For more than a decade, they've been separated. One has spent much of the long wait in a chilly stupor. The other has been finding its way to their assignation, a journey of more than 3.5 billion miles.
Now the meeting draws near. Next Wednesday, barring disaster, the steadfast spacecraft called Rosetta will finally join the object of its devotion, an icy and mysterious comet. After that, the two will travel the solar system together, making Rosetta the first spacecraft to spend quality time with a comet.
Other probes have paid fleeting visits to comets, but Rosetta will spend a year and a half, maybe more, at the comet's side even as the couple approach the sun and the comet begins to spew dust and ice. Rosetta will take pictures, sample the comet's chemicals and even drop a lander onto the comet's surface this fall.
Rosetta "gives you a front-seat, ride-along vision of what the comet's going to do and how a comet works," says Rosetta project scientist Matt Taylor of the European Space Agency, which is mounting the mission. "This is really a big leap forward."
That big leap required patience and $1.7 billion in mission costs and a lot of cooperation. It also took a lot of coordination, because Rosetta's scientific instruments were built by scientists in the USA and across Europe. Mission planning took roughly a decade , and after its launch in 2004, the spacecraft needed another decade to reach the frigid reaches of the solar system where the comet awaited.
As the two head toward the sun together, the growing warmth will waken the comet. Its glowing halo will expand, it will sprout several tails and Rosetta will be there to watch the whole process – another first.
Rosetta's quarry is Comet 67P/Churyumov-Gerasimenko, named after its discoverers and nicknamed Comet C-G. Scientists thought of it as a garden-variety comet in many ways, but recent pictures taken by Rosetta show Comet C-G is not the potato-shaped lump that Taylor and others expected. Instead, it's a weird lobular thing reminiscent, some say, of a rubber duck.
"Rosetta is not even at the comet yet, and we already have the first surprise," says the University of Michigan's Tamas Gombosi, a scientist on the Rosetta team. The spacecraft "can't help discovering new things."
Wednesday, Rosetta will make its final major maneuver to align itself with the comet, then, hovering at a distance of roughly 60 miles, it will settle down to the serious business of mapping the comet's surface. Over the next six months, it will gradually draw closer and closer, dipping to 20 miles, maybe less, from the comet's surface.
Once close to the comet, Rosetta is in for a challenge. The pressure of the gas wafting off the comet as it approaches the sun could blow the spacecraft off course. The lander could bounce back into space after trying to settle onto the comet's fluffy surface.
Rosetta is "a real advance, and they have to be very careful how they do it," says Daniel Scheeres of the University of Colorado-Boulder, who is not associated with the mission. "It'll take a lot of skill on their part to pull this off successfully."
If Rosetta does succeed, scientists expect a trove of data like nothing ever gathered before. The spacecraft that conducted previous comet missions blew by their targets at more than half a mile per second or far faster, whereas Rosetta will travel at a leisurely 3 feet or so per second. Rosetta bristles with 11 clusters of instruments, which will help shed light on an object made from the primordial stuff that formed the planets 4.6 billion years ago.
Past missions have been "very quick snapshots in time," says Rosetta scientist Lori Feaga of the University of Maryland, but this time, "we'll be watching the comet's every move."