information as Rosetta, but not in the same detail

Other researchers are tackling the same question from different angles. One group used OSIRIS — Rosetta’s high-resolution camera system — to analyze the shape, size, and movement of the comet’s nucleus, a pair of lobes separated by a short neck. Using mass data from another instrument, they estimated the density as less than half that of water, suggesting that it could be loosely packed and highly porous. The results corroborated other, less certain findings from other comets, providing a better sense of their makeup. Another examined OSIRIS images to model the comet’s surface, creating a terrain map and finding variations that could indicate different materials or environments in different parts of the nucleus, and others used microwaves or infrared imaging to measure subsurface temperatures or the material on the comet’s surface — the latter suggests that despite the icy overall nature of comets, there’s little visible ice on its sunlit side.

One of the major questions in this research is whether the comet’s two lobes were originally separate bodies, or whether the "neck" in the middle has simply been hollowed out over time — although researchers have found notable differences between the parts, the issue remains unsettled. Partly, this is because there are still so many gaps in our understanding of comets. A study led by Myrtha Hässig of the Southwest Research Institute, for instance, looked at the combination of water, carbon monoxide, and carbon dioxide in 67P’s coma. The results were surprising: over Rosetta’s first two months in orbit, its instruments found that the gas composition changed significantly as it moved. "We see big variations of water and CO/CO2" as the comet rotates under the spacecraft, says Hässig, "and that we had not expected. We expected to see a steady increase once we entered the coma."

Rosetta

Rosetta's GIADA instrument measures comet dust.

These variations could be happening because the nucleus itself isn't uniform; if that's true, it might mean that the different pieces come from different parts of the solar system. But Hässig says interpreting the data isn't easy. For one thing, the sun’s heat produces the coma, and it doesn’t hit all parts of the comet equally. At least some of these changes are the equivalent of a seasonal effect; water, for instance, is only released at a higher temperature, so it’s more common on the comet’s "summer" side. It’s not clear right now whether a comet’s makeup also plays a part. The paper compares 67P to another comet called Hartley-2, which also had variations that could come from a heterogeneous nucleus. But Hartley-2 was only measured briefly, during a flyby. "It was basically just a snapshot of the coma and the nucleus," says Hässig.

Rosetta's mission will continue until the end of 2015

Rosetta scientists, though, have at least another year to collect data. The craft has a planned life of 12 years, though the official mission ends in December of 2015, once the comet has reached its closest point to the sun and continued on its 6.5-year orbit. They’ll be able to observe how the coma’s composition changes, as well as other properties — like the development of a "magnetosphere" that insulates comets and other celestial bodies from solar winds. A project led by Hans Nilsson of the Swedish Institute of Space Physics is tracking the way that ultraviolet light excites water molecules as the comet heads towards the sun, creating ions that will eventually form a barrier against the winds. Like the coma, the magnetosphere grows near the sun and shrinks away from it, and Nilsson says this mission is the first time we’ll be able to actually watch one develop.

Renno says that despite the benefits of Rosetta’s lander, there are downsides to analyzing material in space. The equipment, for instance, has to be lightweight and low-power enough to run off large solar panels, which means it may be less accurate than the state of the art on Earth — it’s just not worth the tradeoff of having to get samples home. The biggest limit to these studies, though, is the relatively short time Rosetta has been in orbit. "There is a lot of stuff we didn't expect, certainly," says Hässig. "I think we will be further surprised by Rosetta and the results. And we will have to rethink, or wrap our mind around, how comets actually formed in a new way."