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Leonids 2003: After the Storms

'98 fireballs

The 1998 " attack of the fireballs" as seen by an all-sky camera at the Modra Astronomical Observatory in the Slovak Republic.

The great storms are over, but the Leonid meteor shower still isn't ready to settle down. This year, astronomers expect two modest outbursts as our planet grazes several streams of comet debris between November 13 and 23.

Think of it as an encore.

Dusty encounters

The Leonids produced extraordinary shows between 1998 and 2002, when Earth encountered material freshly ejected by comet Tempel-Tuttle. The comet swings around the sun every 33 years, shedding dust at every pass. Outbursts of hundreds of meteors an hour — and storms of thousands an hour — occurred when Earth swept through the dense trails created in the past 200 years.

Gravity and other forces nudge the particles comprising each stream until, over centuries, the trails blur together into a broad swath of dust enclosing the comet's orbit. When Earth passes through the most ancient cometary debris each year around November 18, we see a smattering of 10 to 15 meteors an hour. This is the annual Leonid meteor shower, so called because the meteors appear to radiate from a spot in the constellation Leo.

" The annual shower comes from the oldest stuff, and the storms come from the youngest stuff," said Bill Cooke, an astronomer with the Space Environments Group at NASA's Marshall Space Flight Center in Huntsville, Alabama. " We'll be seeing material of an intermediate age this year."

Nov. 19 anim

Skygazers from Brasilia to Boston will be in the best position to enjoy the Nov. 19 Leonid outburst. The crescent moon, Jupiter, and the constellation Leo (top, background) will all be visible in the east after 2:00 a.m. local time (07:00 UT), shortly before astronomers expect Earth to pass through the thickest part of the 1533 stream (bottom). The meteor rate may exceed 120 per hour (right).

The image and associated animation do not show the encounter with the 1733 stream.

Leonids 2003 animation (GIF, 1.3 Mb). Also available as Windows AVI or Quicktime MOV.

Leonids 2002 animation (GIF, 0.9 Mb)

Taking both moonlight and anticipated shower activity into consideration, the best display will occur on November 19 as twilight comes to Europe and North America turns into view. That's when Earth intercepts the trail expected to produce the brightest meteors. A waning crescent moon will not significantly hamper observations.

" What we're expecting is something along the lines of a good Perseid or Geminid display," Cooke said.

The Perseid and Geminid showers, which occur in August and December, respectively, are the " old reliables" for skywatchers, producing 60 or more meteors per hour at their annual activity peaks. " If you're not impressed with the Perseids," Cooke warned, " you won't be impressed with this year's Leonids."

While dual Leonid outbursts have occurred before, notably in 1998 and 2002, fewer than twelve hours separated them. This year, the times of peak activity spans a full six days.

" That's a consequence of our encountering older material," said Cooke. Five years have passed since comet Tempel-Tuttle's last return, so the trails must be relatively old and diffuse in order to intersect Earth at all. Since the smallest particles spread out the fastest, both outbursts will likely be rich in faint meteors.

While astronomers have had great success in determining when past Leonid outbursts would occur, their forecasts of meteor intensity have been much less successful. Last year, predictions by Finnish researcher Esko Lyytinen and the French team of Jérémie Vaubaillon and François Colas came the closest to matching what was actually observed

Twin peaks

Nov. 13–14: The 1499 Trail. This year, both Lyytinen and Vaubaillon agree that the Earth will pass through Tempel-Tuttle's 1499 dust trail. Vaubaillon's more recent work indicates that the stream has split into three individual " mini-trails." Earth will brush past one of them on Nov. 13–14 between 13:00 and 19:00 UT, resulting in several hours of enhanced meteor activity. Vaubaillon forecasts a peak Zenithal Hourly Rate, or ZHR, of 120 meteors during that period. Lyytinen estimates a ZHR of 100.

While the timing favors observers located in and around the Pacific Ocean, a nearly full moon will wash out many of the meteors.

Interestingly, the moon will pass even deeper into the 1499 mini-trail than Earth, entering the densest part of the stream at 11:15 UT on Nov. 13. In 1999 and 2001, lunar observers recorded several faint flashes from the impact of large Leonid meteoroids. Such detections probably won't happen this year, said Cooke, since most of the particles will be striking the sunlit portion of the moon's disk.

Nov. 18–19: The 1533 Trail and the " Filament." Earth grazes the 1533 trail six days later, on Nov. 19. Vaubaillon's computations reveal that this stream has also split into separate mini-trails. We encounter one of them, but in simulations only a very small percentage of particle orbits actually intersect Earth's. Such poor statistics make intensity estimates especially difficult. Vaubaillon originally forecast a display peaking at a ZHR of 400, but earlier this month he reduced that figure to 100. He expects an hour-long spike in activity centered on 07:27 UT, or 2:27 a.m. EST.

Lyytinen, however, predicts a ZHR of 30 for the same stream.

Nov. 19 ZHR profile

Profile of expected meteor activity for the Nov. 19 Leonid outbursts.

We will first encounter the " Filament," which consists of larger debris and is expected to produce the brightest meteors. Then Earth will carry us through one of three strands from the 1533 trail, from which astronomers expect comparatively faint meteors.

A more uncertain encounter occurs ten hours later, when Earth grazes the 1733 trail.

The Zenithal Hourly Rate, or ZHR, is a highly idealized estimate and observers always see lower numbers of meteors than indicated by that value.

The encounter favors observers located in the Atlantic, Caribbean, and the eastern half of the Americas. For skywatchers in western Europe, the display is expected to reach its height just as morning twilight begins to interfere. See 2003 Leonid Activity Predictions for North America and Europe for observing tips and forecasts for specific cities.

Astronomers expect predominately faint meteors from both the 1499 and 1533 trails. But Earth passes through another shower component — one that should produce exceptionally bright meteors mdash just before it enters the 1533 stream. Peter Jenniskens of NASA's Ames Research Center in Moffett Field, California, and Dutch researcher Hans Betlem have dubbed this feature the " Filament."

The Filament gave rise to an impressive and unexpected fireball shower in 1998 and has been present — if less prominent — in all years since. According to Jenniskens, the dust from many old trails has merged to form a broad structure unusually rich in large particles. During the 1998 outburst, observers reported fireballs much brighter than Venus — and even brighter than the moon. That corresponds to particles with masses of 100 grams or more. Jenniskens and Betlem have argued that the Filament contains nearly a trillion grams of matter, or about half of the total mass in the annual shower.

1533 stream

Comet Tempel-Tuttle's orbit extends to beyond Uranus. The 1533 stream, shown here as seen in numerical simulations by Jérémie Vaubaillon, has broken into three " mini-trails."

Jenniskens predicts Filament activity to peak at about 50 meteors an hour at 05:30 UT, November 19. Because the Filament is much broader than any individual dust trail, observers around the world should be on the lookout for a few bright meteors between Nov. 17 and 20.

" Will the 1533 peak stand out to visual observers, or will it be lost among the brighter Filament activity?" Cooke wonders. " Forecasters don't know what to expect."

A third encounter draws the curtain on the 2003 Leonids. According to Vaubaillon, Earth grazes two streams, one created in 736 and another in 636, on Nov. 22 and 23, respectively. He expects no significant increase in meteor numbers from these trails.

In the twilight zone

This year's long span of low-level activity is problematic for satellite operators. Predicted intensities for the 2003 peaks are now smaller than the errors between forecast and observed meteor rates during the last few years.

" We're in the twilight zone now," Cooke said.

During past meteor storms, spacecraft controllers took care to " batten down the hatches," reorienting their satellites to point sensitive areas away from the Leonid radiant.

In 1993, controllers directed the Hubble Space Telescope away from the Perseid radiant when astronomers predicted an outburst with a ZHR of 300.

This year Vaubaillon originally forecast a ZHR of 400 for the 1533 peak. But with the expected intensity downgraded, satellite operators are left to wonder how much of a forecast error they can tolerate without exposing their spacecraft to undue risks.

Yet another Leonid peak?

In a paper released earlier this month, Vaubaillon, Lyytinen and other meteor forecasters discussed the different approaches used in creating predictions for 2003. The appeared in WGN, the journal of the International Meteor Organization, and was made freely available by IMO.

One result involved the 1733, or eight-revolution, Leonid stream. Neither Vaubaillon nor David Asher of Armagh Observatory predicted significant activity from this trail, but Lyytinen argued for a ZHR in the " dozens."

On November 11, he announced a corrected interpretation of his results on the MeteorObs e-mail list. " The encounter with this trail was noticed at a quite late state of the WGN paper development. . . . I wanted to include it even though I didn't have time to study it more closely before submitting the paper," he wrote.

To his surprise, Lyytinen had been interpreting the results in one reference frame while the model results had actually been produced in another. Correcting the error changed the timing of the trail encounter by nearly three-quarters of a day, pushing it back to 16:50 UT on Nov. 19.

Although Lyytinen admitted that there was " barely enough computed data" for a detailed analysis, he noted that the stream shows a maximum ZHR " of about 70." He also believes that the stream should be considered a part of the Filament.

If it materializes, Lyytinen's 1733 peak would favor observers in Alaska, the central Pacific, Japan, and northeastern Asia. With less interference from moonlight, this date becomes the best opportunity for Asian and Pacific observers to catch Leonid meteors.

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