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SOHO Prepares To Weather The Leonid's Meteor Storm

ESA Science News
http://sci.esa.int

11 Nov 1999

SOHO prepares to weather the Leonid's meteor storm

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 atmosphere.

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.

USEFUL LINKS FOR THIS STORY

* ESA Leonids99 page
  http://sci.esa.int/leonids99
* More about SOHO
  http://sohowww.nascom.nasa.gov/
* SOHO near-real time image screen saver
  http://www.estec.esa.nl/spdwww/soho/SohoScreens.exe

[NOTE: An image supporting this release ia available at
http://sci.esa.int/missions/newsitem.cfm?TypeID=12&ContentID=7591]


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