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Leonids 1998: Will the Lion Show Its Fangs?

Nights to Remember

"... It would seem as if worlds upon worlds from the infinity of space were rushing like a whirlwind to our globe... and the stars descended like a snowfall to the earth." "... I heard a faint voice near my door, calling my name... beseeching me to rise, and saying, 'Oh my God, the world is on fire!'..." On the night of November 12, 1833, the residents of the United States were thrown into a state of panic by a spectacular celestial fireworks display. To those out in the night, it appeared that almost every star in the sky was falling from heaven; even those asleep indoors were awakened by the brilliant flashes of meteors and peered fearfully out of their windows, sure that the world was coming to the end. However, there were cooler heads, some belonging to the Pawnee, who watched the meteors without fear, for they remembered the story of the man Pahokatawa. After being killed by enemies and left as animal fodder, he was revived by the

"… a tempest of falling stars broke over the earth… The sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers… were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much diminished rate, that 240,000 must have been visible during the nine hours they continued to fall."

Observations like these, especially those of Denison Olmsted, gave birth to meteor science. The Leonids would storm again in 1866 and 1867, but with a diminished intensity compared to the major displays of 1799 and 1833. The 1866 apparition is especially notable, for it was then that a greater understanding of the nature of meteor showers was attained with the realization by the Italian astronomer Schiaparelli (famous for his drawings of the Martian canals) that the Perseid meteors were caused by particles ejected from Comet Swift-Tuttle. The source of the Leonid meteors was soon determined to be the newly discovered Comet Tempel-Tuttle, which completes one orbit about the Sun every 33 years. This being established, astronomers looked forward eagerly to 1899, when the Leonids were expected to roar once again.

However, mid-November of 1899 did not manifest a meteor storm, there being only a modestly enhanced shower (normally the Leonids have a rate of about 10-15 meteors per hour; in 1899 there were about 40 per hour). The same thing happened in 1932, when the Leonids barely managed about 200 meteor per hour. Astronomers forgot about the Leonids, thinking that the great meteor storms of the 18^th and 19^th centuries would not be seen again. 1965 proved them wrong, for the Leonids once again reached storm levels, achieving a rate of some 5000 per hour. The following year, 1966, was when the Lion really showed its teeth, for 150,000 meteors per hour were seen in the greatest Leonid display of all time.

1998-2000

Past Leonid meteor storms have been no cause for concern, for the meteors are so small that they never make it anywhere near the ground, vaporizing at altitudes greater than 90 km. However, nowadays we have hundreds of active satellites in Earth orbit and in near-Earth space, none of which have the protection of the atmosphere. Can these satellites be damaged by a storm of Leonid meteors, which, due to the orbit of the parent comet relative to that of Earth's, sweep by us at a blazing 72 kilometers per second? This question is not easy to answer, for it involves several factors, some of which are the probabilities of Leonid storms in the next few years, the expected intensity of the storm(s), and the vulnerability of a given satellite to the effects of a meteor impact. The latter can only be answered by the various satellite operators and designers, so the following paragraphs shall attempt to address the other two factors mentioned.

The storm probabilities for the next few years can be obtained via one of two methods - an analysis of past Leonid activity or by the generation of computer models of the stream of meteoroids, taking into account planetary perturbations, radiation pressure, and the characteristics of the meteoroids ejected from the comet. The results are shown in Table 1, below. It should be noted that both methods suffer from incomplete data, almost to the point of absurdity. However, both techniques are consistent in that they give high probabilities of Leonid storms in 1998 and 1999, with a lesser chance in 2000. The computer models also indicate a significant chance of a storm in 2001.

The intensity of the meteor storm is also difficult to estimate, though based on the closeness of the comet orbit to that of Earth, we can estimate that the Leonids will show a rate somewhere between 500 and 10,000 meteors per hour, with the best guess being about 5000 per hour. This is, of course, for visual observers on the ground. Satellite operators are more interested in the fluence of Leonid meteoroids, which varies as a function of mass. Table 2 shows the expected Leonid fluences for 1998 and/or 1999, given our current knowledge of the Leonid meteor stream. Note that the table only goes down to 10^-7 grams; because of solar radiation pressure, we expect to see no Leonids with masses less than 10^-8 grams. Satellites near the Earth-Sun L₁ point should see about an order of magnitude greater fluence, due to the closer proximity of this location to the comet orbit. If the satellite exposes 10 m² to the meteor stream, then it has the probabilities of no impact given in Table 3. Observe that for the smallest mass, the probability of impact is about 4%, a small, but not ignorable, number. The risk at the L₁ point is much greater, about 34%.

Fortunately, the peak times of the Leonid showers are known with a fair degree of precision (uncertainty of about 15 minutes). The peak of the 1998 shower or storm is expected at about 19:45 UT on November 17, that for the 1999 event should occur at 1:50 UT on November 18, and that for the 2000 Leonids should be on November 17 at 8:05 UT. One should expect the duration of the storms (when they occur) to be about 7 hours or so, centered on the peak.

Conclusion

One of the greatest frustrations of satellite operators is the estimation of the impact of probable future events on their vehicle, be it an X class solar flare or the increase in atmospheric density at the peak of the solar cycle. The Leonid storms are yet another such set of events, save that in this case the uncertainties lie not in the event timing, but in the magnitude of the occurrence. It is probably safe to say that the probable error bars on the fluence numbers presented in this article span at least an order of magnitude – almost rendering the estimates meaningless. However, it has been made manifestly clear that the satellite community needs some idea of what to expect in mid-November of the next four years, and that is what the Space Environments and Effects Program has attempted to provide. The results of the latest observations and the interpretations of Leonid computer models have been incorporated into a computer program, the Leonid Fluence Calculator (available on the SEE web site at http://see.msfc.nasa.gov). The program computes the fluences on various spacecraft surfaces and the amount of time the Earth shields the spacecraft from the meteor stream, given the satellite orbital elements at the time of the Leonid peak. Being based on FORTRAN and JAVA, the calculator is available for various platforms, including Windows 95/NT, OS/2, and Sun Solaris.

Operators are advised to start planning for the 1998 storm and should consider re-orienting their spacecraft to expose the minimum area if at all possible. Power should not be a problem, for the sun will be at an angle of ~88° with respect to the Leonid radiant (a = 153° , d = +22° ). Therefore, a shift of only about 2° will enable the spacecraft to receive essentially full power and present the solar arrays edge-on to the meteor stream. Also, do not assume that there will be no storm in 1999 if nothing significant materializes in 1998.

On a final note, the numbers indicate that while the odds of any single spacecraft being struck are small, the odds of an impact on an active satellite out of the several hundred currently in space are not negligible, approaching unity. If these numbers are to be believed, it then boils down to whose satellite will be hit, which then begs the question of whether or not it will be a benign, disabling, or fatal event. Will the Lion strike with its fangs? The answer may be known just a few hours after 2:45 PM EST on November 17, 1998.