Cosmic Light Pollution

see caption July 10, 2000 -- 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 on July 6, 2000, 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.

The balloon carried an experiment called NIGHTGLOW, a collaboration between NASA's 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.

Sign up for EXPRESS SCIENCE NEWS delivery

Above: The NIGHTGLOW balloon is pictured just after launch (left) beside a diagram showing the trajectory of the July 6th test flight (right).

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 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 2001 from Alice Springs in Australia.

Send this page to a friend!

Below: This composite of more than 200 visible light images collected by Earth-orbiting satellites shows what the United States looks like at night. Bright spots correspond to urban centers where stars are hard to see through the glare of city lights. Not all sky glow is caused by urban light pollution, however. Researchers from NASA and the University of Utah are conducting sensitive experiments to study background radiation from sources including bioluminescent squid and high-energy cosmic rays.

"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."

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."

Right: At energies greater than a few times 10¹⁹ eV, the number of cosmic rays is expected to decline sharply because they interact with the omnipresent 2.7 K cosmic microwave background. Observations show an initial drop off (known as the ankle), but then the spectrum becomes more shallow, meaning there are a lot more particles at these energies than was expected. These are subatomic particles with more kinetic energy than a major league fastball! Where do these ultra energetic particles come from? It's a mystery that NIGHTGLOW measurements will help solve. [more information from Goddard Space Flight Center]

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 given time is less than a square mile.

Above: The spectrum of ultraviolet nightglow recorded during a rocket flight in 1962. (J. P. Hennes, 1966, JGR 71, 763.) The data has been adjusted to zenith and binned in 15 Angstrom bins. The total integrated emission from approximately 3000 to 4000 Angstroms is ~400 Rayleighs. [more information]

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.

Web Links

IT MUST HAVE BEEN NIGHTGLOW, THE BALLOON - NASA Goddard Press Release

The NIGHTGLOW home page - learn more about the experiment to characterize UV emissions from the sky at night.

NASA's Orbiting Wide-angle Light collector (OWL) - this mission will image cosmic ray air showers (from particles with energies greater than 10^19 eV). NIGHTGLOW will measure the component of the UV light (from sources other than the ultra-high energy cosmic ray showers) which constitutes a background for OWL.

Cosmic Rays: What Are They? -- from Goddard's "Imagine the Universe" web site.

Cosmic Light Pollution

A series of unmanned balloon flights will measure the subtle ultraviolet glow of the night sky and help unravel one of the most perplexing mysteries of astrophysics.