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Rosetta sees surprising shape to spinning comet

Posted: July 17, 2014

Closing in to begin a thorough investigation in August, a camera on Europe's Rosetta comet-chasing probe has revealed its target has a few surprises in store for scientists.

Comet 67P/Churyumov-Gerasimenko rotates in a view captured July 14 by Rosetta's OSIRIS imaging system at a distance of approximately 12,000 kilometers. This movie uses a sequence of 36 interpolated images each separated by 20 minutes, providing a 360-degree preview of the complex shape of the comet. Photo credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Scientists say the comet is a contact binary, which consists of two bodies stuck together. They have compared the comet's shape to a duck, with one part larger than the other.

Other spacecraft have visited contact binary comets and asteroids, but the imagery from Rosetta shows a comet with sharp ridges and extreme topography.

"This is unlike any other comet we have ever seen before," said Carsten Guttler, project manager for Rosetta's OSIRIS imaging system, from the Max Planck Institute for Solar System Research in Gottingen, Germany. "The images faintly remind me of a rubber ducky with a body and a head."

Rosetta approached within 10,000 kilometers of comet 67P/Churyumov-Gerasimenko on July 15. It is due to arrive at a distance of 100 kilometers from the comet Aug. 6, when it will become the first spacecraft to enter orbit around a comet.

"The two blocks likely formed 4.5 billion years ago, collided at low speed, stuck to each other and have since been moving together," said Ekkehard Kuhrt, a comet researcher at the German Aerospace Centre, in a press release. "Scientifically, it is now of course very interesting to find out whether the two components differ in their composition."

Scientists used a technique called interpolation to create a smoother image from the pixelated raw pictures observed by the OSIRIS camera.

"There is, of course, still uncertainty in these processed, filtered images and the surface will not be as smooth as it now appears," Guttler said. "But they help us the get a first idea."

The images were officially released Thursday, two days after they were prematurely unveiled by CNES, the French space agency. CNES later removed the images from its website.

In a blog posting on the European Space Agency's website, officials wrote that Rosetta operates differently from other missions, such as NASA's Mars rovers, where raw images are released to the public immediately.

Authored by Rosetta mission manager Fred Jansen, Rosetta project scientist Matt Taylor and ESA senior scientific advisor Mark McCaughrean, the web post said most NASA and ESA missions do not follow the same "open data" policy employed by NASA's Mars rovers.

Rosetta's data releases must be coordinated between ESA, the member states which fund the probe's science instruments, and the scientists who operate them.

Complex agreements signed when Rosetta was developed nearly two decades ago set the rules for the publication of data.

Mission officials wrote that measurements and imagery from the 21 instruments on the Rosetta mothership and Philae lander, which will descend to the comet's surface later this year, are subject to a six-month proprietary period.

"This period ... gives exclusive access to the data to the scientists who built the instruments or to scientists who made a winning proposal to make certain observations," the blog post said.

Read the complete blog post explaining the Rosetta data distribution policy.

"The aim of a proprietary period is to ensure that the academic teams who spent decades developing and running the sophisticated scientific instruments on-board the spacecraft are able to calibrate and verify the data, as well as reap the rewards of their efforts: their scientific careers depend on it. Otherwise, it would be very hard to engage people in this long and difficult process," officials wrote.

The $1.7 billion Rosetta mission launched in March 2004, taking a meandering course around the inner solar system before spiraling away from the sun and entering a nearly three-year hibernation phase to conserve power.

Rosetta awakened from its deep sleep in January to begin final preparations for its encounter with Churyumov-Gerasimenko.

Scientists estimate the comet has a diameter of about 4 kilometers, or 2.5 miles.

As the craft moves closer to the comet -- colloquially known as 67P/C-G -- Rosetta's cameras are getting a better picture of the unexplored world.

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.

"Because no one has ever been to 67P/C-G before, each new piece of data from Rosetta has the potential for a scientific discovery," ESA officials wrote on the agency's website. "It's only fair that the instrument science teams have the first chance to make and assess those discoveries. At the same time, it's exactly because 67P/C-G is unknown territory and because there is an exciting journey underway that some are clamoring to see everything as soon as possible, in near 'real-time.' We understand that, but a balance must be struck, which is why only some of the data are being released now."

Follow Stephen Clark on Twitter: @StephenClark1.