The Earth will have another close encounter with Comet Tempel-Tuttle’s dust trail in the early hours of 18 November, and
the resulting meteor storm, called the Leonids, could be spectacular.

But the storm so eagerly awaited by astronomers is also making spacecraft controllers take precautions. Like a ship caught
in a tempest, ESA’s Solar and Heliospheric Observatory (SOHO) will try to stay as safe as possible during the meteor storm.

The scintillating effect produced by the friction between the comet’s dust particles and the upper layers of our atmosphere
should be visible in North America, Europe, Africa, and part of Asia.

With the Earth remaining at a distance of about 1 million kilometres from the comet’s path, the danger for SOHO and other
satellites may be considered minimal, say scientists. But given that the flow of dust particles is hundreds of thousands of
kilometres wide, space agencies and satellite operators have decided the risk should not be disregarded.

The loss of ESA’s Olympus communications satellite in the early 1990s was thought to have been caused by the Perseid
meteor shower.

The relative velocity between our planet and the particles left behind by comet Tempel-Tuttle will be about 70 km/s, that
is more than 250 000 km/h. “At that speed, if you had a hit by a discrete particle, it would pierce right through the
spacecraft like a bullet,” says Bernhard Fleck, SOHO Project Scientist.

During the coming peak of activity of the Leonids, as many as 1000 meteors per hour are expected to strike the Earth’s

To avoid the danger of a sandblasting effect on the optical systems of its instruments, the protective doors on SOHO will be
commanded to close.

In 1986, the camera on ESA’s Giotto spacecraft suffered — as expected — severe damage as it passed in front of Halley’s
comet nucleus. Since cometary dust could cause similar problems to SOHO’s vital star trackers, ground controllers will
command the spacecraft to rotate 120 degrees around its axis. The star trackers are used by the spacecraft to determine its
position in space as it observes the Sun from its vantage point 1.5 million kilometres from Earth.

Ten of SOHO’s 12 scientific instruments will also be turned off, to minimise the risk that any instrument hit by a particle
would be severely damaged by the combination of incandescent debris from the impact and the high-voltage power supply.

During the peak of the storm, SOHO will be placed in a safety configuration, known as the Coarse Roll Pointing mode. The 10
instruments will remain switched off for a period of almost 70 hours starting 16 November.


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