The Earth's Orbit periodically intercepts the path a comet has travelled. Debris left over from the comet's last passage may enter the Earth's atmosphere, creating a meteor shower. Such concentrations of meteors are named for the name of the star field from which they appear to radiate.
Leonid Meteor Shower, 1966
The Leonid Meteor Shower of 1966 was one of the most active in history, with up to 40 meteors visible in a second.
21k GIF - 22k JPEG --Photo courtesy of Dennis Milon.
This photo is a 3 1/2 minute time exposure taken on November 17, 1966, at the height of the Leonid Meteor Shower. Because of the long exposure, stars appear as short streaks that extend from left to right across the photo. The Leonids are the radiating streaks that appear to originate at the constellation Leo. Two bright meteors in the center of the photo do not show streaks because they are heading directly towards the camera.
138k GIF - 59k JPEG
Photo courtesy of Dennis Milon.
The stars here have colored streaks, whereas the meteors that cross the star trails show areas of different brightness.
Pieces of Comets
In the 1950s, Dr. Fred L. Whipple predicted that very small particles from comets could survive entry into the Earth's atmosphere. These fragments are less than 2/1000ths of an inch across, and gradually descend through the atmosphere to land on Earth. Such particles were first collected using high altitude balloons, but because the particles are so rare in the upper atmosphere, only 10 samples were collected.
Beginning in the 1970s, Lockheed U-2 aircraft were used to collect samples of the upper atmosphere. Plates coated with silicone oil were mounted in airtight containers on the wing tips, and were exposed to the air stream when the plane reached a cruising altitude of 60,000 feet.
257k GIF - 57k JPEG --Photos courtesy of Donald Brownlee, Univ. of Washington.
Because the particles are so small, their surfaces can only be studied in detail using electron microscopy. This scanning electron microscope photograph shows a grain about 10 microns (.0004 in) wide, which has a very open structure, presumably once filled by icy comet material. As these particles are ejected from comets, the ice immediately sublimes (turns from solid to gas), leaving a framework of dust similar in comparison to ancient carbon-rich meteorites.
LINKS to more information:
- Comets and Meteor Showers Page - descriptive catalog on Meteor Showers.
- STARDUST - mission to collect cometary dust from Comet Wild-2 in 2003.
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