Is the solar system full of interstellar vagrants, passing unseen? For decades, it was all conjecture, interrupted only by the brief glimpses of objects such as 1I/’Oumuamua and 2I/Borisov. But with the Vera C. Rubin Observatory’s Large Synoptic Survey Telescope (LSST) now up and running on Cerro Pachón, Chile, the age of accidental discovery may be over.
The 3.2-gigapixel Rubin Observatory LSST camera, the largest astronomy has ever constructed, will scan the southern sky every few nights and record a digital tapestry of unprecedented depth and width. Within its first 10 hours of operation, Rubin had spotted over 2,000 new asteroids, indicative of its sensitivity and reach. But the true revolution is its ability to detect fast-moving, faint interstellar objects (ISOs) that used to be out of reach because they were too fast and too faint.
The challenge is daunting. ISOs, by definition, follow hyperbolic trajectories and will stay in the inner solar system for only weeks or months at most. Typical surveys, with smaller fields and slower cadences, have too frequently missed such visitors. Rubin’s design alters the math: its broad field of view, deep limiting g-band magnitude of +24.5, and high frequency of revisits make it particularly well-suited to detect the faint, characteristic motions of ISOs.
However, estimating the number of ISOs that Rubin will actually discover is a subject of vigorous controversy. Estimates depend upon three significant unknowns: the number density, size distribution, and albedo of ISOs. As a 2022 study by Hoover et al. has pointed out, LSST could potentially discover between 0.9 and 1.9 ISOs per year, or some 15 in a decade though these are pessimistic lower limits, very sensitive to whatever assumptions one makes about the underlying population. More optimistic models, such as those by Marčeta and Seligman, suggest as many as 0 to 70 ISOs annually, if smaller, darker objects are more common than currently believed.
The detection process itself is a technical marvel. Each night, Rubin’s automated pipeline will generate up to five million alerts, flagging moving or variable objects. Machine learning classifiers specifically gradient boosting machines (GBMs) and random forests (RFs) have also proven to be very effective in separating ISO “tracklets” from the general haze of main-belt asteroids and near-Earth objects. The GBM classifier, in simulated LSST data, reached precision and recall rates of more than 99.8%, a performance that sets the stage for a reliable, real-time ISO discovery engine.
One of the key elements in this automated pipeline is the Digest2 short-arc orbit classifier, which labels candidate objects with pseudo-probabilities of motion. Digest2 features, including measures of high-eccentricity or slow-moving orbits, have proven to be the most significant predictors for ISO classification, even surpassing direct measurements like right ascension or magnitude.
But detection is not the end of it. The possibility of catching an ISO to closely study it has energized mission planners. The European Space Agency’s Comet Interceptor mission, which departs for the stable L2 point in 2028, will hover in space for a good target preferably an unaltered long-period comet or an interstellar visitor. But the chances are low: Hoover et al. put the probability of LSST detecting an ISO within reach of Comet Interceptor at only 0.07%, due to its low maneuverability. More ambitious ideas, such as Bridge, with higher velocity change, might encounter as many as seven ISOs during the LSST survey.
Early detection is crucial for ISO rendezvous. Rubin’s skill to detect ISOs years in advance of perihelion, possibly out beyond Saturn, might give the lead time for spacecraft to intercept these intergalactic messengers. The scientific dividend would be enormous: in situ observations would provide definitive answers to the very basic questions on the composition, structure, and genesis of extrasolar planetesimals, as exemplified by the exhaustive set of instruments on Comet Interceptor, such as high-resolution cameras, infrared spectrometers, and plasma analyzers that can probe the nucleus as well as the coma.
As Rubin’s survey expands, the universe of astronomy is presented with a tantalizing question. The observatory will it tell us ISOs are scarce, or that there is an unseen population, each bearing messages from far-off planets? The answer, in the form of the world’s best survey telescope’s data streams, might redefine our view of the solar system’s position in the universe soon.
