Correction: An earlier version of this story incorrectly described the particles in the Van Allen radiation belts are “radioactive.” This version has been corrected.

The spacecrafts will orbit the Van Allen radiation belts, two donut-shaped rings of radioactive particles encircling the earth, for two years. (NASA)

A pair of armored NASA spacecraft will soon head into one of the most treacherous regions of outer space on a mission to understand how radioactive particles that surround the Earth affect our satellites and astronauts.

The two will journey into the Van Allen radiation belts, doughnut-shaped rings of high-energy particles that encircle Earth. The particles, which are expelled from the sun, are pulled into these belts by Earth’s magnetic field — the same force that swings a compass needle toward the North Pole.

“We are trying to go to a place that everyone else tries to avoid,” said Rick Fitzgerald, the project manager of the Radiation Belt Storm Probes mission at the Applied Physics Laboratory of Johns Hopkins University. The spacecraft are scheduled to launch together at 4:08 a.m. Thursday from Cape Canaveral Air Force Station and fly for two years, during which time they will traverse the belts in their entirety.

The Van Allen radiation belts are America’s earliest Space Age discoveries, found in 1958 by the nation’s first satellite, Explorer 1. Yet the belts largely remain a mystery, their environment too harsh to accommodate satellites for long. The new $686 million twin storm probes are different.

“This is the most rugged and best-instrumented mission ever flown to measure radiation,” Fitzgerald said. The octagonal spacecraft — about six feet across, three feet tall and 1,475 pounds each — are shielded in aluminum as thick as a slice of bread. Each storm probe holds five instruments to measure radioactive particles — mostly protons and electrons — as the spacecraft plow through the belts and endure harsh storms of radiation from the sun, which scientists call “space weather.”

Project scientist Barry Mauk compared flight in the radiation belts to navigation in the deep sea.

“You don’t want to be in a submarine in the most dangerous depths of the ocean and have your safety systems malfunction,” he said. “In the same way, if a satellite’s shielding fails in one of these belts, it will be in big trouble.”

The particles that these spacecraft will capture are expelled from the sun and hurtle 93 million miles toward Earth in million-mile-per-hour waves called solar storms. The data from these probes will help scientists build models that explain how different solar disturbances blowing through the cosmos could change belt size and shape. Those models could then become predictive tools that allow scientists to guess how the belts will behave before a solar storm is in Earth’s range.

The space weather predictions will be critical for communication satellites that orbit in the heart of these belts, Mauk says. Global positioning systems, cellphones and cable television satellites that fly near the belts are especially at risk of malfunctioning. Accumulations of highly charged particles on their surfaces can cause their computer chips to go haywire. That can lead to glitches in GPS devices and cellphone service, as well as power grid failures, scientists say.

“There are a lot of technologies that fly in and out of these belts that we don’t know about or take for granted — and only when the belts cause problems do people want to know more about them,” Fitzgerald said.

According to a 2000 study, the Department of Defense has estimated that disruptions to government satellites due to space weather add up to nearly $100 million each year.

A particularly brutal solar storm in January 1994 caused two of Canada’s main broadcast satellites traveling in the belts to spin out of control, shutting down telephone, radio and television services for days around the country and costing media companies millions of dollars in losses.

Fitzgerald imagines that the probes could serve as front-line “space-weathermen,” sending data that could one day help scientists give warning to communication companies and astronauts before solar storms hit. That way, satellites could shut off their electronics and “go into safe mode” when the belts become especially dangerous.

The radiation belts can expand, shrink and even multiply. While the outer belt can extend more than 25,000 miles above the Earth, the inner belt can come as close as 125 miles above the surface, where astronauts on the international space station would be susceptible to radiation.

A full understanding of how these belts morph during solar storms is only achievable with two spacecraft, Fitzgerald said. That’s because they will be sent into slightly different orbits, allowing scientists to take simultaneous measurements of radiation levels in different regions of the belts and parse the effects of time and space on belt activity.

That capability, he added, has been a “major impediment” to understanding the belts thus far.

All strongly magnetized planets in our solar system have radiation belts, Mauk added. That includes Jupiter, Saturn, Uranus and Neptune — though not Mars, where NASA’s Curiosity rover landed Aug. 6. Scientists suspect that Mars cannot hold an atmosphere that sustains life like Earth’s because it lacks a magnetic field.

“We are using the Earth’s radiation belts as a natural laboratory to study how other radiation regions, which are plentiful, are created in space,” he said.