Comets at close range
26 September 200115 years after ESA's Giotto spacecraft achieved an historic close range reconnaissance of Comet Halley, another probe from Earth has successfully followed in its footsteps.
At 23:30 CET on 22 September, NASA's Deep Space 1 (DS1) spacecraft glimpsed the solid nucleus of Comet Borrelly, only the second occasion in the annals of space exploration that such a close encounter with one of these small ice worlds had been achieved.
"We congratulate our American colleagues on their remarkable success," said Gerhard Schwehm, who was deputy project scientist for Giotto during its March 1986 flyby of comet Halley and is now project scientist for ESA's Rosetta comet orbiter/lander mission.
"Comets are the building blocks of the Solar System, and carry clues about how the planets, including our Earth, were born," he said. "So it is very important that we obtain as much information as possible about these mysterious bodies."
"Images and other data from spacecraft such as Giotto and DS1 also give us vital information about what to expect when Rosetta reaches comet Wirtanen," he added. "This will help us to plan for Rosetta's landing on the nucleus of Wirtanen and its two-year-long survey of the comet."
DS1 passed within 2200 kilometres of Comet Borrelly, the closest that any spacecraft have come to the heart of a comet, apart from Giotto's two skimming encounters with Comet Halley and Comet Grigg-Skjellerup in 1986 and 1992 respectively.
The NASA spacecraft returned a series of black-and-white pictures, infrared spectrometer measurements, ion and electron data, and measurements of the magnetic field and plasma waves around the comet. These results will help scientists understand the nature and overall composition of the comet's surface and the dusty coma that surrounds the nucleus.
Further transitory comet encounters are planned during the next decade, culminating in Rosetta's in-depth exploration of Comet Wirtanen - the most detailed examination ever made of one of these intruders to the inner Solar System.
Particularly exciting will be the unique opportunity to study a comet from extremely close quarters over a period of many months. Starting in November 2011, Rosetta will edge closer and closer to the nucleus as its battery of instruments studies every conceivable aspect of Comet Wirtanen. Eventually, as the spacecraft closes to within 1 km of the pock-marked surface, features only a few cm across will be visible to the orbiter's cameras.
While the main spacecraft maps the comet in unprecedented detail, a 100 kg lander will drop gently onto the nucleus to complete the first soft-landing on a comet. Another suite of instruments will swing into action, taking panoramic pictures of the barren landing site, measuring its composition and 'weather', probing the comet's internal structure and even drilling into its frozen crust.
Up above, the orbiter will continue to observe the dramatic changes that occur as the Sun looms ever larger and the ices of the nucleus start to sublime. During 18 months of orbiting the nucleus, Rosetta will bring Wirtanen in from the cold as it catalogues in minute detail for the first time every characteristic of a comet.