Can Rubin Spot an Incoming Asteroid in Time? Testing LSST’s Early-Warning Power

Asteroid impacts pose one of the most serious natural hazards to Earth, even though large impacts are rare. In this paper, first author Qifeng Cheng and collaborators investigate how well the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) can discover asteroids that are actually on a collision course with Earth, and, crucially, how much warning time such discoveries would provide. While LSST is designed to survey the entire visible sky deeply and repeatedly, its observing cadence was not optimized specifically for planetary defense. The authors therefore ask a focused question: given LSST’s real survey strategy, how many Earth-impacting asteroids would it detect before impact, and how early would those detections occur?

Building a Realistic Population of Impactors

To answer this, the authors begin by constructing a large and realistic population of synthetic Earth impactors. Instead of generating impactors from scratch, a computationally expensive process because true impacts are extremely rare, they take orbits from an existing near-Earth object population model (NEOMOD3) and apply minimal changes to the timing of each orbit so that the asteroid and Earth arrive at the same orbital intersection point at the same time. This approach preserves realistic orbital properties such as inclination and eccentricity, while efficiently producing a statistically large sample of impact trajectories. In total, the study generates 17,424 synthetic impactors spanning a wide range of sizes, from Chelyabinsk-scale objects (~10–20 m) to continental-scale hazards larger than 140 m.

Simulating LSST Observations

The simulated impactors are then passed through Sorcha, a detailed survey simulator that models LSST’s actual pointing history, depth, noise, and moving-object processing requirements. An asteroid is counted as “discovered” only if it is detected multiple times and successfully linked into an orbit by LSST’s Solar System Processing Pipelines. The authors divide the objects into four size regimes, small (10–20 m), lower mid-size (20–50 m), upper mid-size (50–140 m), and large (>140 m), reflecting common planetary-defense hazard categories. This framework allows them to measure both discovery efficiency and the pre-impact warning time for each size range.

How Many Impactors Are Discovered?

The results show a strong dependence on asteroid size. LSST performs very well for large impactors, discovering about 79.7% of objects larger than 140 m before impact. However, completeness drops quickly for smaller objects: only 50.3% of 50–140 m impactors, 26.8% of 20–50 m impactors, and 10.5% of 10–20 m impactors are discovered at all. These trends reflect the fact that smaller asteroids are fainter and only become visible shortly before impact. The authors emphasize that while LSST excels at detecting faint objects in general, imminent impactors represent a particularly challenging subset of near-Earth objects.

How Much Warning Time Is Available?

Warning times, the interval between discovery and impact, show an equally strong size dependence. Large impactors are typically discovered months to years in advance, with a median warning time of over three years, though even in this regime about 60% fail to reach a one-year warning threshold. Upper mid-size objects are usually discovered only a few months before impact, while smaller objects are often found mere weeks or even days beforehand. For the 10–20 m population, the median warning time is just 12.4 days. This means that for most small and mid-sized impactors, LSST discoveries would come too late for deflection, leaving only civil-defense responses as viable options.

Why Are Impactors Missed?

To understand why impactors are missed, the authors analyze “loss modes” within the survey. They find that almost all impactors pass through LSST’s field of view at least once, so pointing is not the main limitation. Instead, small objects are primarily lost because they are too faint (photometric losses), while mid-sized and large impactors are most often missed because LSST’s cadence does not revisit them frequently enough to link detections into a confirmed orbit (linking losses). This reveals an important tension: LSST is deep enough to see many hazardous objects, but its observing pattern is not optimized to confirm them as impactors in time.

The Case for a Multi-Survey Planetary Defense System

Finally, the authors explore whether a complementary survey could fill these gaps by simulating an Argus-like all-sky, high-cadence system. Although such a survey is much shallower than LSST, its near-continuous coverage makes it very effective at linking rapidly moving, late-stage impactors once they brighten enough to be detected. The comparison highlights a key conclusion of the paper: LSST alone cannot guarantee long-lead warning across the full hazardous size spectrum. A coordinated, multi-survey strategy, combining LSST’s depth with high-cadence, all-sky monitoring, will be essential for robust planetary defense in the Rubin Observatory era.

Source: Cheng