Dusting Off 3I/ATLAS: Interstellar Travelers Must Clean their Windshields!
In a new paper posted here, Matthew Belyakov (from Caltech) and collaborators present the first spectroscopic mineralogical analysis of the dust shed by the interstellar object 3I/ATLAS. The analysis pertains to mid-infrared spectroscopy of 3I/ATLAS by the Webb telescope. The object exhibited a strong spectral feature at a wavelength of 10-micrometers, implying a dust composition dominated by amorphous silicates. Unlike Solar System comets which show significant crystalline silicate dust, the composition of the dust shed by 3I/ATLAS after perihelion is more similar to the dust in the interstellar medium. This adds a new unexpected property of 3I/ATLAS to the list of 22 anomalies I already posted here.
The simplest interpretation of this anomaly is that 3I/ATLAS formed in a distant part of its home planetary system out of interstellar medium-like material, without substantial incorporation of silicates condensed near its host star. Such a formation process differs markedly from the mixing scenarios commonly hypothesized for Solar System comets.
After reading this preprint, I was intrigued to calculate the amount of interstellar dust that may have accumulated on the surface of 3I/ATLAs as a result of its long journey through the disk of the Milky-Way galaxy.
The dust disk of the Milky Way galaxy has a radius of order 30,000 light-years and a thickness of order 300 light-years. The interstellar dust accounts for about a percent of the gas mass in the Milky Way, totaling of order a hundred million Solar masses. The average mass density of dust in the Milky Way disk is obtained by taking the ratio between its total mass and the disk volume.
The age estimate of 3I/ATLAS is about 10–12 billion years, based on its high speed — characteristic of old stars, the anomalously high isotope ratio of carbon-12 to carbon-13 and the extraordinarily high deuterium abundance (as derived here).
The path length traversed by 3I/ATLAS through the Milky-Way disk is obtained through the product of its speed relative to the Local Standard of Rest of the Milky-Way (about 60 kilometers per second) times its estimated age (about 10 billion years). Multiplying this path length with the mean mass density of the dust disk gives a net column-density of accumulated dust on its surface. Before my morning jog at sunrise, I calculated the result to be about 0.4 grams per square centimeter. This suggests a surface layer of accumulated interstellar dust with a thickness of order a centimeter.
Given that the estimated radius of 3I/ATLAS is about a kilometer, the total mass of interstellar dust that it may have collected is about 60,000 metric tons. This makes only a small fraction — of order 30 parts per million — of the total mass of 3I/ATLAS. Indeed, a centimeter thick layer of accumulated interstellar dust makes a small fraction of the total meter-scale layer of dust that was shed by 3I/ATLAS near perihelion based on the new preprint.
The practical lesson from my early morning calculation is clear and simple. Interstellar travelers must dust off their windshield or else they would accumulate a centimeter thick layer of opaque dust over a travel period of 10 billion years.
ABOUT THE AUTHOR
Avi Loeb is chair of the UAP Science Advisory Council to the White House, Pentagon, FBI and intelligence agencies, director of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former 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.
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