International Meteor Organization - Science note on the Leonids 2000
Introduction
From, mainly western, Europe and Africa, as well as from large parts of North America, Central America and parts of South America, people may see a lot of meteors - "shooting stars" - between midnight and dawn of the night of 17 to 18 November, provided skies are clear. These meteors belong to the so-called Leonid shower.
A first peak, visible from western Europe and Africa (including central Europe) and NE South America, is expected around 3:44 a.m. Greenwich Mean Time, which is 4:44 a.m. local time for most of the favoured continental European and African locations, 3:44 a.m. for the British Isles, mainland Portugal, and the Canary Islands, and 1:44 a.m. for eastern Brazil.
A second peak, visible from large parts of North America, Central America, and NW South America, is expected around 7:51 a.m. Greenwich Mean Time, which is 3:51 a.m. Atlantic Standard Time, 2:51 a.m. Eastern Standard Time, 1:51 Central Standard Time, and 0:51 a.m. Mountain Standard Time. This peak falls too early for Pacific Time Zone locations, unfortunately.
At the times mentioned above, an observer at the indicated locations may expect to see 50 to 100 meteors per hour. A veritable meteor storm with several tens of meteors per minute as last year is much less likely this year, but not ruled out. Therefore, vigilance is called for!
The International Meteor Organization, who collects meteor observations worldwide for the purpose of analysis, wishes to point the attention of the public to this spectacular natural phenomenon.
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, Tempel-Tuttle. The orbit of the Leonid particles happens to intersect the Earth's orbit. Each year around 17 November, when the Earth is at this intersection, Leonid particles may enter the Earth's atmosphere and cause meteors. 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!
Leonids in the past
Old chronicles from all over the world (European, Arab, Chinese, Korean, Japanese, and 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 Judgement Day had broken and many who witnessed these celestial fireworks compared it to a snowstorm! 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 splendour: observers at Kitt Peak in Arizona reported no less than approximately 40 meteors each second when the storm was at its best. This corresponds to a frequency of 150 000 meteors per hour!
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 28 February 1998 marked the beginning of a five-year period (1998-2002) during which strongly increased Leonid meteor activity is again possible (and has already occurred). Whether or not a meteor storm actually materialises in any or all of these years depends on several circumstances, on which we will briefly elaborate.
When do storms materialise?
Meteor showers 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 gases), in the case of Comet Tempel-Tuttle every 33 years. The particles released during a single passage form a so-called "dust filament". Not all particles follow exactly the same path, as a consequence of which these dust filaments gradually fade away until they can no longer be distinguished from the dust around the comet that was released much longer ago. In the case of Comet Tempel-Tuttle, a veritable Leonid meteor storm will occur only if the Earth passes through a dust particle filament released by the comet at most 7 or 8 revolutions ago, which is less than about 270 years.
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 a few tens and more than thousand meteors per minute, 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.
When do we see Leonid meteors?
Around 17 November, Leonid particles may enter the Earth's atmosphere from a direction - called the radiant 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.
What happened up to now?
In 1998, no veritable Leonid storm was seen, but a - then - unexpected and very impressive fireball shower developed during the first morning hours of 17 November. Subsequent calculations convincingly demonstrated that large dust particles released by the Comet 6 to 7 centuries ago were responsible for this phenomenon. Normally, dust particle filaments that old have completely faded away and are incapable of producing significantly enhanced activity, but, because of the particular geometry in this case, the gravitational pull of the giant planet Jupiter managed to keep the larger dust particles of the filament together.
The unexpected events of 1998 encouraged astronomers to develop more reliable models for predicting meteor shower activity. Especially the model of astronomers David Asher and Robert McNaught attracted a lot of attention. Their prediction for a meteor storm in 1999 proved to be correct, both with respect to the time of maximum and the order of magnitude of meteor activity.
The 1999 Leonids peaked around 2:02 a.m. Greenwich Mean Time. Several tens of meteors per minute were visible at that time, corresponding to a frequency of 3700 meteors per hour as obtained from averaging reports from hundreds of observers at various locations. Many favourable observing locations suffered from clouds, unfortunately. The most successful observations were carried out in the Middle East, the French Provence, Southern Spain, and the Canary Islands.
Two striking features of the 1999 Leonids deserve mentioning. First, there were a lot of local variations in the activity profile. In order to understand these variations, it is essential to have observations from as many locations as possible and analyse them collectively and comparatively. Precisely this is the most important aim of the International Meteor Organization.
Second, the brightness distribution was very atypical. The shower was poor in both very bright and very faint meteors. Normally, it is either one or the other. Finally, it must be noted that observers in Hawaii and the Far East witnessed a secondary peak around 16h Greenwich Mean Time on 18 November, yielding 180 meteors per hour.
What may be expected this year?
The calculations made after the 1998 Leonid event, comparisons with observations of previous Leonid events, and the successful prediction of the 1999 Leonid storm have demonstrated that it is possible to accurately predict whether or not the Earth will pass through a young dust particle filament of Comet Tempel-Tuttle.
The calculations for 2000 indicate that the Earth will pass through a dust filament of 267 years old, released by the Comet eight revolutions ago, and through a dust filament of 134 years old, released four revolutions ago. The Earth will pass through the first filament around 3:44 a.m. Greenwich Mean Time and through the second around 7:51 a.m. Greenwich Mean Time, in both cases in the night of 17 to 18 November. The first encounter is visible from, mainly western, Europe and Africa, and from NE South America; the second encounter from large parts of North America, Central America, and NW South America. Unfortunately, the Earth will not pass through the core of these filaments, but rather through their outer regions. In addition, results on past encounters of the Earth with these particular dust trails are scarce. It is therefore very difficult to predict the level of activity. Asher and McNaught's best estimate is 100 meteors per hour around the above mentioned times. However, the real frequency may be either higher or lower! A storm comparable to the 1999 event is unlikely, but not ruled out.
Finally, it must be noted that the Earth will pass the outer regions of a filament of 68 years old, eject two revolutions ago. This is expected to happen around 7:53 a.m. Greenwich Mean Time the night before, 16 to 17 November. A secondary peak around this time is possible, but not certain! Interested observers who can see the 7:51 a.m. Greenwich Mean Time peak on 17 to 18 November, are in principle able to watch out for this possible secondary peak too, almost exactly 24 hours earlier.
Where and when to look
As explained above, peak activity of the Leonid meteor shower this year will only be visible from mainly western Europe and Africa, North America except for the Pacific Time Zone, Central America and parts of South America. European, African and NE South American observers should watch around 3:44 a.m. Greenwich Mean Time, North American, Central-American, and NW South-American observations should watch around 7:51 a.m. Greenwich Mean Time (conversions to local time are given in the Summary at the beginning this note). To see any activity, people there must either stay up long on 17 November or rise early on 18 November. Expect to see 50 to 100 meteors per hour around the peak times, but be prepared for both lower and higher activity!
However, much is going to depend on the weather, which is generally unstable in northern locations at that time of year. There, the best strategy to avoid bad weather is closely following the weather charts and travel one day in advance to the location within your "action radius" that offers the most favourable 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 favourable - 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. Generally, Leonid meteors are not that bright: the more light pollution, the fewer meteors you will see! So, choose a dark spot!
As explained above, Leonid meteors cannot be seen before midnight. Hence, there is no point in starting an observation earlier. People who cannot afford to watch the entire second half of the night should focus on a period of, say, one and a half-hour centred around the peak time mentioned for their location. On the other hand, dedicated observers should also consider to cover also (the second halves of) the nights before and after, to watch for the possible secondary peak mentioned above or any unexpected activity.
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 important to emphasise that 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. This year, we explicitly advice NOT to look in the direction of Leo, as the Last-Quarter Moon, a considerable source of undesirable light will then be in your field of view and ruin the dark-adaptation of your eyes! So, watch the sky in a direction in which Moon does not disturb you!
For comfortable observing, use a reclining chair, and install yourself in a suitable sleeping bag or under several blankets.
Leonids after 2000
Leonid meteor storms may occur in 2001 and 2002 as well. According to Asher and McNaught's predictions, three meteor storms with peak rates varying between 2000 and 15 000 meteors per hour may materialise in 2001. One or more of these peaks are visible from America, eastern, southeastern and central Asia, and Australia, the highest frequencies being expected over western Australia and eastern, southeastern and central Asia. As the date line intersect this area, American observers should watch during the night of 17 to 18 November, while Asian and Australian observers should watch during the night of 18 to 19 November. There will be no interference of the almost new Moon, which will be well below the horizon during the second half of the night.
The year 2002 may prove even more spectacular with a meteor storm of 15 000 meteors per hour over western Europe and Africa, northern Canada, and NE South America, and a meteor storm of 30 000 meteors per hour over North America, both events occurring during the night of November 18-19. Unfortunately, they will suffer from the bright light of the Full Moon; therefore 2001 may be our best bet for the current Leonid epoch.