June 20, 2014
ESA Expects The Unexpected As Rosetta Nears Comet Rendezvous
April Flowers for redOrbit.com - Your Universe Online
The European Space Agency's (ESA) Rosetta spacecraft is the first mission in history to attempt to rendezvous with a comet. The spacecraft will deploy the Philae lander to the surface of the comet and accompany it as it orbits the Sun. The comet in question is 67P/Churyumov-Gerasimenko, which is currently approximately 267,189 miles from the spacecraft.
The Rosetta team released a series of images last May that detailed the extended dust cloud developing around the comet's nucleus. A new full-resolution image, captured on June 4 by Rosetta's scientific narrow-angle camera, reveals that there is no longer any sign of this dust cloud, and that the comet has had a significant drop in its brightness since then. The team used this new image to help fine-tune Rosetta's navigation toward the distant comet.
"The comet is now almost within our reach – and teaching us to expect the unexpected," says the camera's Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research in Germany.
"After its onset of activity at the end April, our images are currently showing a comet back at rest."
Comets are known to display varying levels of activity, but this is the first time scientists have been able to witness dust production changes from such a close distance.
As a comet moves along its orbit, progressively moving closer to the Sun's warmth, the surface ices sublimate and gas escapes from the ice-rock nucleus to form the "coma." The gas flows away from the nucleus, carrying tiny dust particles which expand to create the coma.
Activity rises, and the warming continues, as the comet moves closer to the Sun. The long tail usually associated with a comet is formed when pressure from the solar wind causes some of the coma material to stream behind the comet.
Comets are typically lumpy, non-spherical objects, which makes the process of coma and tail creation unpredictable. Comet activity waxes and wanes as the comet warms. The Rosetta scientists were able to observe, in the six-week period from late April to early June, how quickly the activity of a comet can change.
After a long hibernation, Rosetta's scientific instruments were reactivated earlier this year. Since then, the scientific and navigation cameras have been acquiring images to correct Rosetta's trajectory to the comet.
This information has resulted in Rosetta making adjustments that will slowly bring it in line, aiming for a rendezvous with the comet in the first week of August.
So far, Rosetta has performed four maneuvers already, and another six are planned before the rendezvous. The final sequence of adjustments will be made August 6, when Rosetta reaches a distance of 62 miles from the comet. The final complex sequence will bring the spacecraft even closer to the comet's surface.
Rosetta has already started gathering data about the comet's environment and evolution, despite the six-week and 102,000-mile distance. Examples of such data include measuring the coma and determining the rates at which water and gases (like carbon dioxide) are being produced. Rosetta is even capable of determining how those rates change over time. The scientists will use this data to understand the chemical makeup of the comet's surface and interior.
Rosetta will also assess the plasma environment of the comet as the coma develops and interacts with the particles of the solar wind. As Rosetta draws closer to the comet, it will start collecting gas and dust particles from the coma to analyze in its miniaturized on-board laboratories.
"It's great to have started regularly receiving science data, especially after a long 10 year journey towards our destination," says Matt Taylor, ESA's Rosetta project scientist. "The variable activity of the comet shows it definitely has personality, which makes us all the more eager to get there to learn just how it ticks."
At the current distance, the 2.5-mile-wide comet occupies only one pixel in the narrow-angle camera. This means that the camera cannot yet discern any details of the nucleus. Within a few weeks, however, Rosetta will be close enough to see far more. The scientists believe that the comet will expand to five pixels by early July and 500 pixels by the start of August.
The team intend to release images more regularly from this point forward, with the next release scheduled for July 3. This release will be followed by a weekly update until the August 6 rendezvous. The new images will be published in the Rosetta gallery and through the Rosetta mission blog.