Avi Loeb

Avi Loeb

Follow

6 min read

·

Jul 12, 2025

On July 1, 2025, a new interstellar object, 3I/ATLAS, was spotted at a distance 4.5 times the Earth-Sun separation. At that distance, I immediately calculated that it was anomalously bright, implying an object with a diameter of ~20 kilometers for the typical albedo of asteroids.

A day later, I posted a note explaining that this size estimate makes little sense for an interstellar asteroid because the interstellar object 1I/`Oumuamua was 200 times smaller, and based on the statistics of asteroids in the Solar system, we should have discovered a million objects of the scale of 1I/`Oumuamua before spotting one interstellar object that is ~20 kilometers in diameter. We know that 20-kilometers asteroids are rare, because non-avian dinosaurs were killed by an asteroid half that size 66 million ago, whereas meter-scale asteroids impact the Earth every year.

On July 4, I wrote a new paper about the size anomaly of 3I/ATLAS, posted it immediately on my website and the arXiv preprint server, and submitted it for publication in the journal Research Notes of the American Astronomical Society (RNAAS). In my paper, I explained that detecting a 20-kilometer asteroid in the inner solar system over the survey period of 5 years has a probability of 0.0001 based on the total reservoir of interstellar asteroids. This strongly argues in favor of 3I/ATLAS being a comet whose brightness stems from a plume of gas and dust that reflects sunlight from a tens of kilometers region around a compact nucleus with a diameter smaller than a kilometer.

But facts do not always comply with expectations. A week after the discovery of 3I/ATLAS, two preprints (posted here and here) reported that its observed spectrum does not show the spectral fingerprints of atomic or molecular gas. Instead, the spectrum only shows evidence for reddening of reflected sunlight. Such reddening could be indicative of dust, or it might otherwise be related to the surface properties of 3I/ATLAS. For example, Kuiper belt objects in the outer Solar system are reddened when organics on their icy surface are exposed to ultraviolet light or cosmic rays for billions of years. This is caused by Tholins, a wide variety of organic compounds formed by ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO_2), methane (CH_4) or ethane (C_2H_6), often in combination with nitrogen (N_2) or water (H_2O). In this “red surface” interpretation, there is no plume of dust or gas around 3I/ATLAS and the object has a diameter of order ~20 kilometers. The mild elongation of the fuzz around 3I/ATLAS in current images is along its direction of motion with an extent that is comparable to its speed, ~60 kilometers per second, times the exposure time of the telescopes used to obtain them, a few hundreds seconds.

If 3I/ATLAS is not an asteroid — based on the interstellar reservoir argument in my paper, nor a comet — based on the lack of the spectral fingerprints of carbon-based molecules around it, then what is it?

As 3I/ATLAS gets closer to the Sun, it will get brighter. If it is a solid object without a cometary plume of gas or dust around it, then its brightness will increase inversely with the square of the decreasing distance from the Sun times the square of the distance from Earth. Future data from the largest ground-based telescopes including the Rubin Observatory, as well as the Hubble and Webb space telescopes, will likely reveal its nature.

The simplest hypothesis is that 3I/ATLAS is a comet and we are missing the spectral features of its gaseous coma because of its large distance from Earth. However, in case future data will indicate the absence of a cometary tail, we will be faced with the tantalizing possibility that it did not inherit a random velocity in interstellar space but instead was sent towards the inner solar system by design, being a member of a rare population of massive interstellar objects. I mentioned this possibility in the last sentence of my paper, but it was surgically removed by the editor of RNAAS before the paper was accepted for publication, along with data updates, on July 8.

As I noted in an essay posted on July 7, the anomalous scenario is reminiscent of the science-fiction novel `Rendezvous with Rama,’ in which Arthur C. Clarke described the entry of a cylindrical 50-by-20 kilometers alien spaceship, not far from the inferred size of 3I/ATLAS, to the inner solar system. Interestingly, 3I/ATLAS will pass nearest to the Sun on October 29, 2025, when the Earth will be located on the opposite side of the Sun, making terrestrial observations of it difficult at that time. Under these circumstances, it would not be easy to “search for the keys closest to the lamppost”. A technological probe which is aware of observers eager to watch it from Earth might favor these circumstances. 3I/ATLAS is on a retrograde orbit within 5 degrees of the orbital plane of the Earth around the Sun, an exceptional coincidence with a probability of ~0.002 for a random orientation of its orbital angular momentum vector from interstellar space.

Needless to say, when the details of 3I/ATLAS were summarized on Wikipedia a few days after its discovery, the editors of that entry omitted any reference to anomalies of 3I/ATLAS. They learned about my paper from colleagues on July 4 but responded that this paper must be published in a journal before being referenced on Wikipedia. For context, the Wikipedia site for 3I/ATLAS included at that time only references to unrefereed scientific announcements and news reports. This practice by the Wikipedia gatekeepers provides yet additional evidence for the thesis presented in a new paper that I posted with the psychologists Omer Eldadi and Gershon Tenenbaum on July 9, explaining the psychological reasons for the suppression of paradigm-breaking evidence by the scientific community.

The size anomaly of 3I/ATLAS will be easily clarified by upcoming data. Science is better served by a discourse which is open minded to anomalies, because awareness of them motivates the collection of new data to resolve them. The effort of gatekeepers to hide anomalies and maintain traditional thinking will ultimately fail. Placing Galileo Galilei in house arrest to suppress the dissemination of anomalies about the Moons of Jupiter did not stop modern science but only delayed it — until even the Vatican eventually admitted that Galileo was right. We deserve to stay ignorant if we support a closed-minded culture where gatekeepers deny the dissemination of information about anomalies that contradict prevailing paradigms.

Let us instead maintain our childhood curiosity and seek evidence rather than pretend to be the adults in the room that know the answers in advance. Science does not need to feel like a lecture in a classroom, summarizing past knowledge. It could be far more exciting if the teachers would be willing to learn something new!

(http://www.autoadmit.com/thread.php?thread_id=5752739&forum_id=2/#49117177)