09-29-2025

Earth’s invisible exosphere will soon get a close-up from space

ByEric Ralls

Earth.com staff writer

There’s a massive, ghostly halo around Earth. You can’t see it. Most people have never heard of it. But it’s there – a loose blanket of hydrogen atoms that stretches far out into space. It starts about 300 miles above us. This cloud is called the exosphere, and it forms the very outer edge of Earth’s atmosphere.

The exosphere is where Earth’s air slowly fades into space. It’s where hydrogen atoms escape our planet forever. It’s also the first place that gets hit by energy blasts from the sun. And now, for the first time, we’re sending a spacecraft to take a long, hard look at it.

Watching Earth’s exosphere fade into space

The new spacecraft is called the Carruthers Geocorona Observatory. It’s going to take pictures of Earth’s exosphere – not just snapshots, but full-on movies. These moving images will show how Earth’s outer atmosphere shifts and reacts to solar storms and other space weather.

Back in the early 1970s, few people realized how big Earth’s atmosphere actually was. The exosphere was largely a theoretical concept – a hydrogen-gas cloud that floated high above, with some of it drifting into space. Because hydrogen glows in ultraviolet light, scientists speculated it might appear as a sort of faint halo around the planet.

One scientist, George Carruthers, made it his mission to find that glow. He built a special ultraviolet camera that flew to the moon on Apollo 16 in 1972. Astronauts placed it on the lunar surface and aimed it back at Earth. What it captured surprised everyone.

“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 Earth could extend that far from the surface.”

Today, more than 50 years later, NASA is paying tribute to Carruthers by naming a spacecraft after him. Waldrop, who oversees the mission from the University of Illinois Urbana-Champaign – Carruthers’ alma mater – is picking up where he left off.

Why study this thin, quiet layer?

Scientists now believe the Earth’s exosphere extends at least halfway to the moon. That makes Carruthers’s cloud not simply large – it is enormous. But this isn’t just about measuring how far it goes.

When the sun releases solar flares or other eruptions, the exosphere takes the hit first. These solar storms can wreak havoc on satellites, blast out power grids, and even harm astronauts. By understanding the response of the exosphere, scientists hope to be able to forecast better when things are likely to go wrong.

Hydrogen also escapes into space via the Earth’s exosphere. That matters because hydrogen is found in water (H₂O). Determining how Earth retained water, but other planets lost theirs, may be able to tell us which exoplanets – planets beyond our solar system – would possibly have water today.

“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. “It’s really exciting that we’re going to get these measurements for the first time.”

Capturing the entire halo around Earth

The Carruthers spacecraft is about the size of a loveseat and weighs 531 pounds (240 kilograms). It’s launching from NASA’s Kennedy Space Center in Florida no earlier than Tuesday, September 23, on a SpaceX Falcon 9 rocket.

It won’t be flying solo. Two other spacecraft – NASA’s IMAP and NOAA’s SWFO-L1 – will be tagging along. All three will cruise for about four months to a spot called Lagrange Point 1, or L1. That’s a stable place in space about 1 million miles (1.6 million kilometers) closer to the sun than Earth.

From that far out – nearly four times the distance to the moon – Carruthers will have the perfect view to capture the entire halo around Earth. It will use two ultraviolet cameras: a near-field imager and a wide-field imager.

The exospheres of other planets

“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 Earth’s surface.”

These instruments will track hydrogen atoms as they float away from Earth’s exosphere. But this mission isn’t just about Earth. It’s about every planet that ever had – or still has – an atmosphere.

“Understanding how that works at Earth will greatly inform our understanding of exoplanets and how quickly their atmospheres can escape,” Waldrop said.

Carruthers’ mission will begin full scientific operations in March 2026. For the first time, we’ll have moving pictures of Earth’s invisible edge – and a better idea of what it means to live on a planet that can keep its air and water while still facing the power of the sun.