A midnight balloon ride high above the farms and fields of the American West to view the gentle glow of the Earth at night may sound rather romantic. The ride from Palestine, Texas, July 5, however, was pure science, the first of a series of unmanned NASA scientific balloon flights that will measure the ultraviolet (UV) light illuminating the night sky and that may ultimately help solve the mystery of high-energy cosmic rays, which seem to travel through space further than physicists deem possible.

Project logo. Photo: NASA

The balloon carried an experiment called NIGHTGLOW, a collaboration between NASA Goddard Space Flight Center (Greenbelt, Md.) and the University of Utah, Salt Lake City. NIGHTGLOW is designed to detect background radiation produced by a variety of sources, including moonlight and starlight, the interaction of oxygen and nitrogen molecules in the atmosphere, human-made lighting, and even the bioluminescence of squid and other animals.

NIGHTGLOW flew for nearly 9 hours at a height of 100,000 feet (30,480 meters) beneath a 4 million cubic foot (113,000 cubic meter) scientific balloon, landing at 3:55 a.m. local time. While this maiden flight for NIGHTGLOW collected a modest amount of data about the UV background, the goal was to test the mechanics of the instrumentation in preparation for a global, long-duration flight in February from Alice Springs in Australia.

"The flight went off without a hitch, and all the instruments tested out fine," said Dr. Louis Barbier, the NIGHTGLOW principal investigator and an astrophysicist at Goddard. "We launched under a warm, cloudless sky heading into a waxing, crescent moon hanging in the western sky, a perfect night for observing the Earth's UV glow. The payload flew west and landed among rattlesnakes near the small Texas town of Stiles."

A map shows the balloon's trek. Photo: NASA

There have not been many measurements of the UV nightglow background, Barbier said. Most scientists have concentrated on measuring the dayglow instead. Nightglow is less intense than dayglow, and sensitive instruments are needed to accurately measure it. Interestingly, understanding background radiation levels of lower-energy UV is a key component in finding the origin of the mysterious high-energy cosmic rays.

Cosmic rays are atomic particles flying nearly at light speed, constantly bombarding the Earth. Lower-energy particles come from the Sun. Mid- and higher-energy cosmic rays, such as protons and heavier atomic nuclei, may be produced in stellar explosions. What is most perplexing are the highest-energy cosmic rays. Not only is their origin unknown, these particles possess an energy level that seems implausible.

"These highest-energy cosmic rays are a Catch-22," said Dr. Robert Streitmatter, a Goddard astrophysicist who works on NIGHTGLOW. "Anything that energetic had to have come from within 150 million light-years of Earth, because anything traveling farther would have lost its energy during the long trip. Yet there are no obvious sources within 150 million light years that could produce a particle this energetic."

When these highest-energy cosmic rays strike the Earth's atmosphere, they produce low-energy UV radiation in the NIGHTGLOW range. A proposed NASA satellite mission called OWL (Orbiting Wide-angle Light-collectors) would detect this radiation from a low-earth orbit and help us understand their origins. The highest-energy cosmic rays are rare, and a device such as OWL is needed to search for them simultaneously over wide stretches of the atmosphere, as wide as 400,000 square miles.

NIGHTGLOW will lay the groundwork for OWL by precisely measuring the background UV radiation at nighttime. When an energetic cosmic ray strikes, OWL would be able to differentiate between the cosmic ray-induced UV radiation and ordinary background radiation. NIGHTGLOW itself could not search for such rare cosmic rays because its field of view at any give time is less than a square mile.

A composite of over 200 images made by satellites orbiting the Earth showing the United States at night. The scans were made by the U.S. Air Force's Defense Meteorological Satellite Program Operational Linescan System. Man-made lights also emit in the near UV, and cities can be a large source of "light pollution" in this range just as in the visible. The image of the United States at night in visible light demonstrates this problem. Photo: USAF

The NIGHTGLOW instrument comprises three telescopes, each with a 14-inch (355 mm) diameter mirror and a 28-inch (711 mm) focal length instrument with two photomultiplier tubes (PMTs). A PMT is a very sensitive device for converting light into an electronic signal. One of the three telescopes looks down at all times while the other two rotate to view the UV glow at higher altitudes, above 55 miles (88.5 kilometers).

Aside from cosmic-ray work, NIGHTGLOW UV data is also valuable for meteorological studies about wind and lightning.