Thursday, July 28, 2005

Comet Dust Drives Summer Shower

Comet Dust Drives Summer Shower
by Dave Adalian

(July 28, 2005) Anyone who attempted to observe Comet Tempel 1 earlier this month when NASA’s Deep Impact probe slammed it with an 814-pound copper bullet moving at 6.3 miles a second knows first-hand comets are an unpredictable lot.

Hopes were high this visitor from the outer reaches of the Solar System would brighten from an almost undetectable smudge in small telescopes to a naked-eye delight. The mission succeeded, but the brightening didn’t happen.

Fortunately, next month holds good promise of a sort for would-be comet observers.

August, as seasoned meteor observers know, is time for the year’s most reliable meteor sky show. The Perseids began lighting the night sky early this week and continue until late August with the peak expected to produce between 40-80 meteors an hour on the night of Aug. 11-12 and perhaps again on Aug. 12-13. Meteors from other minor showers will add 15-20 an hour.

Perseus, the constellation from which the Perseids stream, rises in the northeast around 10 o’clock. The best meteor viewing is after moonset--around 11 the first night and nearer midnight the next--until dawn. The darker sky you find yourself under, the better the viewing will be.

What does this have to do with comets? The Perseids are tiny pieces of Comet Swift-Tuttle.

Meteors aren’t really stars falling from the sky. Instead, they are bits that have broken off of other bodies--usually ice and dust the consistency of ash or more rarely solid rock--colliding with Earth’s atmosphere at several thousand miles an hour, ionizing the air and producing that magical glow.

Comet Swift-Tuttle is a periodic comet, returning again and again to swing around the Sun before heading off to the dark and frozen outskirts of our star system on a 135-year circuit. While Swift-Tuttle was first discovered in 1862, the shower it produces is known throughout history. During the 11th century, astronomers in China recorded an intense display, and Perseids are sometimes called St. Lawrence’s Tears to honor his martyrdom in coincidence with the shower of 258.

Swift-Tuttle returned most recently in 1992, freshening its trail of debris significantly as the Sun’s gravity drew the comet in, warming its surface and causing it to shed potential meteors. Therein lies this comet’s uncertainty. This year Earth enters the 1992 debris stream for the first time, and the usual rates of 40-80 meteors an hour on nights near the shower’s peak could be far higher than normal.

Or maybe not... We’ll just have to see.


This column originally appeared in the Visalia (Calif.) Times-Delta on July 28, 2005.