Marschall Space Flight Center
June 22, 1999
Leonids on the Horizon
The Leonid meteor shower takes place every year around November 17 when Earth passes close to the orbit of comet Tempel-Tuttle. Usually not much happens. The Earth plows through a diffuse cloud of old comet dust that shares Tempel-Tuttle's orbit, and the debris burns up harmlessly in Earth's atmosphere. A typical Leonid meteor shower consists of a meager 10 to 20 shooting stars per hour.
If this were always the case, the Leonids would be known to a small number of meteor enthusiasts only. Instead they are famous. At roughly 33 year intervals the Leonid meteor shower can blossom into what astronomers call a meteor "storm," when hundreds of thousands of shooting stars per hour rain down from the sky.
Leonid storms occur at intervals separated by multiples of 33 years, the period of comet Tempel-Tuttle's orbit around the sun. Whenever the comet swings through the inner solar system it brings a dense cloud of debris with it, so that for 3 or 4 years after its passage the Leonids can be very active. Curiously, there isn't a full-fledged storm every time Tempel-Tuttle passes by.
Sometimes there's simply a stronger-than-average shower, never quite rising to the level of a storm, and sometimes nothing much at all happens to mark the comet's passage. This capricious behavior makes predicting Leonid meteor storms a bit tricky.
The last great Leonid meteor storm was in 1966. It was, predictably, somewhat unexpected. The comet had passed by Earth's orbit in 1965, so astronomers were aware that something might happen. But, judging by the paucity of the 1899 and 1932 showers, it was widely thought that the orbit of the debris stream had been deflected so much by gravitational encounters with other planets (mainly Jupiter) that a close encounter with Earth's orbit was no longer possible. The best predictions suggested a strong shower over western Europe with 100 or so meteors per hour.
Instead, there was an stunning display of shooting stars over western North America. This recollection by James Young at JPL's Table Mountain Observatory in California gives a sense of what the storm was like:
"This very noteworthy  meteor shower was nearly missed altogether... There were 2-5 meteors seen every second as we scrambled to set up the only two cameras we had, as no real preparations had been made for any observations or photography. The shower was expected to occur over the European continent.The 1966 return of the Leonids was one of the greatest displays in history, with a maximum rate of 2400 meteors per minute or 144,000 per hour.
The shower peaked around 4 a.m., with some 50 meteors falling per second. We all felt like we needed to put on 'hard hats'! The sky was absolutely full of meteors...a sight never imagined ... and never seen since! To further understand the sheer intensity of this event, we blinked our eyes open for the same time we normally blink them closed, and saw the entire sky full of streaks ... everywhere!"
Tempel-Tuttle visited the inner solar system most recently in late 1997 and early 1998. The subsequent Leonids display, in Nov. 1998, was marvelous as observers all over the world were treated to a dazzling display of fireballs (shooting stars with magnitudes brighter than -3). Nevertheless, the 1998 Leonids were a shower, not a storm. The maximum rate of meteors last year was about 250 per hour. Scientists have learned that if Earth crosses the orbit of Tempel-Tuttle too soon after the comets passage, then there is no storm, just a strong shower. Apparently that's what happened in 1998. In recent history no Leonid storm has ever occurred less than 300 days after Tempel-Tuttle passed by Earth's orbit. In 1998, Earth followed the comet to the orbit-crossing point by only 257 days [ref].
The period of maximum activity during the 1998 Leonid shower took place about 12 hours before the earth crossed Tempel-Tuttle's orbital plane. The early activity caught many observers by surprise, but it was business as usual for the unpredictable Leonids. Rainer Arlt of the International Meteor Organization noted that while the maximum activity came early, there was a secondary maximum when the Earth passed the comet's orbit (see left). This pattern is similar to that observed in 1965, the year that preceded the great Leonids storm of 1966. In his report Bulletin 13 of the International Leonid Watch: The 1998 Leonid Meteor Shower, Arlt wrote:
[T]he radar, visual, and photographic records of the 1965 Leonids indicate an activity profile which resembles that of the 1998 Leonids. Even the low population index seems comparable. Judging from these phenomenological facts, we may expect 1999 to show a similar shape of activity as in 1966. The actual maximum meteor numbers are hardly predictable. [ref].Joe Rao, a Leonids expert who lectures at New York's Hayden Planetarium, also advocates 1999 as possibly the best year for a storm during this 33 year cycle. Writing for Sky &Telescope he says:
Based on what happened last November, I will venture a prediction. If a meteor storm is to take place at all, 1999 would appear to be the most likely year for it to happen. But even if this year's Leonids are richer in number, observers should not expect the same high proportion of fireballs that were seen in 1998. Instead, a more even mix of bright and faint meteors is likely. [ref]Rao bases his argument on historical precedent and the Earth-comet geometry. During the seven most recent Leonid storms when Earth crossed Tempel-Tuttle's orbit soon after the comet, the average distance between the comet and Earth was 0.0068 astronomical unit. The average number of days between the comet's passage and the Earth's arrival at the plane of the comet's orbit was 602.8 days. With the 1999 values of 0.0080 a.u. and 622.5 days, Rao says we ought to be in a prime position to see significant, if not storm-level, activity.
Rao is also a meteorologist for News 12 Westchester, which seems a suitable occupation for predicting meteor showers.
In 1999, the Earth will pass nearly three times as far from the comet's orbital path as it did in 1966 and more than six times further than it did during the great storm of 1833. If the peak of the Leonids arrives exactly when the Earth passes through the comet's orbital plane, Donald Yeomans of JPL gives 01:48 UT on November 18, 1999 as the most likely time for the 1999 maximum [ref]. That would make Europe and North Africa the best places to watch the show. However, Leonid meteor showers frequently arrive much earlier or later than predicted, so any place on the globe could be favored.
According to David Asher of the Armagh Observatory and his colleagues, the intensity and duration of this exceptional event indicated that the Earth must have passed through an extremely dense, narrow stream of large dust grains and particles. The timing implied that these debris particles occupied an orbit somewhat different from the main stream, and that they left the comet's nucleus many hundreds of years ago. But in that case, how did the stream has hold together so tightly for so long?
To solve the problem, Dr David Asher and his co-workers calculated the motion of large dust grains ejected from comet Tempel-Tuttle at each of the last 42 occasions when it made its closest approach to the Sun. They checked to see whether any of the particles could explain the fireballs seen in 1998, and identified September 1333 as the time when most of the observed particles must have been released.
The particles were kept in a tight stream by a process known as "gravitational resonance." A similar phenomenon gives rise to the fine structure seen in Saturn's rings. In this case, grains ejected from the comet in 1333 were kept in step by the gravitational influence of Jupiter. Instead of spreading around the whole orbit, they were nudged by periodic "kicks" from Jupiter's gravity into a rather short arc of large particles, distinct from the 'normal' stream of small particles ahead of and behind the comet. Their calculations showed that in November 1998 most of the resonant arcs missed the Earth by a wide margin, but the arc of particles released in 1333 cut right through the Earth's orbit, and the calculated time for when this happened matched the observed fireball maximum to the hour.
Armagh and co-workers are not expecting a repeat performance of bright fireballs in November in 1999. All the resonant strands in the meteoroid stream will be well past Earth in space. However, a strong 'normal' display is likely, peaking at about 2 a.m. on November 18th, due to meteoroids ejected from Comet Tempel-Tuttle in the years 1866, 1899 and 1932, which have not yet had time to disperse around the comet's orbit.
Rob McNaught of the Research School of Astronomy and Astrophysics at the Australian National University and colleagues have examined the motions of Tempel-Tuttle debris particles ejected from the comet within the last 200 years. Their calculations predict that the maximum hourly rate of meteors in 1999 will be 1000 to 1500 per hour -- not exactly a major storm, but still a remarkable display -- and that the best years to observe could be in the next millennium when hourly rates might reach 10,000 - 35,000 in 2001 and 25,000 in 2002. These conclusions differ from most other recent studies which predict greatest activity during the years 1998 - 2000, but the Leonids do not always adhere to conventional wisdom.
All sorts of conjectures were made by all sorts of people ... We may learn of this that, when men are in a high state of excitement, their testimony must be taken with many grains of allowance.There seems to be plenty of room for debate about the upcoming Leonid meteor showers. The exact timing of the display, the number of fireballs vs. fainter meteors, and the best observing sites are all uncertain. Nevertheless, even the most pessimistic predictions for 1999 presage a memorable show.
First-hand account of the 1833 Leonid Meteor Shower.
Elder Samuel Rogers
One thing seems sure, no matter where you live: The Leonids are coming and, on Nov 17, 1999 the place to be is outside, looking up!
Background information on the Leonid 1998 event may be found at the following sites:
Marshall's News Center Web site
November 12, 1998
Scientists will launch the balloon sometime near midnight from Marshall's Atmospheric Research Facility, dependent upon weather conditions. Both still images and low-resolution television captured by the on-board camera will be available online at the Space Sciences Laboratory Web site at:
Live downlink television also will be carried on cable TV channel 58 over amateur radio frequency 426.250 Mhz. A special set-up must be followed, and viewing instructions will be available at the Space Sciences Laboratory Web site.
ROYAL ASTRONOMICAL SOCIETY
10 november 1998
According to the best data available, Europe is likely to experience a good meteor shower, but not a truly exceptional one - perhaps up to 100 meteors per hour if we are lucky. The best time to look will be between 1 a.m. and dawn in the early hours of 18th November. A storm of many thousands of meteors per hour could occur, but it is much more likely to be seen in the Far East - China, Thailand, Japan - than in Europe.
But forecasting meteor showers is not a precise business, unlike predicting eclipses, for example, for which the exact times and circumstances can be calculated in advance. The time when a meteor shower will peak, and the maximum rate at which meteors will appear to rain down, can never be anticipated with great certainty. They are something of a celestial lottery.
For that reason, it is well worth looking out for meteors in the early hours of the 18th, if skies are clear, even from the UK. There is a slim chance of something exceptional, but a modest display at least is on the cards, and meteors are easy to observe. They are best seen with the naked eye and, during a shower, they can streak across almost any part of the sky, as long as the radiant point is above the horizon.
If a Leonid storm takes place, it is unlikely to last more than an hour or so, but the gentler background shower carries on for a day or two. According to the experts the expected peak time of any storm is most likely to be about 7.45 p.m. (GMT). If this is correct, the storm would be finished several hours before the constellation Leo rises above the horizon in the UK.
Leonid meteors are dust particles that have come off Comet Tempel-Tuttle. Most of this dust is still following the comet fairly closely in space. The comet takes 33 years to complete an orbit around the Sun, and planet Earth ploughs through its main dust cloud when the comet returns to our vicinity every 33 years. In the years when this happens, a strong shower or storm takes place. In the years in between, a very small number of Leonid meteors are seen in mid-November.
Some meteor showers produce about the same rate of meteors around the same date every year. Regular annual showers happen when the dust from a comet has spread around the whole of the comet's orbit, something that takes place gradually over a long period of time. An example is the Orionids, a shower in late October each year caused by dust from Halley's Comet.
Looking ahead to 1999, Comet Tempel-Tuttle will still be relatively nearby and some astronomers are predicting that the Leonid meteor display could be better next year than this. If that were to happen, then Europe is expected to be the ideal location.
Since the velocity of the meteor impacts is affected by a spacecraft's motion as it orbits the Earth, hits could occur at any speed between 65 and 80 km (40 and 50 miles) per second. These could result in some physical damage in sensitive areas as well as electrical short circuits, plasma discharges, and computer malfunctions, which may be sufficiently serious to disable a satellite. A form of sand-blasting can erode outer surfaces such as thermal blankets, mirrors and solar cells. Large impacting particles may even knock a satellite out of its normal position, as happened to the European Space Agency's Giotto spacecraft during its 1986 flyby of Haley's Comet.
"These microparticles could penetrate a fairly weak spacecraft skin," said Professor Tony McDonnell of the Unit for Space Sciences and Astrophysics at the University of Kent in Canterbury. However, the most likely form of damage is to vulnerable power systems. "Perhaps a handful of satellites could have unusual electrical anomalies," said McDonnell.
Past evidence suggests that the risks are fairly low. During the past four decades, only one spacecraft, the European Space Agency's Olympus satellite, is known to have been disabled by a (Perseid) meteor. Furthermore, no spacecraft were damaged by the 1966 Leonid storm. On the other hand, there are now more than 500 spacecraft orbiting the Earth, over 10 times as many as in the mid-1960s.
"The biggest uncertainty is the hourly rate (of arrival)," said Professor McDonnell. "If this reaches 150,000 per hour, there will be all sorts of damage, but there may only be 1,800 per hour."
While the probability of any satellite being hit is thought to be less than 0.1%, many spacecraft operators are taking no chances. The Space Shuttle mission that carried John Glenn was deliberately timed to avoid the Leonid shower. Cosmonauts on the Mir space station do not have the luxury of choosing their flight window. While the Mir station presents a large target for the Leonids, no serious damage is expected. However, the two crewmen may play safe by moving into the Soyuz lifeboat at the peak of the shower.
Fortunately, the direction from which the particles approach the Earth is almost perpendicular to the direction of the Sun. This means that the chance of a direct hit will be substantially reduced since most satellites will already have their solar panels aligned edge-on to the shower.
Further damage-limitation measures have been recommended by the European Space Operations Centre operated by the European Space Agency. These include turning spacecraft so that their most vulnerable systems are not in the direct line of fire; switching off high voltage systems; and putting a team of ground controllers on alert in case of emergencies.
In the case of the Hubble Space Telescope, its all-important mirror will be turned away from the shower during observations of distant galaxies. Most of the scientific instruments on the European ERS-1 and ERS-2 Earth observation satellites and the Solar and Heliospheric Observatory (SOHO) will be powered down and placed in 'sleep' mode during the shower. SOHO and the American Advanced Composition Explorer (ACE), which are located 1.5 million km sunward of the Earth, will be particularly at risk since the main stream of meteors is expected to pass much closer to them than any of their Earth-orbiting brethren. Indeed, the trail of Leonids will actually travel between the Earth and these two solar observatories.
NASA Headquarters, Washington, DC
Ames Research Center, Moffett Field, CA
November 12, 1998
The Leonid meteors originate from a trail of dust and debris in the wake of the comet Tempel-Tuttle, which orbits the Sun every 33 years. The Earth crosses this trail every November, but every 33 years the debris trail is especially dense, sometimes resulting in a meteor storm. The "shooting stars" streak through Earth's upper atmosphere, sometimes at rates of up to thousands per hour. The storm's peak lasts approximately one hour. This year, Earth is expected to pass a region just behind the comet and outside of its orbit, a favorable set of conditions for a larger-than-normal storm event. The best viewing of this storm will be in eastern Asia and the western Pacific region.
NASA's mission consists of two research aircraft that will carry a broad array of scientific instruments to observe and explore the meteors. Operating simultaneously, the aircraft will provide three-dimensional views, making high-resolution stereoscopic images and spectrographic observations of meteor dynamics and chemistry. A team of interdisciplinary scientists -- astronomers, atmospheric physicists and meteor specialists -- will use state-of-the-art-sampling techniques to provide a "window on the sky" over Japan during the storm.
"The central theme of this mission is astrobiology," said Peter Jenniskens, mission principal investigator and astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute, Mountain View, CA. "We are especially interested in learning the composition of Tempel-Tuttle's debris, the molecules that are created during the meteor's interaction with the Earth's atmosphere, and the composition and chemistry of the atoms, molecules and particles detected in the meteor's path. This may help us understand how extraterrestrial materials helped create the conditions on Earth necessary for the origin of life."
The Leonid mission is NASA's first operational astrobiology mission. Astrobiology is the study of the origin, evolution and destiny of life in the universe. The mission may provide important clues about what extraterrestrial materials were brought to Earth by comets, and what part that may have played in the beginnings of life on Earth, as well as clues on how biogenic compounds formed in stars are eventually incorporated into planets.
A modified L-188C Electra aircraft from the National Oceanic and Atmospheric Administration's (NOAA) National Center for Atmospheric Research in Boulder, CO, and sponsored by the National Science Foundation, will act as the mission "spotter" and recorder. It will carry a two-beam Lidar, a type of radar with light pulses that measures the altitude of neutral atom debris in the meteor trails. Other instruments include airglow, visible wavelength imagers and high-definition TV cameras.
Scientists aboard the first aircraft are seeking to learn how a meteor's mass compares to its brightness and to the mass of the resulting comet. Currently, they can only guess how much material enters the atmosphere during a meteor bombardment. Researchers will compare the meteor's image with information from the dual Lidar, providing an indication of the chemical evolution of the meteor debris.
The second aircraft, a U.S. Air Force-owned FISTA (Flying Infrared Signatures Technology Aircraft) from Edwards Air Force Base, CA, will have 20 upward-looking portholes to observe the meteors. It will carry imagers and infrared and visible-light spectrometers to dissect the meteor's light in search of the fingerprint of atoms and molecules.
The mission will fly out of Kadena AFB in Okinawa, Japan, over the East China Sea. The FISTA aircraft will fly as high as 39,000 feet to be above the lower atmosphere's water vapor layer, while the Electra will maintain an altitude of about 22,000 feet, just above the clouds.
NASA's Ames Research Center, Moffett Field, CA, is collaborating in this international effort with the SETI Institute, the National Science Foundation and several other science organizations. Aircraft and other support are being provided by NOAA and the U.S. Air Force. Instruments are being contributed by the University of Illinois at Urbana; the Aerospace Corporation; the Air Force Research Laboratory; the Japanese Broadcasting Company (NHK); Kobe University, Japan; the Ondrejov Observatory (Czech Republic); Mt. Allison University (Canada); the SETI Institute; and the University of East Anglia, England.
Additional information on the Leonid meteor storm and the mission can be found on the worldwide web at: http://www-space.arc.nasa.gov/~leonid/. During the mission, video animation and images will be available at http://leonid.arc.nasa.gov.
National Science Foundation
November 6, 1998
An L-188C Electra, owned by the National Science Foundation (NSF) and operated by the National Center for Atmospheric Research (NCAR) in Boulder, Colo. will be joined by an Air Force KC-135 in the night skies over Okinawa, Japan, during the meteor storm.
"The NSF Electra is an ideal platform to participate in the Leonids meteor experiment," says Cliff Jacobs, program manager in NSF's division of atmospheric sciences, which funds NCAR. "Its ability to accommodate multiple state-of-the-art, upward-looking instruments will provide an exceptional opportunity to study these meteors."
The meteor storm will occur when Earth enters the dense debris behind Temple- Tuttle on November 17, 1998, and again on November 18, 1999. Although the comet returns every 33 years, its orbit crosses Earth's only once every hundred years. This century's crossing offers scientists a close look at the trails of unusually fresh and large (millimeter- to centimeter-size) meteors entering the earth's atmosphere at the fastest possible speeds -- 72 kilometers per second (160,000 miles per hour). Best observations will be from East Asia (China and Japan). Next year, Europe and North Africa will offer the best viewing. From the ground, the source of the storm appears in the constellation Leo.
The National Aeronautics and Space Administration is heading the experiment, which is the first mission in NASA's Astrobiology Program, created to study the origin and prevalence of life in the universe. The Leonid Multi- Instrument Aircraft Campaign is also supported by NSF, the U.S. Air Force, and NHK Japanese television.
The two aircraft are needed to take the observing instruments into clear skies above the weather-laden lower atmosphere. The Air Force's FISTA (Flying Infrared Signatures Technology Aircraft) will circle the NSF/NCAR Electra in a racetrack pattern between 30,000 and 40,000 feet while the Electra flies back and forth (north-south) about 10,000 feet lower within the loop. At these altitudes (7 to 10 kilometers, or roughly 4 to 6 miles) both planes will be safe from the meteors above, which will burn up at 100 to 120 kilometers (60 to 75 miles) above the ground.
A major scientific goal of the mission is to determine how a meteor's mass compares to its brightness. To date, scientists can only guess how much material enters the atmosphere during a meteor shower. The Electra will carry a dual-beam lidar (laser-based radar) built this year to detect iron vaporized from the meteors in the upper atmosphere. Says NCAR project manager Bruce Morley, "We know very little about iron in the atmosphere and even less about the iron contribution from meteors. Observing just one meteor accurately from the sky would make a big difference to our understanding."
Griffith Observatory Press Releases
Every November 17, when the earth passes near the orbit of Comet Tempel-Tuttle, we pass through a sparse swarm of comet debris and experience a minor meteor shower that generally goes unnoticed. Every 33 years, however, the earth passes through a dense knot of this cometary material, and there is the possibility of a dramatic meteor storm when the sky might fill with thousands of "falling stars." This year there may be such a brief meteor storm lasting no more than a few hours. Although the shower is predicted to be strongest over Asia, enough meteors may fall over California to make the night very interesting. This is a meteor shower that sky watchers should not miss.
The best time to observe the shower from the United States will be the few hours before dawn on Tuesday morning, November 17. There is less likelihood of a shower on the morning of the 18th. Few meteors will fall before 1 a.m. Although it is difficult to estimate the actual rate, an observer in a dark location will likely see dozens of meteors per hour. There may be brief periods lasting several minutes when quite a few meteors fall, followed by periods of relative calm.
The meteors radiate from the direction of Leo, the Lion, and for that reason they are called the Leonids. Leo is low in the east before dawn, but the meteors will appear all over the sky. Leonid meteors strike the earth's atmosphere at high speed, 44 miles per second, and often leave smoke trails. The moon is almost new and out of the way.
Because of the early hour, Griffith Park will not be open for meteor observing. Observers should plan to be far from city lights. The Observatory is often asked to recommend an observing location, but the answer is -- go away from the city to where it is dark.
Jet Propulsion Laboratory/California Institute of Technology
August 7, 1998
Each November when the Earth runs into the dusty debris from periodic comet 55P/Tempel-Tuttle, some Leonid meteor shower activity is noted. These annual displays of meteors, or shooting stars, seem to originate in the constellation Leo so they are termed Leonid meteors. Normally, the observed rate of the Leonid meteors is about 15 per hour under ideal observing conditions. However, every 33 years or so when the parent comet Tempel-Tuttle returns to the Earth's neighborhood, there is a possibility that the Leonid meteors rates can get substantially higher. In some years such as 1799, 1833, and 1966, when the Earth passed particularly close to the tube of debris following in the comet's wake, there were Leonid meteor "storms" noted of up to 150,000 meteors per hour. Periodic comet Tempel-Tuttle passed closest to the sun (reached perihelion) most recently on Feb. 28, 1998 and a month later on March 5, the comet passed through the plane of the Earth's orbit about the sun.
Another way of saying the same thing is to note that the comet passed through the ecliptic plane from north to south or it passed through its descending node. We can expect the maximum Leonid meteor shower activity when the Earth arrives close to this nodal crossing point on November 17, 1998 at 19 hours 43 minutes Universal Time (UT). The peak Leonid meteor shower activity takes place within one hour but some activity can be observed for a few hours on either side of this peak. Unfortunately for observers located in the United States, the Nov. 1998 shower maximum will occur during daylight hours (2:43 pm local time on the east coast and 11:43 am on the west coast). While some enhanced 1998 Leonid activity may be visible just before dawn for U.S. observers, the Leonid shower maximum should be best observed by those located near the regions of Japan and eastern Asia. In November 1999, the Leonid shower will be best observed from the regions near Europe and North Africa.
Table 1. Predicted Leonid Shower Circumstances.
Although slightly enhanced meteor shower activity was evident in 1996 - 97, impressive meteor showers are most likely in 1998 and/or 1999.
Predicted time of Observed time Leonid shower peak of shower peak ZHR Good observing Date (UTC) HH:MM (Hours) meteors/hr Locations ------------------ -------------- ----------- -------------------- 1996-Nov-17 07:20 05 - 10 60 Eastern U.S. 1997-Nov-17 13:34 12 - 14 40 Western U.S., Hawaii 1998-Nov-17 19:43 200 - 5000? Japan, Asia 1999-Nov-18 01:48 200 - 5000? Europe, North Africa
As noted in Table 1, the predictions for the times of the 1996 and 1997 maximum shower events were rather accurate and there is no obvious reason to doubt that the 1998 and 1999 predictions will be seriously in error. What sort of Leonid meteor rates can we expect in 1998 and 1999? Meteor shower rates are often expressed in terms of the so-called zenith hourly rate (ZHR) or the hourly rate of meteors an observer would witness under ideal conditions with the meteors appearing directly overhead (at the zenith). The geometric circumstances between the comet's orbit and that of the Earth for 1998 and 1999 are most similar to those circumstances during the Leonid showers in 1866-67 and 1931-32. Since the observed Leonid meteor rates in 1866-67 and 1931-32 were approximately 5000 and 200 per hour respectively, we might anticipate a zenith hourly rate in 1998 and 1999 bounded by the rates witnessed in the earlier events - between 200 and 5000 meteors per hour.
Like the weather, it is extremely difficult to predict the hourly rates of meteor showers. Table 1 is meant only as a rough guide. Peter Brown, a respected researcher of the Leonid meteor phenomena, has suggested a more optimistic prediction of between 1000 and 9000 meteors per hour in 1998 (zenith hourly rate). In any case, it is well worth the effort to observe the upcoming Leonid meteors since it will be another century after the 1998-1999 events before significant Leonid meteor displays are once again likely.
Suggestions for further reading:
Royal Astronomical Society Press Notices
Tuesday 8th April 1997
Meteor streams are formed from the dust grains released by the nucleus of a comet when it gets near to the Sun and becomes active. If the Earth's orbit crosses through the meteor stream, we experience a meteor shower. Over time, the dust grains spread slowly round the comet's orbit. When they are distributed around the whole of the orbit, the same number of meteors are seen at about the same time each year. However, this is not the case with the Leonids.
The Leonids are dust from comet Temple-Tuttle, discovered in 1865, but that dust is still in a large clump close to the comet. It has not had enough time to spread all round the orbit. That means that the Leonids produce large numbers of meteors when Earth is near the comet -- about every 33 years -- and are rather sparse or non-existent at other times.
Professor Williams says that his calculations suggest 'the display will be spectacular but not awesome, that is several thousand meteors per hour rather than tens or even hundreds of thousand as has been the case on some occasions in the past. He also thinks 1998 will possibly be the best year rather than 1999 since the comet's closest approach to Earth is between the two dates but closer to November 1998.
The start of the serious scientific study of meteors is generally attributed to the spectacular display produced by the Leonids in November 1833. It was studies of this shower that led to the observation that the meteors appeared to originate from a single point, or radiant, in the sky. This was correctly interpreted as signifying that the meteoroids move on essentially parallel tracks in the solar system, as a stream. In the case of the Leonids, the radiant is in the constellation Leo -- hence the name of the shower.
The mathematician John Couch Adams (1819-1892) (who also predicted the existence of the planet Neptune) studied changes in the Leonid stream and concluded that the orbital period of the Leonid meteors was about 33.25 years. This was remarkably similar to that of the newly discovered Comet Temple-Tuttle. It was also soon realized that spectacular displays of the Leonid stream had occurred at the same interval. Good displays were seen in 1799, 1833 and 1866. Since then, spectacular displays occurred around 1900 and in 1966.
November 5th, 1998. Using a total of 12 different models for the ejection of meteoroids from comet Tempel-Tuttle, a preliminary "best" guestimate for the location of the strongest peak in activity and its associated ZHR for the 1998 Leonids has been found.
The 12 model approach involves using three major variations in meteoroid density (0.1, 0.8 and 4.0 g/cm^3 for bulk density of the meteoroid). For each of these three densities, four different variations in the initial ejection velocities are also employed - one follows the distributed production model of Crifo which produces broad distributions in initial ejection velocity which has a mean velocity lower than the classical Whipple/Jones ejection model. In addition to Crifos distributed production model, a Whipple/Jones ejection velocity model is used, as well as a second variant of the same with a heliocentric velocity dependance of r^-0.5 in place of the usual r^-1. The fourth model is again a variant on the Jones/Whipple model in which the ejection velocity at a given heliocentric distance is not single-valued in the monte carlo generating routine, but rather has a parabolic distribution of probable velocities about the average Jones/Whipple velocity for the chosen heliocentric distance. See Brown and Jones (1998), Icarus, v. 133, pp. 36 - 68 for more details.
The results of the modelling for the Leonids, using ejections at all perhelion passages of the comet back to 1499 AD (ie 15 revolutions of the comet prior to the current epoch). A simple summation of the meteoroids which are then visible at Earth at the present time from this ensemble and which would produce visually observable meteors (mass 1 mg) was then computed from all ejecta. A meteoroid is defined as being Earth-intersecting if its nodal radius is within 0.005 AU of Earth at the longitude of its descending node. All models suggested a steep increase in activity beginning in December, 1997/early 1998 accompanying the passage of Tempel-Tuttle. The resolution of the modelling is of order 2 months and thus all models suggest that this November will show significantly increased activity relative to 1997 (when the peak ZHR reached just short of 100), and likely activity approaching meteor storm levels (ZHRs of order 1000). Using 1997 as a baseline and taking the peak ZHR to have been 96 +/- 13 at 235.22 +/- 0.02 (J2000) in 1997 we have extrapolated the relative model difference between the activity strength predicted by the model in 1997 to that observed and that predicted for 1998. Using a mean of all models, produces a predicted location for the peak in 1998 of 235.26 +/- 0.04 (J2000) with a peak ZHR of 1200 +/- 280. This solar longitude corresponds to Nov 17 at 19:20 UT with a 1-sigma uncertainty of 60 minutes. We emphasize that due to the model results sensitive dependance on density of the meteoroids, the range of possible ZHRs extends from slightly lower than the bound given above to nearly 10 000 (the higher values associated with the models using the least dense meteoroids and lowest ejection velocities).
The use of relative modelling difference between 1997 and 1998 implies
that the veracity of the prediction in 1998 relies entirely on the
accuracy of the magniude of the ZHR reported in 1997 under full moon
conditions. As well as the above, the models suggest that broad
activity, persisting for of order a full day centred about this peak
should be noticeably above normal Leonid background levels and should be
rich in larger meteoroids in 1998 most notably after the time of the
peak. The model suggests ZHRs of order 100 or greater in the 3-4 hour
window prior to the peak and ZHRs of order 100-200 persisting for many
hours after the peak.
The mass index near the time of the peak over the visual magnitude range will be near 1.6 +/- 0.1. It is worth noting that a significant decrease in the mass index from 1.8 +- 0.1 several hours prior to the peak to this lower value and then upward again after the peak is visible in most models.
For future planning here are the predicted times of greatest Leonid meteor activity in 1998 and 1999:
1998: Peak date / time, November 17, 19:45 UTC
Most favored area: Asia.
Moon phase: New Moon, 28 days
1999: Peak date / time, November 18, 01:50 UTC
Most favored areas: Eastern Atlantic, Europe, Africa, Asia.
Moon phase: Waxing Gibbous Moon, 9 days