660-pound NASA spacecraft is hours away from crashing into Earth after more than a decade studying the sun

An old piece of NASA equipment is flying back to Earth decades after it launched – and is just hours away from crashing onto the planet's surface. 

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched into space in 2002, where it spent years studying solar flares and coronal mass ejections from the sun. It was retired 16 years later, in 2018, and officials say it's finally going to make it back to Earth on Wednesday. 

"The 660-pound spacecraft will reenter the atmosphere at approximately 9:30 p.m. EDT on Wednesday, April 19 with an uncertainty of +/- 16 hours," NASA said on Monday. "NASA and the Defense Department will continue to monitor reentry and update predictions."

Most of RHESSI will "burn up" as it crashes its way through the atmosphere, the agency said, "but some components are expected to survive entry." 

It's unlikely, however, that any of the falling pieces will cause an issue for people on Earth. NASA said the risk of harm is "low – approximately 1 in 2,467." 

Throughout the course of its tenure, RHESSI recorded more than 100,000 X-ray events and at the time, was the only observatory providing imaging spectroscopy of the electrons in solar flares. According to NASA, it has the "finest angular and the spectral resolution of any hard X-ray or gamma-ray instrument ever flown in space." The instrument also helped improve the measurements scientists have of the sun's shape.

NASA retired the spacecraft due to communications difficulties. 

It may be possible to see the spacecraft make its final descent. 

"As with any re-entry, we expect that visual observations of the fireball will be possible for a very lucky observer," researchers said. "Ideally one would have clear skies, a dark night, and the great luck of having ... just the right moment along the orbital track."

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