The unmanned rockets took off Thursday before dawn. The extra tough satellites are designed to withstand an onslaught of cosmic rays over the next two years. It's the first time two spacecraft are flying in tandem amid the Earth's radiation belt.
The Johns Hopkins lab built the radiation belt probes for NASA, and is operating them from Maryland following a week of launch delays.
Scientists expect the $686 million mission to shed light on how the sun affects the Van Allen radiation belts, named after the astrophysicist who discovered them a half-century ago. The goal of this mission is to improve space weather forecasting.
It took NASA three tries to launch the spacecraft. Last week's attempts were thwarted by trouble with a tracking beacon and then stormy weather. NASA opted to wait until the passage of Hurricane Isaac before trying again.
The Associated Press contributed to this report.