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Rosetta mission one month from historic comet arrival

Posted: July 6, 2014

The European Space Agency's Rosetta spacecraft is one month from pulling alongside an icy comet and beginning the most comprehensive survey of a comet yet attempted.

Artist's concept of Rosetta at comet Churyumov-Gerasimenko. Photo credit: ESA-C. Carreau/ATG medialab
Rosetta has been fine-tuning its approach to comet 67P/Churyumov-Gerasimenko since early May, using its rocket thrusters to bend the spacecraft's orbit to rendezvous with its target in the first week of August.

Arrival at Churyumov-Gerasimenko is scheduled for Aug. 6, when Rosetta will become the first spacecraft to ever enter orbit around a comet.

On July 6, Rosetta's distance to the comet was less than 35,000 kilometers, or about 22,000 miles. As the craft's range to Churyumov-Gerasimenko decreases, Rosetta's cameras are getting a better picture of the unexplored comet.

More details about the comet's shape, rotation, and surface features will be resolved by Rosetta's narrow angle camera in the coming weeks. The data will help scientists plan observations and give controllers input on the conditions awaiting Rosetta, such as gas, dust and ice particles, when it passes inside 50 miles of the comet in early August.

"Our comet is coming out of its deep space slumber and beginning to put on a show for Rosetta's science instruments," said Matt Taylor, Rosetta's project scientist at ESA.

Other observations from the $1.7 billion mission's suite of science instruments have detected water being ejected from the comet. A microwave sensor on Rosetta from NASA looked at Churyumov-Gerasimenko in early June, measuring the rate at which water vapor is released from the comet.

Data showed the 4-kilometer-wide (2.5-mile-wide) comet is releasing the equivalent of two glasses of water into space every second, surprising scientists who did not expect to see outgassing of water from so far away.

"At this production rate, comet 67P/Churyumov-Gerasimenko would fill an Olympic-size swimming pool in about 100 days," said Sam Gulkis, principal investigator of the MIRO instrument at the Jet Propulsion Laboratory in Pasadena, Calif. "But, as the comet gets closer to the sun, the gas production rate will increase. With Rosetta, we have an amazing vantage point to observe these changes up close and learn more about exactly why they happen."

Comet 67P/Churyumov-Gerasimenko, seen in Rosetta's OSIRIS narrow angle camera on June 28, from a distance of about 86,000 kilometers. The comet covers about four pixels in this image and appears, as expected, rather fuzzy from this distance. Photo credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Five rocket burns during the next month will put Rosetta in position for capture by the comet's feeble gravity field, beginning more than a year of close-up mapping and scientific research, including the release of a German-built lander to touch down on the comet's nucleus in November.

Most of the maneuvering work was accomplished in five burns beginning May 7.

The last rocket firing July 2 adjusted Rosetta's velocity relative to Churyumov-Gerasimenko by 58.7 meters per second, or 131.3 mph.

More than 90 percent of the required velocity change -- known as delta-V in the space business -- to reach Churyumov-Gerasimenko has been accomplished following the successful maneuver July 2.

The next thruster burns are scheduled for July 9, July 16, July 23, Aug. 3 and Aug. 6, according to ESA.

Rosetta will need occasional rocket firings throughout its mission to stay close to the comet, which officials expect will have an irregular, lumpy gravity field, making trajectory planning a challenge for control teams.

When Rosetta arrives at the comet in August, it will be beyond the orbit Mars. The comet's orbit will take it to just outside the orbit of Earth at perihelion -- its closest approach to the sun -- in August 2015.

The probe will escort the comet on its journey into the inner solar system.

Scientists say comets contain the primordial building blocks of life, and missions like Rosetta aim to yield clues on the role they played in the early solar system.

Follow Stephen Clark on Twitter: @StephenClark1.