The list of space firsts got a little longer this week. On August 6 at 11:30 am CET, ESA's Space Operations Centre (ESOC) in Darmstadt, Germany received radio signals confirming that Rosetta had begun its approach and was going into orbit around comet 67P/Churyumov–Gerasimenko, making Rosetta the first spacecraft to go into orbit around a comet.
The culmination of a 10-year journey, this marks the start of a year-long mission by Rosetta to study Churyumov–Gerasimenko to gain new insights into the early history of the Solar System, as well as the first attempt to land on a comet in November. The probe has already traveled 6.4 billion km (3.9 billion mi), and is now 450 million km (280 million mi) from Earth, orbiting Churyumov–Gerasimenko as it plummets toward the inner Solar System at about 55,000 km/h (34,000 mph).
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Launched in 2004, Rosetta reached Churyumov–Gerasimenko by a circuitous route involving three flybys of Earth, one of Mars, and a long detour out beyond Jupiter as it built up enough speed to catch up to the comet. During this time, it passed close to the asteroids Šteins and Lutetia, and went into a 31-month hibernation to conserve resources until the comet rendezvous.
In January, Rosetta was awakened by a radio signal from mission control and, over the course of several weeks, its scientific instruments were brought back online, including a NASA package that started examining Churyumov–Gerasimenko in June. Since May, the spacecraft has been carrying out a series of 10 critical course corrections, the failure of which would have resulted in the probe flying past the comet with no hope of recovery.
Like most comets, Churyumov–Gerasimenko has an elliptical orbit – in this case, circling the Sun once every 6.5 years as it travels beyond Jupiter and back inside the orbit of Mars. Astronomers believe that comets, which are “dirty snowballs” composed of ice, rock, and dust, are the remnants of the early Solar System and can tell us a great deal about how the planets formed. In addition, many believe that it was from comet impacts that Earth received its water.
During its approach, Rosetta has trained its navcams and OSIRIS camera at the comet, which sent back dramatic pictures of Churyumov–Gerasimenko; revealing it to be craggier and sharper edged than the asteroids and moons previously explored that had been worn down by billions of years of meteor impacts. The most striking feature is a pronounced neck in the comet nucleus, which makes it look a bit like a rubber duck that’s gone a few rounds with a terrier.
In addition, ESA says that observations from the Microwave Instrument for the Rosetta Orbiter (MIRO) show that Churyumov–Gerasimenko is losing water at a rate of 300 milliliters (10 oz) per second, and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) shows that the comet has an average surface temperature of -70° C (-94° F), which means it’s a very dark and dirty snowball that absorbs rather than reflects sunlight.
Rosetta is currently about 100 km (62 mi) from Churyumov–Gerasimenko and will remain in orbit around it for over a year until the comet swings back toward Jupiter. Because the gravity is so low, the spacecraft isn't making neat circles around the comet, but travels in a series of triangular orbits as it nudges closer to the nucleus over the next six weeks. Eventually, it will go into a circular orbit at a distance of 30 km (18 mi).
As it approaches Churyumov–Gerasimenko, Rosetta will scout out five candidate spots to set down the Philae lander for the historic first attempt at landing on a comet. After the final decision is made sometime in October, Philae will leave Rosetta and head for a touchdown on Churyumov–Gerasimenko on November 11.
"Over the next few months, in addition to characterizing the comet nucleus and setting the bar for the rest of the mission, we will begin final preparations for another space history first: landing on a comet," says Matt Taylor, ESA’s Rosetta project scientist. "After landing, Rosetta will continue to accompany the comet until its closest approach to the Sun in August 2015 and beyond, watching its behavior from close quarters to give us a unique insight and real time experience of how a comet works as it hurtles around the Sun.”
The video below contains highlights of Rosetta’s rendezvous with Churyumov–Gerasimenko.