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(meteorobs) Goddard Spaceraft Prepared For Encounter With Leonid Meteor Storm



Mark Hess/Jim Sahli
Goddard Space Flight Center			Nov. 9, 1998
Greenbelt, MD 20771
(Phone: 301-286-8955)

RELEASE NO: 98-184

GODDARD SPACECRAFT PREPARED FOR ENCOUNTER WITH LEONID METEOR STORM

Flight controllers are laying plans to prepare an orbiting fleet of 22
Goddard spacecraft for the upcoming Leonid meteor storm, predicted to be the
fiercest in more than three decades.

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 (for more on this, check out
http://www.stscidot edu/ftp/proposer/leonid.html).

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.

More information on the Leonid meteor storm can be found at these web sites:

   http://www.aero.org/leonid/index.html

   http://www-space.arc.nasadot gov/~leonid/

   http://leroy.cc.ureginadot ca/~astro/Leonids/Leo_1.html


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