Perseids Provide Practice for September Shower

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

OnSept. 1, 2007, that trail of dust from Roman times will wander in the Earth'spath again, causing an extremely raremeteor shower during the short time ittakes the Earth to travel through the stream of dust. The meteors radiate fromthe constellation of Auriga, and are called Aurigids. Only three people alivetoday are known to have seen this shower before in 1935, 1986, and 1994. Afterthe 2007 encounter, the Aurigids will not be seen again in our lifetimes.

Wehope that the public will attempt to take digital photos and camcorder moviesof this rare Aurigid shower of "shooting stars" and thus contributeto the study of comet Kiess.

The best timeto practice is during the upcoming Perseid meteor shower. At their peak, thePerseids are nearly as good as the Aurigids, but the Perseids are much easierto observe. And, unlike the Aurigids, the Perseids will be great all night longand appear in a dark sky on Saturday night August 11 and Sunday night August12.

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

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

Wepredict 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, ahalf hour on either side of 4:36 a.m. PDT. The whole event will last no longerthan one-and-a-half hours. Twenty-five minutes long rates will be above halfthe peak rate.

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

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

Iam leading a team of scientists and astronomers to study the Aurigid showeraboard 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 locationhigh above the Pacific Ocean to view the meteor show in the wee hours Saturdaymorning, September 1.

Theprimary goal of the mission is to count the meteors efficiently over the largearea visible from altitude and measure the exact duration and peak time of theshower. The participating researchers will also examine the colors and way ofbreaking of the meteors to learn about the materials that formed the solarsystem.

Notonly is the shower rarely seen, the Aurigid meteors also may be very unusual. Somecould 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, wherecosmic rays baked its crust over the age of the solar system. Kiess could haveshed some of this pristine crust 2000 years ago. Comets that return morefrequently to the sun have long lost this pristine crust.

Ifso, the meteors are expected to penetrate 5 km deeper than normal in theatmosphere and lack a very specific color of yellow light from the elementsodium. Such unusual meteors were seen once before during the alpha-Monocerotidmeteor shower in 1995, caused by an unknown long-period comet.

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

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

Astronomerssuggest that photographers go to remote public parks or other safe locationsthat are far away from the haze of cities. Dust in the atmosphere inmetropolitan areas will scatter moonlight and make the sky too bright tophotograph or see the meteors well.

Peoplewho wish to contribute digital images to scientists should first set the cameraclock 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 SanFrancisco 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 picturesof the sky for periods of 10 seconds. If you are lucky, a meteor will streak byat the right time. Set the camera's light sensitivity to ISO 1600.

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

Donot alter the digital images because we will use photo-editing programs toanalyze the different colors in the images to learn about the meteors' compositionand way of fragmentation.

Peopleinterested in videotaping the meteors also should first set their camcorderclocks to the correct time. Set the camera so that the time is recorded on thevideo picture (date not needed).

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

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

Othersinterested in your observations include the American Meteor Society and theInternational Meteor Organization, who specialize in amateur observations ofmeteor showers. The public may also upload their images and data to the AurigidMulti-Instrument Aircraft Campaign Web site at: http://aurigid.seti.org

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

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