Two characteristics of the Van Allen Probes make it stand out from all previous missions to study the radiation belts. First, by relying on two spacecraft instead of one, the Van Allen Probes can take simultaneous measurements at different locations and determine whether an event occurs at the same time throughout the belts or instead travels across the belts, changing over time and space. Second, the instrument suite is unusually comprehensive, able to measure an enormous range of energies, particles, and waves.
One of the Van Allen Probes with location of instruments indicated. Credit: JHU/APL
Two Spacecraft In an Elliptical Orbit
The Van Allen Probes are two spacecraft carrying identical instruments. Each eight-sided satellite is approximately 6 feet across, 3 feet high, and weighs 1,475 pounds (including 62 pounds of propellant). One spacecraft follows the other along nearly identical orbits. The orbits lie nearly in the Earth's equatorial plane and are highly elliptical, coming in as close as 375 miles and reaching out almost to 20,000 miles above Earth’s surface, thus traveling through diverse areas within the belts.
By comparing observations from both spacecraft, scientists can distinguish between events that occur simultaneously throughout the belts, those that happen in only a single point in space, and those that move from one point to another over time.