Weeks after a rare interstellar comet made its closest pass to the Sun, it will make its closest pass to Earth.
Comet 3I/ATLAS was first discovered in July by a regular survey of the sky by the Asteroid Terrestrial Impact Alert System (ATLAS), after which the comet was named. The name “3I” means “third interstellar” because it is only the third confirmed interstellar object to pass through our solar system – after 2I/Borisov in 2019 and 1I/’Oumuamua in 2017.
NASA’s Psyche mission obtained four observations of the interstellar comet 3I/ATLAS over the course of eight hours on September 8 and 9, 2025, when the comet was about 33 million miles (53 million kilometers) from the spacecraft. The data, captured by Psyche’s multispectral imager, is helping astronomers refine their 3I/ATLAS trajectory and learn more about the faint coma, or gas cloud, surrounding its core (shown in enlarged image). (Courtesy/NASA/JPL-Caltech/ASU)
The comet traveled billions of miles through other systems in space. Its orbit is highly hyperbolic, meaning it’s moving fast enough to avoid our sun’s gravitational pull — even when it reached its closest point to the Sun on Oct. 30, about 130 million miles away. For reference, Earth is about 93 million miles from the Sun.
When does comet 3I/ATLAS pass Earth? Can I see the comet?
The astronomical wonder will reach its closest point to Earth Friday, Dec. 19about 168 million miles away. Experts say it will be closest in the pre-dawn hours between 2:30 a.m. and 5 a.m. in the east-northeast sky near Regulus, the bright star in the constellation Leo.
Unfortunately, comet 3I/ATLAS will not be visible to the naked eye. Viewers will need a small telescope or powerful binoculars to see what appears to be a faint, hazy spot.
Don’t worry though, online space exploration publication Space.com is hosting a live stream December 18-19 courtesy of the Virtual Telescope Project starting at 10pm CT on Thursday, December 18.
What we know about Comet 3I/ATLAS
The interstellar origins of Comet 3I/ATLAS make it an extraordinary discovery for astronomers. Such objects can give scientists the opportunity to sample material from other star systems and compare it to our own.
“[Interstellar objects are] like cosmic time capsules, which provide samples of distant exoplanetary systems that we might not otherwise be able to visit and study directly,” states Space.com. “The discovery of 3I/ATLAS opens the door to a whole new branch of study in astrophysics.”
However, this opportunity is fleeting – upon its discovery, 3I/ATLAS passed through our solar system at approximately 130,000 miles per hour, which according to NASA is “the fastest speed ever recorded for a visitor to the solar system.” It is believed to have moved faster and faster in the months since, and will soon leave our system and continue its journey through interstellar space.
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A few weeks after the comet was identified, NASA’s Hubble Space Telescope gathered enough information to give astronomers a better understanding of its size and composition. 3I/ATLAS has a “solid, icy core” believed to be between 1,000 feet and 3.5 miles in diameter. David Jewitt of the University of California, the leader of the science team for the Hubble observations, explained the difficulty in drawing deeper conclusions about the object.
“No one knows where the comet came from,” Jewitt said. “It’s like glimpsing a rifle bullet for a thousandth of a second. You can’t project it back with any precision to figure out where it started on its way.”
The comet’s chemical composition suggests that it formed in a cold, distant region of its parent system’s disk under conditions of temperature, radiation, or disk environment much different from those of our solar system. Experts say 3I/ATLAS is probably very old, between three and 14 billion years old, according to the Planetary Society. For comparison, our solar system is about 4.6 billion years old.
Timeline of comet 3I/ATLAS by NASA images
This image shows the observation of comet 3I/ATLAS when it was discovered on July 1, 2025. The NASA-funded ATLAS survey telescope in Chile first reported that the comet originated in interstellar space. (Courtesy/ATLAS/University of Hawaii/NASA)
Hubble captured this image of the interstellar comet 3I/ATLAS on July 21, 2025, when the comet was 277 million miles from Earth. Hubble shows that the comet has a teardrop-shaped cocoon of dust protruding from its solid, frozen nucleus. (Courtesy/NASA/ESA/David Jewitt UCLA)
NASA’s James Webb Space Telescope observed the interstellar comet 3I/ATLAS on August 6 with its Near Infrared Spectrograph instrument. (Courtesy/NASA/James Webb Space Telescope)
NASA’s Psyche mission obtained four observations of the interstellar comet 3I/ATLAS over the course of eight hours on September 8 and 9, 2025, when the comet was about 33 million miles (53 million kilometers) from the spacecraft. The data, captured by Psyche’s multispectral imager, is helping astronomers refine their 3I/ATLAS trajectory and learn more about the faint coma, or gas cloud, surrounding its core (shown in enlarged image). (Courtesy/NASA/JPL-Caltech/ASU)
Is 3I/ATLAS an alien ship? The Harvard professor’s theory has been criticized
The rumor that 3I/ATLAS might not be a comet at all came from Avi Loeb, an astrophysicist at Harvard University and head of its Galileo project. Loeb, along with two others, wrote a paper published in July that speculated the object could be “hostile” alien technology. The publication justified the claim in part by pointing out its unusual features, such as its hyperbolic trajectory, alignment with the ecliptic plane (the plane of Earth’s orbit), and “unnatural” chemical and physical structure.
Many experts and agencies such as NASA shared doubts about Loeb’s theory, as The Guardian reported.
“It looks like a comet. It does comet things. It looks very, very strongly, in almost every way, to the comets we know,” said Tom Statler, a NASA principal scientist. “It has some interesting properties that are a little different from comets in our solar system, but it behaves like a comet. And so the evidence overwhelmingly points to this object being a natural body. It is a comet.”
The theory that 3I/ATLAS is related to extraterrestrials is not Loeb’s first such claim. In 2017, he made a similar claim about 1I/’Oumuamua, the first interstellar object detected in our solar system. This theory has since been debunked, including by world-class astrophysics programs such as the one at the University of Hawaii.