THE LEONID METEORS
THE LEONID METEORS
What we saw at Parkes Observatory - 18 November 1998.
Maurizio Toscano, Stuart Anderson (two visiting astronomers) and I, had gathered at the Parkes Observatory around 1:00 am (AEDT) to view the meteor shower. Thin, high cloud at first obscured the sky but it soon cleared and all seemed ready for the show. A hail storm over the town of Orange (a hundred kilometres to the east) gave us a fantastic display of lightning, with some of the strikes lighting the entire countryside. We were fortunate that the storm stayed away from us.
Shortly after Leo had risen, we spotted our first fireball at approx. 2:50 am (AEDT). It was low in the north-eastern sky and it displayed a bright green tail of about 10 degrees in length, which persisted for a second or so. The head of the meteor had a bright red colour. Following this, we spotted a further four fireballs within a space of five minutes. This rate of approx. 1 fireball a minute continued for the next 40 minutes, before cloud rolled in from the north, obscuring the sky for about 15 minutes. All of the fireballs we saw sported bright green tails, which persisted for a second or so. All seemed to have negative magnitudes, and had either deep red or white coloured heads. Some tails were estimated to be over a quarter of the sky in length - Wow ! Also, some of the tails seemed to be divided ie. they appeared to have a gap running through their centres along the full length of the tails.
Possibly the most spectacular fireball we saw, rose from out of Leo, low in the north-east, to well past the zenith above us. It's bright green tail persisted for a breathtaking three seconds, and was about half the sky in length. Its head was a deep red in colour. Another fireball, seen in the east, sported a 70 degree long tail. It travelled at a shallow angle down toward the horizon. Shouts of "Look at that !" and "Wow !" reverberated through the surrounding farm fields with each fireball sighting. They appeared so quickly, that it was easy to miss fireballs if you weren't looking in the right direction at the time.
The meteors were seen in all parts of the sky. Toward the north-east, they appeared to rise toward the zenith. Behind us, in the south-west they appeared to fall, and toward the north and north-west, and the east and south-east, they appeared to travel more or less parallel to the horizon. This was all consistent with the radiant being in the direction of Leo, rising in the north-east. Ocassionally, we saw faint meteors, moving in directions inconsistent with the above, which indicated they weren't associated with the shower.
The sky cleared again around 3:45 am (AEDT) and the fireballs reappeared. The meteor rates seemed to be down slightly from the previous period. In both periods, there seemed to be several minutes without any fireballs, then suddenly we would see two or three in quick succession. This suggested that there may have been some minor structure in the debris trail of the shower.
Around 4:00 am (AEDT) we were joined by Dr. David McConnell, Dr. Ray Norris, Dr. John Whiteoak and Dr. Ron Ekers. By this time the rates were well down from an hour earlier (about half), but we still saw many very beautiful and spectacular fireballs. Around 4:30 am (AEDT) the clouds again rolled in and obscured our view. Maurizio and I, remained while the others retired to bed. About 4:50 am (AEDT) in a small break in the cloud cover, and with a pale blue sky as a backdrop, we spotted a bright fireball, which looked to have a dull white coloured tail about 20 degrees in length. With the increasingly bright sky encroaching, we finally called it a night at about 5:15 am (AEDT).
In the hour and half of clear skies, I estimate we saw approx. 40-50 bright fireballs. Although the predicted "storm" never eventuated, the above average meteor shower was a spectacular sight never to be forgotten.
John Sarkissian - 20 November 1998.
Find out what happened elsewhere
Article by Robert McNaught
What may be one of the most spectacular meteor showers for the next century is predicted to occur next Wednesday morning, November 18. Occasionally throughout history, the Leonid meteors have fallen out of the sky like rain. Whilst such an intensity is unlikely this year, this is one of the best opportunities we will have to witness a meteor "storm".
A meteor is more commonly known as a shooting or falling star and is caused by a tiny piece of dust, only a millimetre or so across. It may seem unusual that such a small particle can be seen some 100 km or so above the Earth's surface, but the reason is that the dust is hitting the Earth's atmosphere at very high speed. In the case of the Leonids, this is 71 km/sec (250,000 km/hr!). At that speed, the particle, and molecules in the Earth's atmosphere,are pulled to pieces, giving the short flash of light called a meteor. Each Leonid meteor lasts from a tiny fraction of a second to a couple of seconds.
The Leonid meteors are dust from comet Tempel-Tuttle which orbits the Sun every 33 years. This orbit passes very close to the Earth's orbit, and the resulting celestial crossroads is the cause of the collisions. The meteors "burn up" harmlessly in the Earth's upper atmosphere, but if the Earth were to collide with the comet (something we know cannot happen in the next few centuries), it would be a very different situation, but more on that later.
The point in the Earth's orbit where this crossroads exists, is reached each year around November 18 and every year a few Leonid meteors can be seen. Just why should this year be any different? Basically, there is more dust close to the comet as the dust is shed from it's surface every time it passes close to the Sun. Comets contain ice and dust, and as the ice is heated by the close approach to the Sun, it turns to gas, releasing some of the dust. Thus the dust tends to be thickest close to the comet. In February this year, the comet rounded the Sun once again, largely without fanfare. It was poorly placed for observation and was barely visible at all from the southern hemisphere. However, we will probably witness the consequences next Wednesday when the Earth passes through this thick vein of dust in the comet's wake.
The dust in the comet's orbit moves with a common motion, all the particles moving in almost parallel paths at virtually identical speeds. When they hit the Earth's atmosphere, the meteors appear to radiant outwards from one point in the sky. In the case of the Leonid meteors, this point is in the constellation Leo. When Leo is not in the sky, you cannot see the Leonids. An analogy may make this easier to understand. When rain is falling on a wind free night, each droplet falls straight down on parallel paths. As we drive through the rain with headlights on, the forward motion of the car adds to the motion of the rain, and the drops appear to radiate outwards from a point in front of and above the car. This is solely an effect of perspective, the droplets still having parallel paths.
As with many predictions in astronomy, there is much uncertainty in what to expect. Perhaps nothing special will happen, which at the worst will mean you lose a few hours sleep. If however the shower is a truely spectacular "once in a lifetime event", you will be glad you made the sacrifice. The two uncertainties are the time at which the Leonids will give the highest number of meteors and just how high these rates will be.
The time can be calculated with a uncertainty of only a few hours, but the current predictions place Australia right in the firing line! The meteors only hit one side of the Earth, and eastern Australia rotates into view of the meteors between 2:30am and 3:00am daylight savings time on the morning of Wednesday Nov. 18. Initially the rates are likely to be low with the meteors coming from the direction of the constellation Leo low on the north-east horizon, but visible anywhere in the sky. Towards dawn at around 5am, the Leonids are likely to be more numerous as Leo rises higher towards the north. A sharp peak could occur at any time during this period or an hour or two either side. If it occurs early, the Leonids will be over the horizon and we will not see the peak. If later, the peak will occur in morning twilight or daylight, but if that were the case, the rate of Leonids will probably increase sharply towards dawn. Some of the brightest Leonids will be visible in daylight. When these occur at night, the flash of light can rival or exceed the full moon in brightness.
 The Leonid Meteor storm as seen in 1833. |
Next year will produce another opportunity for a Leonid storm, but the predicted time favours Europe, the Middle East and Africa with Australia unlikely to see rates comparable to this year. In 33 and 66 years time, the Earth will pass even further from the dust stream making storms unlikely on those occasions. Whilst the dust close to other comets can produce good meteor showers, or occasionally storms, the Leonids this year is our best chance to witness very high rates from Australia, possibly for the next century.
There is no danger to us on the ground from the Leonids. Even the biggest ones will burn up many tens of kilometres above the Earth's surface. The situation in space is rather different, with up to 600 active satellites and the Mir cosmonauts potentially in danger. The larger, less common particles, could physically destroy components on a satellite or puncture the pressurised hull of the Mir space station, but the main danger is from the much more numerous tiny particles. On hitting a satellite, they create a highly charged gas plasma which can disrupt sensitive electronic components rendering the satellite inoperative. Some predictions suggest that several satellites could be damaged in this way during the peak of the Leonids. Even the Mir cosmonauts will not risk being caught out by a major malfunction on Mir. They will ride out the "storm" in the re-entry capsule that is designed to withstand a much stronger bombardment than the space station itself. Many satellites, including the Hubble Space Telescope (HST) will take precautions by turning their least sensitive surfaces away from the direction of the incoming particles. The HST will keep operating however, and Australian astronomer Michael Drinkwater will be using it during that period. The direction in which the HST will be "parked" allows Michael to study some interesting quasars, an field in which he is an international expert.
No really large particles (boulders) have been witnessed amongst the Leonids before. The dust also seems to be very fragile, so even a boulder would disintegrate in the atmosphere. To put the collisions of these particles in perspective, it is like a mosquito hitting a car windscreen. You see the effect, but it does no damage. Were comet Temple-Tuttle (a few kilometres in diameter) to hit the Earth, the consequences would be more akin to hitting a 'roo at high speed. The Earth may not be a write off, but it will be badly damaged. It is this idea, of the potential threat of the Earth to cosmic bombardment, that has led to several groups searching the skies for asteroids or comets on collision course with the earth (and to several movies as well). The Australian effort at Siding Spring that ran from 1990 to 1996 had its funding cut by the incoming government in the latter year. It would seem that international and media pressure has caused a change of heart and it would appear that funding will recommence for an Australian program. However there is still no program in the southern hemisphere since the demise of the Australian group, and thus much of the sky is going unobserved. There has also been a proposal by a University of Arizona group to set up an automated search telescope at Siding Spring. Work on that may start next year.
Having said all that, what is the best way to witness the Leonids? It would be better to get away from any street lights and preferably to an open location to give the widest possible view of the sky. You could have a "dry run" on the days leading up to Wednesday morning as Leonid meteors can be seen at low rates in the early morning hours from Nov 14 to 20. There is no advantage in using binoculars or a telescope to view the meteors, the naked eye is the best instrument, but binoculars will be useful to examine the luminous glow left behind some of the brightest meteors. These can last many tens of seconds and can be seen distorting in shape in the upper atmospheric winds. Those who are really keen, might like to follow the weather forcasts and travel to where the sky has the best chance of being clear. The meteors can be photographed using a fast film (400 ISO or faster), a long time exposure (ten or more minutes) and a wide aperture (f1.4 to f2.8). Use of a wide angle or standard lens is preferred over a telephoto and use fresh batteries in electronic cameras. To prevent disappointment, try some exposures a few days before and check the results.
Observing meteors is a field of astronomy that doesn't require expensive telescopes, just a deck chair! If the skies are clear and the shower performs, then celebrate with a champagne breakfast. If it's cloudy or the rates are not high, have a champagne breakfast anyway!
Good luck.
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Enquiries jsarkiss@atnf.csiro.au John Sarkissian