The Delta Aquarid meteor shower is bringing its light show to Earth this weekend (July 28 and 29), courtesy of two (or one, or three) comets that regularly orbit the sun and shed the astronomical ammunition used in this annual atmospheric strafing run.
As the Earth's orbit brings it through the path of debris ejected from comets in their long and lonely sweeps around our sun, the usually tiny objects called meteoroids (ranging in comparable size from dust motes to rice grains to peas) slam into our atmosphere at enormous speeds.
Moving at between 40,000 and 250,000 kilometres per hour, the debris (called meteors after they enter the atmosphere) vaporizes when it comes into contact with air molecules, sometimes leaving a temporary, seconds-long trail of ionized gas about 100 kilometres above the Earth.
(It should be noted that the heaviest recorded meteorite—the name given to what remains of meteoroids that survive the fiery friction—was 60 tons, found in South Africa in 1920. Comet debris, though, would probably never get that large, unless a comet broke up entirely.)
The Delta Aquarid debris hits our airspace at about 150,000 km/h. Ejected and boiled off particles from two comets named Marsden and Kracht are thought to provide the bulk of the 10 to 20 meteor sightings per hour normally associated with the annual spectacle (with about five to 10 percent of these displaying a bright, glowing trail).
Lately, though, some scientists are starting to think that cosmic debris fron Comet 96P Machholz, only discovered in 1986, may be responsible for the Aquarids. Macholz's relatively short orbital path swings it out past Jupiter's orbit before it heads back in to circle the sun about every five years.
The Delta Aquarids—so named because the meteors seem to radiate from the direction of the constellation Aquarius, though they can be seen in any quadrant of the predawn sky (about 3 a.m. is the best time to view them)—are up against some competition for viewing opportunities this year, though. In the first place, they are relatively dim in the Northern Hemisphere, compared to southern latitudes, and they are up against a full moon, which will mean viewers should try to get outside of cities to have the best chance of seeing a few per hour.
The good news is that the shower chugs along for about five weeks, so as the moon wanes and its interfering light dims heading into early August, your chances of catching some fireworks increase, right up until (and possibly spilling over into) the often spectacular annual Perseid shower (which should be impressive this year with the dark sky).
Happy viewing, and good luck.