MEXICO: Modern deep space probes may be among the most sophisticated pieces of hardware the 21st century can produce, but that doesn’t mean they aren’t susceptible to the age-old problem of dust. The European Space Agency’s (ESA) Rosetta space probe was thrown into safe mode recently when it was unable to take a simple star fix due to comet dust.
Since it left orbit around comet 67P/Churyumov–Gerasimenko, Rosetta has been executing a series of flybys at various altitudes to learn more about the comet as it approaches the Sun. On March 28, it came within 14 km (8.6 mi) of the surface when it had problems navigating and began to lose its radio link with Earth, which resulted in the spacecraft going into safe mode.
According to ESA, the cause was Rosetta’s star trackers. These are standard equipment on spacecraft, which use them to orient themselves. Since a compass is useless in deep space and gyros tend to drift, space probes keep on the straight and narrow by taking sightings on the stars in a manner that any mariner of the 18th century would understand. By identifying various key stars and triangulating their positions, a spacecraft can calculate its trajectory and its attitude.
Star trackers have been a reliable technology for half a century, but comets set challenges that make them much less so. As Rosetta flew close to 67/P, the dust particles blown away from the comet caught the sunlight and appeared to the trackers as stars. This confused them and the Attitude and Orbit Control and Measurement Subsystem couldn’t lock onto the target stars and keep Rosetta on course or aim the high-gain antenna properly at Earth.
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