By Tolga Sahin | Intern
The third-ever interstellar comet is on its way out of the solar system forever.
Astronomers are tracking a visitor called 3I/ATLAS, the third confirmed interstellar object ever observed. The two previous instances were 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. 3I/ATLAS is currently passing through the solar system on a one-way route, and it will continue outward rather than settling into a repeating orbit.
In this case, “interstellar” means the object was formed around another star and later drifted into the solar system. NASA describes 3I/ATLAS as following a hyperbolic trajectory, an open path independent of the sun.
Dr. Benjamin Rose, assistant professor of physics and astronomy, said interstellar objects are rarer than recent glimpses might suggest.
“Space is huge, so interstellar objects are rare, but they are also extremely hard to observe, so it is difficult to know their true rate at this time,” Rose said via email. “Interstellar objects are formed from different gas clouds. Therefore, with enough observations, we can learn if the material composition of our solar system is typical for stellar systems in our neighborhood.”
The discovery came on July 1, 2025, when the NASA-funded ATLAS survey telescope in Río Hurtado, Chile, reported a faint object to the Minor Planet Center, and additional observations arrived as astronomers refined the orbit.
“There are a lot of unanswered questions around comets, planet formation and the rate of interstellar objects,” Rose said. “Objects like 3I/ATLAS are able to contribute to our understanding of planet formation since it is a ‘rejected’ object from the planet formation processes.“
The first days of discovery were marked by uncertainty because a small number of measurements couldn’t precisely define an orbit. The first findings suggested that 3I/ATLAS was not a threat to Earth; its closest approach to Earth is about 1.8 astronomical units, or roughly 170 million miles, according to NASA’s reports.
As the orbit solution strengthened, the evidence for an origin outside the solar system became clear. A strongly hyperbolic orbit indicated that the object arrived with extra speed relative to the sun. NASA notes that it did not pass close enough to any planet to gain that speed through a gravitational slingshot.
Hasbrouck Heights, N.J., sophomore Evan De Cicco said many debated what the object was, some even considering that it might be an alien spacecraft.
“3I/ATLAS was a comet, many thought it might be an alien spacecraft due to its unusual properties, concentrations, speed and trajectory,” De Cicco said. “While this did not end up being the case, 3I/ATLAS is still a topic of much interest as it is merely the third known interstellar object to visit our solar system.”
While 3I/ATLAS may be the third-ever recorded interstellar object, it’s not a reliable measure of how many exist. The larger problem is detection because they are small and fast. Surveys can spot them only after they are already moving through the inner solar system, where the best observing window is already shrinking.
“To most people, this may not be interesting; however, with how infrequently Earth and our solar system as a whole come into contact with interstellar objects, this rare occurrence is extremely fascinating,” De Cicco said.
Early images did not immediately show certain comet features, so the object’s nature was initially undetermined. Observers later on identified cometary activity.
Once activity was clarified, the central question shifted from “what is it?” to “what is it made of?” In August 2025, the James Webb Space Telescope used infrared spectroscopy to identify a coma rich in carbon dioxide, along with water-related signatures and carbon monoxide. In addition to these, the carbon dioxide to water ratio was among the highest measured for comets, according to NASA.
NASA’s timeline places the comet’s closest approach to the sun around Oct. 30, 2025, at about 130 million miles, near the orbit of Mars. During an early October window, the European Space Agency reported that ExoMars Trace Gas Orbiter and Mars Express observed 3I/ATLAS as it passed close to Mars, with Trace Gas Orbiter capturing images.
A July 2025 mission-feasibility study evaluated whether a spacecraft could reach 3I/ATLAS for a flyby on short notice. The authors found that launching from Earth after July 1 would require an exceptionally large change in speed, while departures from Mars during a narrow window from July to September 2025 could have enabled an early October flyby with a much smaller change in speed.
A spacecraft encounter with 3I/ATLAS did not happen because there was no prepared and funded rapid response program for an interstellar visitor.
