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The Webfooted Astronomer - December 2001

 

The 2001 Leonid Meteor Storm Down Under

By Fred Quarnstrom

THIS year the Leonid meteor shower took place on November 18 or 19 depending on which side of the International Date Line you were standing when you pointed your nose toward the stars. I watched the storm from Alice Springs, Northern Territories, Australia.

This meteor shower is a result of the dust trails left by comet 55P/Tempel/Tuttle. Comet Tempel/Tuttle sweeps through the inner solar system every 33 years. As it approaches the sun, it brightens and ejects small particles. This rubble orbits with the comet. The solar wind pushes these particles outside the orbit of the parent come. The cloud of microscopic dust particles falls behind the comet because of their slightly more distant orbit. This cosmic sand box collides with our atmosphere at 71 km/sec. The shower takes place every November. On the years following the passage of this comet, the counts usually increases and some passages have resulted in the most spectacular meteor storms ever recorded. Meteor storms are defined as counts of more than 1000 meteors an hour. There have be less than 10 such meteor showers in recorded history. Seven of these were the Leonids.

A group of 35 of us assembled on a dry lakebed to observe this year’s shower. We had been in Australia for 8 days. The first 4 days were spent in the Pt. Douglas area of NE Australia to let us turn our clocks around and to give us a few days in the Coral Sea. Unfortunately our timing was the same as the box jellyfish’s yearly migration to the beaches. The sting of the box jellyfish can be fatal, so the beaches were closed. We did tour the rain forests and were able to dive and snorkel on the Great Barrier Reef where the jellyfish are not a problem.

Thirty-two of us were amateur astronomers. Three were professionals, Dr. Eric Hooper, a post doc fellow at the University of Texas doing work coordinating visual observations with the x-ray images of the CHANDRA X-ray Observatory. Dr. Don McCarthy, of the Arizona State University, is an astronomer with the Seward observatory. Eric and Don were old friends from the University of Arizona astronomy camps. We were fortunate to also have Dr. David Levy who was one of the co finders of comet Shoemaker-Levy 9, the comet that impacted Jupiter in 1993, and an author of many books on comets. These three gave us a background information on the history of the Leonid meteor shower, the methods that went into the predictions for this year, the reason for coming to Alice Springs, and the reports of past showers. They are fun folks who can make the most complex astrophysical concept understandable. We were fortunate to have them with us.

We arrived in Alice Springs after a 2-hour airplane trip from Cairns. Pt. Douglas had been overcast most of the time we were there. We did a little observing from the beach but were usually clouded out after a short time. A 50% cloud cover plagued Alice Springs the day we arrived.

The first evening we enjoyed an outback BBQ and had the stars and constellations identified for us. The local astronomy club had set up a star party, which we joined after the BBQ. It was an enjoyable evening of seeing the objects of the Southern Sky. Three of our group had telescopes, and there were several provided by the local club for us to enjoy. We also met some folks from NASA who were in the area to count meteors. Their leader was a friend of the Levys. The NASA folk had set up 8 chairs in a circle facing out in all directions. Each chair was equipped with a counter button. They were refining their counting skills that evening and checking the computer recording hookup. The goal was to count the meteors and record rates of impact with the atmosphere.

Most of us called it quits early, as we wanted to be well rested for the next day. Four of our group drove out of town for more observing and had the sky cloud over about 1 a.m. The skies opened up and we had a downpour about 4 a.m.

The next morning the skies were clear. We had arranged for a bus to pick us up at 2 p.m. and take us as far as 800 kilometers, if necessary, to get away from any clouds. It is an unpublished astronomical fact that the more important the astronomical event, the greater the chance the sky will be overcast. One of our professionals, Eric, spoke frequently with the local meteorologists to get any new weather information and the latest weather predictions.

The weather forecast looked good so we boarded the bus at 2 p.m. to visit the Henbury Meteor impact field 68 miles SW of Alice Springs. This field was created by the impact of 8 objects about 50,000 years ago. It was a bit sobering to observe impact craters knowing that we would be looking at similar extraterrestrial objects later that evening. The bus delivered us back at the hotel at 7 p.m. and plans were made to load up at 11 p.m. and head out of town to get away from the sky glow of Alice Springs.

At 11 p.m. the decision was made to go southeast to the dry lakebed. The weather forecast was good, but south seemed to be the preferred direction to put the slight sky glow of Alice Springs to the North. The meteors would emanate from the East in the constellation Leo that would arise at about 2 a.m. on the 19th. By the time we boarded the bus, several of the group had called the U.S. to get reports on the earlier peak there. The information was encouraging:; several astronomers in California had reported multiple meteors in the sky at one time. This earlier meteor cloud was predicted to have about half as many counts per hour as the one we would observe.

The bus headed south at 11:45 p.m. on highway 87, which goes to Adelaide more than a thousand miles to the South. Fifteen minutes later we turned onto a dirt road arriving at the “dry lake” by 12:45 a.m. As it turned out the rain the night before had turned it into a “not so dry lake.” Our location was South 24° 00.753” East 133° 57.334” at 1920 feet elevation, 21 miles SE of Alice Springs, Northern Territories, Australia.

Our drivers advised us to use our flashlights for the 100-yard walk to the flats so as to displace any creatures of the night that might think the lake was theirs. By 1:30 a.m. we pulled out ground sheets, mats, pillows and cameras in place. Leo was still below the horizon.

As if by script, the first two meteors arrived on time at 2 a.m. These two were born on the Eastern horizon, streaked overhead emitting particles much like a Fourth of July sparkler and died near the horizon to the East. Transit time was probably close to 10 seconds. The time of their arrival corresponded to the predicted dust trail of the 1699 passage of comet Temple-Tuttle. These were easily the most spectacular meteors I have ever seen. Our professionals said the same. The only phrase that seemed appropriate was WOW! Wow was uttered by every person on the flats many times that night.

The first two meteors were the scouting party for the coming storm, checking us out to see if we were worthy of the rest of the display. These first two liked what they saw and for the next three hours from 2:00 a.m. until 5 a.m. it was rare to go 10 seconds without a sighting. Many times there were as many as 5 meteors in the sky at a time. David Levy and his wife Wendee faced different directions and counted over 2,200 in those three hours. The Levys tallied only the meteors they saw to avoid any chance of double counts. John Fox counted over 1,300 by himself. Most of us just watched in awe as they streaked overhead, to the right, to the left, low on the horizon, and behind us. It was frustrating that our visual fields cover only about 1/6 of the sky. We would hear exclamations and turn only to see a trail. Many of the Meteors broke up as they entered the atmosphere creating 2 or 3 tracks. Others exploded in a flash becoming a bolide. Our group had seen more than 1,000 meteors an hour for several hours. There was probably twice that many. This shower clearly qualified as a meteor storm and one of the most memorable in recorded history.

Several meteor experts had made predictions of a 1,000 to 15,000 ZHR (Zenith Hourly Rate) being the number adjusted for what a single observer would see if the showers radiant had been at the zenith with a dark sky that would have allowed seeing 6.5 magnitude stars). Our sky was that dark but the radiant started on the horizon and only approached the zenith with the arrival of dawn.

From 2-3 a.m. the meteors were larger, brighter and longer lasting. The early larger meteors were probably from the 1699 tail. Many of these split into 2 or 3 as they entered the atmosphere. From 3-5 a.m. there were more meteors but smaller and dimmer. These were probably from the 1866 passage. Often there would be 5 or more in the sky at the same time. It was very obvious where the radiant was in Leo as they all came from that direction. By 4:30 a.m. the sky to the East was starting to lighten but the meteors kept coming. We started carrying our equipment to the bus at 5 a.m. Although we were clearly now in twilight the meteors were still very visible.

One of the reasons this year was predicted to be so good was a new moon that set before the shower, or more properly “Storm.” With no moon even the fainter meteors would be visible. Next year there should be a good show, but it will be during a full moon. Judging by number of really good trails we saw in the early morning twilight, next year should also be a good year even with the moon.

My advice to those observing next November, station yourself on high ground. I found when I was sitting at my camera taking time exposures, I missed many meteors that were low on the horizon that were visible when I was standing up. Had we been on a hill rather than a lake bed, we would have seen even more. I did get several bright meteors on film. One 5-minute exposure showed 7 faint streaks with a 27 mm lens and 200 ASA film.

We all slept in the next morning and then left the following day for Melbourne. Melbourne is a fun city. We spent time in the Victoria Market with its 1000 stalls selling everything from meat and nuts to clothes, and toys. We saw the penguin parade, kangaroos, and koalas. However none could compare to the show from the heavens.

This trip turned out to be even better than we could have hoped. The folks who made up our group were as diverse as could be, from Joan who retired from the New York Museum of Art, to a fireman, CPA, dentist, football referee, carpenter, engineer, pistachio nut farmer, just to mention a few. We had one common interest and that was astronomy or love for one interested in astronomy. It was a great group. We had as much fun with the group as we did observing. Our professionals gave us an appreciation of what we were about to see, what we saw and how we fit into the Universe. A good time was had by all.

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