Tune-Up For The Leonids

October 07, 1998

Perhaps the most anticipated event in astronomy this year is the upcoming Leonid meteor storm. Sometime during the early morning hours of November 17th, observers could be treated to a spectacular display of shooting stars as Earth passes through the debris trail of periodic Comet 55P/Tempel-Tuttle. Experts are cautiously predicting a repeat of the great meteor storm that broke over the Earth in November 1966 when more than 10,000 meteors per hour were observed in some locations. Despite all the talk about Leonids, a lesser known shower called the Giacobinids could steal the show this week. Every year Earth passes close to the orbit of comet 21P/Giacobini-Zinner. Usually not much happens because the comet isn't nearby. This year could be different. On October 9th Earth will pass by the comet's orbit just 49 days before the comet itself arrives. In 1933 Earth sailed by Giacobini's orbital path 80 days after comet passage, and there was a historic meteor storm. In some places over 20,000 meteors per hour were observed. We will be closer to the comet in 1998, but no one is certain what will happen because Earth will reach the vicinity of the orbit before the comet arrives. If there's plenty of debris flying ahead of the comet, then the resulting meteor shower could rival the predicted Leonid storm. If not, it will be another lackluster year for the Draconids with just a few, faint shooting stars per hour.

Amateurs can make a difference in 98
One reason for the uncertainty about the 1998 Giacobinids is that scientists still have a lot to learn about meteor streams. When comets visit the inner solar system, they are warmed by the sun, and ablated by the solar wind, which produces the familiar tails that we see. This debris is left in space, and is comprised of particles of ice, dust, and rock. When Earth encounters these particles on its journey around the Sun, they strike the atmosphere with tremendous speed and become shooting stars. Astronomers know that comets leave debris behind them, but does comet debris also precede the comet? This week we have a good chance to find out, because Earth will be passing by Giacobini's orbit in advance of the comet.

For this year's Giacobinid meteor storm Science@NASA will be collecting meteor counts from amateurs and lay observers to probe the structure of the meteor stream ahead of comet Giacobini-Zinner. If you would like to participate simply follow the simple instructions about how to observe and keep records. Then, after the shower, return here to submit your data. Even if you don't observe any meteors, your null result is valuable. It tells us how little debris is flying ahead of the comet. Cumulative results will be posted on SpaceWeather.com before the Leonid meteor shower in November.

How to View the Giacobinids

Normally the best time to view meteors is after midnight when Earth's rotation aligns our line of sight with the direction of Earth's motion around the Sun. Then we're heading directly into the stream of meteors. The Giacobinids are an exception. The best time to see them is during the early evening when the constellation Draco is still high in the sky, well above the horizon. The bright gibbous moon rises around 9 p.m. local time. After moonrise the sky will be bright and it will be difficult to see the fainter meteors.

To find the Giacobinids, go outside and face North. The radiant, indicated by a red dot on the sky map, is near two of the most familiar asterisms: the Big Dipper and the Little Dipper. You don't need binoculars or a telescope, the naked eye is usually best for seeing meteors which often streak more than 45o across the sky. The field of view of most binoculars and telescopes is simply too narrow for good meteor observations.

Experienced meteor observers suggest the following viewing strategy: Dress warmly as the autumn nights are likely to be cold. Spread a thick blanket over a flat spot of ground. Lie down, look up and somewhat to the north. Meteors can appear in any part of the sky, although their trails will tend to point back toward the radiant. A reclining chair is also handy.

Giacobinids at a Glance

Thursday evening, October 8th is the best time to watch.
Earth crosses the orbital plane of comet Giacobini-Zinner at 20:53 UT on October 8th.
The radiant is at RA=17h23m, DEC=+57o

This week's bright gibbous moon will make observations of the Giacobinids more difficult after 9 p.m. local time when the moon rises. The best observing conditions will be before moonrise, in the early evening when the constellation Draco is still high in the sky.

Tuning In to the Giacobinids
In 1946 the Earth passed through the orbit of comet Giacobini-Zinner just 15 days after the comet. The result was a remarkable meteor shower with hourly rates exceeding 3000 in many locations. It also marked an important event in serious meteor astronomy: the first radar detection of meteors. Just after World War II many countries had sophisticated radar installations, and over 20 were trained on the Giacobinid radiant during the predicted storm. Radar receivers in London, the Soviet Union and the United States all detected echoes. Radar measurements of meteor showers are important because radar is able to detect meteors even when they are very small, or when bright moonlight or sunlight makes visual observing impossible. In 1956 radar was responsible for the detection of a surprisingly strong Giacobinid storm. The Earth passed by the comet's orbit nearly 200 days before the comet, so astronomers weren't expecting much of a shower. However, radio astronomers at Jodrell Bank detected radar echoes from an intense meteor burst during the day on October 9th that lasted nearly 2 hours.

Modern amateur astronomers are tuning in to meteors through radio echoes. Radio echoes are not the same as radar echoes, but they are related. When a meteor burns up in the atmosphere it leaves behind a trail of ionized gas. The ionization rapidly dissipates, but transmissions from distant radio stations are briefly reflected from the ionized trail back down to Earth. During an intense meteor shower, a simple shortwave receiver can detect many echoes per minute from stations thousands of kilometers away. Interested amateurs can visit the International Meteor Organization for more information.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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