A simple yet effective method to differentiate a spacecraft from a rock lies in its large non-gravitational acceleration. A natural icy rock, such as a comet, experiences propulsion due to its mass loss, which can be observed through the cometary plume of gas and dust enveloping its nucleus. By measuring the rate of mass loss and the characteristic ejection speed of gas and dust, scientists can calculate the momentum change over time, or the non-gravitational force acting upon the nucleus.
This mass loss primarily occurs on the dayside of the comet, which is warmed by sunlight, pushing the nucleus away from the Sun. However, once a comet travels to a significant distance—typically several times the Earth-Sun separation—the surface of its nucleus does not receive enough sunlight to release volatile ices and dust, leading to diminished cometary activity.
In contrast, a technological object has the capability to operate an engine and maneuver independently of solar influence. Such an object could be propelled towards the Sun or any planet of interest, exhibiting a non-gravitational acceleration of arbitrary magnitude or direction. Observations of non-gravitational maneuvers could influence the categorization of an interstellar object on the "Loeb scale," shifting it from `0’—the baseline for natural comets—to `10’—indicating a definitively artificial object.
Given this context, measuring the acceleration of the new interstellar object, 3I/ATLAS, along its path through the Solar System becomes incredibly significant. Scientists aim to determine if any deviations from the expected gravity-driven trajectory occur, especially following its closest approach to the Sun on October 29, 2025. If 3I/ATLAS fails to adhere to its predicted path, there may be significant ramifications, including fears of an alien technological visitation, potentially impacting the stock market.
Recent imaging of 3I/ATLAS by the Hubble Space Telescope reveals a glow ahead of the object, yet lacks the bright tail of gas and dust typically associated with comets. Additionally, spectroscopic measurements show no signs of molecular or atomic gas accompanying this glow. A plausible interpretation of these findings is that 3I/ATLAS is a dust-rich comet that primarily releases large dust particles, which are not significantly influenced by solar radiation pressure or solar wind due to their small surface-to-mass ratio.
From the observed plume of dust, researchers can estimate the expected non-gravitational acceleration of 3I/ATLAS. Analysis suggests a mass loss rate of up to 60 kilograms per second for 100-micron-sized dust particles, with an ejection speed of approximately 2 meters per second directed towards the Sun. For 1-micron particles, the mass loss rate decreases to 6 kilograms per second, while the ejection speed increases to 20 meters per second. The non-gravitational force exerted on 3I/ATLAS remains consistent across these scenarios and is unaffected by the size of the ejected dust particles.
The brightness distribution in the glow preceding 3I/ATLAS has also provided limits on the diameter of its nucleus, estimated to be between 0.32 and 5.6 kilometers. This size range implies a nucleus mass of approximately 30 billion to 200 trillion kilograms. Applying the calculated non-gravitational force to this mass results in a non-gravitational acceleration range of 3x10-14 to 2x10-10 AU per day squared, with AU representing the Astronomical Unit, or the distance from Earth to the Sun. This range translates to values between 6x10-11 and 4x10-7 centimeters per second squared, directed away from the Sun.
For context, the first interstellar object, 1I/`Oumuamua, exhibited a non-gravitational acceleration of 1.4x10-7 AU per day squared on October 25, 2017, equivalent to 2.7x10-4 centimeters per second squared. This value is significantly higher than the expected non-gravitational acceleration of 3I/ATLAS, by factors ranging from a thousand to 10 million. If 1I/`Oumuamua had been a typical comet, it would have needed to lose about a tenth of its mass during its close pass by the Sun.
As 3I/ATLAS approaches the Sun, its outgassing could increase, potentially confirming its status as a natural comet. A study conducted by my student, Sriram Elango, and me prior to the discovery of 3I/ATLAS demonstrated that using localization data from the Webb Telescope, in combination with terrestrial telescopes, could precisely track the trajectory of an interstellar object through parallax measurements.
A significant deviation in the measured non-gravitational acceleration from the expected cometary range would imply that 3I/ATLAS might be technologically propelled. Currently, we cannot confidently determine whether 3I/ATLAS is a natural dust-rich comet with an unusual trajectory or a technological object designed to align with the ecliptic plane of the Solar System's planets.
What is known is that 3I/ATLAS displays a rare alignment (with a 0.2% probability) of its retrograde path with the ecliptic plane within 5 degrees. Its timing is ideally suited for a close encounter with Mars, Venus, and Jupiter, presenting a 0.0005% probability that could allow a mothership to deploy mini-probes into those planets' orbits as they intersect with the mini-probes' paths.
However, as 3I/ATLAS will obscure itself behind the Sun at perihelion on October 29, 2025, observing any potential mini-probe releases into Earth's orbit will be impossible. Exquisite measurements of the non-gravitational acceleration of 3I/ATLAS will provide crucial insights into its true nature, as the ultimate verdict will rely on accurate scientific data rather than public opinion. Just as the VAR protocol in FIFA relies on recorded data to resolve controversies, determining the origins of 3I/ATLAS will depend on precise measurements rather than speculation.
