* Astronomy

On Nov. 18-19th, 2005, sky watchers somewhere will see a dazzling storm of Leonid meteors.
Though not as spectacular an event as the "1998 Leonid fireball storm", which wasn't really a storm at all, with meteor rates that night never exceeded a few hundred shooting stars per hour, the 2005 shower is still one to watch...
We define a meteor storm to be times when observers can see 1000 or more per hour. The Leonids of '98 -- as spectacular as they were -- were not a full-fledged storm.
In 1966 North Americans enjoyed a Leonid storm numbering 100,000 shooting stars per hour!In 2001 we ran into relatively young clouds, richer in small meteoroids.Leonid meteor storms happen when Earth passes through clouds of dusty debris shed by comet 55P/Tempel-Tuttle as it orbits the Sun every 33 years. Last year our planet had a close encounter with two such clouds. They bubbled off Tempel-Tuttle in 1333 and 1733.
The leonids 2005 may peak from the 1167 trail at 1:40 UT 21 November.
Unfortunately, the almost full Moon will mask the fainter meteors.



Is there a risk from the annual Leonid meteor shower?
During the Leonid meteor shower, particles smaller than the head of a pin are very common while large particles are rare.
Most burn up high up in the Earths atmosphere.
So on the ground there is no risk - but what if you were in space?
For the 2005 Leonid shower the actual risk of a satellite being hit by a particle that could cause damage is approximately one-in-one-thousand to one-in-ten-thousand per square meter of exposed spacecraft area.

Physical mechanical damage to a spacecraft can occur as a result of "sandblasting," direct impact, or through "spalling" or chipping as larger particles hit the spacecraft and break up. Electrical damage to the satellite can result because of electrostatic discharges (ESDs) and electromagnetic pulse (EMP).

Mechanical damage consists predominantly of sandblasting, which all spacecraft experience during the Leonids. Sandblasting causes general surface degradation and pitting as the smallest particles hit the spacecraft. Because most particle sizes are very small, the impact on the overall health of a satellite is usually minimal.

Impacts and spall that result from larger particles hitting a satellite are of greater concern, however. Kinetic impacts can punch holes in a spacecraft's solar panel or wall. Spall produced by secondary particles created by the initial impact can affect the internal mechanisms of the spacecraft in a graver manner than even the initial hit.

ESDs and EMPs can result because of the Leonids impact on the electrical properties of the satellite and the surrounding plasma--the charged oxygen ions and electrons created by the interaction of solar radiation with the Earth's upper atmosphere, where satellites orbit. The high velocities of the impacts vaporise particles into plasma, adding to its electrical activity.

ESDs are caused by a build-up of charging over the satellite surface. If the charge becomes greater than the spacecraft can contain, a sudden discharge of accumulated electrical charge occurs. EMPs are created from the direct vaporization of impacting particles into plasma. Both ESDs and EMPs can cause electrical and communications problems. Erroneous signals in telemetry and short circuits can occur.

Satellite operators employ a number of techniques to avoid many of the potential dangers that arise during the Leonids. Most often, the cross-sectional area of the satellite that is exposed to the meteor shower is minimized, lowering the probability of the spacecraft getting hit. This is done by "feathering" the solar panels so that they point edge-on into the meteor stream, or by reorienting the spacecraft body to present a minimal target area to the meteors.

Another technique is to "safe" vehicle components by turning off equipment that is particularly sensitive to ESDs.

Whatever options are considered or implemented have to be weighed against their impact on mission operations. "Safing" a vehicle or reorienting the solar panels or spacecraft body can result in mission downtime. Any potential downtime must be balanced against the risk posed by the Leonids.

There are approximately 650 satellites on orbit.

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