After a journey of 6.4 billion kilometers (4 billion miles), Europe's unmanned Rosetta probe reached its destination Wednesday, a milestone in mankind's first attempt to land a spacecraft on a comet.
The decade-long trip was successfully completed with a seven-minute thrust that allowed Rosetta to swing alongside comet 67P/Churyumov-Gerasimenko somewhere between the orbits of Mars and Jupiter.
"This is your only chance to have a rendezvous with a comet," Jean-Jacques Dordain, director-general of the European Space Agency, told scientists and spectators at the mission control center in Darmstadt, Germany.
The goal of the mission is to orbit 67P at a distance of 100 kilometers (60 miles) and observe the comet as it hurtles toward the sun. If all goes according to plan, Rosetta will attempt the unprecedented feat of dropping a lander onto a comet in November.
Scientists hope that the information they collect will help them learn more about the origins of comets, stars and planets, said David Southwood, who oversaw the scientific part of the mission until his recent retirement.
"Comets are the stuff of which the solar system was originally made," he said. Some scientists have suggested that water, an essential element for the development of life, arrived on Earth from comets.
Plans to bring material extracted from the comet back to Earth were canceled when NASA pulled out of a joint mission at an early stage, but the U.S. space agency contributed three of the 21 instruments aboard Rosetta and its Philae lander.
Scientists have already made a number of exciting observations even as Rosetta hurtled through space at about 55,000 kph (34,000 mph) — a speed that required three loops around Earth and one around Mars to gain pace.
Recently-released pictures taken by Rosetta show that 67P has an uneven shape that some have likened to a giant, four-kilometer (2.5-mile) long duck. This could mean that the comet is made up of two formerly distinct objects, or that it was heavily eroded.
The images, which have a resolution of 2.5 meters (eight feet) per pixel, also show steep 150-meter (490-feet) cliffs as well as smooth plains and house-sized boulders.
Scientists will spend the coming months analyzing the pictures Rosetta sends home to determine the best place to drop Philae. The lander will glide down to the comet before shooting a harpoon into its porous surface to avoid drifting off again.
Apart from the unprecedented landing, the orbiter section will also be the first to accompany a comet on its journey toward the sun, when 67P will begin to fizz and release the cloud of dust and ice that most people associate with comets. Measurements show that the comet is already losing the equivalent of two small glasses of water each second, an amount that will increase thousand-fold over the coming months.
"We're going to have a ringside seat to see, for the first time, a comet turn into a comet, to develop its tail and explain what for centuries mankind has been puzzled by," Southwood said.
While comets have long been associated with superstition, there is a real, albeit slim chance that such an object could one day hit Earth, causing a global catastrophe. Learning more about the nature of comets might help prevent that, Southwood said.
The spacecraft and its lander won't survive much beyond the end of next year, but the data they collect are expected to keep scientists busy for at least a decade. Rosetta, which has so far cost €1.3 billion ($1.74 billion), is one of the most high-profile missions for ESA, which is lobbying governments for its next four-year budget in December.
The European mission is different from NASA's Deep Impact spacecraft, which fired a projectile into a comet in 2005 to study the resulting plume of matter. NASA also landed a probe on an asteroid in 2001, but comets are much more volatile places because they constantly release dust and gas that can harm a spacecraft.
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