Fireball Finale: RHESSI’s Fiery Reentry After 16 Years of Solar Observation

NASA’s retired RHESSI spacecraft, which observed solar flares and coronal mass ejections, is expected to reenter Earth’s atmosphere on April 19. The risk of harm to anyone on Earth is low. RHESSI’s 16-year mission contributed valuable data on solar flare physics and made discoveries about the Sun’s shape and terrestrial gamma-ray flashes. (Illustration of RHESSI satellite.) Credit: NASA

NASA Retired Solar Energy Imager Spacecraft to Reenter Atmosphere

NASA’s retired Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft is predicted to reenter Earth’s atmosphere on April 19, with an uncertainty of +/- 16 hours. Launched in 2002, RHESSI observed solar flares and coronal mass ejections, providing vital data for scientists to understand the physics behind these energetic events. The risk of harm to anyone on Earth is estimated to be low, at 1 in 2,467. During its 16-year operation, RHESSI recorded over 100,000 X-ray events and documented a wide range of solar flare sizes, while also making discoveries about the Sun’s shape and terrestrial gamma-ray flashes.

NASA’s retired Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft is expected to reenter Earth’s atmosphere in April, almost 21 years after launch. From 2002 to its decommissioning in 2018, RHESSI observed solar flares and coronal mass ejections from its low-Earth orbit, helping scientists understand the underlying physics of how such powerful bursts of energy are created.

As of Monday, April 17, the Department of Defense predicted the 660-pound spacecraft will reenter the atmosphere at approximately 9:30 p.m. EDT (6:30 p.m. EDT) on Wednesday, April 19 with an uncertainty of +/- 16 hours. NASA and the Defense Department will continue to monitor reentry and update predictions.

NASA expects most of the spacecraft to burn up as it travels through the atmosphere, but some components are expected to survive reentry. The risk of harm coming to anyone on Earth is low – approximately 1 in 2,467.

An artist’s representation of RHESSI. Flying up above Earth’s radiation-blocking atmosphere, RHESSI could observe X-rays and gamma rays from solar flares. Credit: NASA

The spacecraft launched aboard an Orbital Sciences Corporation Pegasus XL rocket with a mission to image the high-energy electrons that carry a large part of the energy released in solar flares. It achieved this with its sole instrument, an imaging spectrometer, which recorded X-rays and gamma rays from the Sun. Before RHESSI, no gamma-ray images nor high-energy X-ray images had been taken of solar flares.

Data from RHESSI provided vital clues about solar flares and their associated coronal mass ejections. These events release the energy equivalent of billions of megatons of TNT into the solar atmosphere within minutes and can have effects on Earth, including the disruption of electrical systems. Understanding them has proven challenging.

Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) explored the basic physics of particle acceleration and explosive energy release in solar flares. RHESSI imaged solar flares by capturing energetic photons from x-rays to gamma rays; RHESSI also performed spectroscopy on these captured particles. Credit: NASA

During its mission tenure, RHESSI recorded more than 100,000 X-ray events, allowing scientists to study the energetic particles in solar flares. The imager helped researchers determine the particles’ frequency, location, and movement, which helped them understand where the particles were being accelerated.

Over the years, RHESSI documented the huge range in solar flare size, from tiny nanoflares to massive superflares tens of thousands of times bigger and more explosive. RHESSI even made discoveries not related to flares, such as improving measurements of the Sun’s shape, and showing that terrestrial gamma-ray flashes – bursts of gamma rays emitted from high in Earth’s atmosphere over lightning storms – are more common than previously thought.

After 16 years of operations, NASA decommissioned RHESSI in 2018 due to communications difficulties with the spacecraft. RHESSI was a NASA Small Explorers mission, managed and operated by the agency’s Goddard Space Flight Center in Greenbelt, Maryland.