3I/ATLAS, an interstellar object believed to be a comet, brightened sharply at perihelion and developed long jets and an anti-tail. Avi Loeb calculates that the observed mass loss would require a surface area far larger than earlier estimates, leading him to suggest the object may have fragmented into at least 16 pieces. Alternatively, Loeb notes a non-natural explanation would fit the data if the object remained intact; astronomers await follow-up observations during its upcoming close approaches to Earth and Jupiter.
Interstellar Visitor 3I/ATLAS Brightens — Avi Loeb Says It May Have Fragmented at Perihelion
3I/ATLAS, an interstellar object believed to be a comet, brightened sharply at perihelion and developed long jets and an anti-tail. Avi Loeb calculates that the observed mass loss would require a surface area far larger than earlier estimates, leading him to suggest the object may have fragmented into at least 16 pieces. Alternatively, Loeb notes a non-natural explanation would fit the data if the object remained intact; astronomers await follow-up observations during its upcoming close approaches to Earth and Jupiter.
04:05 AM, 11/11/2025Science
Mysterious Interstellar Object 3I/ATLAS Brightens Dramatically at Perihelion
Mysterious interstellar object 3I/ATLAS brightened sharply as it passed closest to the Sun (perihelion), producing an extraordinary outflow of gas and dust visible in telescopic images. Observers widely treat 3I/ATLAS as a comet, but the rate of mass loss inferred from recent images is unusually large and has prompted renewed debate about the object's nature.
New images and striking jets
British astronomers Michael Buechner and Frank Niebling captured post-perihelion images showing a pronounced "anti-tail" and a separate smoky trail. Harvard astrophysicist Avi Loeb notes these features extend roughly 620,000 miles (≈998,000 km) toward the Sun and about 1,860,000 miles (≈2,993,000 km) in the opposite direction.
Loeb estimates that a natural-comet jet at 3I/ATLAS's solar distance would have an outflow speed near 0.248 miles per second (≈0.40 km/s). At that velocity, the jets would need to have been active for about one to three months to create the observed structure.
Energy and surface-area puzzle
Using an energy-balance argument, Loeb calculates that the Sun supplied roughly 700 J/m²/s (700 W/m²) at the object's perihelion distance. To sublimate the large amount of carbon dioxide and water ice required to explain the inferred mass loss, he concludes the absorbing surface area would have to be at least 617 square miles (≈1,600 km²) — roughly equivalent to a sphere about 14.3 miles (≈23 km) across.
That estimate is significantly larger than Loeb's earlier lower bound (a minimum diameter of about 3.1 miles, with a mass of at least 33 billion tons). It also appears inconsistent with constraints from a Hubble image taken July 21, 2025, and with a James Webb Space Telescope observation on August 6, 2025, which indicated a much lower loss rate (about 330 lb/s, ≈150 kg/s).
Fragmentation versus alternative explanations
Loeb highlights a stark contrast in inferred mass-loss rates: roughly 330 lb/s (≈150 kg/s) in August versus an estimated ~4.4 million lb/s (≈2.0×10⁶ kg/s) near perihelion. He argues that breakup into many fragments would increase the total exposed surface area and could account for the dramatic surge in outgassing. Based on his calculations, he suggests 3I/ATLAS could have split into "at least 16 equal pieces, and likely many more," characterizing the event as an explosion at perihelion.
"Was the dramatic mass loss and brightening of 3I/ATLAS at perihelion evidence that it disintegrated?" Loeb asks. "Breakup into fragments would have increased the surface area of its material."
Loeb also reiterates a more speculative alternative: if follow-up observations show the object remained intact, it could be "something other than a natural comet." He has previously proposed provocative hypotheses — including that the object might be an engineered (non-natural) artefact — and notes that engineered thrusters with much higher exhaust speeds would require far less mass loss to produce similar jets.
It is important to stress that many astronomers urge caution. The object’s unusual behavior is intriguing, but the data are still limited; fragmentation, exotic compositions, or observational biases may all play roles. Additional observations are needed to test competing hypotheses and to determine how much of the nucleus, if any, survived perihelion.
What’s next?
Observers will have additional opportunities to study 3I/ATLAS: it is expected to make a close approach to Earth on December 19 and pass near Jupiter in March, offering chances to measure its brightness, shape, and any surviving fragments. These follow-up observations should help clarify whether the object truly disintegrated or whether another explanation better fits the data.
Credits: New images by Michael Buechner and Frank Niebling; analysis and public commentary by Avi Loeb. Reported observational constraints include images from the Hubble Space Telescope (July 21, 2025) and the James Webb Space Telescope (August 6, 2025).