Comet dust sends Rosetta into safe mode

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Rosetta's star trackers (red) are used to orient the spacecraft (Image: ESA/ATG medialab)

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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.

Plotting track of recent Rosetta flybys (Image: ESA)

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.

Another factor is Rosetta's unusually large solar panels designed to capture the maximum energy while traveling far beyond the orbit of Mars. As 67/P blows out more gas and dust, these panels act like airfoils and generate drag, which makes the spacecraft even harder to orientate.

Between these problems, Rosetta was unable re-establish tracking for 24 hours. As the high-gain antenna skewed away from Earth, the spacecraft's systems went into safe mode until mission control could reestablish proper contact. Unfortunately, the dust continued to play havoc with the tackers and on the 29th Rosetta went back into safe mode to protect the spacecraft's instruments.

As of the latest reports, Rosetta has been restored to full operations but ESA says that resuming its scientific mission will take a while. On April 1, mission control was able to successfully execute a maneuver that brought the probe from an altitude of 400 km (250 mi) down to 140 km (87 mi).

Rosetta has been studying 67P since it first went into orbit around the comet in August of last year. For much of that time, it was mapping the surface in anticipation of the Philae probe making the first soft landing on a comet in history. Since then, the spacecraft has been making a detailed study of the comet and its coma in the hopes of gaining a better understanding of the mechanisms that shape comets as they approach the Sun.

Source: ESA

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