Thunderstorms have ruined NASA's second attempt to launch a pair of science satellites.

For the second day in a row, NASA had to halt the countdown for its Radiation Belt Storm Probes.

Lightning and thick storm clouds prevented the unmanned rocket from taking off early Saturday from Cape Canaveral. On Friday, a tracking beacon on the rocket held up the flight.

NASA says it will try again Sunday.

The twin satellites are designed to study Earth's harsh radiation belts. Scientists say the two-year 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.

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Online:

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