For practically as long as NASA‘s Hubble Space Telescope has circled Earth, astronomers have wanted to use it to see a comet as it’s breaking apart.
Many researchers have proposed such observations in the past, but because comet breakups are hard to anticipate, no one had achieved it — until now. By a stroke of luck, a team of scientists has, at long last, caught one in the act.
The researchers looked at the comet, C/2025 K1 (ATLAS), only because there were some technical issues preventing them from observing their first choice. When John Noonan, the co-investigator on the study, saw the images Hubble had taken, he was in for the ultimate surprise. Instead of one space snowball, he saw four.
“We knew this was something really, really special,” said Noonan, a research professor at Auburn University, in a statement.
Astronomers have sought a fragmenting comet for years because it’s one of the few opportunities they have to see what’s hidden inside these ancient icy objects. Such observations could be compared to a geologist taking a core sample of the early solar system.
Comets formed about 4.6 billion years ago, when planets were still developing around the sun. Scientists think of these relics as time capsules because their interiors can still hold relatively unchanged ice, dust, and chemicals from that era.
Scientists are tracking over 4,000 known comets, but countless others are likely flying in orbits beyond Neptune in a disk known as the Kuiper Belt or from an even more distant realm about 50 times farther, referred to as the Oort Cloud. The region remains a theory because the comets there have been too faint to be directly observed.
Astronomers normally only observe the outside of a comet, which has seen the abuse of heat and radiation. That makes it difficult for them to know whether they’re looking at primitive properties from the solar system’s birth or properties changed by time and environment.
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“By cracking open a comet, you can see the ancient material that has not been processed,” said Dennis Bodewits, also a professor at Auburn and the lead investigator.
In the new Hubble images, each of the fragments of the comet, called K1 for short, had its own glowing halo of gas and dust — a coma — that forms when sunlight heats the icy surface and instantly turns the material into gas.
The telescope observed K1 over the course of three consecutive days in November. That was about one month after the comet swung closest to the sun — even closer than Mercury. The team’s findings were published in the journal Icarus.
Before K1 shattered, it was probably a bit larger than an average comet, perhaps the size of Key West, Florida, or the Los Angeles International Airport.
The inner edge of the main part of the Oort Cloud, a theorized region surrounding the solar system, could be as close as 1,000 astronomical units from the sun, with one astronomical unit equal to the distance between Earth and the sun. The outer edge is estimated to end about 100,000 AU away from the sun.
Credit: NASA Goddard infographic
The team believes the breakup began roughly eight days before Hubble’s first image, then caught another chunk splitting as it happened.
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The timing revealed a new mystery. Telescopes on Earth saw the comet brighten later than predicted. The delay may happen because a comet’s brightness is more closely linked to sunlight reflecting off dust rather than on newly revealed ice. Scientists think fresh ice may need time to warm up, build pressure below the surface, or accumulate a dusty cover that eventually blows away in space.
Early measurements also suggest K1 contains unusually low amounts of carbon compared with other comets. Further studies of its gases could help explain why.
“Sometimes the best science happens by accident,” Noonan said.
Now reduced to scattered bits about 250 million miles from Earth, K1 is heading out of the solar system and will probably never return.
