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The Leonid Meteor Outburst of 1997

Newly released video shows a flurry of Leonids in 1997 that briefly rivaled the great meteor storm of 1966.

July 16, 1999: On Nov. 17, 1997, as comet Tempel-Tuttle was approaching the inner solar system, meteor watchers around the world were scanning the heavens for shooting stars. Tempel-Tuttle is the source of debris that gives rise to the well-known Leonid meteor shower that peaks around November 17th each year. Normally the Leonids produce just 10 to 20 shooting stars per hour, but every 33 years when Tempel-Tuttle passes near Earth, the usually mild Leonids "shower" can become a fierce "storm" with hundreds of meteors streaking across the sky every second!

Above: This newly-released video clip of a Leonid outburst was captured on Nov. 17, 1997 by amateur astronomers Masao Kinoshita, Takuya Maruyama and Toru Sagayama of the Nippon Meteor Society from an observing site on Mauna Kea, Hawaii. The outburst lasted 1.57 seconds. The circular field of view was 55° in diameter centered approximately between the constellations Leo and Ursa Major. Images are copyright Masao Kinoshita, all rights reserved. Click for more information and additional video.

Observing conditions on the night of Nov. 17, 1997 were not ideal. The moon was nearly full so that only the brightest meteors would be visible through the blinding glare. Furthermore, although Tempel-Tuttle was in the neighborhood of Earth, it had not yet passed perihelion (its closest approach to the Sun). Historically, the best Leonid showers have taken place after the comet has passed perihelion, not before.

Nevertheless, many amateur and professional astronomers were watching the skies. With little hope of seeing anything significant, three Japanese observers in Hawaii were rewarded with a spectacular outburst that briefly rivaled the best Leonids storms ever.

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Armed with an array of cameras and video recorders, amateurs Masao Kinoshita, Takuya Maruyama and Toru Sagayama chose to observe the 1997 Leonids from one of the world's best astronomical viewing sites: Mauna Kea, Hawaii at 3500 m elevation. Their equipment included three 35 mm cameras with optical and infrared film, and two 8mm video recorders equipied with image intensifiers. The video recorders were sensitive to meteors brighter than about magnitude +4. During most of their observing session, which lasted from 11:00 UT to 15:40 UT, the video cameras recorded between 10 and 40 meteors per hour. Other observers around the world reported similar numbers.

However, the video cameras also captured something that eluded naked-eye observers. At 13:31:51 UT, just as Earth was passing through the descending node of Temple-Tuttle's orbit, there was a 2-second outburst of meteors numbering between 100 and 150.

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According to M. Kinoshita et al, who reported their observations in the January 1999 issue of the Geophysical Research Letters, the outburst occupied 47 video frames corresponding to 1.57 seconds in time. Surprisingly, the flurry of meteors was not recognised as an outburst by visual observers or cameras watching the same patch of sky. T. Maruyama, who was observing visually, saw the brightest meteor of the group (magnitude -2) but no others. Similarly, the photographic cameras recorded only a couple of meteors.

M. Kinoshita et al speculated that the outburst could have been caused by the breakup of a larger meteoroid high in the earth's atmosphere, or it may have been due to a clump of smaller, more typical grain-of-sand sized meteoroids in the highly structured cometary debris stream. They estimated that such a meteoroid clump must have been spread about 100 km along the orbit of Temple-Tuttle to produce the observed outburst.

Video Rules

The surprising Leonids outburst in 1997 might never have been noticed if not for the use of low-light video cameras. Inexpensive CCD video cameras are a fairly recent innovation that could greatly improve meteor observing for professionals and amateurs alike.

"This field is wide open," commented Leif Robinson, editor of Sky & Telescope magazine, at the recent Partners in Astronomy meeting in Toronto hosted by the Astronomical Society of the Pacific. "These new video cameras could open the door for amateur astronomers to make important research contributions to meteor studies that were difficult or impossible in the past."

Potentially, amateurs can use the new breed of video hardware to study the composition and breakup dynamics of meteoroids by examining meteor spectra and their light curves, and they can explore the substructure of meteoroid debris streams through observations of outbursts like the one over Hawaii in Nov. 1997.

Even the simple task of visually counting meteors can be greatly improved through video imaging. For example, during the 1998 Geminid meteor shower astronomer Tony Phillips observed a 20 degree-wide patch of sky centered on the constellation Orion for 3 hours using an Astrovid 2000 CCD video camera. Video images were continuously recorded on tape while Phillips counted meteors by eye and recorded the counts in a notebook. The next day, a comparison of the video tape with Phillips's notebook revealed that the camera had captured twice as many meteors as the astronomer!

Above: Clicking on the image above will activate a video clip of two meteors streaking through the constellation Orion. They were filmed by Dr. Tony Phillips in Aspendell, CA at approximately 1110 UT on December 13, 1998, the night before the maximum of the 1998 Geminid meteor shower. He used an Astrovid 2000 CCD video camera with a 12 mm f1.2 lens, and a shutter speed of 1/60 sec. The 30 frame sequence spans 2 seconds of actual time.

"I have 20/20 vision and the limiting magnitude was +6 for both me and the video recording system," says Phillips. "It was a fair competition, but the camera recorded many more meteors than I did. The ones that I missed tended to be faint, short and fast moving. When I played back the tape they were there, as clear as day. If lots of amateurs begin using recording devices like this we may discover all sorts of new things about meteor showers. "

Right: A short video segment showing a Leonid fireball as seen from the stratosphere. It was recorded by a digital video camera carried aloft by a NASA weather balloon on Nov. 17, 1998. More images and video may be seen at LeonidsLive.com.

The video cameras atop Mauna Kea in 1997 saw about 150 meteors in less than 2 seconds. That corresponds to an hourly rate of 180,000 - 270,000 per hour, which is comparable to the activity seen over western North America during the great Leonids meteor storm of 1966. The 1966 storm lasted for hours, but the 1997 outburst was over in less than two seconds. The 1997 event can hardly be considered a "storm," but it could be a sneak preview of the 1999 Leonids when many experts expect a spectacular show. Hopefully, more observers than ever will be monitoring the skies on November 17 with video recorders to capture whatever surprises the Leonids have to offer. Web Links

Leonids Live! -site of the live webcast of the 1998 Leonids

Preliminary activity of Leonid meteor storm observed with a video camera in 1997 - by Masao Kinoshita, Takuya Maruyama & Toru Sagayama, Geophysical Research Letters volume 26:1, pages 41-44 (1999 January 1).

PDF VERSION: Preliminary activity of Leonid meteor storm observed with a video camera in 1997 - by Masao Kinoshita, Takuya Maruyama & Toru Sagayama, Geophysical Research Letters volume 26:1, pages 41-44 (1999 January 1).

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For more information, please contact: Dr. John M. Horack , Director of Science Communications

Author: Dr. Tony Phillips Curator: Linda Porter NASA Official: Ron Koczor

Topics:

• Solar System
• Stars