[meteorite-list] Rosetta Probe Disoriented by Comet Dust

[Ron Baalke]

http://spaceflightnow.com/2015/04/05/rosetta-probe-disoriented-by-comet-dust/

Rosetta probe disoriented by comet dust
By Stephen Clark
Spaceflight Now
April 5, 2015

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 March 28 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 March 28 flyby, Rosetta zipped past the comet's icy core at 
a distance of about 14 kilometers, or 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 Aug. 13, 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 March 28 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."

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.

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-kilometer (3.7-mile) flyby Feb. 14. Rosetta's star trackers experienced 
similar issues then, but the spacecraft weathered the flyby without defaulting 
to safe mode.

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