Comet 3I/ATLAS is leaving the solar system with a dramatic light show
After months of unprecedented observations, astronomers bid farewell to the beloved comet 3I/ATLAS. First spotted in July 2025, this icy, dusty space rock is only the third known interstellar object to pass through the solar system, giving researchers the rare opportunity to examine a visitor from deep space. Among other discoveries, scientists have since confirmed that the interstellar comet is the fastest comet ever recorded and covered in ice volcanoes – and certainly not extraterrestrial tourists.
But even as it continues to move away from Earth at a speed of about 130,000 miles per hour, astronomers continue to learn new information from 3I/ATLAS. In a recently published research note, NASA describes a recent and striking turn of events on the comet’s surface. In December 2025, the agency’s SPHEREx space observatory recorded a massive peak in brightness from 3I/ATLAS. The exposure took place about two months after the comet reached its closest distance from the sun and allowed researchers to better catalog more of the comet’s organic molecules, including cyanide, methane and methanol.
“Comet 3I/ATLAS was erupting into space in December 2025, after its close flyby of the sun, which brightened it considerably. Even water ice quickly sublimated into gas in interplanetary space,” explained astrophysicist and lead author of the study Carey Lisse. “And because comets are about one-third water ice, they were releasing an abundance of new carbon-rich material that had remained locked in the ice deep below the surface.”
Previous SPHEREx observations taken in August 2025 recorded a cometary coma containing large amounts of carbon dioxide, as well as lower levels of carbon monoxide and water. 3I/ATLAS came closest to the sun in October. In December, SPHEREx detected a diverse amalgam including organic matter and rock debris as well as antecedent materials.
At first glance, this delay in sublimation (the transition from a solid to a gas) may not make sense. After all, why wouldn’t the process start when 3I/ATLAS is closest to the sun? Although this is certainly when exposure to solar radiation peaks, it still takes time for all of this energy to reach the deeper layers of the comet. In this case, the materials did not begin to sublimate until two months later. Part of this delay is probably also due to the comet’s ancient origins.
“The comet spent centuries passing through interstellar space, being bombarded by highly energetic cosmic rays, and likely formed a crust that was treated by this radiation,” said Phil Korngut, Caltech mission instrument scientist. “But now that the Sun’s energy has had time to penetrate deep into the comet, the pristine ices beneath the surface are heating up and erupting, releasing a cocktail of chemicals that have not been exposed to space in billions of years.”
It remains to be seen when – or even if – another interstellar object will visit the solar system in our lifetime. But tools like SPHEREx already provide astronomers with mountains of data to sift through long after the light from 3I/ATLAS has faded.
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