The European Space Agency launched the Rosetta spacecraft in 2004, beginning a decade-long mission to catch the comet 67P/Churyumov-Gerasimenko.
Despite being in forced hibernation since its historic landing on a comet last November, the Philae space probe has given scientists their first closeup look at what these ancient and mysterious objects are made of, and how they were born during the solar system’s infancy.
The information beamed back to Earth before Philae went to sleep was enough to generate seven scientific papers, which appear in a special issue of the journal Science. Among the tantalizing results: Comet 67/P Churyumov-Gerasimenko, where Philae set down, has a surface that varies from hard as asphalt to soft as a sandy beach. Its interior isn’t like a rocky snowball, as scientists thought it might be, but rather a smooth mixture of dust and ice. And it's home to organic compounds that not only have never been seen on a comet before, but also support the idea comets brought the building blocks of life to Earth billions of years ago.
Scientists have even managed to narrow down where the lander came to rest, which has been a mystery since the bouncy landing eight months ago. “It’s a pity we don’t have even more information,” says Wlodek Kofman of the Institute of Planetology and Astrophysics in Grenoble, France, author of one of the seven papers. “But I’m really happy we have any.”
That’s an understatement. Rosetta was the first spacecraft ever to orbit a comet, and Philae the first to attempt a soft landing on one. Things didn’t exactly go as planned.
Instead of anchoring itself immediately to the landing spot, Philae bounced a half-mile up and to one side. Then it clipped the edge of a crater, spun, bounced again, and finally came to rest in the shadow of a cliff, where it it was too dark for solar panels to recharge the probe’s batteries. After about nine hours, Philae went silent.
In those hours, however, it managed to do a lot. The depth and shape of the holes Philae’s legs left at the site of the first bounce, for example, along with the force measured by the lander, let scientists calculate that the surface was granular, but more solid a foot or two down.
Surface Like Asphalt
“That was reassuring,” says Jens Biele of the German Aerospace Center in Cologne, lead author of a paper that analyzed the landing in detail, “because it confirmed our predictions.” It also put to rest the idea that Philae might sink three feet or more into a soft, grainy surface.
Philae’s final resting place, by contrast, has a surface more like asphalt in texture. The crust might be only a few inches thick: The solar heat that scientists think welds the original dust into a hard crust penetrates only a short distance into the comet. Until and unless mission controllers can get Philae’s built-in drill to work, that question will remain open.
A working drill would also let mission scientists study the chemical composition of 67/P’s subsurface. But they’ve managed to get at least a whiff of what lies on top. When Philae took its first bounce, it raised a cloud of dust. “We think that this dust must have entered our exhaust system,” says Fred Goesmann of the Max Planck Institute for Solar System Research in Göttingen, Germany, lead author of another of the Science papers.
Comets Key to Life
Once there, the dust must have warmed up, sending out gases read by one of Philae’s mass spectrometers. It’s an instrument that separates molecules by weight, letting scientists deduce what they’re made of. This lucky accident allowed Goesmann and his colleagues to finger 16 organic chemicals, including four—methyl isocyanate, acetone, propionaldehyde, and acetamide—that have never before been detected on a comet. The suite of compounds, says Goesmann, “is the sexiest mass spectrum in the world.”
That’s not because the chemicals are especially surprising, he says. It’s because nobody has ever taken a chemical sample from a comet before. Still, the discovery is consistent with the longstanding idea that a rain of comets long ago brought life's building blocks to Earth, kickstarting the process that led to humans smart enough to visit a comet. Comets may also have delivered much or all of Earth’s water, a second key factor in the origin of life.
What might be called a CAT scan of the interior of the duck-shaped comet's "head" also held a surprise. Radar beams passed back and forth through the comet show a surprisingly uniform interior. That's quite different from the loose aggregation of rock and ice that many textbooks point to.
Intriguing as these discoveries are, they’re only a tiny fraction of what Rosetta scientists had hoped to get from Philae. They were thrilled, therefore, when the lander abruptly woke up and phoned home in June, presumably thanks to changes in the comet’s orientation that allowed the solar panels to work after seven months in darkness.
The lander is still having problems communicating with the mother ship, but that might improve as Rosetta descends to a lower orbit in coming weeks.
If Philae comes fully back to life and drills into Comet 67/P, this week's discoveries could be just the tip of an iceberg holding the secrets of comets and their role in how our solar system—and perhaps life itself—came into existence.