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November 09, 1999

A woodcut depicts the impression people had of the 1799 Leonids. The woodcut appears to show everything the artist saw during the entire shower rather than the view at any given instant.

Full size image available through contact

Nov. 10, 1999: Meteorites have become popular enough in recent years that they have worked their ways into disaster films and finally into commercials that parody the films. In the ads, what appears to be a massive rock aimed at the planet turns out to be a small cinder by the time it hits.

That's surprisingly close to the truth. Most of what we see blazing through the upper atmosphere - including the Nov. 17-18 Leonids meteor shower - poses no hazard to us.

"Leonids never make it to the ground," said Bill Cooke, an engineer for CSC Corp. working in the Space Environments Team at NASA's Marshall Space Flight Center. "They vaporize at heights of 100 to 160 km," or 62 to 100 miles.

"If they never hit the ground, then what's the big deal?" he continued.

The answer is that satellites in Earth orbit are at a slightly greater risk of damage since they are above Earth's protective atmosphere. The chances of getting pulverized by "the big one," or even a small one, are minimal.

What poses a far greater hazard is the potential for electrical short circuits when the impact from a dust grain forms a plasma cloud around a satellite.

So to help satellite operators keep an eye on what's going on during this year's Leonids, a special Leonids Environment Operations Center will operate at NASA/Marshall for 90 hours centered on the expected peak influx of Leonid meteors. Cooke will be part of the team that also includes members of the U.S. Air Force 50th Space Wing and Canadian meteor scientists. NASA, the Department of Defense, the European Space Agency, and commercial satellite companies like Iridium all support the project.

It won't be a meteor storm equivalent of the weather bureau, Cooke cautioned, but an effort to provide "situational awareness" to satellite operators.

The Leonids are the remains of comet Tempel-Tuttle which orbits the sun every 33 years in a direction opposite that of the planets and most other objects in the solar system. That means that objects shed by the comet run into the Earth at a relative speed of 72 km/s (150,000 mph).

That means that even small grains from the Leonids can form large, energetic plasma clouds when they strike a satellite. The risk is greater than from most meteor showers, because most comets and asteroids (and their debris streams) orbit the sun in the same direction as the Earth and so have lower impact velocities.

The Leonids happen every year, but peak every 33 years after the comet has shed a fresh load of litter for Earth to sail through. Older material is moved sunward by tidal effects of Jupiter and Uranus.

Records of the Leonids go back as far as 1799, and possibly 1100. The meteor storm of 1833, with more than 100,000 per hour, "marked the birth of meteor science as we know it," Cooke said. The 1866 storm inspired the song, "Stars Fell on Alabama." Storms skipped 1899 and 1932, then roared back in 1966 with an incredible storm that peaked at 150,000 meteors an hour.

"These things can surprise you," Cooke said.

Partly because of the 1966 showing, a few experts in 1998 issued dire warnings that satellites would be sandblasted.

Nothing happened.

"This year there is concern but much less panic," Cooke said. "There is a degree of concern because this is the first meteor storm of the modern space age." Only a few dozen satellites were operating in 1966. Today, several hundred are in orbit, and much of Earth's communications network is tied through them.

Yet Cooke has reason to be calm about the potential for harm. He went back to the historical record. During the Apollo lunar landing program, NASA wanted to know the meteor hazard to the Apollo manned spacecraft.

NASA/Marshall outfitted the upper stages of three Saturn I rockets (the two-stage predecessors of the massive Saturn V) as Pegasus satellites with immense wings that spread in orbit. The wings were equipped with thin metal plates to act as discharging capacitors when struck by a meteoroid to record impacts. Both Pegasus 2 and 3 were still operating during the 1965 and 1966 Leonids.

In 1965, they recorded two impacts, and in 1966, one impact.

"But not a single impact occurred when the satellites were exposed to the radiant," the direction of the Leonids, Cooke said. They all happened when the spacecraft were on the other side of the world and thus shielded by the Earth.

"One impact when you're seeing 150,000 an hour gives you an idea of the relative threat," Cooke said. "That is, there's not much of a threat at all." Cooke estimates that the chances of any one satellite getting hit range from 1 in 1 million to 1 in 10,000.

Still, as any statistician or gambler will tell you, long odds don't translate into an absolute zero chance. In August 1993, during the Perseids, an Olympus communications satellite was lost to a meteor hit. The impact formed a plasma cloud that caused electrical discharges in the spacecraft and zapped its attitude control system. By the time operators could stabilize it, they had depleted all of its attitude control propellant and the satellite was a loss.

The only sure meteor protection for a satellite is to keep it on the ground. But measures can be taken to mitigate the risks for those in orbit. These include turning off all but the most crucial electronics to reduce the risk of short circuits in case of an impact, and pointing sensitive optics and solar arrays away from the Leonids. The degree of risk will vary with location. Those on the opposite side of the Earth will be shielded from the Leonids (but can still get hit from behind). Those in geostationary orbit will be less exposed if their orbit has them on the dayside of Earth since the peak flux is expected on the night side.

This year's Leonid watch will be a larger version of the 1998 storm watch that deployed teams equipped with radar and optical instruments to Mongolia and northwest Australia on a line where the Leonids were expected to peak.

Unfortunately, the estimate of peak rate was off by 16 hours and the Canary Islands were the best place to observe (this year, Eastern Europe and the Near East are expected to be the best places). Cooke said the only fireball seen by the Mongolian team was their tent burning after it caught fire when their hosts used a wood stove to warm a truck's gasoline that had congealed in the extreme cold. (No one was seriously injured.)

For 1999, a broader array of gear will be deployed, including radar in (appropriately) Alert, Canada. At 82 deg. N latitude, the source of the Leonids will never set. Other observing stations have been set up in Israel, the Canary Islands, Hawaii, Key West, and Kwajalein Atoll in the Central Pacific. In addition, NASA's NC-135 Flying Infrared Signature Technology Aircraft and an Air Force EC-18 Advanced Range Instrumentation Aircraft will be airborne with optical instruments to make stereo images of Leonid fireballs.

Data will be collected at the Leonids Environment Operations Center at NASA/Marshall every 15 to 60 minutes - depending on activity - and provided to the satellite user community over the web. The web site will not be available to the public, Cooke said, to keep it from being swamped by requests.

That's OK because the center won't paint a pretty picture, literally. Cooke said that most of the data will be tabular, with the most important products being the flux numbers. A special Leonid fluence calculator has been available since July that lets operators input satellite position data and get an idea of what's flying through their neighborhood. This fluence calculator is used before the Leonids to estimate the risk to a particular spacecraft.

There is no doubt that satellite operators are glad the chance of damage is small, even if this year produces a great show for sky watchers. Cooke noted that not enough is known about the Leonids to predict exactly what they will do, as witnessed by last year's disappointing show.

"We may be dealing with a very asymmetrical stream of material," he said.

And we probably are dealing with the show of the decade, or perhaps the next century, if it materializes as expected.

"After 2000, you won't see another one for about 100 years," Cooke said. "This year is your last chance for a long time."
-end-


NASA/Marshall Space Flight Center--Space Sciences Laboratory

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