Strong Non-Gravitational Braking by Interstellar Objects is a Technological Signature!
Interstellar objects are identified by their positive energy relative to the Sun. This is not a metaphor but rather a physical characteristic, formulated as: E > 0. It means that interstellar objects move faster than the escape speed from the Solar System, which is dominated by the Sun’s gravity.
Far from the Sun, their energy is purely in the form of kinetic energy (per unit mass):
E = (1/2) U²
where U is their velocity in interstellar space. Since energy is conserved under the Sun’s gravity, their local velocity v evolves as a function of their changing distance r from the Sun according to the relation:
E = -GM/r + (1/2) v²
Altogether, in the presence of gravity:
v² = U² + 2GM/r
The local escape speed v_e at a distance r from the Sun is defined as the value of v for U=0, namely:
v_e² = 2GM/r
The Solar escape speed v_e represents the minimum speed needed to carry an object out of the Solar System. Any object observed moving faster than v_e at a heliocentric distance r is flagged as interstellar in origin. At the orbital radius of Earth, the escape speed is 42.1 kilometers per second.
However, the situation changes under the action of a non-gravitational force, such as the rocket effect from outgassing. Let us restrict our attention to the simple case of a non-gravitational acceleration, A[r], that scales similarly to gravity as 1/r² (as was the case for 1I/`Oumuamua or 3I/ATLAS) and always points opposite to the object’s velocity, namely away from the Sun before perihelion. In this case, the energy E of the interstellar object will be reduced by A*r as the object arrives from interstellar space to a distance r from the Sun. The reduction in energy results from the fact that this non-gravitational acceleration allows down the object and reduces its positive kinetic energy. The effect is equivalent to pumping the brakes on a vehicle.
If the energy change exceeds the positive value E that the interstellar object possessed to start with, then the net energy value will turn negative and the object will become gravitationally bound to the Sun.
The condition for an interstellar object to be trapped by the Sun owing to its non-gravitational acceleration is A*r > (1/2) U², or equivalently:
A > U²/2r
This can be compared to the gravitational acceleration at a distance r from the Sun, g=(GM/r²) = (v_e^/2r).
The above requirement for trapping an interstellar object in the Solar System is therefore:
A/g > (U/v_e)²
Sublimation of ice by sunlight typically results in outgassing limited by the thermal speed of a few hundred meters per second, which is a hundred times slower than v_e at the Earth-Sun separation. This means that the resulting non-gravitational acceleration of natural icebergs near Earth can only reach values as small as: A/g < (0.01)²=0.0001.
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Consider the example of the interstellar object 3I/ATLAS which entered the Solar System with an interstellar speed of U = 58 kilometers per second. The escape speed at its perihelion distance of 1.36 times the Earth-Sun separation is v_e = 36 kilometers per second. In order for 3I/ATLAS to slow down enough and stay in the Solar System, the object had to brake with a non-gravitational acceleration that is larger than the gravitational acceleration by a factor of
A/g > (58/36)² = 2.6
The actual non-gravitational acceleration that was measured for 3I/ATLAS, as discussed in the recent paper I co-authored with Valentin Thoss and Andi Burkert here, is merely:
A/g ~ 0.0001
Clearly, 3I/ATLAS did not slow down at the level needed for it to stay in the Solar System. The required threshold for staying of A/g > 2.6 applies to any fragments released by 3I/ATLAS, since the acceleration condition for any object to stay in the Solar System does not depend on the object’s mass.
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Today, the NSF-DOE Rubin Observatory released its data preview here. The full Rubin database over the coming decade is expected to reveal dozens of new interstellar objects. If any of them appears to slow down enough to become gravitational bound to the Solar System, this braking should be regarded as a strong enough technological signature to elevate its rank close to 10 on the Loeb Classification Scale of interstellar objects (as discussed here, here and here).
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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