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(meteorobs) The Leonid Meteor Outburst of 1997

The Leonid Meteor Outburst of 1997
Marshall Space Flight Center
http://science.nasadot gov/newhome/headlines/ast16jul99_1.htm

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!

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.

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.

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!

"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. "

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.
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