e-Pulsar - Dec 98

e-Pulsar Dec 98 - Jan 99

Astronomical Feature of the Month
METEOR SHOWERS

The Great Leonid Shower of 1998
By Don Davis

The Leonids are the 'holy grail' of the meteor showers, a once in a 33 year passage of the Earth through a band of gritty rubble spreading from the comet Tempel-Tuttle out along the orbital path. More sparsely populated cometary orbits are common, with Earth passing through a dozen or so of these yearly. meteor showers accompany each passage, named for the constellation they appear to emerge from.

A 33 year cycle of massive periodic showers was recognized then anticipated, with uneven performance lulling most observers to expect little more than one of many annual showers.

The sight that greeted Western U.S. observers in November 1966 will be carried in their minds as long as they live. Rates of 1800 meteors per MINUTE were reported! It must have looked like driving fast in a snowfall with your high beams on! The storm was intense but brief, the peak lasting less than 2 hours before decaying swiftly to more traditional levels.

A composite picture of the orbit of Tempel-Tuttle in relation to the yearly path of the Earth was constructed using the accumulated observations of the last few cycles, modeling the situation enough to suggest the Asian Pacific rim regions might be facing the dense stream during the critical 2 hours, thought to be due at about 1943 Hrs UT on the 17th. The International Meteor Organization issued an 'Alert' circa 0900Hrs UT on Nov 16, stating that "a rapid rise in activity is in progress".

Before dawn that morning numerous brilliant fireballs were seen in the western U.S., some rivaling the brightness of the full Moon! An account from the San Diego area read: "The trains (of the numerous fireballs) lasted quite long for a lot of them... a -15 and a -12 had persistent trains that were of magnitudes of around -8 for a few seconds it seemed. They snaked and twisted all over the place. It was hard to keep count of their duration because often another fireball would show up. The -15 occurred above and behind us...the stars totally disappeared and the sky looked like daylight blue for a second or two. It was pure old excitement."

By 0800 UT on the 16th, reports came in from Mongolia of numerous fireballs, with brightnesses of -8 to -16, bright enough to make moving shadows! As Europe began to be exposed to the shower, 1100 UT, many echoes were detected on the FM band by someone listening in France. Meteor trails can reflect distant FM stations like the ionosphere routinely does for AM radio waves, bringing clear fragments of music and speech out of the static. At about that time, an observer in Jordan reported seeing meteors despite wretched observing conditions. Observers lucky enough to be under clear skies near Glasgow, Scotland, and in Northwest Europe were treated to a fireball rich shower mounting hourly in intensity.

The Cosmonauts on Mir observed the event, from orbit wide regions of the night side of Earth glimmered like a carpet of disturbed fireflies in a midwest field, each a dying flare of comet dust slamming head-on into the atmosphere.

The dark Atlantic seas danced with the reflections of the frenzied sparks careening overhead, with the only witnesses aboard ships and aircraft until the Canary Islands were exposed to the incoming stream. By this time, 0Hrs UT on the 17th, the hourly count of meteors visible at any one place had risen well into the hundreds, and still climbing! Alan Fitzsimmons sent a message, 'on behalf of a bunch of happy astronomers': "Dear All, We're at the summit of the island of La Palma in the Canary islands, at the UK Isaac Newton Telescope. We are having an amazing display here. From doing 2-minute counts, we were seeing roughly 1000 meteors per hour at 03:30 UT, climbing to roughly 2000 per hour at 04:30 UT. These are estimates of the all sky rate uncorrected for the height of Leo above the horizon. It may be an underestimate, as we are mere professional astronomers and are not used to this kind of activity - we feel that we may be missing a number of faint meteors (less than 3rd every couple of minutes you get a bright flash behind you and you turn around to see the ion trail fading. The brightest meteors have bright green trails, often with bright red heads. The rate still seems to be increasing - we've just gone outside the control room of the telescope and we are approaching one meteor per second. It looks like rates over the Eastern US may be very high. However the zodiacal light is now quite bright, so twilight is approaching."

The densest core of the column of debris brushed against the Earth somewhere in the Atlantic at about 0600 hrs UT, some 16 hours before the widely circulated prediction. In retrospect this was well within the uncertainties known to exist in such forecasts. The most intense moments of the meteor storm flared and died quickly, by the time North America was reached it had settled down to a still impressive hundreds per hour. Unfortunately much of the United States was clouded out, but here and there exited reports were added to the gathering wave of internet accounts.

From NW North Carolina : An observer reported "Between 0730 - 1030 UT I logged an average of well over 100 Leonids per hour that exceeded +1 mag. The majority were -1 or better and nearly all left short ionized trails. I didn't even bother to count the fainter meteors! The brightest observed was estimated at -8 and just happened to be coming straight out of the radiant... it was like someone setting off a flashbulb in the sky! No apparent motion - just a blinding flash! But the highlight was when I took a 'time out' from counting and sneaked a peek at M-42 with my 20 X 125 binoculars. Suddenly a very bright Leonid flashed through the middle of the field! I could see its ionized trail for about 30 seconds after its passage... just like a greenish arrow straight through the heart of the nebula!"

From 0830 UT to 1030 UT: "Over 200 meteors were observed over two hours through about 10% spotty cloud cover. At least 10 left trails lasting up to 30 seconds. Approximately 15 ended with extremely bright flashes which, when happening behind me, would lit up the landscape like lighting."

As North America rolled into the storm, accounts came in from the scattered clear spots, such as northern Texas: "Was watching early. Amazingly, activity started at 12 midnight on the nose. These first meteors to me were the best as they seemed to stream up from the horizon traveling nearly half the sky."

"Leonids was AWESOME in north-central Tennessee! We started watching at midnight, saw 66 meteors the first hour, mostly very bright and with long tails... The most SPECTACULAR event of our viewing came at about 1:30 am, when suddenly at least fifteen meteors AT ONCE shot out of a central spot in the sky. It was like it was raining! There were maybe five or six meteors side by side, with a second volley of five or six right behind them, and then a third, they just POURED out of the sky. Immediately afterward, there were seven or eight meteors all over the sky, one right after another (some simultaneously). It was incredible!"

From Lowell Observatory, Flagstaff, Arizona: "At 12:25 AM (0725 UT), I was in the telescope dome working when I heard the others exclaim in unison, "Wow!" They had seen a -5 magnitude meteor which was visible for 140 degrees of flight. It left a train that lasted for 10 minutes, getting kinked by the upper atmospheric winds. The display prompted us to developed our own meteor nomenclature. 'Puff Daddies' were the ones that went pffft. Then we had the 'Whoas' 'Double Whoas' and the 'Triple-Double Whoas.'"

The Western United States, most of which was cloudy, was the last region to see the spectacular phase of the shower. Many got a look through holes in the clouds, a few such as myself drove out from under the them.

Los Angeles was socked in under a thick drizzly fog. I drove Northeast through the mountains to the Lancaster area, and finally out from under the low coastal clouds. Several times while driving I saw fireballs in the periphery of my vision. Finally I found a decent exit, in a cold and reasonably dark vista point not far from Lancaster.

The wide faint stretch of Milky Way near Orion was clearly seen, and Leonids were everywhere. All one had to do was look up and shortly a bright streak would grab your attention, and when you looked away as often as not you would immediately see another. In all perhaps 200 meteors per hour were seen between 4 and 5 am Pacific time on the 17th. At times I saw a meteor every few seconds, other times a few minutes would elapse between sightings.

Nearly half of all the meteors I saw were very bright, most of these leaving persisting trails. Some of these brighter fireballs were very short, others streaked across a quarter of the sky. Once I saw two bright but short trail producing ones appear simultaneously near the radiant point in the head of Leo, spreading 'down' from each other and diverging at about a 30 degree angle to each other. On two other occasions I saw a meteor, then another, paralleling but not exactly following the path of it's immediate predecessor.

Once I saw a bright one, then a moment later a faint slow moving meteor roughly paralleling the path of the bright one but going in the opposite direction!

A couple were very bright, perhaps -10, one along the distant horizon ended in a bright gold burst that made the sky nearby glow. A couple ones whizzing overhead also tended to appear golden at their bright tips, with a cool gray trail initially remarkably bright then quickly fading, but visible for at times several minutes. At one time I watched two such glowing trails at once twist and spread among the stars while other meteors flashed by. I saw one very slow moving second magnitude meteor near the radiant point, like a swiftly moving satellite winking on and off. The bitter cold wind steadily picked up, and the crescent Moon rose above the horizon along with the first light of dawn.

The apparent ambiguities in the model of the swarms encounter with Earth rendered it useless to travel to some other continent to try to catch the best moments of the storm. The possibility exists that this was only the prequel to a larger display next year! It is suggested fewer fireballs but many, many more smaller meteors may be seen. A preliminary reshuffling of the data with next year in mind suggests Western North America may be favored, but we know how such predictions can go!

(Anybody interested in a workshop then?... Ed.)