Scientists and astronomers are racing to study only the third-ever known interstellar visitor to the solar system, but with a powerful new observatory coming online, these enigmatic objects may soon become routine discoveries.
A comet, now known as 3I/ATLAS, with 3I short for “third interstellar,” sparked immediate excitement on July 1 when it was detected by the Deep Random Survey remote telescope in Chile, exhibiting a hyperbolic and highly eccentric orbit.
It is the third confirmed interstellar visitor, following 1I’Oumuamua in 2017 and 2I/Borisov in 2019. But fleeting visits of high-speed guests from outside our solar system are likely to be detected much more regularly now, thanks to the new Vera C. Rubin Observatory.
The Rubin observatory is located on the mountain of Cerro Pachón in Chile, and saw first light in June after a decade of construction. While it is only in its early commissioning phase, in just 10 hours of observations, Rubin discovered 2,104 new asteroids. Its science objectives include understanding the structure and evolution of the universe, mapping the Milky Way and observing transient astronomical events, but it is also set to revolutionize the detection of interstellar objects (ISOs).
This is thanks to Rubin’s gigantic Large Synoptic Survey Telescope (LSST) camera— the largest digital camera ever constructed for astronomy, with a staggering 3.2 gigapixels. LSST will scan giant swaths of the sky at once and observe the entire southern sky every few nights. Due to its wide field, depth, and how frequently it observes the same regions of sky, Rubin is uniquely capable of catching fast, faint objects like 1I/’Oumuamua or 3I/ATLAS.
ISOs like 1I/’Oumuamua or 3I/ATLAS move quickly and can easily pass through our sky unnoticed if the sky is not being scanned often and everywhere. Rubin will be looking constantly and broadly, giving astronomers the best chance yet to catch these fleeting visitors, while also being able to detect objects fainter than nearly any ground-based survey before it. Rubin’s powerful imaging and automatic image comparison, coupled with an automated alert system — with millions triggered and filtered every night — means it will pick up telltale motion and flag a potential ISO.
So how many interstellar objects might Rubin actually detect? The answer varies widely depending on which assumptions scientists use.
We are in the early days of detecting ISOs, so it is difficult to estimate how many Rubin is likely to pick up; we know little about their overall frequency, size range, brightness, if they exhibit cometary activity, and how LSST performs.
However, a few recent papers on the topic provide some useful context for how many ISOs LSST might be able to detect, depending on a range of variables.
In a 2022 paper, Hoover et al. estimate that LSST will detect on the order of between 0.9-1.9 ISOs every year, or around 15 such objects across Rubin’s 10-year observational campaign. It notes that these are lower limits, which can be updated when there is more data on the number density and size frequency of interstellar objects.
Additionally, Hoover et al. estimate the chances that Rubin will find an ISO reachable by the Comet Interceptor and Bridge mission concepts, which would fly by an interstellar object as it passes through our solar system. These missions would be launched to lurk in wait, ready to intercept and rendezvous with a passing ISO. The researchers concluded that there is just a roughly 0.07% chance that LSST would identify an ISO target available to Comet Interceptor, which has limited capability to change its velocity, while LSST could detect around three to seven ISOs reachable by Bridge, a more capable but yet-to-be-approved mission concept.
Another estimate, from a 2023 paper by Ezell and Loeb, expects LSST to detect one small ISO 3 to 164 feet (1 to 50 meters) wide every one to two years.
A more optimistic assessment comes from Marceta and Seligman in a 2023 paper. They find, based on a simulated suite of galactic populations of asteroidal interstellar objects and their trajectories and kinematics, that Rubin should detect between around 0 and 70 asteroidal interstellar objects every year. Again, one of the main factors is how many objects of different sizes actually exist in the population of ISOs, as well as their albedo, or how much light they reflect.
With just three confirmed interstellar visitors so far, much remains unknown about the number, size, and diversity of ISOs. But with the Rubin Observatory coming online, sightings of these fast-moving cosmic messengers may soon shift from rare events to regular science, offering unique insights into the galaxy beyond our solar system.