NASA’s Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft is expected to re-enter Earth’s atmosphere in April, about 21 years after launch. From 2002 until 2018 when it was decommissioned, RHESSI observed solar flares and coronal mass ejections from low Earth orbit, helping scientists understand the physics behind how such powerful bursts of energy were created.

On Monday, April 17, the Department of Defense estimated that the 660-pound spacecraft would re-enter the atmosphere at approximately 9:30 ET on Wednesday, April 19, with an uncertainty of +/- 16 hours. NASA and the Department of Defense will continue to monitor the re-entry and update the estimates.

NASA expects most of the spacecraft to burn up during its journey through the atmosphere, but some components are expected to survive reentry. The risk of harming anyone in the world is low, about 1 in 2,467.

The spacecraft was launched on the Orbital Sciences Corporation Pegasus XL rocket with the mission to image the high-energy electrons that carry most of the energy released during solar flares. This was achieved with the Imaging Spectrometer, a single instrument that records X-rays and gamma rays from the Sun. Before RHESSI, neither gamma nor high-energy X-ray images of solar flares had been taken.

The RHESSI data provided vital clues about solar flares and related coronal mass ejections. These events release energy equivalent to billions of megatons of TNT into the solar atmosphere within minutes and can affect Earth, including disrupting electrical systems. It turned out to be difficult to understand them.

During its mission, RHESSI recorded more than 100,000 X-ray events, allowing scientists to study the energetic particles of solar flares. The thermal camera helped the researchers determine the frequency, position and motion of the particles, which helped them understand exactly where the particles were accelerating.

Over the years, RHESSI has documented a wide variety of solar flare sizes, from tiny nano-flames to massive super-flames tens of thousands of times larger and more explosive. RHESSI has even made discoveries unrelated to flares, such as improving measurements of the Sun’s shape and showing that terrestrial gamma-ray bursts — bursts of gamma rays emitted at high levels in Earth’s atmosphere during thunderstorms — are more common than previously thought.

After 16 years of operation, NASA retired RHESSI due to communication problems with the spacecraft. RHESSI was a NASA Small Explorer mission managed and operated by the Goddard Space Flight Center in Greenbelt, Maryland.