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Hubble Astronomers Develop a New Use for a Century-Old Relativity Experiment to Measure a White Dwarf's Mass
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By NASA
5 min read
New NASA Mission to Reveal Earth’s Invisible ‘Halo’
A new NASA mission will capture images of Earth’s invisible “halo,” the faint light given off by our planet’s outermost atmospheric layer, the exosphere, as it morphs and changes in response to the Sun. Understanding the physics of the exosphere is a key step toward forecasting dangerous conditions in near-Earth space, a requirement for protecting Artemis astronauts traveling through the region on the way to the Moon or on future trips to Mars. The Carruthers Geocorona Observatory will launch from NASA’s Kennedy Space Center in Florida no earlier than Tuesday, Sept. 23.
Revealing Earth’s invisible edge
In the early 1970s, scientists could only speculate about how far Earth’s atmosphere extended into space. The mystery was rooted in the exosphere, our atmosphere’s outermost layer, which begins some 300 miles up. Theorists conceived of it as a cloud of hydrogen atoms — the lightest element in existence — that had risen so high the atoms were actively escaping into space.
But the exosphere reveals itself only via a faint “halo” of ultraviolet light known as the geocorona. Pioneering scientist and engineer Dr. George Carruthers set himself the task of seeing it. After launching a few prototypes on test rockets, he developed an ultraviolet camera ready for a one-way trip to space.
Apollo 16 astronaut John Young is pictured on the lunar surface with George Carruthers’ gold-plated Far Ultraviolet Camera/Spectrograph, the first Moon-based observatory. The Lunar Module “Orion” is on the right and the Lunar Roving Vehicle is parked in the background next to the American flag. NASA In April 1972, Apollo 16 astronauts placed Carruthers’ camera on the Moon’s Descartes Highlands, and humanity got its first glimpse of Earth’s geocorona. The images it produced were as stunning for what they captured as they were for what they didn’t.
“The camera wasn’t far enough away, being at the Moon, to get the entire field of view,” said Lara Waldrop, principal investigator for the Carruthers Geocorona Observatory. “And that was really shocking — that this light, fluffy cloud of hydrogen around the Earth could extend that far from the surface.” Waldrop leads the mission from the University of Illinois Urbana-Champaign, where George Carruthers was an alumnus.
The first image of UV light from Earth’s outer atmosphere, the geocorona, taken from a telescope designed and built by George Carruthers. The telescope took the image while on the Moon during the Apollo 16 mission in 1972. G. Carruthers (NRL) et al./Far UV Camera/NASA/Apollo 16 Our planet, in a new light
Today, the exosphere is thought to stretch at least halfway to the Moon. But the reasons for studying go beyond curiosity about its size.
As solar eruptions reach Earth, they hit the exosphere first, setting off a chain of reactions that sometimes culminate in dangerous space weather storms. Understanding the exosphere’s response is important to predicting and mitigating the effects of these storms. In addition, hydrogen — one of the atomic building blocks of water, or H2O — escapes through the exosphere. Mapping that escape process will shed light on why Earth retains water while other planets don’t, helping us find exoplanets, or planets outside our solar system, that might do the same.
NASA’s Carruthers Geocorona Observatory, named in honor of George Carruthers, is designed to capture the first continuous movies of Earth’s exosphere, revealing its full expanse and internal dynamics.
“We’ve never had a mission before that was dedicated to making exospheric observations,” said Alex Glocer, the Carruthers mission scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s really exciting that we’re going to get these measurements for the first time.”
Download this video from NASA’s Scientific Visualization Studio.
Journey to L1
At 531 pounds and roughly the size of a loveseat sofa, the Carruthers spacecraft will launch aboard a SpaceX Falcon 9 rocket along with NASA’s IMAP (Interstellar Mapping and Acceleration Probe) spacecraft and the National Oceanic and Atmospheric Administration’s SWFO-L1 (Space Weather Follow On – Lagrange 1) space weather satellite. After launch, all three missions will commence a four-month cruise phase to Lagrange point 1 (L1), a location approximately 1 million miles closer to the Sun than Earth is. After a one-month period for science checkouts, Carruthers’ two-year science phase will begin in March 2026.
Artist’s concept of the five Sun-Earth Lagrange points in space. At Lagrange points, the gravitational pull of two large masses counteract, allowing spacecraft to reduce fuel consumption needed to remain in position. The L1 point of the Earth-Sun system affords an uninterrupted view of the Sun and will be home to three new heliophysics missions in 2025: NASA’s Interstellar Mapping and Acceleration Probe (IMAP), NASA’s Carruthers Geocorona Observatory, and NOAA’s Space Weather Follow-On – Lagrange 1 (SWFO – L1). NASA’s Conceptual Image Lab/Krystofer Kim From L1, roughly four times farther away than the Moon, Carruthers will capture a comprehensive view of the exosphere using two ultraviolet cameras, a near-field imager and a wide-field imager.
“The near-field imager lets you zoom up really close to see how the exosphere is varying close to the planet,” Glocer said. “The wide-field imager lets you see the full scope and expanse of the exosphere, and how it’s changing far away from the Earth’s surface.”
The two imagers will together map hydrogen atoms as they move through the exosphere and ultimately out to space. But what we learn about atmospheric escape on our home planet applies far beyond it.
“Understanding how that works at Earth will greatly inform our understanding of exoplanets and how quickly their atmospheres can escape,” Waldrop said.
By studying the physics of Earth, the one planet we know that supports life, the Carruthers Geocorona Observatory can help us know what to look for elsewhere in the universe.
The Carruthers Geocorona Observatory mission is led by Lara Waldrop from the University of Illinois Urbana-Champaign. The Space Sciences Laboratory at the University of California, Berkeley leads mission implementation, design and development of the payload in collaboration with Utah State University’s Space Dynamics Laboratory. The Carruthers spacecraft was designed and built by BAE Systems. NASA’s Explorers and Heliophysics Projects Division at the agency’s Goddard Space Flight Center in Greenbelt, Maryland, manages the mission for the agency’s Heliophysics Division at NASA Headquarters in Washington.
By Miles Hatfield
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Sep 18, 2025 Related Terms
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Explore Hubble Science Hubble Space Telescope NASA’s Hubble Sees White… Hubble Home Overview About Hubble The History of Hubble Hubble Timeline Why Have a Telescope in Space? Hubble by the Numbers At the Museum FAQs Impact & Benefits Hubble’s Impact & Benefits Science Impacts Cultural Impact Technology Benefits Impact on Human Spaceflight Astro Community Impacts Science Hubble Science Science Themes Science Highlights Science Behind Discoveries Universe Uncovered Hubble’s Partners in Science AI and Hubble Science Explore the Night Sky Observatory Hubble Observatory Hubble Design Mission Operations Science Operations Astronaut Missions to Hubble Hubble vs Webb Team Hubble Team Career Aspirations Hubble Astronauts Multimedia Images Videos Sonifications Podcasts e-Books Online Activities 3D Hubble Models Lithographs Fact Sheets Posters Hubble on the NASA App Glossary News Hubble News Social Media Media Resources More 35th Anniversary Online Activities 5 Min Read NASA’s Hubble Sees White Dwarf Eating Piece of Pluto-Like Object
This artist’s concept shows a white dwarf surrounded by a large debris disk. Debris from pieces of a captured, Pluto-like object is falling onto the white dwarf. Credits:
Artwork: NASA, Tim Pyle (NASA/JPL-Caltech) In our nearby stellar neighborhood, a burned-out star is snacking on a fragment of a Pluto-like object. With its unique ultraviolet capability, only NASA’s Hubble Space Telescope could identify that this meal is taking place.
The stellar remnant is a white dwarf about half the mass of our Sun, but that is densely packed into a body about the size of Earth. Scientists think the dwarf’s immense gravity pulled in and tore apart an icy Pluto analog from the system’s own version of the Kuiper Belt, an icy ring of debris that encircles our solar system. The findings were reported on September 18 in the Monthly Notices of the Royal Astronomical Society.
The researchers were able to determine this carnage by analyzing the chemical composition of the doomed object as its pieces fell onto the white dwarf. In particular, they detected “volatiles” — substances with low boiling points — including carbon, sulphur, nitrogen, and a high oxygen content that suggests the strong presence of water.
“We were surprised,” said Snehalata Sahu of the University of Warwick in the United Kingdom. Sahu led the data analysis of a Hubble survey of white dwarfs. “We did not expect to find water or other icy content. This is because the comets and Kuiper Belt-like objects are thrown out of their planetary systems early, as their stars evolve into white dwarfs. But here, we are detecting this very volatile-rich material. This is surprising for astronomers studying white dwarfs as well as exoplanets, planets outside our solar system.”
This artist’s concept shows a white dwarf surrounded by a large debris disk. Debris from pieces of a captured, Pluto-like object is falling onto the white dwarf. Artwork: NASA, Tim Pyle (NASA/JPL-Caltech) Only with Hubble
Using Hubble’s Cosmic Origins Spectrograph, the team found that the fragments were composed of 64 percent water ice. The fact that they detected so much ice meant that the pieces were part of a very massive object that formed far out in the star system’s icy Kuiper Belt analog. Using Hubble data, scientists calculated that the object was bigger than typical comets and may be a fragment of an exo-Pluto.
They also detected a large fraction of nitrogen – the highest ever detected in white dwarf debris systems. “We know that Pluto’s surface is covered with nitrogen ices,” said Sahu. “We think that the white dwarf accreted fragments of the crust and mantle of a dwarf planet.”
Accretion of these volatile-rich objects by white dwarfs is very difficult to detect in visible light. These volatile elements can only be detected with Hubble’s unique ultraviolet light sensitivity. In optical light, the white dwarf would appear ordinary.
About 260 light-years away, the white dwarf is a relatively close cosmic neighbor. In the past, when it was a Sun-like star, it would have been expected to host planets and an analog to our Kuiper Belt.
Like seeing our Sun in future
Billions of years from now, when our Sun burns out and collapses to a white dwarf, Kuiper Belt objects will be pulled in by the stellar remnant’s immense gravity. “These planetesimals will then be disrupted and accreted,” said Sahu. “If an alien observer looks into our solar system in the far future, they might see the same kind of remains we see today around this white dwarf.”
The team hopes to use NASA’s James Webb Space Telescope to detect molecular features of volatiles such as water vapor and carbonates by observing this white dwarf in infrared light. By further studying white dwarfs, scientists can better understand the frequency and composition of these volatile-rich accretion events.
Sahu is also following the recent discovery of the interstellar comet 3I/ATLAS. She is eager to learn its chemical composition, especially its fraction of water. “These types of studies will help us learn more about planet formation. They can also help us understand how water is delivered to rocky planets,” said Sahu.
Boris Gänsicke, of the University of Warwick and a visitor at Spain’s Instituto de Astrofisica de Canarias, was the principal investigator of the Hubble program that led to this discovery. “We observed over 500 white dwarfs with Hubble. We’ve already learned so much about the building blocks and fragments of planets, but I’ve been absolutely thrilled that we now identified a system that resembles the objects in the frigid outer edges of our solar system,” said Gänsicke. “Measuring the composition of an exo-Pluto is an important contribution toward our understanding of the formation and evolution of these bodies.”
The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
To learn more about Hubble, visit: https://science.nasa.gov/hubble
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White Dwarf Accreting Icy Object (Illustration)
This artist’s concept shows a white dwarf surrounded by a large debris disk. Debris from pieces of a captured, Pluto-like object is falling onto the white dwarf.
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Last Updated Sep 18, 2025 Editor Andrea Gianopoulos Location NASA Goddard Space Flight Center Contact Media Claire Andreoli
NASA’s Goddard Space Flight Center
Greenbelt, Maryland
claire.andreoli@nasa.gov
Ann Jenkins
Space Telescope Science Institute
Baltimore, Maryland
Ray Villard
Space Telescope Science Institute
Baltimore, Maryland
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Science Paper: Discovery of an icy and nitrogen-rich extra-solar planetesimal, PDF (674.84 KB)
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