NASA launched its Interstellar Boundary Explorer, or Ibex, into orbit today for an examination of the solar winds that shield the Earth from harmful cosmic rays.

Ibex will give scientists a better understanding of how the solar wind -- made of magnetically charged particles -- interacts with the larger galaxy. The winds have fallen to the weakest level in half a century.

Much as the Earth's magnetic field repels cosmic rays, the solar wind protects the entire solar system. Ions, expelled by the sun in every direction at 1 million miles (1.6 million kilometers) per hour, create a ''bubble.'' This shield screens 90 percent of the intense, stellar radiation pulsing throughout the galaxy, said the National Aeronautics and Space Administration.

"Why the sun would be putting less flux out, no one knows,'' David McComas, Ibex chief scientist at the Southwest Research Institute in San Antonio, said by telephone. "We don't believe we're in imminent danger, but we've only measured the solar wind for about 50 years.''

In the past decade, the wind's intensity has waned by about 25 percent. The variation could be part of a natural cycle, McComas said.

Ibex, about the size of a living-room ottoman, will examine solar particles that have interacted with the cold gases of space. The probe will capture the particles while orbiting Earth.

Ibex was launched from the mid-Pacific Ocean at 1:48 p.m. New York time aboard a Pegasus rocket, NASA's smallest orbital vehicle.

Rather than blasting off from the ground, the rocket was strapped to the belly of an airplane, which carried it to 40,000 feet. From there, the rocket will carry Ibex to an orbit about 200,000 miles above Earth.