Might a comet be more than ice and dust? That’s what Harvard astrophysicist Avi Loeb’s request for an unprecedented close monitoring of 3I/ATLAS, the third verified interstellar object to pass through our solar system, is about. Loeb has proposed that the comet, now hurtling toward perihelion, may not be purely natural and may even expel tiny probes en route.
Detected on July 1, 2025, by the ATLAS survey telescope in Chile, 3I/ATLAS differs from the two previous interstellar visitors, ‘Oumuamua and 2I/Borisov. It is moving at about 130,000 mph with respect to the Sun the fastest interstellar object ever and has a retrograde orbit nearly as good an alignment as the solar system’s ecliptic plane. Its path will take it just 1.67 million miles from the orbit of Mars and comparatively near Venus and Jupiter, a “remarkable fine-tuning” that Loeb has claimed could be beneficial to an extraterrestrial intelligence looking to witness or send devices.
Size estimates have only added to the enigma. Whereas initial Hubble data hinted at a nucleus of 0.2 to 3.5 miles in diameter, subsequent analyses have pegged its diameter at anywhere from 6.2 to 28.5 miles. If the high end is correct, Loeb writes, the object would be a million times larger than any known interstellar body, an outlier that tests models for the formation and travels through the galaxy of such bodies.
Spectroscopic observations using NASA’s James Webb Space Telescope in August showed a carbon dioxide-dominated coma with weak water and carbon monoxide signatures a rare solar system comet composition. The elevated CO₂-to-water ratio implies that 3I/ATLAS could have originated close to a CO₂ ice line in its parent system or under strong radiation, eventually becoming active far beyond Jupiter’s orbit because of the low sublimation temperature of CO₂. This initial activity was confirmed by Gemini South telescope observations of a developing tail, which will probably split into separate dust and ion components as the comet approaches perihelion.
In addition to the robust cometary signatures, Loeb has encouraged NASA to make use of spacecraft that are already well-positioned for close-up surveillance during perihelion, when telescopes on Earth will be blinded by the Sun. Contenders are the Mars Reconnaissance Orbiter, ESA’s JUICE mission, and NASA’s Psyche probe. JUICE, having just passed an August 31 Venus gravity assist, will be roughly 43 million miles from the comet at perihelion, and Mars orbiters may be as close as 18 million miles much closer than 3I/ATLAS will ever approach Earth. These platforms might have the ability to take high-resolution photos, spectral data, and even search for anomalous emissions or objects detaching from the main mass.
The technical hurdle is quite high. With its current speed and trajectory, interception by a freshly launched spacecraft is not feasible. This opportunistically makes use of assets that already exist essential, particularly if scientists are to be able to capture any transient activity like fragmentation, unstable outbursts, or in Loeb’s more fanciful hypothesis the deployment of “mini-probes” that would go undetected as unidentified aerial phenomena when they arrive on Earth. “We must keep that in mind,” Loeb said to The Sol Foundation podcast. “And we just need to keep our eyes on the ball.”
A majority of astronomers continue to believe 3I/ATLAS is an interstellar comet of natural origin, having been expelled from a different star system billions of years ago. Dynamical simulations indicate that it could be from the Milky Way’s thick disk, and thus potentially 7 billion years old much older than our planets and Sun. Analyzing its content might thus provide a rare insight into the material of the galaxy’s “cosmic noon,” the period of peak star formation. As Andreas Hein of the University of Luxembourg explained, “It’s like an aeon-old fridge, which will open during the next months to release some of its contents.”
Finding such interstellar guests is still in its early stages. The Vera C. Rubin Observatory, now up and running, should significantly grow the list of such objects by monitoring the southern sky every few nights. Until then, every discovery is a treasure. With perihelion on Oct. 29–30, 2025, and nearest approach to Earth in mid-December at 168 million miles, the time for close observation is short. After 3I/ATLAS slingshots back out into interstellar space, it will take its secrets natural or otherwise with it.
