21.10.2019

Ground controllers transmitted commands to shut down the second of NASA’s Van Allen Probes on Friday, three months after turning off an identical satellite to conclude a seven-year mission that explored the donut-shaped radiation belts surrounding Earth.

The Van Allen Probes were running low on fuel after maneuvers earlier this year to lower their orbits, allowing the satellites to naturally fall back into Earth’s atmosphere and burn up in about 15 years.

The two satellites repeatedly flew through the ever-changing clouds of electrons and charged particles surround Earth. Most satellites avoid prolonged exposure to the radiation belts, but the treacherous region was home for the Van Allen Probes.

“This mission spent seven years in the radiation belts, and broke all the records for a spacecraft to tolerate and operate in that hazardous region, all with no interruptions,” said Nelofar Mosavi, Van Allen Probes project manager at Johns Hopkins University Applied Physics Laboratory. “This mission was about resiliency against the harshest space environment.”

The two spacecraft were named for the discoverer of the radiation belts, James Van Allen, who was lead scientist for Explorer 1, first U.S. satellite to reach orbit.

Explorer 1 discovered the presence of the radiation clouds surrounding Earth’s in 1958, but the Van Allen belts were not well understood for decades after the dawn of the Space Age. Physicists learned in the 1990s the belts are not as stable as once thought. Instead, they expand, heat up, and contract when charged particles from solar eruptions reach Earth.

The Van Allen Probes — originally known as the Radiation Belt Storm Probes — launched Aug. 30, 2012, on top of a United Launch Alliance Atlas 5 rocket from Cape Canaveral. The identical satellites each carried five instruments to detect particles, magnetic fields and plasma waves in the radiation belts.

The probes initially circled Earth in orbits ranging between approximately 375 miles (600 kilometers) and 19,000 miles (30,600 kilometers), with an inclination of about 10 degrees.

Just days after launch, the probes discovered the existence of a third radiation belt joining two previously-known Van Allen belts during periods of intense solar activity.

Changing space weather, driven by the sun, cause the Van Allen belts to swell and contract. The Van Allen Probes charted those changes over seven years, yielding fresh insights about the complex plasma waves and magnetic fields behind the fluctuations in the radiation belts.

Magnetic storms in the radiation belts pose threats to communications and navigation satellites, along with astronauts in space.

The Van Allen Probes were the first satellite to simultaneously sample the radiation belts from two different locations.

The charged particles in the radiation belts can damage satellite electronics. The Van Allen Probes were originally designed for a two-year mission because engineers worried the satellites would not survive much longer in such a hostile environment.

“The Van Allen Probes rewrote the textbook on radiation belt physics,” said Sasha Ukhorskiy, Van Allen Probes project scientist at at APL, which also designed and built the spacecraft. “The spacecraft used uniquely capable instruments to unveil radiation belt features that were all but invisible to previous sensors, and discovered many new physical mechanisms of radiation belt acceleration and loss.”

With the Van Allen Probes running low on fuel, mission managers decided earlier this year to adjust the low points, or perigees, of the satellites’ orbits below 150 miles (240 kilometers). That maneuvers will ensure the satellites re-enter the atmosphere in 15 years, and do not remain in orbit as space junk.

“The Van Allen Probes mission has done a tremendous job in characterizing the radiation belts and providing us with the comprehensive information needed to deduce what is going on in them,” said David Sibeck, mission scientist for the Van Allen Probes at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

“The very survival of these spacecraft and all their instruments, virtually unscathed, after all these years is an accomplishment and a lesson learned on how to design spacecraft,” Sibeck said in a statement earlier this year.

Quelle: SN