The outer belt begins at an altitude of about 8,000 miles (12,875 km) and extends to about 26,000 miles (41,843 km).

The solar-powered probes, heavily shielded to operate in the radiation belts, are flying in slightly different, highly elliptical orbits that are inclined 10 degrees to the planet’s equator, allowing them to periodically lap each other. Science operations are scheduled to begin after a 60-day instrument checkout.

The satellites, built and operated by Johns Hopkins University’s Applied Physics Lab, will fly as close as 100 miles (161 km) to each one another, and as far as 24,000 miles (38,624 km) apart.

The dual measurements are key to understanding how the belts puff out and contract over time and in response to solar activity.

“If you imagine sitting on a life raft in the ocean and you suddenly go down and come up again, you don’t know very much about what caused you to go down and come up,” Fox said before the launch.

“If you have a friend who is sitting on a life raft a little way away, you can say ‘Well, did we both go down and up at the same time?’ In which case it’s a big-scale feature like a tsunami. Did one of us go down and then the other one? You can really start to look at the global dynamics of what’s happening in the radiation belts,” Fox said.

United Launch Alliance is a partnership of Lockheed Martin and Boeing. The mission cost $686 million, including the launch vehicle.