CAPE CANAVERAL, Fla. — NASA's effort to launch a pair of satellites to Earth's radiation belts has been delayed until late next week because of Tropical Storm Isaac.

The countdown was halted at the four-minute mark in the wee hours of Saturday — for the second time in as many days.

Thunderstorms prevented the unmanned rocket from blasting off with NASA's Radiation Belt Storm Probes. On Friday, a tracking beacon on the rocket held up the flight.

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NASA initially said it would try again Sunday. But with Isaac bearing toward Florida, launch managers decided to move the Atlas V rocket back into its hangar and sit tight until the storm passed. They're now aiming for a Thursday launch.

The twin satellites are designed to study Earth's harsh radiation belts. Scientists say the two-year, $686 million mission will improve space forecasting; the goal is to better guard against solar storms.

Spacecraft can be damaged, and astronauts hurt, from severe solar outbursts. Life here on the planet also can be disrupted.

Online:

NASA: http://www.nasa.gov/mission_pages/rbsp/main/index.html