In a remarkable display of forethought, the Van Allen Probes have been set on a trajectory that will plunge the two radiation belt satellites into the Earth's atmosphere in about 15 years. In order to keep the pair of spacecraft from becoming a hazard to navigation, Johns Hopkins Applied Physics Laboratory (APL), which designed and built the pair, put them through a series of thruster maneuvers in February that will place them in ever-tightening orbits ending around 2034.

Launched on August 23, 2012 atop an Atlas V rocket from Cape Canaveral Air Force Station, Florida, the two Van Allen Probes, formerly known as the Radiation Belt Storm Probes, were originally tasked with spending two years studying the famous Van Allen radiation belts that encircle the Earth and protect us from cosmic radiation. However, nearly seven years later, the unmanned satellites are still going strong and mission control must arrange to deorbit them while there is still sufficient propellant to make the needed course corrections.

First detected in 1958 by the first successful American satellite, Explorer I, and named after their discoverer James Van Allen, the two radiation belts are produced by the interaction of the Sun and the Earth's magnetic field. When this happens, the charged protons and electrons get caught in the field like iron filings around a magnet, forming zones of deadly radiation.

These fields protect the Earth against radiation coming from space, but they are no-go areas for astronauts and unmanned craft for any prolonged time. The Van Allen Probes, as their name implies, are specially designed to traverse the belts and not be destroyed by the radiation. This involved shielding the electronics to prevent the accumulation of electrical charges and general strengthening that has allowed the craft to act as flying laboratories to understand the effects of radiation on electronics.

"Designing the spacecraft and instruments to withstand a very harsh radiation environment was the toughest challenge for Van Allen Probes during design and development," says Rick Fitzgerald, project manager at APL from 2007 to 2012. "Radiation can cause damage to electronics, leading to erratic behavior or outright failure. We lowered the risk of failure through a rigorous design review process, careful selection of electronics parts, and extensive parts and materials testing."

Thanks to this hardening, the Van Allen Probes have not only enjoyed an extended life, but they have also continued to send back high volumes of data.

"Over the past six and a half years, the Van Allen Probes have completed three full circuits around the magnetosphere, and measured more than 100 geomagnetic storms" says Sasha Ukhorskiy, a project scientist at APL. "The Van Allen Probes verified and quantified previously suggested theories, discovered new mechanisms that can sculpt near-Earth energetic particle populations, and used uniquely capable instruments to unveil unexpected features that were all but invisible to previous sensors."

The drawback of this success is that the probes are running low on thruster propellant, which means that eventually mission control will no longer be able to order the pair to make course corrections. NASA regulations now require all spacecraft to deorbit within 25 years of the end of their missions, so the Van Allen Probes team had to think ahead. The thrusters are not very powerful and unless the probe trajectories are altered now, they could remain in orbit for up to thousands of years.

Though the Van Allen Probe mission is continuing, last month's change in trajectory will put the spacecraft on an ever-tightening orbit as they skim the outer reaches of the Earth's atmosphere. This required firing the thrusters at a specific points in orbit at specific times of year. After that, the laws of physics take over until their orbits decay naturally and the craft burn up in the atmosphere.

"We need to maneuver when the satellites are at their highest orbit point away from the Earth, or what's known as apogee," says Justin Atchison, Van Allen Probes mission designer. "Ideally, we'd do all of the orbit change in a single maneuver on a single day. However, we're limited by the capability of the thrusters, which are typically only used for very small maneuvers to slightly adjust the orbit or the pointing. So we have to divide the maneuver up into smaller segments to achieve the goal."

The video below discusses the Van Allen Probes.

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