Leonids Around the Clock
Olivier's colleagues had predicted a furious meteor storm that year -- and millions of sky watchers were paying attention. Even folks normally indifferent to the heavens were outdoors when the storm was due to erupt. But they saw little. The 1899 Leonids were a conspicuous dud, revealing only how poorly astronomers of the day were able to forecast meteor activity.
Above: Meteors seem to fill the Little Dipper in this 1966 image of a Leonid meteor storm captured by A. Scott Murrell. Like most Leonid storms, this outburst surprised sky watchers.
Now ... fast forward 100 years.
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Times, it seemed, had changed.
"Leonid forecasts have really improved in recent years," says Bill Cooke of the NASA Marshall Space Flight Center. Using computers to model the orbits of comet trails, "we can now say with confidence in which years Earth will pass through the clouds of comet dust that cause Leonid outbursts."
These modern forecasts aren't perfect, though. This year is a good example: Four independent teams each using computer-aided methods have issued Leonid predictions. They all agree that storm-level activity will erupt on Nov. 18th. But they differ on basic questions such as When? Where? and How big?
Above: Colored curves distinguish Leonid forecasts by four groups of experts. Why do their results differ? They all use computers to model the orbits of comet dust clouds -- but the models contain different data and somewhat different physics. For instance, Brown and Cooke assigned little weight to Leonid meteor counts reported in the 19th century, while others relied substantially on such data. The assumed velocity of dust grains ejected from the comet can vary from model to model, as do assumptions about the effect of solar radiation pressure on the orbits of dust clouds, and the size of the clouds themselves. Which assumptions are correct? Perhaps the 2001 Leonids will tell us.
"According to our model, people in Hawaii should see the greatest number of meteors: one thousand or so per hour, weather permitting," says Cooke, who works in collaboration with Peter Brown of the University of Western Ontario. Other forecasters expect outbursts elsewhere: over North America or east Asia. Meteor rates there, they say, could climb as high as 4000 to 8000 per hour. [observing tips]
"We chose these places," explains Suggs, "based on weather and meteor forecasts." They are all likely to have clear skies on Nov. 18th, and each site is favored for at least one of the predicted outbursts.
Above: NASA scientists will monitor the 2001 Leonids from the Maui Space Surveillance Site atop the 10,000 ft Haleakala summit.
The scientists, mostly from NASA, the University of Western Ontario, and the U.S. Air Force, will split into six groups of 3 or 4 observers. "Our teams (except the ones on islands) will be mobile. They can move away from bad weather if necessary," says Suggs. Once they find a clear spot, they will set up low-light video cameras that can detect stars as dim as 8th magnitude -- about six times fainter than what the human eye can see.
Every hour they will transmit their meteor counts to the Leonid Environment Operations Center (LEOC) at the Marshall Space Flight Center. The same data will appear in near-real time on SpaceWeather.com, so anyone with an Internet connection can monitor the global storm.
Below: One of the NASA observing sites will be in the Gobi desert, near Ulaanbaatar, Mongolia. Poor communications could delay reports from this remote spot until long after the storm is over.
"Our goal is to distinguish between the various Leonid forecasts," says Suggs. "By comparing the predictions to the actual meteor counts, we are laying the groundwork to improve forecasts in the future."
Better predictions will not only please stargazers, but also help NASA safeguard Earth-orbiting satellites and spacecraft. Suggs explains: "If we can tell a satellite operator when a meteoroid stream is nearby, they can take timely precautions -- like pointing sensitive hardware away from the incoming space dust."
Space dust looks harmless enough: Typical grains span just a few tenths of a millimeter and weigh less than 10-4 grams. But they move very quickly. Leonid meteoroids are faster than 70 km/s or 157,000 mph. When such a meteoroid hits a satellite -- like a bullet -- the entire meteoroid is usually vaporized along with a tiny bit of the satellite itself. Electrons and ions in those vapors can electrify sensitive components, scrambling software and triggering mistaken control procedures.
Below: This artist's concept of the "plasma effect" illustrates how a meteoroid impact can produce electrified vapors that disrupt spacecraft systems. [more]
The danger to individual spacecraft during the coming meteor storm will be slight, notes Cooke. A satellite in low-Earth orbit, for example, would have to span an enormous area -- nearly a square kilometer -- just to intercept a few Leonid meteoroids.
But there are hundreds of active Earth-orbiting satellites. "When you add them all together the odds of somebody's satellite being hit are about 1% to 5%. That's substantial," says Cooke. "We'll see if anyone is unlucky this year."
Even if satellites escape the Leonids of 2001 unscathed, future storms will surely pose new threats to space-property. Accurate predictions could come in handy.... Just ask any 19th century astronomer: a good meteor forecast can save plenty of embarrassment!
Tune in to SpaceWeather.com on Nov. 18th and see for yourself which of the 2001 Leonid forecasts fares best. Meteor counts will appear hourly from around the world. Among the NASA monitoring sites only the Gobi desert will likely be omitted because of poor communications with that remote outpost.
Web Links
Jaw-Dropping Leonids - (Science@NASA) Don't rely on NASA astronomers to watch the Leonids for you. Go outside and see them for yourself!
2001 Leonid Forecasts - (NASA/MSFC) Leading meteor experts predict the coming meteor storm.
History of Leonid Predictions -- a nice overview of historical successes ... and flops.
Quicktime movies of the 1999 Leonid Meteor Storm over Israel:
Heads Up! - (Science@NASA) NASA helps satellite operators watch the ups and downs of the Leonids in 1999.
NASA engineers to track and share Leonids data -- (NASA/MSFC) From Mongolia to Maui, researchers will use special cameras to scan the skies and report meteor activity around the clock during the Nov. 17-18 Leonids shower.
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