NASA's retired Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft is expected to reenter Earth's atmosphere in April after almost 21 years since its launch. The spacecraft observed solar flares and coronal mass ejections from its low-Earth orbit, providing scientists with vital clues about the underlying physics of how such powerful bursts of energy are created.
RHESSI was launched in February 2002 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 was equipped with a sole instrument, an imaging spectrometer that 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, but RHESSI's mission tenure was essential in documenting 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.
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.
NASA decommissioned RHESSI in 2018 due to communications difficulties with the spacecraft. The RHESSI was a NASA Small Explorers mission, managed and operated by the agency's Goddard Space Flight Center in Greenbelt, Maryland.
As of Monday, April 17, the Department of Defense predicted that the 660-pound spacecraft would re-enter the Earth's atmosphere at approximately 9:30 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.
The data and insights from RHESSI's mission tenure have been instrumental in advancing our understanding of solar flares and their impact on Earth. The spacecraft's legacy will continue to inspire future missions to study our Sun and its complex behavior.