Dr. David Sibeck describes in instruments on NASA’s twin Van Allen probes that are exploring the Van Allen Radiation Belts in the Earth’s magnetosphere. (Courtesy of NASA)

Happy accidents litter the annals of scientific discovery.

Here’s another: NASA probes that are exploring the treacherous twin Van Allen radiation belts encircling the Earth spied a third, unexpected band of radiation that burst into view and then disappeared, scientists reported Thursday.

The NASA probes spotted the temporary band of high-energy electrons just three days after launch.

The discovery has stunned scientists and is forcing them to rethink the radiation environment above Earth, which has implications for proposed human deep-space missions.

In 1958, early NASA satellites recorded two zones of dangerous electron and proton radiation extending 12,000 miles beyond Earth. Named for scientist James Van Allen, the dual doughnut-shaped regions were the first major discovery of the space age.

Last Aug. 30, NASA launched the twin Radiation Belt Storm Probes — since renamed the Van Allen Probes — to provide a deeper understanding of the region. A radiation-detection instrument was scheduled to be switched on a month later, but mission scientists decided to turn it on early.

Three days after launch, and just a day after the instrument powered up, the third belt appeared.

“We initially thought, ‘This looks odd, maybe something’s screwed up with our instrument,’” said Shri Kanekal, a mission scientist at NASA’s Goddard Space Flight Center in Greenbelt. “But as days went by [the third belt] just stood there. We checked our instrument and became more and more confident there was not something wrong.”

The third belt, which formed between the two known belts, disappeared about a month later when a blast of energy from the sun blew past Earth. Such solar weather can warp and compress the outer Van Allen belt, but earlier missions had never seen a third belt form.

Researchers are now puzzling over the temporary band of high-energy electrons. They’re trying to understand why it did not quickly merge with the outer belt, as predicted by current understanding of the physics of the region.

“We don’t know why we haven’t seen this before,” said Kanekal, a co-author of a new report online in Science describing the find. “We don’t know if it’s a rare phenomenon. Even after 50 years, nature is still capable of surprising us.”

Being able to predict such sudden increases in near-Earth radiation could help steer future human space missions away from dangerous doses.

The twin octagon-shaped Van Allen Probes were built for NASA by the Johns Hopkins University Applied Physics Laboratory in Laurel.