Perseid extras in 2004
August 29, 2004: The Perseid outburst happened right on cue. For an update on this story, see the August astro bytes on the Astronomy web site.
With warm August evenings come thoughts of cookouts, camping trips and, for the astronomically inclined, meteors.
Outbursts from the Perseid meteor shower failed to occur in 2000, 2001 and 2002 (chart above), but astronomers don't believe that's the end of the story. Perseid outbursts occurred throughout the 1990s, as shown in this animation. Much of that activity was due to the 1-rev stream, which consists of dust ejected by comet Swift-Tuttle during its 1862 visit. |
Although it has been upstaged in recent years by splendid outbursts from the Leonids of November, the Perseid meteor shower never fails to put on a good show. One might catch a fast, bright Perseid flashing across the sky on any evening from late July to late August. But most of the activity occurs around August 12, when a patient observer might see fifty or more fiery trails each hour.
Colliding with our atmosphere at 132,000 miles an hour, Perseid meteors are among the fastest known. Nearly half of them leave behind glowing smoke trails — called trains *mdash that may remain visible for a minute or more. The meteors seem to radiate from the northern constellation of Perseus, hence the shower's name.
Unfortunately, the 2003 Perseids peak on August 13, when a nearly full moon will wash out much of the show.
But things will be very different in 2004. First, a waning crescent moon will not pose much of a problem to observers. Better still, some astronomers anticipate a brief but intense outburst before the traditional Perseid maximum and possibly enhanced general activity as well.
Three Showers in One
The Perseid shower arises from dust ejected by comet 109P/Swift-Tuttle, which returns to the inner solar system about once every 130 years. When the comet nears the sun, portions of its icy surface turn to gas. This creates dust-laden jets that spray material along the comet's path every time it rounds the sun. We experience the Perseids when Earth passes closest to where the comet's orbit dives below the ecliptic, a point called the descending node.
Each time Swift-Tuttle returns, it takes a slightly different path and creates a new trail of meteoroids. Once ejected by the comet, each particle is subjected to the gravitional forces of the sun and planets. Over time, these forces cause the particles to migrate away from their original orbit, gradually erasing a stream's individual identity by mixing it with others.
We can think of the Perseid shower as having three distinct components. The first is a diffuse cloud that gives rise to background activity over the 40 or so days the shower lasts. Within this lies a dense core of material that creates the August 12 maximum. The third component " turned on" on in 1991 — the first year in which a Perseid outburst was unambiguously detected. It primarily consists of a narrow dust stream ejected by the comet in 1862. Because these meteoroids are completing their first circuit around the sun, scientists say they comprise the " 1 rev" (revolution) stream. Other streams produce smaller peaks in other years.
Perseids: The Animation
Jupiter's pull brings the 1862 Perseid stream inward, where it can intercept Earth (blue line) and create outbursts note the 12-year cycle. Satellites at the L2 position (purple line) may be more vulnerable to Perseid impacts. |
The above animation illustrates several of the most intriguing Perseid features gleaned from detailed computer models.
Outbursts in 1991-1994. Models suggest that these events arose primarily from the 1862 stream, with some additional contribution from material ejected in 1610. Every twelve years, when the 1862 meteoroids make a close approach to Jupiter, the giant planet perturbs them into orbits that will intercept the Earth.
Changing location of the outburst peak. The 1862 outburst maxima hover near the location of Swift-Tuttle's descending node that year (dashed blue line). But after 1994 the outbursts systematically moved closer to the traditional peak. The reason is that the dust streams Earth encountered in those years were dominated by older populations of meteoroids. In 1989 and 1990, we ran through debris from 1610 and 1737 from 1991 through 1994, we encountered mostly 1862 material and from 1995 through 1997, we ran through streams ejected in 1479 and 1079.
Outbursts and the proximity of the 1862 stream. The center of each gray bar (inset and right) indicates the center of the 1862 dust stream each year. A 1991 approach to Jupiter resulted in significant outbursts for years. The stream again encounters Jupiter in 2003, so astronomers expect similar, if less intense, outbursts from 2004 to 2006. Specifically, Finnish researcher Esko Lyytinen predicts an outburst of several hundred meteors per hour at 21h UT on August 11, 2004. Europe and Asia are turned toward the radiant, but some enhanced activity may be observable in the U.S.
The 1862 stream and L2. The 1-rev stream spends more time near the L2 Lagrangian point (purple line on the inset) than it does near the Earth. L2 is a useful, quasi-stable location about 932,000 miles farther from sun than the Earth. It's also the future home of the James Webb Space Telescope, to be launched in 2011. Just how much protection it may need from Perseid meteoroids remains an open question. The first satellite to operate at L2, WMAP, has been stationed there since 2001 and has not experienced problems.
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