WEEK 47: NOVEMBER 15-21

TOPIC: COMETS AND METEOR SHOWERS

     

I first began to show an interest in astronomy when I was 6 years old, although my interests shifted between astronomy and various other scientific fields over the next few years. My father was an early riser, and one morning when I was 8 he was engaged in his normal morning routine when he noticed that an intense meteor shower was going on outside, and he came and aroused his astronomically-inclined son out of bed. Through our living room window we watched the meteors appearing so quickly it almost looked like it was snowing outside.

The date was November 17, 1966. What my father and I had witnessed was the Great Leonid Storm of 1966, one of the strongest meteor showers in all of recorded history. New Mexico, where we lived, and in fact the entire western part of the U.S., was in the right location to witness this event, and the peak rate, which lasted for perhaps twenty minutes, was somewhere between 100,000 and 150,000 meteors per hour.
ABOVE: Twelve-minute exposure of the 1966 Leonid meteor storm, November 17, 1966, taken by Scott Murrell at New Mexico State University. I observed with Scott a few times while I was a graduate student. He passed away in 2004. RIGHT: The night “stars fell on Alabama,” the Leonid meteor storm on November 13, 1833, from the eastern U.S. This engraving was made by Adolf Volmy based on a painting by Swiss artist Karl Jauslin, which in turn is based upon eyewitness descriptions.

While the Leonid meteor shower is normally a pretty weak affair, producing no more than about ten meteors per hour, it has on occasion, like in 1966, produced significantly stronger displays. One such “storm” took place on October 13, 902, when observers in Europe, Egypt, and elsewhere reported that stars were falling “as thickly as snowflakes.” (The difference in calendar dates is due largely in part to the switch from the
Julian Calendar to the Gregorian Calendar in 1582.) An intense Leonid “storm” was visible from parts of Europe and South America on November 12, 1799, and another very strong “storm” – apparently every bit as strong as the one I witnessed – took place on November 13, 1833, as seen from the eastern U.S.; this was the famous night when “stars fell on Alabama.” A somewhat weaker, but still very strong shower with a peak rate of approximately 5000 meteors per hour, was seen from Europe on November 14, 1866. Shortly thereafter the French mathematician Urbain Le Verrier, who had correctly predicted the location of Neptune prior to its discovery two decades earlier, determined that the orbital period of the Leonid meteors was 33 years, which matched the approximate periodicity of the intense showers.

It so happened that in mid-December 1865 the French astronomer Wilhelm Tempel had discovered a 6th-magnitude comet near the “bowl” of the Little Dipper, which in turn was independently discovered by the American astronomer Horace Tuttle in early January 1866 when it was near its peak brightness of magnitude 5.5. The following year the Austrian astronomer Theodor von Oppolzer calculated that that comet – now known as Comet 55P/Tempel-Tuttle – has an orbital period close to 33 years. It was obvious that the correspondence in orbital periods was not a coincidence, and the Italian astronomer Giovanni Schiaparelli soon conclusively demonstrated the relationship between that comet and the Leonid meteors.

Schiaparelli had also just recently demonstrated the relationship between another comet and a meteor shower: the comet now known as Comet 109P/Swift-Tuttle, which had been seen in 1862 (and which returned in 1992, and is a future “Comet of the Week”), and the Perseid meteor shower, which is one of the strongest of the “annual” meteor showers and which peaks during the second week of August each year. Over the years and decades which have elapsed since then, numerous other meteor showers have been found to be associated with various comets.
As a comet approaches perihelion and becomes active, the dust grains that are ejected from the nucleus for the most part do not return to it, but instead keep traveling around the sun in the same basic orbit as the comet's. Over time these dust grains spread out into a “stream” that essentially marks the comet's orbit through the inner solar system. (These dust streams have been detected by infrared-sensitive spacecraft missions.) If these dust streams happen to come close to Earth's orbit, then every year at the time when that close passage occurs the dust grains will enter the atmosphere at speeds of up to several km per second and vaporize, and we see this as a “shower” of meteors. The meteors will all appear to come from the same basic location in the sky, which is an effect of perspective similar to the parallel lines of a road appearing to converge in the distance; this location is called the shower's “radiant.” Most meteor showers are named for the constellation in which the radiant appears, or if there is more than one meteor shower associated with a given constellation, for the star which the radiant might lie near.

The strengths of the various meteor showers very widely, and can be affected by factors such as the activity level of the parent comet and the closest distance between the

Spitzer Space Telescope image of Comet 2P/Encke taken on June 23, 2004. The diagonal glow is the comet's dust stream within its orbit. Courtesy NASA/ JPL-CalTech/University of Minnesota/Michael Kelley.
earth's orbit and the comet's dust stream. These factors can also influence the duration of a given shower; some showers extend over several days, while others exhibit a very sharp peak in strength which might last no more than a few hours at most. Furthermore, since cometary dust streams are affected by gravitational influences just like comets themselves are, the orbits can shift over time and become stronger or weaker over a period of decades.

For some comets, 55P/Tempel-Tuttle being among them, the dust has not had enough time to spread out all along their respective orbits, and thus much of the dust travels in “clumps” close to the comet itself. The result of this is what we see with the Leonid shower: normally quite weak, but extremely intense around times that the comet returns to perihelion. Even here, the gravitational effects on the dust stream can affect the displays we see: while the Leonid shower associated with the comet’s return in 1866 was a strong one, the ones associated with the return in 1899 were much weaker, and only a slight enhancement of meteor activity was exhibited during the comet’s 1932 return. (For what it’s worth, the comet itself was missed during those returns). The situation was reversed for the 1965 return – when the comet was seen again, although it remained distant and faint – and the result was the extremely intense “storm” I witnessed the following year.

There was much anticipation among astronomers as Comet Tempel-Tuttle returned to perihelion again in 1998. It was duly recovered, and the viewing geometry was relatively favorable, with its reaching a peak brightness close to 8th magnitude early that year. Predictions seemed to suggest a strong Leonid display that year – and there indeed was one, although not as strong as originally expected, and in a departure from previous Leonid displays, this one lasted for the better part of a day and was widely viewed from around the world. (It also featured a large number of bright fireballs, and I personally consider it to be the second-best meteor shower I have ever seen, second only to the 1966 “storm.”)

Astronomers began to realize that, in order to make accurate predictions of a meteor shower’s display, it was necessary to take into account the dust streams released during individual returns of the parent comet. Two individuals who took this up were Rob McNaught in Australia and David Asher in Ireland, and together they predicted that a strong but brief display of the Leonids would be visible in 1999 from Europe and the Middle East. They turned out to be correct – with peak rates of between 3000 and 5000 meteors per hour being seen – and one interesting feature of this shower is that some observers recorded at least five bright flashes on the moon’s unlit portion that were apparently due to Leonid meteors striking the lunar surface. Meanwhile, and somewhat surprisingly, Asher and McNaught predicted that the strongest Leonid showers would take place in 2001 and 2002 – and, again, they were correct, although the 2002 shower coincided with full moon and from an observational perspective was not as impressive as it otherwise might have been.

The strongest of the “annual” showers are the Quadrantids in early January, the Perseids in August, and the Geminids in December, all of which can exhibit peak rates in excess of 100 meteors per hour (although the peak of the Quadrantid shower is very brief). While what one might consider a “strong” shower is perhaps somewhat subjective, the below table lists some of the stronger and more notable meteor showers that appear during the course of a typical year (keeping in mind that at least a few will be affected by strong moonlight). The table also gives the parent comet and the “zenithal hourly rate,” or ZHR, which is the peak rate that an observer would see with a perfectly clear and unobstructed sky from a dark rural site; the true observed rate will almost always be somewhat less than this.

SHOWER                  MAXIMUM DISPLAY (2020)    PARENT COMET      ZHR      
       
Quadrantids January 4 (196256) 2003 EH1 120
Lyrids April 22 Thatcher 1861 I   18
Eta Aquarids May 5 1P/Halley   50
Arietids June 7 SOHO P/1999 J6?   30
Delta Aquarids July 30 96P/Machholz 1?   16
Perseids August 12 109P/Swift-Tuttle 110
Draconids October 8 21P/Giacobini-Zinner   10
Orionids October 21 1P/Halley   20
Taurids November 12 2P/Encke     5
Leonids November 17 55P/Tempel-Tuttle   10
Andromedids December 2 3D/Biela     3
Geminids December 14 (3200) Phaethon 150
Ursids December 22 8P/Tuttle   10

The Draconids and Andromedids are “clumpy” showers like the Leonids that have produced intense displays in the past, and are discussed more thoroughly in the “Comet of the Week” presentations for their respective parent comets. While at face value it may not appear to be one of the stronger meteor showers, the Lyrids have on rare occasions produced strong displays (700 per hour in 1803 and 90 per hour in 1922 and 1982); the parent comet, incidentally, has an approximate orbital period of 415 years. The Ursids have also occasionally produced some strong displays, sometimes when the parent comet is near aphelion. The Arietids are the strongest of the daytime meteor showers, observable via radar techniques (described in a previous “Special Topics” presentation), although a few meteors may be visible during dawn. The likely parents are the Marsden “family” of near-sun comets discussed in the upcoming “Special Topics” presentation on comet and asteroid “families.”

     LEFT: A composite of 30 images of the Geminid meteor shower over the European Southern Observatory in Chile. Photo copyright Stephane Guisard (Instagram), used with permission. RIGHT: The 1997 Leonid meteor shower from space. This is a composite image taken (over a span of 48 minutes) by the Midcourse Space Experiment (MSX) satellite on November 17, 1997. Courtesy Peter Jenniskens and colleagues, Applied Physics Laboratory, USIVI, MSX, and BMDO.

It will be noticed, of course, that the parent “comets” of two of the strongest meteor showers are apparent asteroids, and this hearkens back somewhat to the active-comet-to-inert-asteroid discussion in an earlier “Special Topics” presentation. (3200) Phaethon was discovered by the InfraRed Astronomical Satellite (IRAS) spacecraft in 1983 and has an orbital period as small as 1.4 years and a very small perihelion distance of 0.14 AU; it has been observed to exhibit “cometary” activity of sorts when near perihelion and is discussed more thoroughly in an upcoming “Special Topics” presentation on “Active Asteroids.” (196256) 2003 EH1, which was discovered in 2003 by the LONEOS program in Arizona, has an orbital period of 5.5 years and a perihelion distance of 1.19 AU; it has never been seen to exhibit any kind of cometary activity, but on the other hand it has never been observed close to Earth and thus far hasn’t been amenable to any detailed investigations. (Indeed, its next “close” approach to Earth doesn’t take place until 2052, and that is still a somewhat distant 0.33 AU.)
There are many, many more meteor showers that have been cataloged, in fact, almost 800 showers have been identified (with more being added all the time), although only a little over a hundred of these are considered “established” at this time. Most of these are very minor affairs with ZHRs no higher than 1 or 2. The parent bodies for many of these showers have not been identified, and indeed it is quite possible that at least some of these no longer exist. On the other hand, some of the showers appear to be associated with near-Earth asteroids, which suggests that these objects are either possible extinct comets, or that the showers are the result of impact events on the parent bodies.

On any given night, even if it is not around the time of the “major” showers, at least a few of the minor showers are active, and almost all of the meteors that appear --
An Orionid meteor, October 2017. Courtesy Con Stoitsis in Victoria.

even the “sporadic” ones -- are members of some shower or other. Just about any meteor we might see, then, has come to us from the nucleus of some comet at some point in time in the past.

 

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