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UV Imager Catches Comets Targeting Earth

Photonics Spectra
Jul 1997
R. Winn Hardin

BALTIMORE -- Having a winning design for a satellite imaging device can have rewards beyond the recognition associated with winning a NASA contract. Just ask Iowa University's Louis A. Frank, who designed a UV/VIS imager to study the Earth's aurora, which is about one-millionth the intensity of reflected daylight. He has managed to prove his theory that thousands of water-carrying comets punch through the atmosphere every day.
Frank and John Sigwarth have silenced critics who have plagued them since 1986 when the pair first announced that holes in the Earth's atmosphere were caused by a constant bombardment of 20- to 40-ton comets. As the comets disintegrate 600 to 15,000 miles above the Earth, melting water blocks ultraviolet light. These shadows appear as "holes" in the atmosphere to satellites circling above. The water from these comets could be the source of water on Earth, giving Frank's discovery far-reaching implications on how scientists believe life first began here.
The imaging system aboard NASA's Polar spacecraft uses a UV-sensitive charge-coupled device camera as a guide to two highly sensitive visible cameras. An Earth-sensing camera looks at UV light to determine the planet's position relative to the satellite. The information is used to steer the visible cameras.
Without the UV camera, Polar project scientist Robert Hoffman said the visible detectors would burn out in about a second if they were directly exposed to Earth's daylight side.
The visible cameras have resolutions of 11 and 22 km. Their high-resolution photos were part of the proof Frank needed. In 1986, the scientific community had scoffed at Frank and Sigwarth's theory that comets were behind the black pixels shown in photographs from NASA's Dynamic Explorer 1.
Another attempt to image the comets in 1988 from Sweden's VKING satellite also resulted in incomplete data sets, according to Hoffman.

The older satellites had resolutions on the order of 100 m.
An interference filter developed by Frank and Westford, Mass.-based Barr Associates provided the final comet-detecting tool, Sigwarth said. The filter is optimized for 308.5 nm, the band at which water and related hydroxyl molecules block ultraviolet light.

Cometconstituents
Frank and Sigwarth hope to look at water absorption bands in the 8- to 14-µm region to learn more about the incoming comets. A paper on their research is expected to be published in Geophysical Research Letters in the near future.
"We'd be naive to think he didn't build this to look for his comets," said Hoffman. If Frank had not won the NASA contract, it is possible there would not have been an instrument with UV imaging capabilities and the necessary resolution until the year 2001, when the agency has plans to launch another UV imaging device.



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