Nasa's effort to launch a pair of satellites to Earth's radiation belts is delayed until late this week because of tropical storm Isaac.
The countdown was halted at the four-minute mark on 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.
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, US$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.
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