Just days after launching twin spacecraft deep into Earth's magnetosphere, space physicists were rewarded with the discovery of a third region of trapped high-energy particles in the Van Allen radiation belts.
I'm not quite old enough to remember the launch of Explorer 1, the first U.S. satellite, on February 1, 1958. Nor do I recall the news that a Geiger-tube detector on board, the handiwork of James Van Allen (University of Iowa) and his students, had detected two broad belts of high-energy charged particles trapped in Earth's magnetosphere. Van Allen was as surprised as anyone by the discovery — he'd hoped instead to record cosmic rays from deep space — but it made his a household name. Van Allen and his radiation belts even made the cover of Time magazine in 1959.
Because the particles are charged, they're confined to specific doughnut-shaped regions within the magnetosphere, spiraling up and down along field lines at relativistic speeds (which is what makes them harmful). The inner belt, ranging in height from about 1,000 to 8,000 miles, consists of protons and electrons. The outer belt, almost entirely electrons, ranges from 12,000 to 25,000 miles, and the two regions are ordinarily separated by a nearly empty gap or "slot".
Now another set of Van Allen namesakes have altered this simple picture. NASA launched two identical Radiation Belt Space Probes last August 30th, and by the time the agency renamed them the Van Allen Probes in early November they'd already found something remarkable: a third region of trapped radiation high above Earth.
As detailed in last week's online issue of Science, a team led by Daniel Baker (University of Colorado) describes how the Van Allen Probes recorded a third region of energetic electrons that appeared between the two "classic" belts for about a month. During that time the outer belt became distinctly weaker and even disappeared for about a week. Then the mysterious third component waned and the outer belt reformed.
Baker's team doesn't know what caused this series of events — the outer belt is notoriously changeable. For example, it's not clear whether the enhancement seen by the probes represents a distinct third region or instead some unexplained splitting of the outer belt. But the probes will monitor the dynamic goings-on out there for at least two years, perhaps shedding light on how charged particles arise both around other planets and elsewhere in the universe. "We're very lucky," notes Mona Kessel, the mission's program scientist . "It's rather like having a particle accelerator in our backyard."
Here are two interesting sidelights to this story. The first is that the Van Allen Probes recorded these dramatic changes only because their Relativistic Electron-Proton Telescopes (REPTs) were turned on right away. Mission scientists wanted to get some data before another NASA space sentinel called SAMPEX (short for "Solar, Anomalous, and Magnetospheric Particle Explorer") reentered the atmosphere after a 20-year mission. Normally, Baker explains, the REPT instruments wouldn't have been activated for at least a month; instead, the team turned them on just two days after launch. "We were very fortunate that we did," he adds.
The second bit of backstory involves a tale of Cold War secrecy that backfired. It turns out that Sputnik 2, which famously carried the dog Laika into space three months before Explorer 1 reached orbit, also carried a Geiger-tube detector. Built by Russian cosmic-ray expert Sergey Vernov and his colleagues, the detector worked just fine for several days and no doubt registered intense levels of energetic particles each time the craft neared its orbit's 1,000-mile-high apogee.
But, as space historian Don Mitchell recounts, the classified data transmissions from Sputnik 2 could only be received when the craft was over the U.S.S.R. — and near perigee. Had the team been allowed to ask for tracking help from abroad, this story might have been all about the "Vernov Belts."