Avi Loeb

Let me start this quantitative discussion with the conservative assumption that the interstellar object 3I/ATLAS is a natural comet, and work out its properties based on its latest post-perihelion image.

The large-scale image of 3I/ATLAS reported here on November 9, 2029 shows multiple jets reaching out to ~1 million kilometers towards the Sun and ~3 million kilometers in the opposite direction, as discussed here.

For a natural comet, the outflow velocity of the jets is expected to be 0.4 kilometers per second, of order the sound speed of gas at the distance of 3I/ATLAS from the Sun. At that speed, the jets must have persisted over a timescale of 1–3 months.

Given that the jets towards the Sun were stopped by the solar wind at a distance of a million kilometers, I calculated here that their mass density is a few million proton masses per cubic centimeter at a distance of a million kilometers from 3I/ATLAS. The product of this mass density and the outflow speed, implies a mass flux of 5 billion tons per month per area of a million-kilometer on a side. I calculated here that the total mass associated with 3I/ATLAS is at least 33 billion tons based on its early dynamics. Adopting an outer surface area for the jetted material of order a million-kilometer squared, I find that 3I/ATLAS may have lost ~16% of its mass. This is consistent with the mass fraction required by its non-gravitational acceleration at perihelion, as I calculated here.

Are these numbers consistent with the energy provided to 3I/ATLAS by the Sun?

The sublimation of carbon dioxide (CO2) ice requires 600 Joules per gram, nearly 5 times less than the sublimation of water (H2O) ice which is 2,835 Joules per gram. The Webb telescope spectroscopic data (reported here) indicated that when 3I/ATLAS was 2.4 times farther from the Sun than its perihelion distance, 87% of the mass of gas around it was CO2. In order to supply 5 billion tons of CO2 over the perihelion passage period of a month, 3I/ATLAS must have received at least 3×10^{18} Joules to sublimate this much CO2 mass. At its perihelion distance, the Sun provided 700 Joules per square meter per second. This means that the absorbing area of 3I/ATLAS must have been larger than 1,600 square kilometers. This is the area of a sphere with a diameter of 23 kilometers, 4 times larger than the maximum diameter of 5.6 kilometers inferred for 3I/ATLAS from the imaging data of the Hubble Space telescope (reported here). The required diameter is 51 kilometers for water ice.

“Houston, we have a problem” with the natural comet hypothesis! The required surface area of 3I/ATLAS to provide the inferred mass loss from the latest post-perihelion image, is at least 16 times larger than the upper limit derived here from its Hubble image on July 21, 2025.

When the Webb data was taken on August 6, 2025, 3I/ATLAS lost only 150 kilograms per second. The mass loss at perihelion derived above is 4 orders of magnitude larger, about 2 million kilograms per second. This is a dramatic increase, requiring a power-law dependence of mass loss on distance from the Sun with a power-law index of -10.5, consistent with the rapid perihelion brightening of 3I/ATLAS reported here.

Was the dramatic mass loss and brightening of 3I/ATLAS at perihelion evidence that it disintegrated? Breakup into fragments would have increased the surface area of its material. Since the surface-to-mass ratio scales inversely with the characteristic radius of fragments, an increase in surface area by a minimum factor of 16 requires that 3I/ATLAS broke into at least 16 equal pieces, and likely many more. This would mean that 3I/ATLAS exploded at perihelion and we are witnessing the resulting fireworks. In other words, the latest image implies that 3I/ATLAS was decimated by heating from the Sun if it is a natural comet.

The tidal force of the Sun is expected to separate the fragments in the coming weeks, creating an appearance similar to that of the comet Shoemaker-Levi 9 in 1994 near Jupiter. I discussed this possible outcome a month ago here.

However, if upcoming observations were to reveal that 3I/ATLAS was not decimated by the Sun and maintained its integrity as a single body, then we will have to consider that it is something other than a natural comet. On December 19, 2025, 3I/ATLAS will get closest to Earth, allowing ground-based telescopes as well as the Hubble and Webb space telescope to diagnose its integrity.

Separately from the required breakup for a natural comet, the large diameter of 3I/ATLAS highlights the first anomaly that I pointed out in my first paper on 3I/ATLAS, published here. The inferred mass of 3I/ATLAS is over a million times more than the inferred mass of 1I/`Oumuamua. Why had we found such a giant object before witnessing a million objects of the size of 1I/`Oumuamua? As I showed in my paper, there is not enough rocky material in interstellar space to accommodate the delivery of such a giant icy rock to the inner solar system over our survey period of a decade. We would expect an object with a diameter above 10 kilometers to be delivered to our vicinity once per ten thousand years or longer. This anomaly has a likelihood of less than 0.1% if all rocky materials are packaged in large bodies of 3I/ATLAS’ size or less than 0.0005% if there is equal amount of total mass per logarithmic package mass interval. Combine that with the 0.2% probability of the retrograde trajectory of 3I/ATLAS being aligned to within 5 degrees with the ecliptic plane, and you get a chance of one in a hundred million for 3I/ATLAS to originate from a familiar astrophysical origin.

Technological thrusters require a much smaller mass loss in order to produce the observed jets around 3I/ATLAS. Chemical rockets are propelled by an exhaust speed of 3–5 kilometers per second, which is ten times larger than the maximum ejection speed of volatiles sublimated by sunlight from natural cometary surfaces. Ion thrusters reach an even higher ejection speed of 10–50 kilometers per second. Alien-tech thrusters might employ yet higher exhaust speeds, reducing the required mass loss by several orders of magnitude and making the required fuel a small fraction of the spacecraft mass. Upcoming spectroscopic observations will determine the velocity, mass flux and composition of the jets of 3I/ATLAS. Stay curious!

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https://avi-loeb.medium.com/did-3i-atlas-just-break-up-near-the-sun-c27f7479f3e0