The Vera Rubin Observatory and the Hunt for Interstellar Objects

Three interstellar visitors have passed through the solar system since 2017, and the newest telescope built to catch the next one already missed its chance once.

It is hoped that the Vera Rubin Observatory, which started producing images in earnest in June 2025, will help astronomers find interstellar objects as they pass through the solar system.

Between 20 June and 20 July 2025, Vera Rubin Observatory took a total of twelve images of 3I/ATLAS. Yet it was actually discovered by the ATLAS survey network based in Chile. No one on the Rubin team had actually noticed what they had, largely because the automated pipeline that was there to flag fast-moving objects hadn't yet been switched on.

While they weren't able to claim the initial sighting of the comet in the solar system, the team, led by Colin Chandler, were able to measure the comet's coma and confirm it was present earlier than the ATLAS discovery data showed.

The team would probably admit that they were lucky the observatory was pointed at the right patch of sky at the right time. Even though they had the disappointment of not announcing the first sighting, they certainly gained kudos with the extra data that gave further insights into the comet. But it does raise the question, are there any other such findings lurking in the images?

What Is an Interstellar Object?

Any object that originates from outside our solar system is termed an interstellar object. Most of the comets and asteroids we see in the solar system have been formed here and stay here, bound by the Sun's gravitational pull. Interstellar objects, by contrast, are few and far between. Having been created in their home system they can be ejected via a gravitational slingshot, perhaps due to an encounter with a planet during the system's chaotic early formation. We then see them on a hyperbolic orbit as they pass through our solar system once, never to return.

1I/'Oumuamua

There have only been three confirmed interstellar objects seen in our solar system. The first was 1I/'Oumuamua in 2017, which was discovered by Pan-STARRS observatory in Hawaii. Elongated in shape and reddish in colour, it had no coma or visible tail. The most curious aspect was a small non-gravitational acceleration that it displayed. Some thought this might be explained by outgassing from volatile ices in the comet, although the lack of a visible coma makes this slightly awkward. A fringe theory, postulated by Avi Loeb, claimed that this could even have been a spacecraft of some sort.

2I/Borisov

2I/Borisov was the second interstellar object recorded in 2019. Named after the man who discovered it, amateur astronomer Gennadiy Borisov, it was a much more traditional comet than 'Oumuamua. It displayed a visible coma and tail and had a chemical composition similar to comets that originate in our own solar system. Its familiar attributes made it much easier to study and astronomers were able to learn a lot including being able to infer that comet formation in other star systems may share traits similar to those in our solar system.

3I/ATLAS

The interstellar object that opened this article, 3I/ATLAS, is an active comet with a developing coma. Its most striking feature is its age. While research is still being done to try and pin down the precise age, researchers think it may be billions of years old, possibly older than our Sun. They have reached this conclusion partly due to the unusually high ratio of carbon-12 to carbon-13 in the comet. Carbon-13 builds up over time as more stars live and die. So, a higher proportion of carbon-12 suggests the object was formed in the early history of the Milky Way.

Another interesting feature of 3I/ATLAS was its high level of methanol. We associate that particular compound with cold, dense molecular clouds where stars and planetary systems are born. This would suggest that the interstellar object was formed in such a place, not dissimilar to our own solar system, but billions of years earlier. Being such a fragile compound, it is remarkable that it has persisted for so many years.

Naming Convention for Interstellar Objects

Interstellar objects have a particular naming convention that is administered by the International Astronomical Union. You may notice that each one has an "I" in its name. That refers to its interstellar nature. 1, 2 and 3 refer to the order in which they have been discovered. So, the next interstellar object will be 4I followed by its proper name, which will be dependent on the particular circumstances at the time. 'Oumuamua is a Hawaiian word roughly meaning a messenger or scout from afar arriving first. Borisov, as mentioned earlier, was named after the discoverer and ATLAS after the Asteroid Terrestrial-impact Last Alert System.

How Many Interstellar Objects Might the Vera Rubin Observatory Find?

It has been speculated that between 5-100 interstellar objects might be found by the observatory in its ten-year survey. This figure was estimated by Rubin's team as a ballpark figure they were hoping for. Another estimate by astrophysicist Rosemary Dorsey at the University of Helsinki gives a wider number, ranging from 5-500.

Astronomers will be taking a keen interest in what Rubin discovers in the coming months and years. Not only will they be scouring the sky for potential new discoveries, they might choose to cast an eye over the first tranche of data Rubin collected, the images that gave so much more information on 3I/ATLAS, even though they missed the first discovery. Perhaps 4I is already there waiting to be seen.

Frequently Asked Questions