The annual Leonid shower -- this year a storm -- is expected to be unusually intense because the Earth is crossing Comet Tempel-Tuttle's orbital path at a time when the comet has recently passed by. This happens once every 33 years when Tempel-Tuttle makes its closest approach to the Sun. The Sun's radiation boils bits of dust and sand off the comet, littering its path with debris.
Where possible, controllers will change the orientation of satellites to reduce the possibility that one of these tiny particles (1 to 100 microns in size, or about the size of a small sand grain) will strike and disable a spacecraft. However, Leonid storms pose a greater than usual threat to spacecraft not only because of the many tiny meteors (thousands per hour) hitting our atmosphere, but also the tremendous velocities of the particles.
As the Earth moves across the comet's trail, Leonid particles will enter the planet's atmosphere. Like two freight trains hurtling at one another on the same track, the distance between the massive debris cloud and the Earth closes at a mind-boggling 45 miles per second, or over 200 times the speed of sound. In contrast, Perseid meteors reach speeds of about 37 miles per second, and typical daily meteors achieve velocities of about 12 miles per second.
On spacecraft where it is practicable, high voltage systems that supply instruments will be turned off, or ramped down, to safeguard against the potential for electrical damage as a result of the satellite's plunge into the debris cloud. The tiny meteors can hit the spacecraft like a sandblaster and disintegrate, creating a cloud of electrically charged plasma. Under the right conditions, this plasma cloud can set off a chain reaction causing a massive short circuit. The loss of the European Space Agency's Olympus communications satellite in 1993 was attributed to a strike from the Perseid shower, and the resulting plasma discharge that zapped the spacecraft's delicate electronics.
The 22 NASA spacecraft under Goddard's control -- from the 24,500 pound Hubble Space Telescope to the 25-year old, 800 pound IMP-8 satellite -- will be continuously monitored during the peak of the storm, and some maneuvered to provide the greatest protection possible from debris.
"Each individual mission and project team reviewed its procedure for dealing with this annual phenomena, and has a specific implementation plan for the Leonid meteor storm," said Philip E. Liebrecht, Associate Director for Networks and Mission Services. "Each spacecraft has an operating plan that balances the risk of taking specific defensive measures against the risk of taking no action. We've had independent review teams assess our plans, and I think we are doing everything prudent and practicable to ensure the safety of our spacecraft."
The Leonid meteor shower arrives every November. It takes its name from the constellation Leo, the area of the sky where the meteors appear to originate. The shower's small particles are completely vaporized high in the Earth's atmosphere, and present no danger to the Earth's surface or to aircraft.
Historically, the most active Leonid showers occur during the first two years following the comet's closest approach to the Sun. This last occurred on Feb. 28, 1998. This year's outburst is projected to be less severe than that observed in the last 33-year cycle, which occurred in 1966. The peak time for the Leonid meteor storm will be Nov. 17, sometime between 11:43 a.m and 5:43 p.m. Eastern Standard Time.
For the past several weeks, engineers at Goddard have been reviewing the status of all the spacecraft under their control and developing ways to reduce exposure to the meteor storm. In general, the health of these spacecraft will be monitored before, during and after the storm, and commands to a number of the spacecraft will be stopped or curtailed during this period.
The Hubble Space Telescope will be maneuvered so that its mirrors face away from the storm. Its solar arrays will be rotated so only the edges are exposed to oncoming particles. Controllers won't turn Hubble off during the storm, but rather use the 10-hour period that Hubble is maintained in this attitude to take a long-exposure picture.
Some spacecraft, like the Tropical Rainfall Measuring Mission, are already in the ideal orientation for the storm, and only an adjustment to position the solar arrays "edge on" to the storm will be needed. The Rossi X-ray Timing Explorer's instruments will be turned off to protect the spacecraft's high voltage devices from a potential massive short circuit similar to what happened to Olympus.
For the Advanced Composition Explorer, the solar arrays will be rotated, and high voltage supplies for instruments will be ramped down. Since the center of the Leonid stream is closer to the L-1 orbit (1 million miles from the Earth toward the Sun) than to Earth, ACE will see an even more intense storm than Earth-orbiting satellites.
Risk reduction procedures will be followed for other spacecraft including the Extreme Ultraviolet Explorer, Compton Gamma Ray Observatory, Upper Atmosphere Research Satellite, Total Ozone Mapping Spectrometer, Fast Auroral Snapshot, Solar Anomalous Magnetospheric Particle Explorer, Transition Region and Coronal Explorer, WIND, POLAR, Solar and Heliospheric Observatory, Interplanetary Monitoring Platform and Earth Radiation Budget Satellite.
The Tracking and Data Relay Satellites will be maintained in their full operational mode, as these spacecraft are vital to provide the communications link to and from other spacecraft during the peak storm period.
Flight control teams for all of Goddard's operational spacecraft have been briefed on the meteor storm and have developed contingency plans to react to any damage sustained during the storm. In addition, all available command and control capabilities will be on alert for possible use in an emergency, and subsystem engineers will be on standby for consultation if there are any problems resulting from the storm.
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