The largest uncertainty surrounding the interstellar object 3I/ATLAS pertains to the diameter of its solid-density nucleus. Recent observations by the SPHEREx space observatory at a wavelength of 1 micrometer, taken from August 8 to August 12, 2025, indicate that the object may possess a substantial nucleus, estimated to have a diameter of 46 kilometers. This could potentially be due to an opaque dust cloud that scatters sunlight. However, the limited resolution of images captured by the Hubble Space Telescope does not provide a definitive understanding of the sunlight reflected by the nucleus in relation to the surrounding dust cloud.
The theoretical conclusions drawn from the available data are highly model-dependent and do not clarify the existing uncertainty regarding the size of 3I/ATLAS. If it indeed has a solid nucleus, the diameter of 46 kilometers would suggest a mass around 1020 grams, which is a million times greater than the estimated mass of the previous interstellar comet, 2I/Borisov. Since the mass of a nucleus scales with the cube of its diameter, accurately measuring the mass of 3I/ATLAS could provide a tight constraint on its size.
So, how can scientists accurately measure the mass of this intriguing interstellar object? One possible method involves the rocket equation. The force acting on 3I/ATLAS is equal to the excess of its mass loss rate towards the Sun multiplied by the outflow speed relative to its surface. By dividing this non-gravitational force by the object’s non-gravitational acceleration, its mass can be determined. In principle, all three parameters—mass loss rate, outflow velocity, and non-gravitational acceleration—can be measured.
Recent data from the Webb telescope has inferred the mass loss rate of CO2 from 3I/ATLAS to be approximately 129 kilograms per second, with an outflow speed estimated at 0.44 kilometers per second. The product of these measurements yields a non-gravitational acceleration of around 6 x 10-11 centimeters per second squared. This level of acceleration is significantly below the lowest levels measured for solar system objects.
The non-gravitational acceleration will be detectable if the mass loss rate increases as 3I/ATLAS approaches the Sun or if the nucleus diameter is smaller. A diameter of less than a kilometer would be necessary to reconcile the high mass of 3I/ATLAS with the available rocky material in interstellar space. Such a reduction in diameter would suggest a nucleus mass below 1015 grams and a non-gravitational acceleration greater than 6 x 10-6 centimeters per second squared, which is only 50 times smaller than the value recorded for 1I/`Oumuamua.
Since the mass loss rate scales with area and the non-gravitational acceleration inversely with volume, the rocket equation proves to be an effective approach for measuring the mass of small objects. Conversely, for larger objects, gravitational methods might yield better results.
On October 3, 2025, 3I/ATLAS will pass at a distance of 29 million kilometers from Mars. This gravitational influence will impart a 'kick' to Mars, akin to two fuzzy billiard balls colliding. The magnitude of this velocity kick can be calculated using the gravitational acceleration that 3I/ATLAS's mass exerts at the distance of closest approach. For a mass of approximately 1020 grams, a distance of 29 million kilometers, and a relative velocity of about 90 kilometers per second, the resulting velocity kick is around 3 x 10-7 centimeters per second. Unfortunately, this kick is currently unmeasurable due to uncertainties in Mars' orbit and that of other solar system planets interacting with 3I/ATLAS.
Interestingly, the Minimum Orbit Intersection Distance (MOID) of 3I/ATLAS from Mars is remarkably short, just 0.018 AU or 2.7 million kilometers. This proximity raises the possibility that if 3I/ATLAS is a technological mothership, it could easily deploy a mini-probe to Mars with the appropriate ejection velocity. Moreover, a minor maneuver by 3I/ATLAS could reduce its MOID with Mars to zero. As Francis Bacon wisely noted, “If the mountain won’t come to Muhammad, then Muhammad must go to the mountain.”
NASA should consider utilizing all available resources to bring the Juno spacecraft as close as possible to 3I/ATLAS during its pass, which will occur within 34 million kilometers of Jupiter on March 16, 2026. The gravitational deflection introduced by 3I/ATLAS could later be utilized for an accurate mass measurement of this enigmatic object.
In the forthcoming months, we may have the opportunity to measure the mass of 3I/ATLAS by applying the rocket equation to its mass loss or by observing the gravitational kick it imparts to various celestial bodies along its trajectory. It is crucial to remain focused on data collection rather than distractions from social media or premature accolades. The true nature of 3I/ATLAS will ultimately be determined by enhanced observational data, leading to a deeper understanding of this remarkable interstellar visitor.
