The shape of the Van Allen belts can vary widely depending on how energetic the individual electrons are, and general conditions in the Earth’s magnetic environment. During geomagnetic storms (4), all three regions in the belts can balloon.
Credit: NASA GODDARD/DUBERSTEIN

Radiation, of course, also poses a human risk. Astronauts are subject to lifetime radiation limits from their time in space, to reduce any risk of cancer. Since only a few dozen people have spent six months or longer in space, however, it will take decades to understand the long-term effects of radiation on humans.

The astronauts on the ISS do not regularly spend time inside the belts, but from time to time solar storms expand the belts to the orbit of the space station. In the 1960s, several Apollo crews went through the Van Allen belts on their way to and from the moon. Their time in that radiation-intensive region, however, was very short, in part because the trajectory was designed to pass through the thinnest known parts. With more study, astronauts can be better protected for long-term stays in Earth orbit.

"We study radiation belts because they pose a hazard to spacecraft and astronauts," said David Sibeck, the Van Allen Probes mission scientist at NASA's Goddard Space Flight Center in Maryland, in an August 2016 NASA statement. "If you knew how bad the radiation could get, you would build a better spacecraft to accommodate that."

Newer findings from the probes show that radiation in certain zones may be less harsh than scientists thought. In March 2017, the Van Allen Probes made a finding showing there is less radiation in the inner belts that previously theorized, which means less shielding is required for spacecraft and satellites in that region. The most energetic electrons residing in the inner radiation belt are there for less time than scientists thought beforehand. 

The following year, the probes discovered that some communications wavelengths (called very low frequency communications) emanating from Earth are sometimes a sort of a shield against high-energy particle radiation in space. This means that human activity has effects even in the near-space environment around Earth.

As of 2018, the Van Allen Probes are running low on fuel and are expected to finish their mission around 2020. Goddard is working on a CubeSat (small spacecraft) mission called GTOSat that will continue studying the Van Allen belts.

"This mission of firsts will serve as a pathfinder for new radiation-tolerant technologies that could help scientists realize a long-sought dream: deploying a constellation of small satellites beyond low-Earth orbit to gather simultaneous, multi-point measurements of Earth's ever-changing magnetosphere, which protects the planet from the constant assault of charged particles streaming off the sun," NASA said in May 2018.