The cosmos is full of mysteries, and one of the most intriguing is the potential for interstellar objects to influence our solar system. In this article, we explore the fascinating possibility of 3I/ATLAS, an interstellar object, getting closest to Jupiter's irregular moon, Eupheme. But here's where it gets controversial... Will 3I/ATLAS add a new moon to Jupiter's family? Let's dive in and find out.
About 8,000 years after entering the Oort Cloud of the solar system, 3I/ATLAS is now on a journey away from the Sun and towards Jupiter. According to its forecasted gravitational trajectory and non-gravitational acceleration, it is expected to arrive within 30.46 million kilometers of Jupiter's irregular moon, Eupheme, on March 17, 2026. A day earlier, it will be within 53.61 million kilometers of Jupiter, surprisingly close to Jupiter's Hill radius of 53.5 million kilometers, where Jupiter's gravity dominates over the Sun's tidal gravity.
Eupheme, discovered in 2003 from data taken by the Mauna Kea Observatories in Hawaii, has an orbital period of 588 days around Jupiter. It will reach its farthest distance of 27.7 million kilometers from Jupiter on January 23, 2026, just 52 days before its closest passage relative to 3I/ATLAS. This coincidence is not surprising, given that there are 95 known satellites in orbit around Jupiter.
Eupheme is classified as an irregular moon and a member of the Ananke group, a family of 15 Jovian satellites with similar orbits, thought to have a common origin. The Ananke group is believed to have originated from the breakup of a parent body captured by Jupiter's gravity, resulting in multiple pieces due to a collision with another object or Jupiter's tidal forces. The largest relic of the parent body was named Ananke, and the smaller pieces constitute the additional 15 moons in the Ananke group.
The Ananke moons orbit opposite to Jupiter's spin on highly elliptical and inclined orbits relative to Jupiter's equatorial plane, consistent with an origin associated with a captured parent body rather than the primordial Jupiter system. Based on its brightness, Eupheme has a mean diameter of about 2 kilometers, named after the spirit of praise and good omen in Greek mythology, and is the granddaughter of Zeus.
The proximity of 3I/ATLAS's passage to the Hill radius allows it to release daughter objects into bound orbits around Jupiter. These small objects can become gravitationally bound to Jupiter as long as they receive a kick that cancels the relative motion between 3I/ATLAS and Jupiter. This relative speed will be relatively high at the closest approach between the two objects, measuring about 66 kilometers per second.
Discovering a fresh 96th moon around Jupiter after March 16, 2026, would constitute a clear technological signature for 3I/ATLAS. This is because a break-up of a natural astrophysical object much less massive than Jupiter cannot result in a velocity kick as high as 66 kilometers per second. The escape speed from the surface of Jupiter itself is 60 kilometers per second, and gravitational break-up of much smaller bodies cannot generate fragments moving at speeds greater than the parent's escape speed.
Therefore, a kick of the required magnitude and direction to bring a fragment of 3I/ATLAS into a bound orbit around Jupiter is an impossible outcome from the break-up of a comet. Additionally, the velocity kick must be highly fine-tuned, as the escape speed at 3I/ATLAS' perijove distance of 53.61 million kilometers from Jupiter is merely 2.2 kilometers per second, only a few percent of the relative speed. So, the direction and magnitude of the kick must be exquisitely designed for the encounter to result in a bound satellite.
The outcome of the encounter between 3I/ATLAS and Jupiter will be monitored by the Juno spacecraft near Jupiter and the biggest telescopes we have near Earth. Stay curious and get your popcorn ready by March 16. Will 3I/ATLAS add a new moon to Jupiter's family? The answer may surprise you, so keep an eye out for updates!
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.
