This week brings the calendar to the expected annual encounter of the Earth's orbit with the dust trail of a comet--an event that varies in its intensity but which has brought spectacular peaks in viewing meteor showers over the past five years.
The Leonids are grains of dust from comet Tempel-Tuttle colliding into the Earth's atmosphere. Most Leonid particles are tiny and will vaporize very high in the atmosphere due to their extreme speed (about 44 miles per second, or almost 71 km/sec), so they present no threat to people on the ground or even in airplanes. As it progresses in its 33-year orbit, the comet releases dust particles every time it comes near the Sun. Earth intersects the comet's debris trail every year in mid-November, but the intensity of each year's Leonid meteor shower depends on whether Earth ploughs through a particularly concentrated stream of dust within the broader debris trail.
The Leonids get their name from the constellation where they appear to originate; the meteors will be radiating from the Sickle pattern in the constellation Leo the Lion, which will be rising out of the east-northeast sky. Don't look directly at the constellation, but at the area above and around it. And, though you don't need them to see the Leonids, a pair of binoculars could come in handy.
Public observers of this year's meteor shower and fireballs are invited to report their results online with various astronomical organizations.
IMO On-Line Fireball Report Form
American Meteor Society - On-line Fireball Reporting Form
Dutch Meteor Society - On-line Fireball Report form
North American Meteor Network - Snail-mail report forms
The past five years (from 1998 to 2002) have seen some exceptional Leonid meteor showers and storms in some years. The last one occurred in 2002 but, from the results of computations, it appears that no other storm is expected for the coming decade. At present, no Leonid storm until at least 2033 has yet been identified.
|A composite of Leonid meteor images recorded by a CCD camera onboard the MSX satellite
The parent body, comet 55P/Tempel-Tuttle, passes its dust trail through the orbit of the Earth around November 18-22 every year, but the magnitude or peak rate of meteor encounters varies on a regular three-decade cycle. This year however may offer conditions for the Earth to pass through what are otherwise considered old dust streams. At any rate, as such old streams are very perturbed, the peak or zenith hourly rate (ZHR) is expected to be low. This year is expected to be poor in bright meteors. Even if the Leonid meteor storm period is over now, the year 2003 will provide good conditions to observe some showers. The last encounter with such old streams was the famous 1998 one.
Two years ago, "Leonid meteor storm yielded rich research results for NASA astrobiologists," said Dr. Peter Jenniskens, a NASA astronomer based at Ames Research Center and principal investigator for an airborne research mission to collect data unhampered by ground lights and most local weather conditions. "Findings to date indicate that the chemical precursors to life -- found in comet dust -- may well have survived a plunge into early Earth's atmosphere."
Jenniskens and his international cadre of researchers think that much of the organic matter in comet dust somehow survived the rapid heating of Earth's atmospheric entry. "Organic molecules in the meteoroid didn't seem to burn up in the atmosphere," he explained. They may have cooled rapidly before breaking apart, he concluded.
Another finding with potentially important implications for astrobiology is that meteors are not as hot as researchers had previously believed. "We discovered that most of the visible light of meteors comes from a warm wake just behind the meteor, not from the hot meteoroid's head," said Jenniskens. This warm wake has just the right temperature for the creation of life's chemical precursors, he said.
Utah State University researchers found that, during the meteors' demise in the atmosphere, their rapid spinning caused small fragments to be ejected in all directions, quite far from the meteoroid's head. This is an important finding for astrobiology, because it means that meteors may be able to chemically alter large amounts of atmosphere.