November Showers

Around the middle of every November, the earth is involved in a headlong collision; it plows full tilt into a stream of meteoroids that heat into shooting stars as they plunge through the upper atmosphere. Most years, hardly anyone notices. Only astronomers and dedicated amateurs take note of the few brief, blazing arcs that make up the "Leonid showers," named for the constellation Leo, which appears behind them in the sky. This week the celestial fireworks promise to be far more gaudy than usual. Instead of half a dozen or so meteors per hour, the count in the early morning of Nov. 17 may number hundreds or even thousands.

Astronomers are hoping for a spectacle reminiscent of 1833, when about 10,000 meteors per hour were visible over the eastern U.S. at the peak of the shower. That year, awed viewers, aroused from sleep by the bright flashes, described shooting stars "falling from the sky like snowflakes." Many thought that the end of the world had come.

Dispersed Debris. Search for a more realistic explanation eventually led scientists to a minor comet called 1866 I, which circles the sun every 331 years.

Wheeling around a highly elliptical path, 1866 I is being gradually broken up by solar radiation, and the gravity of the sun and the larger planets. In the process, it has left a trail of countless meteoroids that now litter its entire orbital track.

As the earth circles the sun, it cuts through 1866 I's trail every November slicing into the thin stream of widely dispersed debris that produces the Leonid showers. In 1833, the earth's course took it through the middle of the main cluster of Leonids that follow closely behind the parent comet; it encountered a vastly larger number of meteoroids than usual. Just 33 years later, in November 1866, there was another fiery but less spectacular shower; the main cluster orbiting the sun once every 33 1/4 years was still three months away. In 1899 and 1932, at the time of the November encounter, the main cluster was even farther away. Both times there were only disappointingly modest increases in the Leonid showers--partly because of the meteoroids' 33 1/4-year orbital period and partly because the main swarm had probably been pulled into a slightly different orbit as it passed close by Jupiter and Saturn.

This November, a full four orbits after 1833, things should be different. The main swarm of Leonids should be back at the same point where they were intercepted by the earth 133 years ago. Astronomers who have predicted a substantial, if not spectacular shower, are hopeful that the earth will again pass directly through 1866 I's biggest clump of orbiting debris.

Comet Composition. During the shower, the Air Force will launch an Aerobee rocket equipped with a "Venus Flytrap" nose cone. While the rocket is rising to a peak altitude of 117 miles, four arms will extend out of the nose cone to catch the Leonid meteoroids, entering the earth's atmosphere at a speed of 162,000 m.p.h.; then the arms fold into the nose cone, which will fall back to earth carrying specimens that will help scientists determine the composition of the comet.

Unlike 1833, when the height of the shower was visible in the U.S., this week's spectacle should reach its peak at 8 p.m. (E.S.T.), Nov. 16, when it will be visible only in the Eastern Hemisphere. But astronomers expect it to be still going strong after midnight, when Leo rises over the U.S. East Coast and the flashy Leonids come into view.

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