Washington — Radiation belts, rings of highly energized particles surrounding Earth, are the target of an unmanned NASA mission launching August 23.

One of the first U.S. discoveries of the Space Age, radiation belts are influenced by solar activity and the space weather the sun creates. Pilots and astronauts in flight risk exposure to dangerous levels of radiation during periods of volatile activity, and the belts also have the potential to play havoc with satellites and communication systems.

“We still don’t understand how the belts behave” more than 50 years after they were first recognized and described, said Lika Guhathakurta, a NASA program scientist. “Nor do we have the ability to make any predictions, which is really very important.”

The space agency will send two spacecraft — Radiation Belt Storm Probes (RBSP) — into the harsh environment of near-Earth space to expand scientific understanding of the radiation belts, known as the Van Allen belts after their discoverer. They are influenced by solar winds and space weather, which can influence human activities as much as surface weather like cyclones and hurricanes. We need to learn more about how the radiation belts behave in the onslaught of solar cycles, Guhathakurta said.

“So that we can be better prepared for making better satellite designs, for astronauts, communications, all of this,” said Guhathakurta, who works in the NASA program Living with a Star, which focuses on increasing knowledge about the behavior of the sun, its influence on the solar system and its effects on humans.

The radiation belts make a double circle around the Earth. The first begins about 1,600 kilometers above the surface and extends to almost 13,000 kilometers. The other belt begins at just more than 15,000 kilometers and stretches out 24,000 kilometers in space.

Active periods of solar weather bring violent, seething eruptions of solar material. This weather affects the radiation belts, causing them to expand, coming as close as 207 kilometers to the Earth’s surface, said Mona Kessel, RBSP program scientist.

“That means that the International Space Station and low-orbiting satellites pass through that region,” she said, “so they are susceptible to the energetic particles that are trapped there.” Communication satellites that maintain a geosynchronous orbit (moving with the rotation of the Earth, stationary above the same point ), also hold positions in space where radiation belts might engulf them.

The two probes to be launched will monitor the dynamic and volatile space weather, downloading data on conditions to ground stations around the world, Kessel said.

Meteorologists have learned to predict Earth’s surface weather with recognition that a given set of atmospheric conditions will follow a pattern in producing certain results in weather. Radiation belts don’t behave that way — the sun stirs up geomagnetic storms that send bursts of energy surging through space, according to Barry Mauk, a scientist with the Johns Hopkins University Applied Physics Laboratory (APL) and a partner in the RBSP project.

“Sometimes it’s big, sometimes it’s small, and we just do not understand why that occurs,” Mauk said.

Another goal of the RBSP mission is to get a better understanding of radiation in space. Mauk said that’s important because radiation is found throughout our solar system and beyond. Earth’s radiation, he said, will serve as a natural laboratory “for understanding how radiation is created in space, how it varies in space and why the creation of radiation in space is so common.”

The radiation probes will move through the belts in elliptical orbits around Earth, taking separate observations of the same phenomenon. Collection of separate observations will allow data comparison, leading to a better understanding of how factors of space and time may influence the observed activity.

A challenge is the difficulty of collecting data in the midst of a radiation field, which can skew or contaminate measurements. The RBSP instruments will be measuring the energy levels of particles in the belts, along with ionized gases found in the belt environment. The probes will also gather data about the magnetic and electric fields in the radiation belts that control the behavior of the energized particles.

The RBSP's two-year mission begins August 23.