I N T E R N A T I O N A L M E T E O R O R G A N I Z A T I O N The 2001 Leonids ================ Dear Meteor Friend, As you know, 2001 may well be the most exciting Leonid year in the current epoch, with three major peaks, the first one visible from America around 10 UT on November 18, and the other ones visible from Eastern Asia and Australia around 18 UT on November 18 (morning of November 19 local time). Rainer Arlt already forwarded to you important observing and reporting information. For your information, we provide you in attachment with the following documents: 1. Press Release; 2. Science Note. It is not unlikely that your local press may ask you for information about the upcoming Leonid event. For this purpose, Documents 1 and 2 (a short press release and more expanded elaboration) may be useful. Provided proper credits are given, you are welcome to use these documents in any manner that suits you. Finally, note that the International Meteor Organization is currently making arrangements to ensure, weather permitting, that reliable first results will be available within hours after the above times. Press releases based on these results will also be forwarded to you. Kind regards, Marc Gyssens Council Member email: wgn@imo.net phone: +32-477-64 05 48 I N T E R N A T I O N A L M E T E O R O R G A N I Z A T I O N Press release November 18: the day of the Leonid meteors ========================================== From North and Central America, as well as from Eastern Asia and Australia, people may see a lot of meteors - "shooting stars" - between midnight and dawn of the nights of November 17 to 18 or 18 to 19, provided skies are clear. These meteors belong to the so-called Leonid shower. A first peak, visible from North and Central America, is expected around 9:55 a.m. Greenwich Mean Time on November 18, which is 4:55 a.m. Eastern Standard Time. An activity equivalent to several hundreds to over one thousand meteors per hour (or 10-15 meteors per minute) is expected around the peak time. A second and a third peak are visible from Eastern Asia and, mainly, Western Australia, around 5:24 and 6:13 p.m. Greenwich Mean Time on November 18, which is 1:24 and 2:13 a.m. on November 19. The weaker first peaked is expected to flow over into the stronger second peak, for which an activity equivalent to several thousand meteors per hour (2 meteors per second!) is expected. Notice that American observers should watch in the second part of the night of November 17 to 18, while Asian and Australian observers should watch in the second part of the night of November 18 to 19! The International Meteor Organization, who collects meteor observations world-wide for the purpose of analysis, wishes to point the attention of the public to this spectacular natural phenomenon. The Leonids are caused by a stream of predominantly very small particles, less than 1 mm in size, which orbit the Sun with a period of 33 years, together with their parent comet, Tempel-Tuttle. The orbit of the Leonid particles happens to intersect the Earth's orbit. Each year around November 18, when the Earth is at this intersection, Leonid particles may enter the Earth's atmosphere and cause meteors, popularly called "shooting stars." This year, the Earth will pass through three dense dust trails ejected by the Comet, in addition to several fainter ones. As the predications above are based on models, peak times and peak rates may vary somewhat from the ones quoted above. In particular, there are indications that the first, American, peak may be stronger than quoted here, whereas the second and third, Asian/Australian may be weaker than quoted here. Whereas Europe, Western Asia, and Africa will miss both storms, observers there may still see several tens up to one hundred meteors per hour in the second half of the nights of November 17 to 18 and 18 to 19. Actually, Leonid meteors can be seen every year around November 17. Along the larger part of Comet Tempel-Tuttle's orbit, however, Leonid particles are scattered sparsely, so that, in most years, we see only a few Leonid meteors per hour. Only in the vicinity of the Comet, the density of Leonid particles is much higher. Therefore, we observe much higher Leonid activity every 33 years during a couple of years, when Comet Tempel-Tuttle revisits our region of the Solar System. In some instances, we even see a real meteor storm! Old chronicles contain references to past Leonid meteor storms back to the 10th century A.D. The best-known Leonid meteor storms are those of 1833 and 1966, when tens of meteors per second darted across the skies during the peak hour! The 1833 meteor storm was so spectacular that it in fact launched meteor research as a branch of astronomy. Since the 1966 meteor storm, Comet Tempel-Tuttle has completed another revolution around the Sun. The passage of the Comet through its closest point to the Sun on February 28, 1998 marked the beginning of a five-year period (1998-2002) during which strongly increased Leonid meteor activity is again possible. Although 1998 gave us an unexpected (but meanwhile convincingly explained) fireball shower, the first storm in the present Leonid epoch occurred in 1999, with a peak activity around 60 meteors per minute (yielding an equivalent hourly rate of almost 4000). In 2000, no storm was seen, but several peaks with a few hundred meteors per hour occurred. Observations in 1999 and 2000 matched the predictions by astronomers David Asher and Robert McNaught really well, so that there is more than good hope that the predictions for this year are reliable, too. In order to see meteors, the sky must be clear and the selected observing site should preferentially be free of light pollution; the less light, the more meteors will be seen! Notice that Leonid meteors occur in the second half of the night. Hence, there is no point in starting an observation much earlier. Die-hards who do not want to miss anything of the show should then continue to watch until dawn. People who cannot afford to stay up that long should focus on a period of, say, one tot two hours centered around the predicted peak time for their region. Mind that it can be very cold in mid-November: warm clothing adapted to the local climate is essential! For comfortable observing, use a reclining chair, and install yourself in a suitable sleeping bag or under several blankets. While observing, do not fix a particular star, but look relaxedly and patiently to a wide area of sky and wait for shooting stars to appear. ------------------------------------------------------------------------ More information on the Leonids can be found in the International Meteor Organization's bimonthly journal WGN and on the internet, at http://www.imo.net and http://www.amsmeteors.org/imo-mirror. For questions, contact Marc Gyssens at wgn@imo.net or +32-477-64 05 48. Notice that the International Meteor Organization will send out a new release with first results on the Leonids during the European early morning hours of November 18, immediately after the event. All recipients of the present release will automatically receive the new release. I N T E R N A T I O N A L M E T E O R O R G A N I Z A T I O N Science Note The Leonid Meteor Shower in 2001 ================================ SUMMARY - From most of North and Central America, from Eastern Asia, and from Western Australia, people may see a lot of meteors - "shooting stars" - between midnight and dawn of the night of November 17 to 18 (America), respectively the night of November 18 to 19 (Asia and Australia), provided skies are clear. These meteors belong to the so-called Leonid shower. Three major peaks are expected. The first one, visible from North and Central America, occurs at 10 a.m. Greenwich Mean Time on November 18, which is 5 a.m. Eastern Standard Time. American observers may expect to see the equivalent of several hundreds up to over one thousand meteors per hour near the peak time. The second and third peak occur near 5:30 and 6:15 p.m. Greenwich Mean Time on November 18, which is 1:30 and 2:15 a.m. Chinese Standard Time on November 19. The activity of the first, weaker, peak will flow over into that of the second, stronger one; near the peak time, the equivalent of several thousand meteors per hour is expected. Europe, Africa, and Western Asia will miss both meteor storms; however, observers there may still see from a few tens up to one hundred Leonids per hour in the second half of the nights of November 17-18 and 18-19. The International Meteor Organization, who collects meteor observations world-wide for the purpose of analysis, wishes to point the attention of the public to this spectacular natural phenomenon. [Numbers between brackets refer to the glossary section.] 1. THE LEONIDS ----------- The Leonids are caused by a stream of predominantly very small particles, less than 1 mm in size, which orbit the Sun with a period of 33 years, together with their parent comet (1), Tempel-Tuttle. The orbit of the Leonid particles happens to intersect the Earth's orbit. Each year around November 17, when the Earth is at this intersection, Leonid particles may enter the Earth's atmosphere and cause meteors (2). Along the larger part of Comet Tempel-Tuttle's orbit, Leonid particles are scattered sparsely, so that, in most years, we see only a few Leonid meteors per hour. Only in the vicinity of the Comet, the density of Leonid particles is much higher. Consequently, every 33 years, during the years that Comet Tempel-Tuttle revisits our region of the Solar System, much higher Leonid activity is recorded. In some instances, this Leonid meteor shower develops into a real meteor storm! 2. LEONIDS IN THE PAST ------------------- Old chronicles from all over the world (European, Arab, Chinese, Korean, Japanese, American, ...) contain references to past Leonid meteor storms back to the 10th century A.D. Well-documented observations of Leonid meteor storms go back only to 1799, when the great German explorer and naturalist Alexander Von Humboldt, rather coincidentally, witnessed a Leonid meteor storm from Venezuela. The same spectacular phenomenon was also observed from Florida. However, the 1833 Leonid meteor storm had a far greater impact on the public and the scientists alike, mainly because it was visible in a much more densely populated area, namely New England. At its peak, tens of meteors crossed the sky each second! Pious Christians believed that Judgment Day had broken and many who witness this celestial fireworks compared it to a snow storm! Because of the interest it had sparked, this particular Leonid meteor storm turned out to be very instrumental for the development of meteor astronomy. A somewhat less spectacular Leonid meteor storm occurred in 1866; around 1899 and 1933, there was increased Leonid meteor activity, but no storm. In 1966, however, the Leonids returned in full splendor: observers at Kitt Peak in Arizona saw a Leonid meteor storm peaking with no less than approximately 40 meteors each second, which amounts to a frequency of 150 000 meteors per second! 3. LEONIDS TODAY ------------- Since the 1966 meteor storm, Comet Tempel-Tuttle has completed another revolution around the Sun. The passage of the Comet through its closest point to the Sun on February 28, 1998 marked the beginning of a five-year period (1998-2002) during which strongly increased Leonid meteor activity is again possible. Whether or not a meteor storm actually materializes in any or all of these years depends on several circumstances, on which we will briefly elaborate. 4. WHEN DO STORMS MATERIALIZE? --------------------------- Meteor showers (3) are caused by small particles orbiting the Sun, in most cases released by comets. Each time a comet passes the Sun, it releases "dust" particles (as well as gasses), in the case of Comet Tempel-Tuttle every 33 years. As a first approximation, we may compare this dust production to the condensation trail of a jet plane. Like a condensation trail, a dust filament released by a comet fades away over a period of a few centuries until it can no longer be distinguished from the dust around the comet that was released much longer ago. Only if the Earth passes through a dust particle filament released by the comet at most 7 or 8 revolutions ago, in the case of Comet Tempel-Tuttle less than about 250 years ago, will a veritable meteor storm occur. Every 33 years, when the Comet passes the Sun, there is a "window" of about 5 years in which the Earth may pass through one or more "young" dust particle filaments. When this happens, we see a storm of between about one thousand and more than one hundred thousand meteors per hour, lasting at most one hour. The actual peak of the activity is often of even shorter duration. The precise frequency depends on the age of the filament and whether the Earth goes straight through the core of this filament, or only through its outer regions. If the Earth misses all young dust particle filaments, the older dust particles around the comet will give rise to a more modest meteor shower producing 50 to 100 meteors per hour. 5. WHEN DO WE SEE LEONID METEORS? ------------------------------ Around November 18, Leonid particles may enter the Earth's atmosphere from a direction - called the radiant (4) - located in the head of the constellation of Leo, the Lion, from which the shower derives its name. Because Leo is below the horizon in most of the first half of the night, we can only see Leonids past midnight. From one particular location, a possible Leonid meteor storm is only visible if peak activity occurs between midnight and dawn. In addition, you need a clear sky, which is not for granted around mid-November ... 6. WHAT HAPPENED UP TO NOW? ------------------------ The current Leonid window was "opened" during the morning hours of November 17, 1998, when early birds witnessed a veritable fireball (1) storm: during a typical hour, 100 to 200 very bright meteors appeared. These meteors were even so bright, that the spectacular show could be followed well into dawn. This fireball storm actually came as a complete surprise. The embarassment it caused to scientists was an important incentive to develop the current prediction model that proved very reliable up to now. In the morning hours of November 18, 1999, a veritable meteor storm of up to 4000 meteors per hour (though much fainter than in 1998) was witnessed mainly from Southern Europe and the Middle East. For 2000, no storm was predicted. Several peaks of a few hundred meteors per hour were observed, in reasonable accordance with predictions. 7. WHAT MAY BE EXPECTED THIS YEAR? ------------------------------- Scientists Robert McNaught and David Asher predict the following peaks for November 18, 2001, all given in Greenwich Mean Time, also known as Coordinated Universal Time (UTC): Where to see When Expected activity ---------------------------------------------------------------- North and Central America 09:55 UTC 800 meteors/hour (13 meteors/min) East Asia/West Australia 17:24 UTC 2000 meteors/hour (33 meteors/min) 18:13 UTC 8000 meteors/hour (2 meteors/second) The first peak is visible around 5 a.m. EST in the morning of November 18 from North and Central America. Both other peaks combined (it is not sure whether they can be distinguished) around 2 a.m. Chinese Standard Time in the morning of November 19. Notice there may be minor differences between the real and predicted peak times; also, the rates may vary. Similar models than the one of Asher and McNaught tend to increase the rates expected in America and to decrease the rates expected in Asia and Australia. 8. WHERE AND WHEN TO LOOK ---------------------- As explained above, you must be at the indicated region to see a Leonid meteor storm this year. From America, you must watch in the second half of the night of November 17 to 18; from Asia and Australia, you must watch in the second half of the night of November 18 to 19. However, much is going to depend on the climate that is unstable over many places in the world in that time of year. However, climate is only what you expect, and weather is what you get! Therefore, the best strategy to avoid bad weather is closely following the weather charts and travel a few days in advance to the location within your "action radius" that offers the most favorable prospects. Whether you choose to travel or stay at home, you will have to wait and see until the last moment if weather conditions will be favorable - a patch of clouds or a clearing at the right time can create a world of difference! Besides weather, light pollution is an important factor in choosing an observing sight. Most Leonids are not that bright. The more light pollution, the fewer meteors you will see! So, choose a dark spot! As explained above, Leonid meteors should be observed in the second half of the night. Die-hards who do not want to miss anything of the show should continue to watch until dawn. People who cannot afford to stay up that long should focus on a period of one to two hours around the predicted peak times. 9. HOW TO WATCH? ------------- Mind that it can be very cold in mid-November: warm clothing adapted to the local climate is essential! Since you can never tell in advance at what precise time at which direction in the sky a meteor will appear, you should never fix a particular star, but rather patiently watch a wide area of sky in a relaxed way until a meteor appears. It is not necessary to look in the direction of the constellation of Leo: you will see meteors all over the sky, in all directions. For comfortable observing, use a reclining chair, and install yourself in a suitable sleeping bag or under several blankets. 10.LEONIDS AFTER 2001 ------------------ Two Leonid meteor storms may occur in 2002 as well, the first one visible from European longitudes and the second one from American longitudes. Activity may even be higher than in 2001. Unfortunately, it will be Full Moon when these storms occur, in the morning hours of November 19, and, therefore, a lot of the fainter meteors may be lost to the moonlight. The best prospects for the 1998-2002 Leonid "window" are, therefore, for 2001. GLOSSARY AND ADDITIONAL EXPLANATIONS ------------------------------------ (1) COMETS - Comets are small celestial bodies (with a diameter varying from a few kilometers to at most a few tens of kilometers) that revolve around the Sun in long elliptical orbits. Comets consist mainly of ice and dust. When a comet approaches the Sun, part of the ice will evaporate and, because of the resulting pressure, the gas will find its way through cracks and fissures in the thin comet crest and be ejected under the form of "geysers." The evaporated ice of these geysers will feed the coma and the tail of the comet. Together with the evaporated ice, a lost of dust is released. This dust eventually spreads along the entire orbit of the comet, but remains densest in its immediate vicinity. Meteoroid streams (4) usually consist of cometary dust. (2) METEORS - Dust particles orbiting the Sun and capable of "colliding" with the Earth are called meteoroids. Such a meteoroid has usually the size of a sand grain or a tiny stone. When it enters the atmosphere, with typical velocities of a few tens of kilometers per second - several then thousands of kilometers per hour! - not only the meteoroid but the surrounding air experience enormous friction. This friction causes the air surrounding the meteoroid to give light, in much the same way as an electric current causes the gas in a TL lamp to give light. Meteors typically light up at heights of 90 to 110 kilometers. The resulting light is called a meteor or a shooting star. Usually, the enormous friction causes a meteoroid to disintegrate into the molecules it is composed of: the meteoroid "evaporates" completely. Only the larger and stronger meteoroids may survive traversing the atmosphere. The remainder of the meteoroid that impacts on the Earth is called a meteorite. Leonids are too fragile to produce meteorites, even if they are meter-sized. Occasionally, meteors are exceptionally bright, brighter that the brightest planets, and sometimes even brighter than to Moon. These meteors are called fireballs. (3) METEOROID STREAMS AND METEOR SHOWERS - The collection of particles released by a comet (or comet-like asteroid) is called a meteoroid stream. The meteor display in the sky caused by a meteoroid stream is referred to as a meteor shower, or, in case of extremely high activity, a meteor storm. Meteoroid streams and their associated meteor showers are named either after the comet from the particles originate, or, as is the case for the Leonids, after the constellation in which its radiant (4) is located. The Leonids are not the only meteor shower we can see. In fact, their are dozens of other meteor showers, but most of them never produce more than a few meteors per hour. Two notable exceptions are the Perseids, active around August 12, and the Geminids, active around December 14. Every year, both showers produce several tens of meteors per hour at their respective peak times. Finally, we must mention that the Solar System contains a lot of dust particles that do not belong to any particular meteoroid stream. These particles cause so-called sporadic meteors, which may appear any time. (4) RADIANT - Meteoroids of the same stream (3) orbit the Sun along a common orbit (roughly the orbit of the comet from which they originate). When the Earth crosses a meteoroid stream, our planet is "hit" by a "bombardment" of dust particles which all come from the same direction. The perspective, however, leaves us the impression that the meteor trajectories in the sky, when prolongated backward, originate from a single point, just like the tracks of a long, straight railroad. This point is called the radiant of the meteor shower. Most meteor streams and showers are named after the constellation in which this radiant is located. Even though the backward prolongations of all meteors of the same shower intersect the radiant, the meteors themselves can appear anywhere in the sky. Hence, there is no need to look in the direction of the radiant to observe a meteor shower. To understand better what happens during a meteor shower, picture the stream orbit as a "race track" along which all meteoroids race the same speed. Picture the Earth, with yourself as an observer on the Earth, as an "unexpected" obstacle on this race track with which meteoroids may collide, producing meteors in the process. If you look in the direction of the radiant, you will only see short meteors, caused by meteoroids colliding with the Earth's atmosphere while "running" almost straight toward you. If you look at 90 degrees from the radiant, you will see long meteors, caused by meteoroids colliding with the Earth's atmosphere just when they were about to overtake you. If you watch even further away from the radiant, you will again see shorter meteors, caused by meteoroids colliding with the Earth's atmosphere after they had passed you, and thus moving away from you. ------------------------------------------------------------------------ More information on the Leonids can be found in the International Meteor Organization's bimonthly journal WGN and on the internet, at http://www.imo.net. For questions, contact Marc Gyssens at wgn@imo.net or +32-477-64 05 48. Notice that the International Meteor Organization will send out a new release with first results on the Leonids during the European early morning hours of November 18, immediately after the event. All recipients of the present note will automatically receive the new release.