All about the 2011 Perseids

The Perseid Meteor Shower will be at its peak on 12 and 13 August. One of the annual treats on the astronomical calendar, this display of celestial pyrotechnics thrills and amazes observers. There are several dozen such shows every year but the Perseids are among the most popular and fascinating to watch.

13 August 2009: A perseid flashes through the sky (Image credit: Andreas M†´ller via wikimedia.org)

Shooting stars or meteors whisper through the EarthòÀÙs upper atmosphere continuously, I dare you to look into a dark clear sky for half an hour and not see at least one sudden streak of light. What you have seen is a tiny (somewhere between the sizes of a sand grain and a grain of rice) piece of rock and metal called a meteoroid ploughing into the atmosphere at very high speed (tens of kilometres per second). The little pieces of rock releases its kinetic energy so rapidly it vaporises on the very edge of space (about 100 km overhead), becoming the short white hot streak we see from far below. Each meteor is the death of a meteroid.

Not all meteors are sporadic events, the regular meteor showers occur when the Earth bulldozes through a great swarm of meteoroids drifting in orbit round the Sun. Each of the meteoroids in these shoals share a common origin, having been ejected from the frigid surface of a comet. Each meteor shower is named for the constellation it seems to originate from.

The first references to the Perseids comes from Chinese records dated to 36 AD (the earliest European mention of them was in 811 AD. The Perseids have been called the “tears of St. Lawrence”, as the shower rains down during the festival of that saint on 10 August.ˆà Their modern name is thanks to the Belgian astronomer, mathematician and criminologist Adolphe Qu†ételet (1796-1874) who noted in 1835 that there was an annual meteor shower occurring in August originating from a point in the constellation Perseus. The source of the shower is Comet Swift-Tuttle, or rather a stream of debris called the Perseid cloud which stretches along the cometòÀÙs orbit. This cloud is a blizzard of dusty particles ejected from the comet as it travels on its 130 year orbit around the Sun (note that the comet is not currently anywhere near Earth). Our planet is simply diving through the long and fuzzy stream of debris left by its passing. The individual gritty bits of comet dust hit Earth’s atmosphere at a closing speed of 210 000 km/h (about 132 000 mph, fast enough to pass between the Earth and Moon in a couple of hours). The Perseids are among the fastest moving meteors.

As they tend to be fast, the Perseids are also bright meteors, leaving lingering trails. They are plentiful too, peaking at 50 or more meteors per hour in the pre-dawn hours. Some years have been much more spectacular: the Perseid display of 1993 saw hourly rates of 200 to 500! High rates were seen again in 1994. This happens as there is an uneven size distribution within the stream of meteoroids. This year is the Perseids are not expected to put on so dazzling a light show, but you never know!

Looking south east from Northern Ireland at 1.30 am (BST) on 13 August 2011. (Image credit: Armagh Planetarium)

Astronomers fondly think of the Perseids as being among the most beautiful meteors. ˆàStart watching for the Perseids in the first week of August. To see the Perseids at their peak, find the darkest site you can and go out in the early morning hours after midnight on 12 or 13 August and look south east. They are at their best in the pre-dawn hours, as the face of the Earth turning towards the Sun catches more meteors as the Earth moves through space. Unfortunately, this year the glare of the full MoonòÀÙs light will spoil the PerseidsòÀÙ display. You can still see Perseid meteors in the weeks leading up to and following the peak, but expect a rate of a dozen per hour at best. Why not dress up warmly and watch the skies for this gentle rain of visitors from a comet? If you see the Perseids, report them to the International Meteor Organization!