A Surprise Meteor Shower?

Amateur observers are determined to be ready when the Aurigid meteor shower strikes Earth on the morning of September 1st.

Skywatchers were out in force for the familiar Perseid meteor shower peaking in mid-August at the dark of the Moon. So why bother with a shower that almost nobody has heard of coming two weeks later in bright moonlight?

Because the Aurigids are this summer’s skywatching wildcard.

The Perseids are the "Old Faithful" of meteor displays, whereas most years the Aurigids produce little or nothing. But three times in the last century, skywatchers were surprised by a short-lived burst of bright meteors emanating from the direction of the constellation Auriga early on the morning of September 1st. Circumstances appear excellent for a repeat in 2007, with a small chance that the shower could turn extraordinary. So well-positioned meteor observers -- those in far-western North America and Hawaii -- are going on high alert.

The first known Aurigid outburst came in 1935, when two German astronomers witnessed an unexpected and moderately strong meteor display. It appeared to radiate from a spot near the star Beta Aurigae, not far from Capella. Orbital expert Vladimir Guth immediately suggested that the meteors came from Comet Kiess (C/1911 N1), which followed a similar path through space and last came through the inner solar system in September 1911. The comet has an orbital period of approximately 2,000 years.

Peter Jenniskens (SETI Institute) believes that the Aurigids are likely to come to life again this September 1st. That forecast is based on calculations by himself and Jérémie Vaubaillon of Caltech, and on earlier work done in collaboration with meteor astronomer Esko Lyytinen of Finland. They determined that Comet Kiess last swept around the Sun in roughly 82 BC. At that time the comet should have released a trail of fine rubble following along its path. Some of this debris should only now be completing its first trip around the Sun, decades behind the comet itself. Jenniskens showed that Aurigid outbursts in 1935, 1986, and 1994 all arose when Earth passed through this one lengthy debris trail. And now the stage is set again.

When and Where

On Saturday morning, September 1st, within about 20 minutes of 11:37 UT (4:37 a.m. PDT), Jenniskens and Vaubaillon expect Earth to pass smack through this rubble trail from Comet Kiess. The timing favors far-western North America, as shown on the map. From there Auriga will be very high in the northeast (roughly 60° or 70° up) in a dark predawn sky — ideal circumstances.

Unfortunately, the rest of North America will be in bright twilight or daylight. As with the previous outbursts, the upcoming display should be short-lived, probably lasting no more than an hour or so.

"What makes this shower so special," notes Jenniskens, "is the opportunity to see bits and pieces of the comet’s original crust." Long-period comets have just recently returned from cold storage in the Oort Cloud and are still covered by a crust that resulted from 4.5 billion years of exposure to cosmic rays. When the comet returns to the inner solar system, that crust is crumbled and creates peculiar meteors. According to Jenniskens, "There will be no other chance to study long-period-comet dust trails in the next three decades."

A drawback will be the bright waning gibbous Moon, 84% illuminated in nearby Aries. But because the Aurigids ram into our atmosphere exceptionally fast (41 miles per second), and because the particles are predicted to be relatively large, any shower should be rich in bright meteors, with many in the range of magnitude +1 to 0. "So," says Jenniskens, "the Moon probably won’t dim much of the display."

As to how many you might see, the safest forecast is perhaps 20 to 30 per hour. But Lyytinen is predicting many more, and he also expects the peak to arrive a little earlier than Jenniskens and Vaubaillon do: around 11:20 UT (4:20 a.m. PDT).

Author Joe Rao has been Sky & Telescope’s chief meteor forecaster since 1993. SkyandTelescope.com offers great tips on how to observe meteors. For the complete version of this article, pick up Sky & Telescope's September issue or see the online version at SkyandTelescope.com.

© 2007 Reprinted with permission from Sky Publishing Corp.