On July 1, 2025, the NASA-funded ATLAS telescope in Rio Hurtado, Chile detected 3I/ATLAS, the third confirmed interstellar object to enter our solar system. Traveling through space at approximately 61-68 kilometers per second, this ancient visitor arrived from a distant star system, carrying with it the promise of unprecedented scientific revelations. Unlike previous interstellar visitors, 3I/ATLAS was met by a global network of astronomers equipped with advanced instruments, ready to unlock secrets about the origins and evolution of planetary materials beyond our solar neighborhood.
The Mysterious Composition and Journey
What sets 3I/ATLAS apart is its exceptional chemical makeup. Observations from the James Webb Space Telescope revealed that the object possesses one of the highest carbon dioxide-to-water ratios ever recorded in a comet. This unusual composition suggests that 3I/ATLAS spent nearly a billion years exposed to galactic cosmic rays, forming a processed outer crust. Scientists regard the comet as a rare time capsule, preserving the chemical fingerprints of its home system and offering a window into the processes that shape interstellar objects over immense timescales.​
Size Estimates: A Key Point of Scientific Debate
The comet’s nucleus size remains uncertain and heavily contested. Hubble Space Telescope observations from July 2025 constrained the nucleus diameter to between 0.32 and 5.6 kilometers, with the most likely estimate being less than 1 kilometer. However, Harvard astrophysicist Avi Loeb later argued based on November observations that the nucleus must be significantly larger—at least 5 kilometers in diameter, and potentially 10 kilometers or more. This discrepancy reflects fundamental disagreements about the comet’s physical properties and presents a key scientific uncertainty.​​
Harvard’s Fragmentation Hypothesis
As 3I/ATLAS approached perihelion on October 29, 2025, the comet exhibited dramatic increases in outgassing. Data showed emissions escalating from approximately 330 pounds per second in early August to roughly 4.4 million pounds per second near perihelion.​​
Based on this data and subsequent observations, Loeb hypothesized that the intense solar heating caused 3I/ATLAS to fragment. His calculations suggested the object may have broken into “at least 16 pieces, and likely many more,” leading him to conclude that “3I/ATLAS exploded at perihelion and we are witnessing the resulting fireworks”.​​
However, this interpretation is not universally accepted among astronomers. Qicheng Zhang, a postdoctoral fellow at Lowell Observatory studying the comet, told Live Science: “All the images I’ve seen show a fairly ordinary/healthy-looking comet. There’s no sign at all that the nucleus broke apart”. NASA’s Tom Statler, program scientist for the small-bodies program, similarly stated: “It looks like a comet. It does comet things. It very, very strongly resembles…the comets that we know”.​
Unprecedented Massive Jet Structures Observed
Post-perihelion observations on November 8 and 9, 2025, revealed complex multi-jet structures of unprecedented scale in cometary science. Astronomers from the International Comet Observation group and the British Astronomical Association captured images showing multiple jets streaming from the comet in various directions.​​
According to Loeb’s analysis, sunward-pointing anti-tail jets extend approximately 0.95 million kilometers, while the main tail pointing away from the Sun measures about 2.85 million kilometers. These measurements represent the largest cometary structures ever observed—far exceeding jets seen in earlier supercomets. At 3I/ATLAS’s December 19, 2025 closest approach to Earth—269 million kilometers away—the comet remains roughly one hundred times more distant than the length of these jet structures.​​
The mass density within these anti-tail jets exceeds solar wind density by approximately a million-fold, indicating unusually high concentrations of dust and gas. However, astronomers note that anti-tails—jets pointing toward the Sun—are known optical phenomena in comets. Similar features have been observed in other comets, including Arend-Roland (1957) and Kohoutek (1974), when viewing geometry creates this visual effect. The unprecedented scale of 3I/ATLAS’s jets is what distinguishes them scientifically, not their fundamental physical nature.​
The Mass Calculation and Its Implications
Loeb’s calculations, based on mass loss rates and observed jet structures, suggest the object contains a mass potentially exceeding 50 billion tons—roughly one million times greater than 1I/’Oumuamua, the first recognized interstellar object. If accurate, this would raise significant questions about the formation and propagation of such massive bodies through interstellar space.​
However, these mass estimates depend critically on assumptions about nucleus size, which remains scientifically disputed. The discrepancy between Hubble’s sub-kilometer estimate and Loeb’s 5-10 kilometer calculation reflects a fundamental uncertainty that future observations should resolve. Some scientists have noted that the energy requirements to produce jets of this scale would necessitate an even larger nucleus (approximately 23 kilometers) than Loeb’s estimates, further complicating the picture.​
Looking Ahead: December Observations
As 3I/ATLAS makes its closest approach to Earth on December 19, 2025, ground-based telescopes and NASA’s space observatories—including Hubble and the James Webb Space Telescope—will conduct detailed observations to address lingering questions about the comet’s structure and whether fragmentation occurred. The Juno spacecraft will also observe the comet on March 16, 2026, from a distance of approximately 53 million kilometers, potentially providing additional data.​
These observations will help determine whether 3I/ATLAS represents a natural comet undergoing extreme but explainable solar heating or exhibits truly anomalous properties that challenge current cometary models. The event has demonstrated the power of international astronomical collaboration and the challenges of interpreting rare cosmic visitors based on limited observational windows