Perseids Provide Practice for September Shower

Around the start of our year count, 2000 years ago, comet Kiess passed the sun and ejected a cloud of dust. Kiess completed one orbit in 1911 when it was discovered by Lick Observatory post-doc Carl Kiess. The dust took longer to return, and formed a continuous stream of dust particles that has been passing just outside Earth orbit ever since.

On Sept. 1, 2007, that trail of dust from Roman times will wander in the Earth's path again, causing an extremely rare meteor shower during the short time it takes the Earth to travel through the stream of dust. The meteors radiate from the constellation of Auriga, and are called Aurigids. Only three people alive today are known to have seen this shower before in 1935, 1986, and 1994. After the 2007 encounter, the Aurigids will not be seen again in our lifetimes.

We hope that the public will attempt to take digital photos and camcorder movies of this rare Aurigid shower of "shooting stars" and thus contribute to the study of comet Kiess.

The best time to practice is during the upcoming Perseid meteor shower. At their peak, the Perseids are nearly as good as the Aurigids, but the Perseids are much easier to observe. And, unlike the Aurigids, the Perseids will be great all night long and appear in a dark sky on Saturday night August 11 and Sunday night August 12.

Best Perseid rates for U.S. observers are in the early morning hours of Sunday. That night, we will deploy on a privately owned Gulfstream GV aircraft to observe the Perseid shower from altitude, to practise observing for the Aurigid shower later that month. Twelve scientists are participating in the test flight, with a range of cameras and video camcorders. Each camera uniquely suited to measure the rate of the meteors, their colors, how they break during impact, and how deep they penetrate in the atmosphere.

The Aurigid shower will last only an hour and a half, with a bright Moon in the sky. The Moon is not expected to dim the spectacle much, however, because most Aurigids seen in the past were relatively bright -2 to +3 magnitude meteors. My colleague, Jeremie Vaubaillon of the California Institute of Technology, Pasadena, California, and I have performed detailed predictions of the dust stream's orbital evolution. The August 7 issue of the journal EOS, Transactions of the AGU, gives details of the encounter.

We predict that the shower will be visible by the naked eye from the western United States, especially in California, Hawaii, Alaska, other western states and from Mexico and the western provinces of Canada. Prime viewing time will be on Saturday, Sept. 1, 2007, a half hour on either side of 4:36 a.m. PDT. The whole event will last no longer than one-and-a-half hours. Twenty-five minutes long rates will be above half the peak rate.

Sunlight has pushed the comet's ejected particles into wider orbits around the sun in a thin stream just outside of Earth's orbit. On occasion, the combined gravity of the solar system's planets moves this dust trail into Earth's path. Only when Earth and dust trail collide do we see this meteor shower.

As they collide with the atmosphere, the dust grains of Kiess begin to vaporize at around 80 miles (130 kilometers) altitude, with the bigger ones penetrating down to as low as 50 miles (80 kilometers), before they are completely stopped. This fiery process creates a meteor.

I am leading a team of scientists and astronomers to study the Aurigid shower aboard two aircraft. The airplanes will take off late Friday night, August 31, 2007, from Moffett Field at NASA Ames, and will carry researchers from NASA, the SETI Institute, Utah State University and other organizations to a location high above the Pacific Ocean to view the meteor show in the wee hours Saturday morning, September 1.

The primary goal of the mission is to count the meteors efficiently over the large area visible from altitude and measure the exact duration and peak time of the shower. The participating researchers will also examine the colors and way of breaking of the meteors to learn about the materials that formed the solar system.

Not only is the shower rarely seen, the Aurigid meteors also may be very unusual. Some could be bits of the comet's pristine crust. Comet Kiess returned from the Oort cloud of comets on the outskirts of the solar system only in recent history. Before that, Kiess spent 4.5 billion years in the Oort cloud, where cosmic rays baked its crust over the age of the solar system. Kiess could have shed some of this pristine crust 2000 years ago. Comets that return more frequently to the sun have long lost this pristine crust.

If so, the meteors are expected to penetrate 5 km deeper than normal in the atmosphere and lack a very specific color of yellow light from the element sodium. Such unusual meteors were seen once before during the alpha-Monocerotid meteor shower in 1995, caused by an unknown long-period comet.

Tips for setting up a digital camera or camcorder to take images of meteors

Meteors, or "shooting stars," look like brief flashes of fireworks or sparks flying from a distant campfire. Observers with digital cameras and camcorders can photograph these meteor streaks by pointing their cameras anywhere in the sky away from the moon.

Astronomers suggest that photographers go to remote public parks or other safe locations that are far away from the haze of cities. Dust in the atmosphere in metropolitan areas will scatter moonlight and make the sky too bright to photograph or see the meteors well.

People who wish to contribute digital images to scientists should first set the camera clock to the correct time, precise to within 1 second. Use the "clock set" option. To get the correct time, use the local telephone service. In the San Francisco Bay Area, for example, one can dial "POPCORN". Photographers should place their cameras on tripods and use a "night," "bulb" or similar settings on their digital cameras to shoot pictures of the sky for periods of 10 seconds. If you are lucky, a meteor will streak by at the right time. Set the camera's light sensitivity to ISO 1600.

Choose a fairly small field of view that is no larger than the square of (the constellation) Pegasus. The meteors are not bright enough to be captured efficiently with a wider field of view. Take many successive 10-second duration exposures during the shower. One more tip: Photographers should not re-point their cameras once they are set to take images, in order to measure the rate of the meteors. Later, watch the images to find the meteors and provide a list of times. Also, record your observing location on a map.

Do not alter the digital images because we will use photo-editing programs to analyze the different colors in the images to learn about the meteors' composition and way of fragmentation.

People interested in videotaping the meteors also should first set their camcorder clocks to the correct time. Set the camera so that the time is recorded on the video picture (date not needed).

Mount the camcorder on a tripod, and then point to a region in the sky with many bright stars. Zoom in enough to see those bright stars in the video. Continue videotaping for the duration of the shower. Do not move the camera during the shower. Later, watch the video to find the meteors and provide a list of times, and record your location on a map.

Those people who would like to contribute their images and other observations to researchers should send them by e-mail to: pjenniskens@seti.org.

Others interested in your observations include the American Meteor Society and the International Meteor Organization, who specialize in amateur observations of meteor showers. The public may also upload their images and data to the Aurigid Multi-Instrument Aircraft Campaign Web site at: http://aurigid.seti.org

For images of past meteor observing campaigns, visit: http://leonid.arc.nasa.gov

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• Image Gallery: 2005 Perseid Meteor Shower