Another long-lived NASA mission has come to an end, this one after more than seven years of dancing through the perilous belts of radiation around Earth.
Mission scientists sent the final command to the second Van Allen Probe today (Oct. 18) at 12:30 p.m. EDT (1630 GMT). The twin spacecraft launched in August 2012 to study space weather, the sometimes-dramatic phenomena caused by the sun's influence on Earth's environment. (The other Van Allen Probe received its last command on July 19.)
Space weather can interfere with power grids on the ground, navigation and communications satellites in orbit, and the health of astronauts. Like terrestrial weather, it ought to be predictable; but scientists have much less data about space weather for building and fine-tuning models, and that's where the Van Allen Probes changed the game.
"The Van Allen Probes is the first radiation-belt mission designed to exist in hazardous radiation belts for such a long period of time," Sasha Ukhorskiy, project scientist for the mission at Johns Hopkins University Applied Physics Laboratory, told Space.com. "The data that we collected over this time will be the golden standard in radiation-belt physics for many generations of scientists to benefit from."
Related: NASA's Van Allen Probe Mission in Pictures
The Van Allen Probes were designed to last at least 2.5 years, and they did so, remaining at work for more than twice as long. (The duo, originally dubbed the Radiation Belt Storm Probes, cost NASA $670 million at launch.) Earlier this year, the mission team began to lower the orbits of the pair of satellites, preparing for the day when they no longer had enough fuel to function. Both spacecraft will continue to exist silently for about 15 years as they spiral slowly toward the atmosphere, eventually burning up safely in orbit.
The Van Allen Probes' long tenure in the radiation belts is particularly valuable to scientists because of its timing. Space weather is governed by the sun, which rages and calms over an 11-year cycle. The twin spacecraft launched around the peak of that cycle and have survived to see its lull as well.
Surviving from one extreme to the other allowed the Van Allen Probes to provide scientists with observations of space weather in a full range of moods. And with measurements coming from two different satellites, scientists can distinguish how space weather changes over time and space. That span of data inherently strengthens scientists' models of the phenomena — a vital development for making more-accurate predictions.
Scientists have puzzled over Earth's radiation belts since they were discovered in 1958 based on data from NASA's Explorer 1 spacecraft, its first mission. But for decades, spacecraft observations were limited to brief forays because the region is so hazardous.
The radiation belts, which are doughnut-like thick rings of charged particles around Earth, are formed by the interaction of the sun and Earth's magnetic field. The sun constantly spits out plasma, the energetic charged particles that roil through the star. Earth's magnetic field traps some of that plasma, forming the radiation belts, where some of those particles careen around the world at nearly the speed of light.
Unsurprisingly, spacecraft tend to fare poorly when exposed to that sort of treatment. Spacecraft engineers use "radiation-hardened" parts to withstand the barrage. Many mission plans are designed to reduce the amount of time spacecraft spend in areas that are particularly rich in radiation and to power down the spacecraft during the worst of the battering. But because space weather is dynamic, so is that radiation, making it more difficult to avoid.
The Van Allen Probes never had that luxury; they had to fly through the hail of radiation in order to study it. So before the mission could become a reality, engineers needed to design a spacecraft capable of functioning in that onslaught.
"We always wanted to send a spacecraft to study this harsh environment, and no one actually dared to send a spacecraft," Nelli Mosavi, project manager for the Van Allen Probes at Johns Hopkins University Applied Physics Laboratory, told Space.com. "The legacy is the resilient spacecraft we built to withstand these environments that no one else could have gone to."
Engineering advancements made for the Van Allen Probes can be implemented to protect spacecraft long into the future, regardless of whether they are tackling an equally hazardous assignment. (Right now, NASA doesn't have any plans for a space weather mission comparable to the Van Allen Probes.)
Future missions will also benefit from the newly detailed understanding of the radiation belts themselves, of course — and not just robotic missions. Radiation is also a key threat to astronauts living and working on the International Space Station, and protecting humans from the dangers of radiation is one of the most important challenges NASA will need to tackle as it looks to expand human exploration of space.
Astronauts haven't ventured beyond low Earth orbit since the Apollo era, when scientists had been studying the radiation belts for just over a decade — and they didn't have anything like the Van Allen Probes to interpret the region's hazards.
Before launch, scientists thought there were two stable belts that evolved over the course of hours and days. It took only a few weeks for the Van Allen Probes to revise that vision. They revealed that instead, the belts can fluctuate from three thinner belts to one massive one, depending on the sun's activity, and that their dynamics can change in just seconds or minutes.
"The entire concept of what radiation belts are and how they change in time and space changed quite dramatically," Ukhorskiy said.
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Email Meghan Bartels at mbartels@space.com or follow her @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.
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