Meteors: Showers and Storms

Watching the sky at night, a casual observer may see from 3 to 5 sporadic meteors per hour. However, on some nights this number may increase markedly, and on projecting the paths of the meteors back we find that many appear to radiate from a very small area in the sky. This point or area is termed the radiant of the meteor shower. There are several regular meteor showers that occur each year and typical hourly rates may vary from 5 to 50 (above the sporadic background rate). Very infrequently, and sometimes without warning, an hourly rate in excess of several hundred shower meteors may occur. This is generally termed a meteor storm.

Comets and Meteors

A meteor shower may occur when the Earth passes near the past orbit of a comet (or in rare cases an asteroid cluster - eg the Geminid meteor shower in December). Comets travel in highly eccentric orbits and solar heating causes outgassing and the release of small low density particles along the orbit. Gravitational perturbations from planets, and other effects cause these particles to spread out around and away from the orbit.

A new comet will leave behind a concentrated stream of debris that the Earth may pass by in a few hours. After a very long lapse of time, this debris will spread out into a very broad stream, and it may take the Earth several days to pass through it. Old debris streams tend obviously to have a lower density of particles than newer compact streams.

When the Earth intercepts a particle debris stream, the individual particles travel through the atmosphere. Large frictional forces heat the particle and the surrounding atmosphere and a visible meteor is seen. The meteor typically is formed around 100 km altitude. Few particles or meteoroids survive below 80 km.

The Leonids

The Leonid meteor shower occurs from about 14 to 20 November as the Earth passes through an old debris stream left by past passages of the comet Temple-Tuttle. The maximum rate occurs within a day or so of November 17 and is usually less than 10 per hour. The meteors appear to come from a radiant that lies within the "sickle" of the constellation of Leo (hence the name). An unusual feature of this stream is that it is often associated with some fairly bright meteors that may leave a trail (called a train) behind that is visible for many seconds, and sometimes even minutes. The meteors travel very fast and the brighter meteors may show a golden colour. In fact these meteors are the fastest of any meteor stream so far observed.

Technical details

        Radiant coordinates:  
                Right Ascension - 153 degrees / 10h 12m
                Declination     - +22 degrees
        Radiant:
                Rises    - 01:30 Local Time
                Transits - 07:00 L
                Sets     - 12:00 L
        (Times vary with latitude and are approx for Australasian area)
        Dates of detectable meteors:
                14 - 20 November
                Broad peak of 4 days centred around November 17
        Observed velocity:
                72 km/sec
        Visual hourly rate: 
                10 during 03:00 to 04:00 Local Time
        (This varies enormously with year and observing conditions)

Comet Temple-Tuttle has an orbital period of about 33 years, and this has been associated with a massive increase in meteor numbers. That is, if the Earth passes near this comet's orbit close to the time that the comet itself has passed by the same point, then recent concentrated particle outbursts may be intercepted and cause a meteor storm. Two such famous Leonid storms occurred in 1833 and in 1966, both with peak meteor rates estimated at up to 100,000 per hour!

Predictions from orbital studies and debris simulations indicate that 1998 or 1999 may be years in which significant Leonid activity again occurs. Estimated rates vary from 100 to 100,000 per hour. Such predictions are notoriously unreliable, and in fact two different simulations have predicted two quite different outcomes.

Australasian Observations

In 1998, Australia (and particularly Western Australia) is in the most favoured position to observe any significant increase in Leonid meteor activity. A meteor storm is most favoured to occur during the hours of 1900 to 2000 Universal Time, and this corresponds to 0300 - 0400 Western Australian Standard Time, the best hour in which to observe meteor activity in general. There is no moon to interfere with observations, and even if no storm eventuates, most people should see a reasonable collection of bright meteors during this time.

Implications

Apart from the visual display (and a meteor storm is indeed a very awe-inspiring sight - some have even credited the 1833 storm with inspiring a number of new religions that are still with us today), a meteor storm has a number of implications for the space environment.

Electromagnetic Effects

As well as the visual phenomenon, a meteor also (and in fact primarily) produces a column of ionisation in the upper atmosphere at altitudes of between 150 and 50 km. This ionisation will affect the propagation of radio waves that pass by. Low frequency VHF signals (30 - 100 MHz) will be reflected by the ionisation trail and may enable brief communications over distances up to 2200 km. This propagation mechanism is known as meteor burst communication and is a low cost way of obtaining data from remote sites. Essentially what happens is that the acquiring site broadcasts a pilot tone. When the remote site detects this tone (from a meteor event), it quickly (in a fraction of a second) transmits its data. This method of communication is relatively secure (due to high trail aspect selection), good in an electromagnetically disturbed environment, and very suitable for high latitude sites where satellites are difficult to access.

However, what can be beneficial to one user is interference to another. Normal VHF communication, which is essentially line of site, may receive interference (via meteor propagation) from other more remote sites. High Frequency radars (eg OTH-B) may experience interference due to meteor echoes which can be spread widely in Doppler space. In the instance of a meteor storm, the ionisation created may be so intense that a new low altitude ionospheric layer is created. If this layer is widely distributed it may act to block signals reaching the higher F-layer, and create an effect similar to blocking sporadic-E. It will also undoubtedly allow reflection of VHF signals on a continuous basis, with concommittant interference to TV transmissions, especially in areas of low primary signal strength.

Spacecraft Damage

Apart from the aspect of radio propagation, which may be beneficial, benign or destructive, there is also the element of physical damage to space systems by the particles themselves. These meteoroids may strike a spacecraft and cause varying degrees of damage according to their size and density. Most shower meteoroids are very low density (below that of water) friable or crumbly material. Still, the damage they can cause increases with their velocity and at Leonid velocities complete vapourisation of the impactor and the impacted will result. At the very least, a collision between a Leonid meteoroid and a spacecraft could result in "sandblasting" of optical surfaces (eg solar cells) with a consequent reduction of efficiency. The Hubble Space Telescope will in fact be turned to face the opposite direction to the Leonid particles during times of predicted activity.

In the extreme case, a large Leonid meteoroid could impact a satellite and cause sufficient damage so as to render the craft unusable. During the early days of space travel, the meteoroid hazard in general was considered a significant threat and NASA spent a considerable amount of money in trying to define the hazard nature and extent. Fortunately, the threat proved to be a lot less worrisome than first thought, although for large structures, such as the International Space Station (ISS), which will orbit for lengthy periods, the meteoroid threat does become significant, and shielding against this threat will be carried by the ISS.

If a Leonid storm rate of 100,000 per hour does eventuate, several predictions have been made that at least one if not several geosynchronous and/or low Earth orbiting satellites may be rendered inoperational. It has also been pointed out the debris hazard at the Earth-Sun Lagrange point (normally known as the L1 point) may be even more intense than on the Earth. This area is host to a number of scientifc satellites that observe the Sun and measure parameters of the space environment. If any of these were to be destroyed, we would lose a substantial space weather predictive ability which we have only recently attained.

Meteorites

When a meteoroid is massive enough and of suitable internal strength, a percentage of it may survive the ablation that it suffers in its progress through the atmosphere and impact the Earth's surface from whence it is then known as a meteorite. However, cometary material is generally neither massive enough nor structurally strong enough to survive entry through a planetary atmosphere. We thus do not expect a meteor shower such the Leonids to be a hazard to the biosphere or even to drop any meteorites.

Most meteorites are believed to originate from asteroid fragments. These also produce visible meteors. A one gram piece of material is generally believed to produce a magnitude zero meteor - that is, one as bright as the brightest stars. To drop a meteorite, the general rule is that the parent meteoroid has to have a mass greater than 100 kilogram, and produce a meteor of magnitude -18 (at night this is bright enough to light up all surrounding objects and allow an observer to read a newspaper with ease). The meteorite that might result from such a meteoroid will typically have a mass of only one kilogram, or 1% of its progenitor.

Summary

Visually, the early morning hours of November 18 (Australian local time - which is Nov 17, 1900 UT) may see an intensely spectacular Leonid meteor storm. It is more probable however, that a much less intense shower of a few hundred meteors per hour will eventuate. Some space environmental effects may be noted, as outlined above. Those with cloudy skies may still be able to monitor any activity by tuning to a distant FM station (not normally received), using an outdoor antenna, and listening for bursts of signal. Rising early on this morning is probably a worthwhile activity. whatever eventuates.

Finding Out More

The November 1998 issue of Sky and Telescope has many interesting and well written articles on the Leonids, both past and present. Definitely a collectors item.

Cambridge University Press has just released a book by Mark Littmann entitled "The Heavens on Fire: The Great Leonid Meteor Storms". This is the only book so far written exclusively about the Leonids. 288 pages in length and its ISBN is 0-521-62405-3.

Material Prepared by John Kennewell. © Copyright IPS - Radio and Space Services.