A surprise October meteor shower?
Meteor observing can be relaxing and enjoyable, yet it's also potentially dramatic. One of its fascinations is that meteors are unpredictable. Usually they are few and far between, but you never know for sure what will happen next. There's always a chance that you will observe something new and different, rare or unique, whether it be a new meteor stream, a brilliant fireball, or a long-enduring smoke train.
Currently, however, much anticipation revolves around something more predictable: the return of not just one but two periodic comets, each possibly bringing unusual meteor activity.
The Leonid meteor shower, produced by Comet 55P/Tempel-Tuttle, is the most anticipated. On the morning of November 17th or 18th this year, or on the morning of November 18, 1999, we could get a repeat of the great meteor storm that broke over the Earth in November 1966. Any Leonid outburst will probably last no more than an hour or two. This year East Asia will be having its early morning meteor-watching hours when the Leonids are most likely to hit. Next year the longitudes of eastern Europe and central Asia are favored. But with a few hours of uncertainty in the prediction, any place could have the ringside seats. Full details will appear in next month's Sky & Telescope.
But a different meteor storm just might sneak up on the world a month beforehand and steal the show.
The Giacobinids of October
Spawned from Periodic Comet 21P/Giacobini-Zinner, the Giacobinid shower produced two of the greatest meteor displays in this century, in 1933 and 1946. Lesser showers arrived in 1926, 1952, and 1985.
Most years bring no Giacobinids at all. An intense shower seems to occur only when Earth passes just inside the orbit of Comet Giacobini-Zinner shortly after the comet itself has gone by this point (see the diagram on the next page). Apparently, the meteoroids that cause the shower left the comet's nucleus relatively recently. They have not yet had time to disperse all the way around the comet's orbit. Rather, they remain in a thin ribbon not very far ahead of and behind the comet.
For this reason, meteor-observing groups plan special watches in years when the comet returns and Earth passes near it. In October 1998 this happens again. But, as history has shown, you may be treated to a spectacular meteor display or come away without seeing a single Giacobinid!
We are currently in the 13th observed return of Giacobini-Zinner. The comet was discovered on December 20, 1900, by Michel Giacobini at Nice Observatory in France and again independently on October 23, 1913, by Ernst Zinner at Remeis Observatory in Bamberg, Germany. This year it returns to perihelion (its closest point to the Sun) on November 21st. The comet should be brightest, 9th magnitude, throughout November and early December. During that time it ought to be detectable in telescopes fairly high in the southwestern sky just after dark, moving from Aquila through Capricornus to Aquarius. A finder chart is planned for next month's Celestial Calendar department.
Tugs and Tweaks
Comet Giacobini-Zinner is in a 6.6-year orbit, with its aphelion (farthest point from the Sun) lying just outside Jupiter's orbit. This flags it as a member of Jupiter's family of comets. The gravitational influence of the giant planet has clearly shepherded Giacobini-Zinner into the orbit where we find it today. (In fact, Jupiter has herded about two-thirds of all short-period comets into having aphelia near its own orbit.) Periodically, the giant planet's strong gravity continues to perturb Giacobini-Zinner a little this way or that.
The meteoroids that crumble off a comet's nucleus form a thin sheet in the comet's orbital plane. Whenever Earth plunges through this plane, we have a chance for a meteor shower. Whether we get a strong Giacobinid shower or none at all depends on exactly what part of the plane we go through.
My calculations indicate that during the 20th century Giacobini-Zinner has suffered three significant perturbations that have affected whether Earth could meet the associated meteor stream.
The first, in February 1910, happened when the comet approached to within 1 astronomical unit of Jupiter. This encounter caused the comet's perihelion distance to increase slightly, pulling the orbit outward toward Earth and making meteor activity possible.
Then in January 1958 the comet swept within 0.93 a.u. of Jupiter, an encounter that adjusted the orbit a little closer toward the Sun. As a consequence, the distance .between the comet's orbit and Earth's increased considerably - and the Giacobinids completely vanished from our skies.
In September 1969 Giacobini-Zinner approached to within 0.58 a.u. of Jupiter, its closest Jovian pass this century. This newest perturbation tugged the comet back toward Earth's orbit. And in subsequent years, the Giacobinids once again returned.
In 1926 the orbits of Earth and Giacobini-Zinner almost crossed, being separated by just 0.0005 a.u. (75,000 km). What a cometary spectacle would have blazed in the sky if both bodies had reached this grazing point at the same time! As it happened, Earth passed through this spot 69.1 days ahead of the comet. But the location turned out not to be entirely empty.
Observing for three hours from Stowmarket, England, the assiduous British observer John P.M. Prentice reported a rate of only 17 meteors per hour. What stole the show over the British Isles, however, was a great Giacobinid fireball noted by many hundreds of people. The meteor moved slowly and lit up the sky, leaving a persistent train that lasted about 30 minutes.
The first great Giacobinid meteor storm took place on the European evening of October 9, 1933. On that date Earth crossed the comet's orbital plane 0.0054 a.u. (810,000 km) inside the comet's orbit, 80.2 days after the comet itself came by. Astronomers were unprepared for what was in store for them, but as darkness fell, observers across Europe noted the beginning of something unusual. Within an hour the number of shooting stars increased dramatically. By 20:08 Universal Time one of the best meteor displays of this century was reaching its peak. Rates of 6,000 or more meteors per hour were reported by many observers. The highest reported estimate was an astonishing 480 per minute by R. Forbes-Bentley at Birchircara, Malta.
Within another hour rates had fallen back to a tenth of the extreme. The shower was all but gone about three hours after that when night fell on the eastern United States. The meteors were described as slow, mostly faint, and yellowish.
In October 1939, around Comet Giacobini-Zinner's next return, many hoped for another meteor storm since Earth would pass a mere 0.0013 a.u. (190,000 km) inside the comet's orbit. But this time Earth reached this spot 136.2 days before the comet ... and not even a weak shower appeared.
In 1946 the stage was set for another tremendous outburst. On the evening of October 9th North America time, Earth would pass 0.0015 a.u. (220,000 km) inside Giacobini-Zinner's orbit and would be following the comet to this point by only 15 days!
Unfortunately a full Moon was also on the calendar. "The Moon will probably kill off a majority of the meteors," wrote Roy K. Marshall in the October 1946 Sky and Telescope, adding, "maybe we'll see only a thousand per hour!" This time, as the magazine pointed out, the timing of the Giacobinids seemed best for the Americas. Sky and Telescope printed detailed observing instructions. The impending shower garnered much advance publicity in the press, including a front-page story in the New York Times on Monday, October 7th, two days before the big event.
As it turned out, the weather was the worst problem. The remnants of a Florida hurricane clouded out most of the eastern United States, and scattered to broken clouds plagued the central states. The West enjoyed mainly fair weather. But eager watchers who anxiously awaited nightfall under at least a few holes in the clouds were not disappointed.
Meteor activity shot up after dark and reached a sharp peak near 3:50 UT, with counts of 50 or 100 meteors per minute despite the full Moon! "There was no quarter of the heavens untouched by the fireworks," wrote one correspondent to Sky and Telescope. In Chicago, Wagner Schlesinger, director of Adler Planetarium, counted 149 "flashing projectiles" in 10 minutes. Not bad, considering that the moonlit sky was more than 80 percent cloud-covered! Schlesinger said he even saw the glare of two bright meteors through the clouds.
In St. Louis, Edward M. Brooks and Edwin E. Friton, regional director of the American Meteor Society, saw many flashes of light through holes in an altocumulus doud deck "like white snowflakes in a minor snowstorm." For five hours Canadian observers under the supervision of Isabel K. Williamson counted over 2,000 meteors. Since they observed through specially made rings that allowed viewing of only selected areas, their count was far below what the number would have been using today's standard methods.
ABC Radio had scheduled a live 15-minute broadcast about the meteor shower for the predicted peak time, a gamble that turned out to be right on the mark. Radio listeners nationwide hurried outside to see what they could of the ongoing fireworks. To get above the cloud deck, the Sky and Telescope editors joined Harvard astronomers for two flights aboard Coast Guard seaplanes equipped with clear domes through which observations could be made.
As with the 1933 display, the peak of the 1946 Giacobinids lasted only about an hour. So the stream width was determined to be less than 150,000 kilometers in the direction of the Earth's motion. Observers found that the meteors appeared at abnormally high altitudes in the atmosphere, displayed rapid decelerations, and had abnormally short trails. From these observations we can deduce that the Giacobinids consist chiefly of very brittle, low-density material.
A technical milestone that night was the first detection of a meteor shower by radar-echo techniques. Most of these observations were made on World War II radars that had been specially adapted for the coming shower.
During the war it had been discovered that meteors made characteristic whistles on radio receivers. As a meteoroid rushes through the upper atmosphere it leaves a long train of hot, ionized air behind it. This is the bright streak of light we see. The ionized column is vastly larger than the little bit of debris that produces it. Perhaps a few meters wide initially, it expands within a second or two to a diameter of a kilometer or more. Because ionized gas conducts electricity, it reflects radio waves much like the metal airplane bodies that the radars were designed to detect.
Radar was responsible for the detection of a surprisingly strong Giacobinid display around the comet's next return, in 1952. That year Earth preceded the comet by 195.5 days, so astronomers did not expect any significant activity. Indeed, only the barest hint of Giacobinid activity was detected visually. But at Jodrell Bank in England, radio-echo apparatus indicated a sudden, two-hour burst of activity during the daylight hours of October 9th. It was far less than in 1946, but considering the geometry between the Earth and comet, it was still remarkable.
Giacobinid activity was most recently observed in 1972 and 1985. Hopes were especially high' in 1972. Not only were the orbits of Earth and Giacobini-Zinner a mere 0.00074 a,u. (110,000 km) apart, but Earth would pass this closest point 58.5 days after the comet. Many confidently expected a repeat of 1933 and 1946. The timing was expected to be best for East Asia, but fog and low clouds were widespread there. Nonetheless, the Hiraiso Branch of the Radio Research Laboratory in Japan detected a peak of 84 radar echoes in 10 minutes.
Elsewhere the shower turned out to be a huge disappointment visually. Very few meteors were observed. Perhaps the fact that, on this occasion, the Earth passed ever so slightly outside the comet's orbit made the difference. Meteorolds may tend to work their way inward toward the Sun after they are shed by a comet's nucleus.
In contrast, the outlook for 1985 was unpromising. Although Earth passed through the comet's orbital plane a mere 26.5 days after the comet came by, planetary perturbations had moved the respective orbits six times farther apart than they were in 1933 and 20 times farther than in 1946. Indeed, most of the world saw little activity.
But observers in Japan happened to be in the right place to watch at the right time. They witnessed an impressive outburst near 9:40 UT October 8th. One of the world's most diligent meteor observers, Yasuo Yabu, reported a rapidly declining rate of approximately 200 meteors per hour right after dusk. The rate was down to 100 just 20 minutes later. Corrections for the zenithal hourly rate suggested a sharp peak in the 600 to 800 range! (The zenithal hourly rate is the number that a single observer would see per hour if the sky were dark enough for 6.5-magnitude stars to be visible and the shower's radiant were at the zenith.)
Ichiro Hasegawa of the Nippon Meteor Society later called the 1985 Giacobinid shower "one of the most impressive events I ever saw." Daylight radar results from the United Kingdom also showed significant activity.
Mixed Prospects for 1998
In 1985 Donald K. Yeomans of NASA's Jet Propulsion Laboratory suggested that, in order for significant Giacobinid activity to occur, the following criteria should be met:
* Earth closely follows Giacobini-Zinner to the comet's descending node (where Earth passes through the comet's orbital plane).
* Earth comes close to the comet's orbit.
* Earth passes inside the orbit.
For the 1998 Giacobinids, there's some good news and bad news. The good news is that we are still on the inside of Giacobini-Zinner's orbit at the descending node. The gap has widened since 1985, but not by much, to 0.0383 a.u. (5,730,000 km). The most important difference, however, is that Earth will reach this nodal crossing point 49.5 days before the comet. "It is very difficult to assess what effect this difference will have," notes Dutch meteor expert Marco Langbroek. "It might very well be the difference between all or nothing."
To gain a perspective on what is happening, refer to the diagram on page 102. The distance between the two orbits at the nodal point is plotted vertically, and the time when Earth crosses this point ahead of or behind the comet is plotted horizontally. Seventeen past encounters are plotted. The major showers of 1933, 1946, and 1985 are shown as starbursts, lesser showers as dots, and years of low or zero reported activity as open circles.
The diagram makes clear that we are probing uncharted territory this year - territory that borders the area where good showers have appeared in the past. Even no activity will serve as a useful constraint on the meteor-stream model. And if cometary debris extends well out ahead of the parent comet on the inside of its orbit, some sort of shower is possible.
When and Where to Watch
The evening of Thursday, October 8th, is the time to be watching this year. Europe or western Asia seem positioned to have the best chance.
Unlike most meteor showers, the Giacobinids are at their best in the evening rather than after midnight. Their radiant (apparent point of origin due to perspective) is highest after dusk; it moves lower throughout the night and is near the horizon by dawn.
This year Earth dives through the plane of Comet Giacobini-Zinner's orbit at 20:53 UT October 8th, when it's evening in Western Europe. But in 1985, peak activity occurred more than 3 1/2 hours before the nodal crossing; the meteoroids must have been displaced slightly out of the comet's plane by some perturbation. Were this to recur in 1998 the peak would take place closer to 17:20 UT, good timing for western Asia, eastern Europe, and the eastern Mediterranean.
North Americans get their best shot at any Giacobinid activity several hours later, after nightfall moves across the Atlantic.
The waning gibbous Moon will rise around 9 p.m. daylight saving time, depending on your location. Fortunately, this leaves roughly 60 to 90 minutes of darkness between the end of twilight and moonrise at midnorthern latitudes.
Begin watching the sky overhead as soon as it gets dark. No special equipment is needed - just an open view of the sky, a reclining lawn chair or blanket on the ground, a watch, and a note pad or tape recorder to keep count of any Giacobinids seen. The shower's predicted radiant is at right ascension [17.sup.h] [22.sup.m], declination +57 [degrees], close to Draco's head. A shower member is a meteor whose path, if traced backward far enough, would appear to come from this spot, The meteors themselves can appear anywhere in the sky. For how to make a scientifically valuable meteor count and where to report it, see the August 1997 issue, page 90, or http://www.skypub.com/meteors/per97a.html.
Because of the radiant's location, the shower is sometimes called the Draconids. Notes British meteor astronomer Alastair McBeath: "Poets among us might like to think of these as the 'Dragon's Tears' or as its fiery breath."
All meteors called Draconids (several minor showers also bear the name) come down onto us from the north, almost perpendicularly to the plane of the solar system; the north pole of the ecliptic (in effect, the north pole of the solar system) lies in Draco. The meteors appear to move slowly, their entry speed of 20 km per second being among the slowest of any shower's. Asks McBeath, "Will the Dragon flame in 1998?"
Giacobinid Showers Past and Present
Listed here are the vital statistics for encounters with the Giacobinid meteor swarm during this century. The data were computed by Donald K. Yeomans (Jet Propulsion Laboratory).
The first columns give the date (in Universal Time) when Earth arrives at the descending node of Comet Giacobini-Zinner's orbit. This is when we pass through the comets orbital plane. C-E is the distance of the cornet's orbit from Earth at that point. A minus sign indicates that Earth passes outside the comet's orbit, a plus sign inside.
Next is the number of days before or after the comet that the Earth reaches the node. Lastly, any known meteor activity is noted, based on data compiled by Gary Kronk, A. C. B. Lovell, Paul Roggemans, and Donald Yeomans. Some showers may have gone unobserved.
[TABULAR DATA OMITTED]
Our best chances for Giacobinid showers early in the next century come in 2005, 2018, and 231, based on calculations by Donald Yeomans. These are the years when Earth follows Comet Giacobini-Zinner to its descending node just inside the comet's orbit. The table here gives the circumstances. By far the best scenario seems to be in 2018, when Earth is due to intersect the comet's orbit plane at 0:00 UT October 9th (8 p.m. Eastern Daylight Time on October 8th). There will be no Moon.
Future Giacobinid Showers Year Date (UT) C-E (a.u.) Earth at Node 2005 Oct. 8.70 +0.043 91.8 days after comet 2018 Oct. 9.00 +0.017 22.7 days after comet 2031 Oct. 8.37 +0.076 29.6 days after comet
JOE RAO is a meteorologist for News 12 Westchester and an instructor and guest lecturer at New York's Hayden Planetarium. He wishes to thank John E. Bottle, Edward M. Brooks, Daniel W. E. Green, Brian G. Marsden, and Donald K. Yeomans for their helpful comments and suggestions.
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|Title Annotation:||includes related articles; October 1998|
|Publication:||Sky & Telescope|
|Date:||Oct 1, 1998|
|Previous Article:||The sun, moon, and planets in October.|
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