The seven-year mission, which ended Friday, was one of the most consequential investigations into how space weather affects the environment just outside our planets atmosphere.

The background: The Van Allen radiation belts are zones of charged particles energized by intense solar winds and cosmic rays, stretching out from 400 to 36,040 miles above the surface. Earths magnetic field, which protects the planet from space radiation, actually ends up mostly trapping the particles into two layers within the magnetosphere.

Satellites and spacecraft in high orbits or on their way into deep space can be damaged by prolonged exposure to this radiation. Humans who spend too long in the Van Allen belts can suffer severe health problems. And the belts play a significant role in modulating space weather that could end up damaging satellites, power grids, and other electronic infrastructure on Earth.

The probes: NASA launched two Van Allen probes in 2012 into an elliptical orbit to directly study the radiation belt. Each satellites used a suite of five instruments to detect particles and study the magnetic fields and plasma waves characteristic of the region of space directly around Earth. Probe B was shut down in July after it ran out of fuel, and NASA ceased probe As operations on Friday. The pair should fall back and burn up in the atmosphere in 15 years.

The legacy: Scientists used the mission to better understand the distribution of charged particles throughout the Van Allen belts, which is helping engineers design spacecraft that can better withstand extreme and extended cosmic radiation. Theres also a clearer understanding of how space weather causes the belts to swell and shrink over time, in what we now know is an 11-year cycle. More exciting was the discovery that additional transient belts can form during bouts of extreme solar activity.

But the biggest impact of the Van Allen probes might be in their engineering. The primary mission was set for only two years, because it was feared that radiation would quickly erode the electronics. Yet the duo unexpectedly lingered on, and might have survived into the 2020s with enough fuel. As we consider long-term missions in space, the probes will be a lesson in how to think about spacecraft design.

Whats next: NASA doesnt actually have any plans for a mission that compares to scope of the Van Allen probes, so we will have a dearth of space weather science for the foreseeable future. ESAs Lagrange mission could be a powerful tool if it launches, and NASAs upcoming TRACERS mission will flesh out how charged particles interact with the magnetic field at the poles.

Neither of these, however, will directly study the Van Allen belts. Meanwhile, NASA and NOAA have several spacecraft that monitor space weather and keep the agencies apprised of any solar events that could wreak havoc on modern-day electronic systems.

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NASA's Van Allen probes are no more, and we owe them a lot - MIT Technology Review