Dave Glackin captured this image of the Leonid meteors at the radiant from the Northwest Cape of Australia on the morning of November 19 2001. Green (oxygen) and red (nitrogen) ionization portions of trails can be seen. Also note faint satellite trail diagonally through left center of image. (Click on image to view a larger version).

Understanding the Leonids

This site provides information on the meteor storms associated with the appearance of the comet Tempel-Tuttle. The meteor activities the comet precipitates are called the "Leonids" because the meteors appear to originate from the constellation Leo. Understanding these events is important because of the effect they might have on the estimated 650 satellites on orbit. The Aerospace Corporation collected information on spacecraft anomalies experienced during the 1997 to 2000 Leonid showers and will be gathering similar information on future events.

During the Leonids, particles smaller than the head of a pin are very common while large particles are rare. For the 2001 Leonid event the actual risk of a satellite being hit by a particle that could cause damage was 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.

One technique satellite operators employ to protect satellites from damage during the leonids is orienting the solar panels "edge-on" to the meteor stream.

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 vaporizes particles into plasma, adding to its electrical activity.

ESDs are caused by a buildup 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 toESDs.

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.