Planetary Alignment Provides NASA Rare Opportunity to Study Uranus

Planetary Alignment Provides NASA Rare Opportunity to Study Uranus
by Charles Hatfield for NASA News
Hampton VA (SPX) Apr 23, 2025

When a planet's orbit brings it between Earth and a distant star, it's more than just a cosmic game of hide and seek. It's an opportunity for NASA to improve its understanding of that planet's atmosphere and rings. Planetary scientists call it a stellar occultation and that's exactly what happened with Uranus on April 7.

Observing the alignment allows NASA scientists to measure the temperatures and composition of Uranus' stratosphere - the middle layer of a planet's atmosphere - and determine how it has changed over the last 30 years since Uranus' last significant occultation.

"Uranus passed in front of a star that is about 400 light years from Earth," said William Saunders, planetary scientist at NASA's Langley Research Center in Hampton, Virginia, and science principal investigator and analysis lead, for what NASA's team calls the Uranus Stellar Occultation Campaign 2025. "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."

This data mainly consists of temperature, density, and pressure of the stratosphere. Analyzing the data will help researchers understand how the middle atmosphere of Uranus works and could help enable future Uranus exploration efforts.

To observe the rare event, which lasted about an hour and was only visible from Western North America, planetary scientists at NASA Langley led an international team of over 30 astronomers using 18 professional observatories.

"This was the first time we have collaborated on this scale for an occultation," said Saunders. "I am extremely grateful to each member of the team and each observatory for taking part in this extraordinary event. NASA will use the observations of Uranus to determine how energy moves around the atmosphere and what causes the upper layers to be inexplicably hot. Others will use the data to measure Uranus' rings, its atmospheric turbulence, and its precise orbit around the Sun."

Knowing the location and orbit of Uranus is not as simple as it sounds. In 1986, NASA's Voyager 2 spacecraft became the first and only spacecraft to fly past the planet - 10 years before the last bright stellar occultation occured in 1996. And, Uranus' exact position in space is only accurate to within about 100 miles, which makes analyzing this new atmospheric data crucial to future NASA exploration of the ice giant.

These investigations were possible because the large number of partners provided many unique views of the stellar occultation from many different instruments.

Emma Dahl, a postdoctoral scholar at Caltech in Pasadena, California, assisted in gathering observations from NASA's Infrared Telescope Facility (IRTF) on the summit of Mauna Kea in Hawaii - an observatory first built to support NASA's Voyager missions.

"As scientists, we do our best work when we collaborate. This was a team effort between NASA scientists, academic researchers, and amateur astronomers," said Dahl. "The atmospheres of the gas and ice giant planets [Jupiter, Saturn, Uranus, and Neptune] are exceptional atmospheric laboratories because they don't have solid surfaces. This allows us to study cloud formation, storms, and wind patterns without the extra variables and effects a surface produces, which can complicate simulations very quickly."

On November 12, 2024, NASA Langley researchers and collaborators were able to do a test run to prepare for the April occultation. Langley coordinated two telescopes in Japan and one in Thailand to observe a dimmer Uranus stellar occultation only visible from Asia. As a result, these observers learned how to calibrate their instruments to observe stellar occultations, and NASA was able to test its theory that multiple observatories working together could capture Uranus' big event in April.

Researchers from the Paris Observatory and Space Science Institute, in contact with NASA, also coordinated observations of the November 2024 occultation from two telescopes in India. These observations of Uranus and its rings allowed the researchers, who were also members of the April 7 occultation team, to improve the predictions about the timing on April 7 down to the second and also improved modeling to update Uranus' expected location during the occultation by 125 miles.

Uranus is almost 2 billion miles away from Earth and has an atmosphere composed of primarily hydrogen and helium. It does not have a solid surface, but rather a soft surface made of water, ammonia, and methane. It's called an ice giant because its interior contains an abundance of these swirling fluids that have relatively low freezing points. And, while Saturn is the most well-known planet for having rings, Uranus has 13 known rings composed of ice and dust.

Over the next six years, Uranus will occult several dimmer stars. NASA hopes to gather airborne and possibly space-based measurements of the next bright Uranus occultation in 2031, which will be of an even brighter star than the one observed in April.