From Solar Flares to Sahara Skies: RHESSI’s Fiery Return to Earth

NASA’s RHESSI spacecraft re-entered Earth’s atmosphere after nearly 21 years, during which it observed solar flares and provided crucial data on their underlying physics. The data allowed scientists to study energetic particles, observe various flare sizes, and make discoveries about the Sun’s shape and terrestrial gamma-ray flashes. 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

NASA’s retired Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft re-entered Earth’s atmosphere at 8:21 p.m. EDT on Wednesday, April 19, almost 21 years after its launch. From 2002 to its decommissioning in 2018, RHESSI observed solar flares from its low-Earth orbit, helping scientists understand the underlying physics of how such powerful bursts of energy are created.

The Department of Defense confirmed that the 660-pound spacecraft re-entered the atmosphere over the Sahara Desert region, at approximately 21.3 degrees north latitude and 26 degrees east longitude. NASA expected most of the spacecraft to burn up as it traveled through the atmosphere, but for some components to survive re-entry.

NASA’s retired RHESSI spacecraft, which observed solar flares and coronal mass ejections, re-entered Earth’s atmosphere at 8:21 p.m. EDT on Wednesday, April 19. 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

The spacecraft launched in 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 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 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.

Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) Milestones

• February 5, 2002: RHESSI spacecraft launched aboard an Orbital Sciences Corporation Pegasus XL rocket. The mission aimed to provide high-resolution imaging of solar flares in X-rays and gamma rays.
• 2002-2018, Throughout its mission:
  • RHESSI observed solar flares and coronal mass ejections from its low-Earth orbit, helping scientists understand the underlying physics of these energetic phenomena and their effects on Earth’s space weather and electromagnetic systems.
  • RHESSI recorded more than 100,000 X-ray events, providing vital clues about solar flares and associated coronal mass ejections, which release immense energy into the solar atmosphere within minutes.
  • RHESSI documented a vast range of solar flare sizes, from nanoflares to massive superflares. This information contributed to the understanding of the processes governing solar flare generation and the impact of different flare sizes on the solar atmosphere.
  • RHESSI made discoveries unrelated to flares, such as improving measurements of the Sun’s shape, which is essential for understanding the behavior of the solar atmosphere and the physics of the Sun’s magnetic field. The spacecraft also found that terrestrial gamma-ray flashes—bursts of gamma rays emitted from Earth’s atmosphere over lightning storms—are more common than previously thought, providing insights into the interaction between Earth’s atmosphere and charged particles.
• 2012: RHESSI observed the largest solar flare in six years, an X5.4-class flare, which allowed researchers to study the behavior of high-energy particles and solar radiation during such intense events.
• 2013: RHESSI celebrated its 11th anniversary, surpassing its initial mission lifetime of three years, marking its extraordinary success and resilience in studying solar flares and their effects on space weather.
• 2018: NASA decommissioned RHESSI due to communication difficulties with the spacecraft. The mission was part of NASA’s Small Explorers program, managed and operated by the agency’s Goddard Space Flight Center in Greenbelt, Maryland.
• April 19, 2023 (predicted): RHESSI is expected to reenter Earth’s atmosphere, with an uncertainty of +/- 16 hours. Although most of the spacecraft will burn up during reentry, some components may survive, posing a minimal risk to people on Earth (estimated at 1 in 2,467).