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Comet dust mimics stars, disorients Rosetta

As Comet 67P/Churyumov-Gerasimenko heats up on its sunward journey, it's becoming something of a hazard. All the lovely dust and water ice flying off the nucleus - the stuff that makes comets look hazy - have given mission planners and...

Rosetta star trackers in red
Rosetta's star trackers are marked in red, and part of the high gain antenna can be seen in the background. Credit: ESA/ATG medialab.

Rosetta 67P March 22 jets
This view of comet 67P/Churyumov-Gerasimenko was taken by Rosetta's navigation camera on March 22, and shows a cloud of dust and gas surrounding the nucleus. I've circled some of the troublesome dust particles Rosetta has recently been running into. Credit: ESA/Rosetta/NAVCAM

As Comet 67P/Churyumov-Gerasimenko heats up on its sunward journey, it's becoming something of a hazard. All the lovely dust and water ice flying off the nucleus - the stuff that makes comets look hazy - have given mission planners and the Rosetta spacecraft a headache.

Rosetta ran into trouble on March 28 when the probe tore past the nucleus at a distance of only 8.7 miles (14 km). The spacecraft uses a set of cameras to fix on bright stars to determine its orientation in space. The trouble occurred when it was positioned over the larger of the two lobes of the comet.

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Rosetta star trackers in red
Rosetta's star trackers are marked in red in this illustration, and part of the high gain antenna can be seen in the background. The star trackers are used to automatically pinpoint the craft's position and its orientation to Earth by using the known positions of stars. Credit: ESA/ATG medialab

Rosetta's star trackers, which are supposed to find guide stars to monitor the spacecraft's alignment with Earth and the Sun, mistook the bright, sunlit flecks of comet debris for stars, preventing the craft from getting a proper navigation fix. That triggered the craft to put itself into temporary safe mode, where science instruments and non-essential functions are shut down to prevent them from any damage.

If Rosetta knows how it's oriented with respect to the stars, it can determine its orientation to the Earth and Sun. That's critical because the probe needs to point its high-gain antenna at Earth in order to send data and receive communications. If the antenna drifts away from home base, communication with Rosetta could potentially be lost.

"Attempts were made to regain tracking capabilities, but there was too much background noise due to activity close to the comet nucleus: hundreds of 'false stars' were registered and it took almost 24 hours before tracking was properly re-established," according to a blog posting on the European Space Agency's Rosetta site.

67P long dust jet April 2_2015
In this specially processed image taken April 2, dust extends from the 2.5-mile-wide nucleus (at lower left) up to and out of the frame - a distance of at least 15 miles. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Rosetta experienced similar trials during an even closer flyby of 3.7 miles (6 km) Feb. 14 but weathered the pelting without defaulting to safe mode.

Less serious but still concerning, the dust grains pushed against Rosetta's two 46-foot-long (14 meter) solar arrays, causing drag on the spacecraft.

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"Cross comparisons with other navigation mechanisms showed inconsistencies with the star trackers and some on board re-configurations occurred. While attempting to reconfigure those, the same error occurred again leading this time to an automatic safe mode on Sunday afternoon," according to ESA.

The good news is that once mission control recovered the star trackers, the system auto-corrected and communications returned to full strength. All science operations are now back on line.

But it does bring up the question of what to do during future close flybys of the nucleus especially since the dust and vapor are only expected to thicken as 67P makes its closest approach to the Sun on Aug. 13. The good folks at ESA are right now re-accessing their options in light of Rosetta's disorienting experiences.

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