On April 7, 2025, NASA scientists seized a rare opportunity to study Uranus’ atmosphere during a stellar occultation, when the planet passed between Earth and a star 400 light years away. This cosmic alignment allowed researchers to measure the temperature, density, and pressure of Uranus’ stratosphere, providing insights into its atmospheric dynamics and refining data critical for future exploration of the ice giant.
The event, visible only from Western North America for about an hour, was observed by an international team of over 30 astronomers coordinated by NASA’s Langley Research Center. Led by planetary scientist William Saunders, the Uranus Stellar Occultation Campaign 2025 utilized 18 professional observatories to capture the event. Saunders explained, “As Uranus began to occult the star, the planet’s atmosphere refracted the starlight, causing the star to appear to gradually dim before being blocked completely. The reverse happened at the end of the occultation, making what we call a light curve. By observing the occultation from many large telescopes, we are able to measure the light curve and determine Uranus’ atmospheric properties at many altitude layers.”
The light curve data enables researchers to analyze how energy moves within Uranus’ atmosphere, investigate why its upper layers are unexpectedly hot, and study its 13 known rings, atmospheric turbulence, and precise orbit. Uranus, nearly 2 billion miles from Earth, lacks a solid surface, consisting primarily of hydrogen, helium, and a soft surface of water, ammonia, and methane. Its interior, rich in these low-freezing-point fluids, earns it the “ice giant” designation.
“This was the first time we have collaborated on this scale for an occultation,” Saunders said. “I am extremely grateful to each member of the team and each observatory for taking part in this extraordinary event.” The collaboration included NASA’s Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, where postdoctoral scholar Emma Dahl from Caltech contributed observations. Dahl noted, “As scientists, we do our best work when we collaborate. This was a team effort between NASA scientists, academic researchers, and amateur astronomers. The atmospheres of the gas and ice giant planets are exceptional atmospheric laboratories because they don’t have solid surfaces.”
A preparatory occultation on November 12, 2024, visible from Asia, allowed NASA to refine techniques. Langley coordinated telescopes in Japan and Thailand, while Paris Observatory and Space Science Institute researchers observed from India. These efforts improved timing predictions for the April event to the second and updated Uranus’ expected position by 125 miles, critical given its orbit is only known within 100 miles.Since NASA’s Voyager 2 flyby in 1986—the only spacecraft to visit Uranus—no bright occultation had occurred since 1996, making the 2025 event a pivotal moment.
The data will inform future Uranus missions by enhancing models of its atmosphere and orbit. Over the next six years, Uranus will occult dimmer stars, with NASA planning airborne and possibly space-based observations for a brighter occultation in 2031. The collaborative approach demonstrated in 2025 sets a foundation for these efforts, leveraging diverse instruments to capture unique perspectives.
Uranus’ atmospheric research also benefits from its lack of a solid surface, simplifying studies of cloud formation, storms, and wind patterns. The April occultation’s success underscores the value of global cooperation in planetary science, with applications for understanding ice giants and planning NASA’s exploration strategies.
