ACE Spacecraft Braces For Fierce Meteor Storm

November 10, 1998

Like Key West store owners anticipating a hurricane, flight controllers at the NASA Goddard Space Flight Center, Greenbelt, Md., are doing what they can to prepare the 22 orbiting spacecraft under their command to meet the November 17 Leonid meteor storm, predicted to be the fiercest in more than three decades.

Each November 17 the Earth and its swarm of orbiting satellites passes through the Leonid meteor stream, which originates from the wake of Comet 55P/Tempel-Tuttle. In most years the Leonid shower (so named because the meteors appear to come from the direction of the constellation Leo) is unimpressive. But once every 33.25 years -- this year -- the Earth passes through Tempel-Tuttle's path just after the comet has made its closest approach to the sun, when dust, sand, and other materials have been freshly boiled up from the comet's surface by the sun's radiation and left in its trail.

NASA technicians are changing the orientation of spacecraft and ramping down their high voltage supplies to reduce the risk of damage as they plunge through the massive cloud of Leonid particles at a speed of 45 miles per second for approximately six hours. They are maneuvering the Hubble Space Telescope so its mirrors face away from the storm and adjusting other spacecraft so their solar arrays will meet the storm "edge-on."

At most risk, scientists say, will be the Advanced Composition Explorer (ACE) spacecraft designed and built by The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md. Launched in August 1997, ACE is in orbit at a point 1 million miles from Earth toward the sun, a position that will take it closest to the center of the comet's wake as we pass through.

ACE carries nine instruments designed to study energetic particles from the sun, interplanetary space, and regions beyond. But none of these instruments is designed to look at the tiny dirt and dust particles the spacecraft will soon encounter. In fact, they'll be ducking their heads.

Mary Chiu, ACE Program Manager at APL, says, "We hope for the best, but we really can't predict what will happen. The probability of getting hit by a particle that could cause damage is still fairly small, but, depending on the size of any given particle and where it might hit, the possibility for problems exists."

To the extent possible, NASA will maneuver ACE so that the backs of solar arrays will face the meteor storm and three of the most sensitive instruments -- the Solar Energetic Particle Ionic Charge Analyzer, the Solar Isotope Spectrometer, and the Cosmic Ray Isotope Spectrometer -- will be pointed away from it.

"We're standing by to help the NASA team analyze and assess any problems that might result from the storm," Chiu says. "But at the same time, we're crossing our fingers in hopes for an uneventful day on November 17."

This dramatic encounter with the Leonid meteor storm will not be visible to amateur stargazers in America, who, like sensitive instruments on NASA spacecraft, will be on the back side of the Earth as it roars through the cloud of particles.
The Applied Physics Laboratory is a not-for-profit laboratory and independent division of The Johns Hopkins University. APL conducts research and development primarily for national security and for nondefense projects of national and global significance. APL is located midway between Baltimore and Washington, D.C., in Laurel, Md.

Johns Hopkins University

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