NASA has delayed the launch of its newest science satellites.

The countdown proceeded all the way down to the four-minute mark early Friday morning at Cape Canaveral in Florida. But a problem cropped up with the rocket's tracking beacon, a mandatory safety item.

NASA tentatively is aiming for another launch attempt Saturday for the Radiation Belt Storm Probes - provided the problem can be fixed quickly. Launch director Tim Dunn says engineers need to determine whether the problem is with the beacon itself or ground equipment.

The twin satellites aboard the Atlas V rocket are designed to study Earth's radiation belts. Scientists say the spacecraft will improve space weather forecasting. Solar storms can damage spacecraft and disrupt life on Earth.

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Online:

NASA: http://www.nasa.gov/mission-pages/rbsp/main/index.html