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Before my morning jog at sunrise, I sketched a model for the dust outflow around the new interstellar object, 3I/ATLAS, which appears as a fuzzy glow in the image obtained by the Hubble Space Telescope on July 21, 2025.

One of the remarkable coincidences regarding 3I/ATLAS is the alignment of its trajectory with the ecliptic orbital plane of the planets around the Sun. This means that 3I/ATLAS is passing through the so-called interplanetary zodiacal dust that resides in that plane. As it gets rarefied at large distances, the outflow of dust from the surface of 3I/ATLAS is destined to eventually be stopped by the zodiacal medium.

The glow of scattered sunlight extends to a characteristic distance of about 3,000 kilometers on all sides of 3I/ATLAS. The mass density of the outflow gets diluted inversely with radius squared until it is stopped by the ram-pressure of the ambient medium. Streams of dust particles could interact electromagnetically as dust particles often acquire electric charge. If the radius of the nucleus of 3I/ATLAS is 10 kilometers, as inferred from its brightness for an albedo of 5%, then the outflow density is diluted by a factor of 90,000 at a distance of about 3,000 kilometers relative to its initial value near the nucleus surface.

What is the ejection speed of the outflow from the surface of 3I/ATLAS? The minimum value would be the rotation speed of that surface. The observed rotation period of 16 hours (reported here) gives a rotation speed of about 1 meter per second for a nucleus radius of 10 kilometers. But detailed analysis of the Hubble image (accessible here) suggests an ejection speed of about 20 meters per second for 1-micrometer sized dust particles.

The outflowing gas delivers to the ambient medium a ram pressure equal to its mass density times its ejection velocity squared. As the outflow gets rarefied, the ram pressure declines inversely with distance squared and eventually gets stopped by the zodiacal medium. In the frame of the object, the ambient medium moves at minus its velocity, 60 kilometers per second. Adopting a characteristic mass density for the zodiacal medium of about a proton mass per cubic centimeter and a speed of 60 kilometers per second for 3I/ATLAS relative to that medium, I calculated the implies mass density profile of the outflow so that it will get stopped at about 3,000 kilometers. Extrapolating the mass density of the outflow to the surface of the object and multiplying by the ejection speed of 20 meters per second and by the surface area of its Sun-facing side, gives a mass loss rate of about 10 kilograms per second. This is very close to the value inferred by other considerations from the detailed analysis of the Hubble image.

The inferred mass loss rate is independent of the assumed radius of the nucleus because the inferred mass density at the nucleus’ surface scales inversely with radius squared after being calibrated at the stopping radius. Since the mass loss rate scales as the mass density of the outflow at the nucleus’ surface times the surface area, which scales as radius squared, the derived mass loss rate does not depend on the radius of the nucleus.

The ram-pressure confinement and the small mass density in the outskirts of the dust cloud around 3I/ATLAS explain the lack of a prominent cometary tail behind 3I/ATLAS. The total dust mass required to maintain this cloud in a steady state over six months can be supplied by a surface layer that is only a millimeter in thickness on a 10-kilometer object. This dust layer could have been developed as a result of fragmentation of the surface from bombardment by interstellar dust and gas during a long interstellar journey.

The derived column density of the outflow is too low for it to be opaque. This suggests that the observed reddening of the spectrum of 3I/ATLAS reflects the red surface of 3I/ATLAS, as characteristic for the surfaces of Kuiper belt objects, like Arrokoth which was observed by the New Horizons spacecraft and believed to have organic molecules as a result of being exposed to interstellar cosmic rays and ultraviolet light.

Most importantly, the low opacity of the dust suggests that the reflected light originates mostly from the surface of 3I/ATLAS and not from the dust surrounding it. Given its brightness, the radius of 3I/ATLAS needs to be of order 10 kilometers for an albedo of 5% or a few times smaller for a perfect reflector.

As I pointed out in my first paper on 3I/ATLAS (accessible here), this conclusion raises two puzzles. The previous interstellar objects, 1I/`Oumuamua and 2I/Borisov, were both hundreds of meters in size. Based on the solar system reservoir of rocks, one would expect to find a hundred thousand rocks with a 100-meter diameter for each 20-kilometer rock. Clearly, we did not observe as many interstellar objects on the scale of 1I/`Oumuamua and 2I/Borisov before discovering 3I/ATLAS. In addition, the reservoir of rocky materials in interstellar space can only supply a 20-kilometer rock once per 10,000 years even if we assume that all this material is packaged in such rocks.

If 3I/ATLAS is not a rock made of heavy elements based on these considerations, what could it be? One possibility is that it is an iceberg made a hydrogen or helium, which are more abundant in interstellar space by several orders of magnitude. The problem is that a hydrogen iceberg would be easily evaporated by starlight, as I showed in a paper with Thiem Hoang (accessible here). Alternatively, 3I/ATLAS might have targeted the inner solar system and has nothing to do with the reservoir of rocks on random trajectories in interstellar space. A technological design would also explain the alignment of its trajectory with the ecliptic plane (likelihood of 0.2%) and its close passage to Mars, Venus and Jupiter (likelihood of 0.0005%).

Here’s hoping that the data collected in the coming months will reveal more details about 3I/ATLAS as it approaches the Sun and gets brighter and warmer. Turning the heat on 3I/ATLAS may reveal its nature.

ABOUT THE AUTHOR

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Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

Chinese astronomers may have discovered a never-before-seen triple black hole system.

The team identified this triplet, which is locked in a complex “waltz,” after spotting a hidden supermassive black hole lurking in the background of a peculiar gravitational wave event first detected six years ago.

The melting of sea ice in the Arctic has slowed dramatically in the past 20 years, scientists have reported, with no statistically significant decline in its extent since 2005.

The finding is surprising, the researchers say, given that carbon emissions from fossil fuel burning have continued to rise and trap ever more heat over that time.

They said natural variations in ocean currents that limit ice melting had probably balanced out the continuing rise in global temperatures. However, they said this was only a temporary reprieve and melting was highly likely to start again at about double the long-term rate at some point in the next five to 10 years.

The findings do not mean Arctic sea ice is rebounding. Sea ice area in September, when it reaches its annual minimum, has halved since 1979, when satellite measurements began. The climate crisis remains “unequivocally real”, the scientists said, and the need for urgent action to avoid the worst impacts remains unchanged.

The natural variation causing the slowdown is probably the multi-decadal fluctuations in currents in the Atlantic and Pacific oceans, which change the amount of warmed water flowing into the Arctic. The Arctic is still expected to see ice-free conditions later in the century, harming people and wildlife in the region and boosting global heating by exposing the dark, heat-absorbing ocean.

Dr Mark England, who led the study while at the University of Exeter, said: “It is surprising, when there is a current debate about whether global warming is accelerating, that we’re talking about a slowdown.

“The good news is that 10 to 15 years ago when sea ice loss was accelerating, some people were talking about an ice-free Arctic before 2020. But now the [natural] variability has switched to largely cancelling out sea ice loss. It has bought us a bit more time but it is a temporary reprieve – when it ends, it isn’t good news.”

The research, published in the journal Geophysical Research Letters, used two different datasets of Arctic sea ice levels from 1979 to the present day. The scientists analysed the sea ice area for every month of the year and the slowdown was seen in all cases.

To see if such a slowdown could be a result of natural variation, they examined the results of thousands of climate model runs. “This is not an extremely rare event – over a century, it should happen a couple of times,” said England, now at the University of California, Irvine. Furthermore, all the simulations showed sea ice loss accelerating again after the slowdown.

Prof Julienne Stroeve, of University College London, said: “We know climate records, be it in global temperatures or sea ice, can remain the same for several years in a row as a result of internal climate variability.”

Stroeve’s analysis of the long-term trend from 1979 to 2024 shows that about 2.5 sq metres of September ice is lost for every tonne of CO₂ emitted.

Prof Andrew Shepherd, of Northumbria University, said: “We know that the Arctic sea ice pack is also thinning, and so even if the area was not reducing, the volume still is. Our data show that since 2010 the average October thickness has fallen by 0.6cm per year.”

The rate of the rise in global surface temperature has also slowed down in the past, before resuming a rapid rise. A major El Niño event in 1998 was followed by a decade or so of similar global temperatures, which was nicknamed “the pause”. However, the planet continued to accumulate heat throughout and global temperatures have since risen rapidly.

England rejected any suggestion the sea ice slowdown suggested climate change was not real. “Climate change is unequivocally real, human-driven, and continues to pose serious threats. The fundamental science and urgency for climate action remain unchanged,” he said.

“It is good to explain to people that [the slowdown] is happening, else they are going to hear it from someone who is trying to use it in bad faith as a way to undermine our very solid understanding of what’s happening with climate change.”