At about seven o’clock this morning, after more than ten years of travel, the European Space Agency’s Rosetta probe went into orbit around a comet named 67P/Churyumov-Gerasimenko, which is currently about two hundred and fifty million miles from Earth. No spacecraft has ever orbited a comet before, Matt Taylor, the Rosetta mission’s lead scientist, told me a few months ago. “Previous missions have been flybys, which gave us brief glimpses of comets,” he said. “Rosetta will ride alongside this comet right through its closest approach with the Sun, in 2015.”
As scientists and engineers watched anxiously from a control room at the European Space Operation Center, in Garching, Germany, the probe fell into orbit without a glitch. “After ten years, five months, and four days travelling toward our destination, looping around the Sun five times and clocking up 6.4 billion kilometres, we are delighted to announce, finally, ‘We are here,’ ” Jean-Jacques Dordain, the director general of the E.S.A., said in a prepared statement.
Rosetta’s ride-along should prove invaluable to astronomers and planetary scientists. Comets are understood to be chunks of ice, rock, and dust left over from the formation of the solar system, more than four and a half billion years ago. Since they’ve spent most of that time in the frigid region out beyond Neptune, comets have changed very little since their inception. A comet, then, is something of a time capsule of information about what was going on in the solar system’s earliest days. Rosetta’s job is to read that information.
The probe is well equipped for this task. As Rosetta circles 67P (the comet’s preferred nickname), eleven different instruments will photograph the surface, take its temperature, analyze the light bouncing off the comet for clues about its chemical composition, and take samples from its coma, the cloud of gases and dust that boil off as the comet is heated by solar energy. (If a comet comes close enough to the Sun, its coma elongates into a visible tail, but 67P won’t get that close.) “This will give us a huge insight into cometary activity,” Taylor told me, including “how the influence of the sun on the nucleus works.”
All of these measurements began when Rosetta was about sixty miles away from 67P. During the next few months, the orbiter will inch closer and closer to the two-mile-wide comet, until it comes to within twenty miles. At that point, a secondary probe named Philae will detach from the main spacecraft and descend to land on 67P’s surface, another first. The landing is currently scheduled for November 11th. Afterward, while Rosetta takes measurements remotely, Philae will use ten instruments of its own to study the surface directly. The lander will drill about eight inches into 67P and extract a sample, which will be heated in a small oven so that the gases coming off the sample can be analyzed in detail.
There’s one more experiment that requires the lander and the orbiter to work together. When the main spacecraft is on the opposite side of 67P from Philae, it will send a burst of radio waves down through the comet. The radio waves will hit the lander and then be reflected back through the body of the comet, to hit the orbiter again. “Conceptually, it’s like an X-ray,” Michael Küppers, another Rosetta scientist, told me. Scientists don’t know whether comets are truly solid bodies or whether they’re aggregations of smaller chunks, held together loosely under their mutual gravity. These bouncing radio beams should be able to settle that question by probing 67P’s internal structure.
There’s also a chance that the Rosetta mission will help settle a question of more direct interest to nonscientists. When seen from space, Earth’s most obvious feature, and its ultimate source of life—the reason for nicknames like the Blue Planet and Pale Blue Dot—is its oceans. Nobody knows for sure, however, where all our water came from. It might have been here from the beginning, condensing out of the original gas and dust along with the rock that makes up most of Earth’s substance. But it may also have been delivered by the impact of millions of ice-rich comets soon after the solar system formed. According to Taylor, observations of other comets have suggested that the ratio of hydrogen to deuterium (a heavier version of hydrogen) in their ice is similar to that of Earth’s oceans. Rosetta’s measurements of the hydrogen-deuterium ratio in 67P will be much more accurate, he said, and could thus help settle the question. He went on, “We’re really looking forward to seeing what Rosetta’s in-situ measurement of these values will tell us.”
