In December, the inbound comet will get within 400 million miles of the sun, about the distance from the sun to the main asteroid belt. Here, the sun’s heat will start to bring it alive. Gas jets will belch from its surface, and a hazy atmosphere called a coma will form from ice and dust, and stretch out to become an incandescent tail. For the next month and perhaps longer, Philae will ride Churyumov-Gerasimenko like a bronco buster, “not bothered too much by all this,” Jansen says, and Rosetta will escort the two, transmitting data from its 11 instruments and the 10 on the lander. The instruments will map the comet’s surface, analyze the composition of the nucleus, coma, and tail, measure the size and momentum of the material emanating from the surface, and image the comet from the ultraviolet to the infrared.
The Rosetta mission was conceived in the 1980s, and revised several times over the next two decades into the most ambitious cometary mission to date. If it all works, Rosetta will be the first spacecraft to orbit a comet, the first to land on one, and the first to accompany it from deep space to perihelion, the point at which it is closest to the sun, and back. “We’ve done comet flybys and learned a lot,” says Matt Taylor. “Rosetta is the next level. It’s like the moon: Pre-1960, you could look at it. Then you went into orbit around it. Once you did that, you had to land.”
Meanwhile, in the 20 years that the spacecraft was being designed and built, comets have undergone a scientific rebranding. In the 1990s, they were regarded as nearly identical dirty snowballs created from the gas and dust ejected by the young sun as the solar system was first forming, 4.5 billion years ago. And they were almost untouched, holding the secrets of the solar system from the moment it was formed.
Since Rosetta’s launch, “we have learned that comets are much more diverse,” says Schwehm. NASA’s 2004 Stardust mission, he explains, found Comet Wild 2 to be heavily cratered, with minerals in the coma formed at temperatures much higher than expected. In 2005, NASA’s Deep Impact spacecraft bombarded Comet Tempel 1 with a copper projectile and found what looked like layers in the comet’s surface.
“We still can’t really explain the activity of a comet. We know there are jets, but we don’t know why they come about,” Schwehm says. “This is one of the key problems we hope to solve. We also want to know if a comet is a ball of loose material, or is it one piece, or an accretion of more than one piece?” To make those discoveries, he says, “you really have to get up close to the nucleus and stay there for a long time. That is our plan.”
Deep Impact team leader and University of Maryland astronomer Michael F. A’Hearn points out that up-close comet studies are still fairly new. The questions that scientists had 10 years ago when Rosetta was launched have changed, but fortunately, he says, the spacecraft is “robust enough to answer the new questions.” Rosetta and Philae will be able to give researchers their most detailed look yet at a comet’s interior.
Jansen says the mission team expects the lander to operate nonstop for a month on its own batteries and its solar array before its stored energy is depleted beyond the array’s ability to recharge it quickly. Philae operations will then be restricted to a few hours every few days.
“The spacecraft orbits very slowly, but the winds coming off the comet are traveling at thousands of feet per second,” says Alan Stern, team leader for ALICE, Rosetta’s ultraviolet spectrometer. “We’re going to a very toxic environment.” Worried that dust might scratch ALICE’s mirrors, Stern’s team has fitted the imager with an aperture that will automatically shut and turn off the instrument during nasty episodes until “we determine it’s safe to put our head out of the foxhole.”
Rosetta will continue to orbit until the disturbance from the comet’s powerful gas jets overcomes the spacecraft’s ability to control its angle. Then it will move out of orbit, but keep up with the comet as closely as possible. The comet will make its closest approach to the sun—112 million miles—on August 13, 2015, then head back to Jupiter’s neighborhood, with Rosetta keeping pace and Philae clinging to its back.
The mission is supposed to end in December 2015, but it’s likely Rosetta will be able to continue for a few more months before the spacecraft’s hydrazine runs out. By that time, the mission scientists hope, Rosetta will have sent back enough information to create our first primer for understanding comets and their role in building the solar system, including whether they are the source of water, and even life, on Earth.