An identical pair of NASA satellites launched in 2012 August to study the Earth’s radiation belts, commonly known as the Van Allen Belts. They each carry five instruments to measure the atomic particles and magnetic fields within the belts. Both spacecraft follow the same highly elliptical orbit, ranging from 500 km to 30 600 km from Earth, passing through the entire belts.

http://vanallenprobes.jhuapl.edu Official mission website.

http://www.nasa.gov/van-allen-probes NASA page about the mission.