Woodcut of Leonid Storm in 1833
All of us have seen an occasional shooting star. A piece of debris that is captured by the earth is accelerated through the earth's atmosphere. At these high speeds , the friction generated heats it up, producing a glowing trail. This is a meteor, another name for a shooting star. Every year during certain specific periods, many more meteors are seen during one night. They all appear to radiate out from the same point in the sky. The increase in the frequency and the spray like appearance earned for them the name ,meteor showers'. As they occur at the same time every year they are called annual meteor showers. Each meteor shower is known by the constellation from which they radiate out. So we can guess that the Leonids would appear to radiate out of the constellation Leo. We see them between November 13 and 20 every year. What is special about this year is that the annual shower is likely to rum into a storm for about two hours on the night of November 17/18.
Where does this debris come from? In the last 50 years many man made objects are circling the earth. They can spiral in and burn up in the atmosphere, but an overwhelming majority of meteor trails are made by natural objects. The planets were formed 4.5 billion years ago , out of gas and dust that surrounded the newly born sun. These planets used up most of the material and are circling the sun even today, in an orderly way. The pieces of rocks and loosely bound rubble that were left over, also go round the sun on their own in orbits but these orbits are anything but orderly. Those that orbit the sun at the edge of the solar system are called comets. Some of them zip across from the cold outer reaches of the solar system to the inner most regions, from all directions, crossing the orbits of the planets, as they do so. While the heating due the sun evaporates the icy material and scatters some of the loosely bound matter, a close encounter with a planet can change their orbits completely. Over the past 4.5 billion years, the vicinity of the earth has been littered with so much material from such objects that approximately 9 tonnes of material is raining on the earth every day! While most are too small to leave a trail, the larger ones plunge into the earth's atmosphere becoming the occasional shooting stars that flit across the sky.
The annual shower of meteors have a more specific origin. The debris left by the passage of a particular comet through the inner solar system remains more or less in the same orbit as the comet. It merely spreads out in space along and around the orbit of the comet. Such meteoroid streams may be a few million kilometers in diameter and spread along the entire orbit of the comet. If such an orbit intersects the plane in which the earth is going around the sun, the earth will plunge through this rubble once a year, at the same part of its own orbit and therefore at the same time of the year. At that time many more meteors will be seen, as some of this rubble enters the earth's atmosphere. They will all appear to come from the same direction in space as they all originate from the direction of the orbit of the parent comet. The well known periodic comet Halley is responsible for two annual showers, one in April and another in October. Comet Swift-Tuttle with a period of 130 years is responsible for the Perseid meteor shower of August. Comet Halley has a period of 76 years. Once in 76 years it comes into the inner solar system leaving a fresh supply of rubble in its meteor stream. So for a few years around this passage, the number of meteors in the associated annual meteor shower increases. This is the case with all annual meteor showers.
The comet associated with the Leonids is the periodic comet Tempel- Turtle with a period of 33 years. So it is natural for the annual meteor shower activity associated with it to increase every 33 years. What is special about Leonids is that this increase in shower activity is so dramatic that the 'shower' turns into a 'storm' for a few hours. The fact that the earth and the meteor stream are traveling in opposite direction means that the velocity of the meteors is close to the maximum possible for leonids ie 70.8km/s and ensures a spectacular display of meteors.
So awesome was the view of the 1833 Leonids in America that the historian R.M. Devens listed it as one the 100 most memorable events in the U.S. history. "During the three hours of its continuance," he wrote, "the day of judgement was believed to be only waiting for sunrise,and,long after the shower had ceased, the morbid and superstitious still were impressed with the idea that the final day was at least only a week ahead. Impromptu meetings for prayer were held in many places, and many other scenes of religious devotion, or terror, or abandonment of worldly affairs, transpired, under the influence of fear occasioned by so sudden and awful a display". North America was once again the preferred location in 1966, when the most recent Leonids storm occurred. Although the populace was not pushed to religious frenzy, the reaction was still one of awe.
After a quiet intervening period , the November meteor shower of Leonids has been more and more active since 1994, heralding the arrival of the parent comet to the inner solar system. In February 1998 the parent comet Tempel-Tuttle came closest to the sun and has doubtless added fresh material to the swarm of meteoroids in the stream before it headed out.
During the night of Nov 17/18 1998 the earth is predicted to pass with in 1.3 million km from the freshly replenished meteoroid stream. This distance is larger than the closest distances in 1966 and 1833 . So the storm will not be as intense as in those years, but strong enough for a display of 5000 meteors per hour during the peak period. The sky will be moon less at that time increasing the visibility of faint meteors east and south Asia are correctly located to see the best displays. It should be an event worth going out to view! You should travel to a dark location away from the city lights. Select a place with a clear view of the horizon and a comfortable sitting space as you may be watching for a few hours. Carry a star map and a torch light dulled by red cellophane paper to locate the leo constellation Wear warm clothing and carry plenty of snacks and coffee. Meteor watching is most pleasurable when it is done in a group, so plan a midnight party!
One interesting aspect of the Leonids storm predicted for this year, is that the storm is occurring for the first time in an age with more than 500 active satellites circling the earth. What would happen to them as they plow through the meteor stream along with the earth? In a testimony before the U.S. Congress U.S. scientists have stated that "this meteoroid storm will be the largest such threat ever experienced by our critical orbiting satellite constellations. Furthermore, because of the very high speed of the particles - they will be moving at speeds of over 70 km/sec (over 155,000 miles/hour) - the storm poses an even greater and somewhat unknown threat". The meteoroids range in size from that of a grain of sand to as large as pebbles. There are many more sand grain sized meteoroids than pebble sized ones. Unlike us, our satellites are not protected by the atmosphere, so they will be "sand blasted" by very small particles traveling more than 100 times faster than a bullet. At these speeds, even a tiny particle can cause damage or electrical problems. While major holes and physical damage to solar panels and structures are very unlikely, impacts of small particles will create an electrically charged plasma which can induce electrical shorts and failures in sensitive electronic components. Satellites like the INSAT series parked in geostationary orbits will be the most exposed to the stream both in 1998 and 1999 and possibly in 2001. The probability of impact for a particle of mass 10**-7 with such a satellite has been calculated to be 4%. That means 20 of the 500 satellites are likely to be hit by at least one particle! The Indian Space Research Organisation however, estimates a lower probability of impact for its satellites.
In a conference held early this year in the U.S. to assess this threat, it was recommended that, if possible, satellites be oriented so that sensitive components are shielded from the oncoming stream of particles, and that recovery plans be in place should there be a spacecraft system failure during the storm. So while you are out at a rural site watching one of nature's awesome spectacles, the very foundation of the communication facilities that are so crucial to modem living may be shaking if not quaking! But meteor predictions are notorious for being wrong! Will the Lion really roar this November? Very likely . But will it also strike out, with its fangs? What exactly will happen, will be known as the sun rises on the morning of November 18, 1998!
What is a Radiant ?
If you have seen a meteor shower you would notice that the meteors appear to
originate from a specific region in the sky i.e. if you back trace their paths they
all appear to converge within a small circle in the sky The centre of this circle
is called the radiant. This does not mean that the meteors are actually radiating
from that point. To understand this refer to Figure b. The row of seats are
actually parallel to each other, yet they seem to taper and eventually meet . The
same is the case with the meteor shower. The long visible parallel tracks seem to
diverge from the radiant. This radiant is useful in differentiating between a
sporadic meteor and a shower meteor. The meteors that can he traced back to a
circular region of 81 in diameter belong to the shower meteor. Those meteors whose
tracks do not fall in this intersection zone are, definitely sporadic meteors.
| Date | Meteor Shower | ZHR | RA | DEC | Illum. Frac. | Longitude |
|---|---|---|---|---|---|---|
| 04101/1999 | Quadrantids | 80 | 15h28m | 500 | 0.94 | 2830 |
| 2310411999 | Lyrids | 12 | 18h08m | 320 | 0.55 | 320 |
| 05/0611999 | eta-Aquadds | 35 | 22h20m | -10 | 0.74 | 450 |
| 26/0711999 | Capricomids | 5 | 21h00m | -150 | 0.95 | 1230 |
| 30/0711999 | delta-Aquarids | 20 | 22h36m | -170 | 0.97 | 1260 |
| 13108/1999 | Perseids | 75 | 3h04m | 580 | 0.04 | 1400 |
| 21108/1999 | alpha-Cygnids | 5 | 21h00m | 480 | 0.70 | 1480 |
| 09/OW1999 | Piscids | 10 | 0h36m | 70 | 0.0 | 1660 |
| 23110/1999 | Orionids | 25 | 6h24m | 150 | 0.96 | 2090 |
| 04110/1999 | Taurids | 8 | 3h44m | 14' | 0.15 | 2210 |
| 18/11/1999 | Leonids | 10 | 10h08m | 220 | 0.69 | 2350 |
| 1011211999 | Puppids-Velids | 15 | 9h00m | -480 | 0.05 | 2570 |
| 1411211999 | Geminids | 75 | 7h28m | 320 | 0.32 | 2620 |
| 2311211999 | Ursids | 51 | 4h28m | 780 | 1.00 | 2710 |
| 2611211999 | Puppids-Velids | 15 | OM | -650 | 0.83 | 2740 |
| Date | 17118th Nov 1998 |
| Time | 1998 : 2 : 1 0 I.S.T |
| Storm duration | 1 ~2 hrs |
| Zenith Hourly Rate (ZHR) | 200 ~ 1 0000 |
| Best Viewing location: | 1998 : East and South Asia |
| Meteoroid composition | Silicate and carbonaceous grains such as olivine and pitch |
| 1990 | lnsat 1 D |
| 1991 | IRS 1B |
| 1992 | Sross 3, lnsat 2A |
| 1993 | lnsat 2B |
| 1994 | Sross C2, IRS P2 |
| 1995 | IRS 1C, lnsat 2C |
| 1996 | IRS P3 |
| 1997 | ORS 1D |
This is the sky map for the night of 17/18 Nov 1998 3:00hrs IST. Use a small torch dimmed with red cellophane paper to read the map in the dark.