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.

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.

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.

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.