Rosetta probe disoriented by comet dust

This view of comet 67P/Churyumov-Gerasimenko was taken by Rosetta's navigation camera on March 22, showing a cloud of dust and gas surrounding the nucleus. Credit: ESA/Rosetta/NAVCAM
This view of comet 67P/Churyumov-Gerasimenko was taken by Rosetta’s navigation camera on March 22, showing a cloud of dust and gas surrounding the nucleus. Credit: ESA/Rosetta/NAVCAM

Ground controllers are analyzing a fault aboard Europe’s Rosetta spacecraft after an encounter with comet dust confused the probe’s navigation system, leaving the robot explorer in a temporary safe mode and halting regular science operations.

Rosetta ran into trouble during a 28 March flyby near the nucleus of comet 67P/Churyumov Gerasimenko, the oddly-shaped comet the mission has explored since August 2014.

The spacecraft uses small cameras to locate bright stars in the sky, using the stellar fixes to determine its orientation in space.

During the 28 March flyby, Rosetta zipped past the comet’s icy core at a distance of about 14 kilometres (8.7 miles). The probe aimed for a flyby point over the larger of comet 67P’s two lobes, according to the European Space Agency.

The comet is heating up as it swings closer to the sun, triggering plumes of outgassing water vapor and dust particles. Scientists expect the comet’s awakening to continue past perihelion – its closest approach to the sun – on 13 August, with the period of most activity forecast for September.

ESA officials blame the growing cloud around the comet’s nucleus for the pointing error during the 28 March flyby.

The dust grains pushed against Rosetta’s long power-generating solar arrays, causing increased drag as the craft approached the comet.

But ground controllers observed a more serious effect on Rosetta’s star trackers, which are supposed to find guide stars to self-monitor the spacecraft’s alignment with Earth and the sun. The guidance system confused the flecks of dust for stars, rendering the trackers unable to set a navigation fix.

“During the most recent flyby, a number of issues were reported, starting with the primary star tracker encountering difficulties in locking on to stars on the way in towards closest approach,” officials wrote in a blog post on ESA’s website. “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.”

Artist's concept of Rosetta with comet 67P/Churyumov-Gerasimenko in the background. Credit: ESA
Artist’s concept of Rosetta with comet 67P/Churyumov-Gerasimenko in the background. Credit: ESA

Rosetta initially recovered from the problem after its high-gain antenna drifted away from Earth.

“However, issues with false stars were still occurring,” the blog post said. “Cross comparisons with other navigation mechanisms showed inconsistencies with the star trackers and some on board reconfigurations occurred.”

The spacecraft ended up in safe mode, a state where Rosetta switched off its scientific instruments and halted non-essential functions to ensure the probe’s survival. The safe mode occurred the day after Rosetta’s closest approach to the comet.

This stunning scene was created from two NAVCAM frames acquired at 19.9 km from the comet centre on 28 March. The scale is about 1.7 m/pixel and the image measures 3.1 x 1.7 km. The image has been adjusted for intensity and contrast, and the vignetting has been fixed. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0


Engineers at the Rosetta control center in Darmstadt, Germany, restored the spacecraft to normal status by March 30, but science operations remain mostly suspended as officials gauge the implications of the event.

The March 28 encounter was Rosetta’s closest brush with the comet since a 6-kilometre (3.7-mile) flyby 14 February. Rosetta’s star trackers experienced similar issues then, but the spacecraft weathered the flyby without defaulting to safe mode.

Rosetta performed a rocket burn 1 April  to bring the spacecraft from a distance of 400 kilometres (248 miles) to about 140 kilometres (87 miles) from the comet by 8 April, officials said.

Limited science operations should resume in the coming days and weeks.

Mission managers planned a series of flybys near comet 67P’s nucleus in the next few months, including a targeted trip through one of the comet’s active jets in July. Rosetta’s struggles last week may prompt officials to reconsider how to execute future flybys.

“The science and operations teams are currently discussing the impact of the recent navigation difficulties on the current planned trajectories, possibly resulting in further replanning in order to ensure that the spacecraft can operate safely as the comet activity continues to increase towards perihelion in August,” the blog post said.

Follow Stephen Clark on Twitter: @StephenClark1.