A simple way to distinguish a spacecraft from a rock is through its large non-gravitational acceleration. A natural icy rock like a comet is propelled by its mass loss. That mass loss can be observed through the cometary plume of gas and dust that surrounds the comet’s nucleus. By measuring the rate of mass loss and the characteristic ejection speed of gas and dust, one can calculate the rate of momentum change per unit time, or the non-gravitational force exerted on the nucleus. Since the evaporation occurs on the dayside of the rock which is warmed up by the virtue of it facing the Sun, this force pushes the comet’s nucleus away from the Sun. At a large enough distance, typically a few times the Earth-Sun separation, the surface of the nucleus is not warmed enough by sunlight to release volatile ices and dust and the cometary activity diminishes.
A technological object, on the other hand, could operate an engine and maneuver independently of the Sun. It can be propelled towards the Sun or any planet of interest and exhibit a non-gravitational acceleration of arbitrary magnitude or direction. Observing non-gravitational maneuvers could shift the ranking of an interstellar object on the “Loeb scale”, from `0’ — the default value for a natural comet to `10’ — a definitely artificial object.
Given this perspective, it is of great interest to measure the acceleration of the new interstellar object 3I/ATLAS along its path through the Solar System and check whether it shows any deviation from the expected trajectory, as dictated by gravity alone. If 3I/ATLAS will not continue along its expected path after its closest approach to the Sun on October 29, 2025, then the stock market might crash from worries about an alien tech visitation.
If 3I/ATLAS is a natural comet, what is its expected non-gravitational acceleration?
The recent imaging of 3I/ATLAS by the Hubble Space Telescope shows a glow ahead of the object but no bright tail of gas and dust behind it — as often observed for comets (see related paper here). In addition, spectroscopic measurements show no evidence for molecular or atomic gas accompanying this glow (see related papers here, here and here, as well as the discussion about water ice here). A natural interpretation of these anomalies is that 3I/ATLAS is a dust-rich comet that releases little gas, but mostly large dust particles which are not pushed back by Solar radiation pressure or the Solar wind because of their small surface-to-mass ratio. In this case, we can calculate the expected non-gravitational acceleration of this comet from the observed plume of dust leading it.
A detailed analysis of the observed glow ahead of 3I/ATLAS (see related paper here) suggests a mass loss rate of up to 60 kilograms per second for dust particles of 100 micron size (where a micron is a millionth of a meter) and an ejection speed of ~2 meters per second in the direction of the Sun. The estimated mass loss rate drops to 6 kilograms per second and an ejection speed of 20 meters per second for 1-micron particles. Since the non-gravitational force exerted on 3I/ATLAS equals the mass loss rate times the ejection speed, its value is the same in both cases and does not depend on the assumed size of the ejected dust particles.
The brightness distribution in the glow preceding 3I/ATLAS was also used to set limits on the diameter of its nucleus, inferred to be in the range of 0.32–5.6 kilometers. This implies that the nucleus mass is in the range of 30 billion to 200 trillion kilograms. Applying the resulting non-gravitational force to this mass leads to a non-gravitational acceleration in the range of 3×10^{-14} to 2×10^{-10} AU per day squared, where AU stands for Astronomical Unit which is defined as the Earth-Sun separation. This non-gravitational acceleration range is equivalent to values between 6×10^{-11} and 4×10^{-7} centimeters per second squared, in the direction away from the Sun.
For comparison, the first interstellar object 1I/`Oumuamua exhibited on October 25, 2017 a non-gravitational acceleration of 1.4×10^{-7} AU per day squared, equivalent to 2.7×10^{-4} centimeters per second squared (see related data here). This is larger than the expected non-gravitational acceleration of 3I/ATLAS by a huge factor, ranging between a thousand and 10 million. If 1I/`Oumuamua was a familiar comet, it had to lose about a tenth of its mass during its passage close to the Sun. But despite its large non-gravitational acceleration, 1I/`Oumuamua did not display any cometary evaporation (see observational data here), making its large non-gravitational acceleration a major anomaly concerning its nature (as argued in my related paper here).
If 3I/ATLAS is a natural comet, its outgassing may intensify as it gets closer to the Sun. A measurement of the expected non-gravitational acceleration from its cometary activity would confirm its natural origin. A paper that I wrote with my student, Sriram Elango, before the discovery of 3I/ATLAS, showed that localization data from the Webb telescope in combination with terrestrial telescopes can pinpoint the trajectory of an interstellar object to unprecedented precision using parallax, since the Webb telescope is located 1.5 million kilometers away from Earth at the L2 Lagrange point. A major deviation of the measured non-gravitational acceleration from the expected range for a comet, would suggest that 3I/ATLAS might be propelled technologically.
For now, we cannot assess with any confidence whether 3I/ATLAS is a natural dust-rich comet with no gaseous tail on an extremely rare trajectory, or perhaps a technological object on a path that was designed to align with the ecliptic plane of the planets around the Sun. All we know is that 3I/ATLAS exhibits a rare (0.2% probability) alignment of its retrograde path with the ecliptic plane to within 5 degrees, and its arrival time along this path is perfectly suited for a close encounter with Mars, Venus and Jupiter (with a 0.0005% probability, as discussed here). These coincidences would allow a mothership to release mini-probes that reach planets as they move into the mini-probes’ orbits, taking advantage of the mothership’s retrograde motion. Since 3I/ATLAS will hide behind the Sun at its perihelion on October 29, 2025, we will not be able to observe whether it releases any mini-probes into Earth’s orbit.
Exquisite measurements of the non-gravitational acceleration of 3I/ATLAS would provide an important clue about its nature. The verdict will not be decided by debates on social media, but rather by accurate measurements of instruments. This is the same as the video assisted referee (VAR) protocol used by FIFA, the international soccer organization, to decide whether a goal was scored under controversial circumstances. FIFA rules by viewing data recorded by cameras, rather than by asking soccer players or the goalkeeper for their opinions. We all know that the Earth moved around the Sun for 4.54 billion years before the Vatican placed Galileo Galilei in house arrest for suggesting that. Whether 3I/ATLAS is natural or technological in origin has nothing to do with popular opinions on Earth.
ABOUT THE AUTHOR
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.
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