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Tuesday, August 03, 2004

2004: An Excellent Year for the Perseids part 2:Storm Watch for 2004

A bright Perseid streaks just below the Andromeda Galaxy, M31, during the 2002 shower. Joe Orman took this 5-minute exposure through a 40-millimeter lens at f/2.8 on Ektachrome P1600 slide film


The source of the Perseid shower is comet 109P/Swift-Tuttle, which orbits the Sun in a long, roughly 130-year ellipse. The comet sheds bits of its material each time it returns near the Sun. This debris keeps traveling along near the comet's orbital path, creating a sparse "river of rubble" in space.
The comet swept past the Sun during the summer of 1862 and made a return appearance in December 1992. By no coincidence, in the early 1990s the Perseids performed spectacularly — displaying a new, additional, much briefer peak with outbursts of up to several hundred meteors per hour in 1991, 1992, and 1993. This new peak was slightly offset in time from the weaker, longer-lasting "traditional" peak of Perseid activity.
In the years after the comet's departure, the strong, new Perseid peak decreased. For the last three years it has failed to appear at all. Accordingly, Perseid forecasts have generally returned to normal — with the maximum lasting about 12 hours centered on the time when the Earth is at the place in its orbit marked by the Sun's ecliptic longitude being 140.0° (equinox 2000.0).
In 2004 that time is 11h Universal Time on August 12th (7 a.m. Eastern Daylight Time, 4 a.m. Pacific Daylight Time). So North America, especially the West and Hawaii, is optimally positioned to catch the peak.
The consensus among meteor astronomers is that the particles of the new, 1990s peak were probably shed by Comet Swift-Tuttle during its swing by the Sun in 1862, and that as far as this rich filament of meteoroids is concerned, the show is over. But now, astronomers Esko Lyytinen of Finland and Tom Van Flandern of Washington, DC, suggest that the new Perseid peak could stage a return appearance in 2004. They predict that this year, the one-revolution rubble trail released in 1862 will be passing just 0.00132 astronomical unit (about 200,000 km) inside Earth's orbit at the time of solar longitude 139.440°. That's 20:50 UT on August 11th, favoring meteor watchers in Eastern Europe and eastern North Africa eastward to central Russia, India, and western China.
Lyytinen and Van Flandern base their prediction on the same techniques they used (rather successfully) in forecasting the intensities of the Leonid showers from 1999 through 2002. "It is very uncertain what kind of a shower this will give," notes Lyytinen. "If the numbers of particles released for the 1862 trail were the same as for the Leonid parent comet, I would say a ZHR [zenith hourly rate] of around 100 is possible. But," he adds, "we must also consider that the Perseid parent comet is a lot bigger than the Leonid comet, so there may even be a slight chance of storm-level activity," meaning a ZHR of 1,000 or more. Lyytinen and Van Flandern believe that most of these meteors could be rather faint, 3rd and 4th magnitude. If an outburst happens, it is likely to last no more than about 40 minutes.
Last, because recent perturbations by Jupiter are directing old Perseid meteoroids about 0.01 a.u. closer to the Sun, the core of the broader, "traditional" stream may be shifted closer to Earth،s orbit, resulting in a stronger-than-average annual shower. In fact, Lyytinen and Van Flandern suspect that the Perseids tend to put on stronger-than-average displays at 12-year intervals (12 years being Jupiter's orbital period), and that 2004 is one of the favored years. The 1992 display might not qualify for comparison because the parent comet was nearby — but 1980 brought an excellent show punctuated by many fireballs, and 1968 was rated quite good despite bright moonlight.
Will we be similarly rewarded this year? If so, the enhancement would come into play only around the date of the normal maximum, perhaps even appearing as a new, additional peak.
So there could hardly be more incentive for meteor watchers to be out in force this month. One thing is certain: the Moon will be dark and the meteors bright.

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