In the invisible region of the electromagnetic spectrum, just beyond the range of our vision, is the (near) ultraviolet (UV) light. Much of this invisible ultraviolet light comes to us from the Sun and is partly responsible for sunburns if we stay outside too long. But at night, it turns out there are other sources of ultraviolet light. The molecules and atoms in the atmosphere high above us are constantly colliding with each other and with energetic particles that enter the atmosphere . The collisions excite the atmospheric oxygen and nitrogen compounds; as they lose this energy, they create ultraviolet light.

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.

NASA's NIGHTGLOW mission will carry out measurements of this invisible light, also called "nightglow", on a long duration balloon flight. Surprisingly, there are not very many measurements of the near UV nightglow, and no long-term studies have been undertaken. Most researchers have concentrated on measuring the dayglow rather than the nightglow, and many instruments lack the sensitivity needed to see the nightglow.

Flying at an altitude of about 30 km (100,000 ft), NIGHTGLOW will circumnavigate the world. During the local night, NIGHTGLOW will use its three onboard telescopes to measure the amount of ultraviolet light produced in the atmosphere, as well as that produced by humans on the ground.

In addition, NIGHTGLOW has two cloud sensing systems onboard to monitor the amount of cloud cover beneath the balloon. One system measures the clouds' temperatures with an infrared camera, and the other system measures the clouds' reflecting properties with a blue laser.

Combining the knowledge of the cloud cover and the light production, NIGHTGLOW will provide valuable data for scientists who are interested in measuring the very highest energy cosmic rays, particles of immense energy that come to us from outside our own Milky Way galaxy. These particles strike our atmosphere and create flashes of invisible ultraviolet light. NIGHTGLOW will help in the identification of these particle flashes, distinguishing them from the background ultraviolet light.

About the NIGHTGLOW instrument...