How many asteroids in the solar system have moons? Theoretically, hundreds of thousands, but in practice only a few hundred have been found so far. The Gaia space telescope will help expand this list significantly. Scientists have proven that the accuracy of their observations allows us to automatically search for suitable asteroids. In a new scientific paper, they have presented a list of hundreds of binary asteroid candidates.

Traditionally, binary asteroids have been searched for using photometry, radar, and direct observations via ground-based and space-based telescopes. Taken together, these methods cover a fairly wide range of asteroid pairs in terms of size and distance from each other. However, the results are still not objective enough for statistical analysis of such objects, which is necessary, for example, to find out how they most commonly form.

Direct observations allow searching for binary asteroids, mainly those with a large distance between objects, photometry – for compact systems, adaptive optics – for binary pairs with a bright parent asteroid, radar – for near-Earth objects.

Data from the Gaia space telescope allows us to use astrometry, the motion of objects, to search. The challenge is that asteroids are very small objects and therefore their cultivation is minimal. High-precision data is required to capture this. The attempt to detect this motion in the Gaia DR2 release was unsuccessful.

In the Gaia DR3 version, data on the orbital motion of more than 150,000 asteroids was 20 times more accurate than in the DR2 version. Scientists were finally able to use this to filter out “training” objects. The results of this work have been published in the journal Astronomy and Astrophysics

First, the authors of the new study selected 30,000 asteroids from more than 150,000 that had a suitable set of consecutive observations for analysis. After analyzing their motions, 3,038 previous binary asteroid candidates remained. The scientists then checked these objects for their physical parameters. The “inclination” had to coincide with the calculated inclination based on the usual size and density of such asteroids. The result was a list of 358 promising candidates for binary asteroids.

The list includes six known binary asteroids: (1509) Escanglona, ​​​​​​(5817) Robertfrazer, (2871) Schober, (4337) Arecibo, (317) Roxana and (18301) Knyukhiv. Such a small number was expected. First, only objects found in the corresponding position during a few series of Gaia observations were added to the list. Second, the criteria for further automatic screening were too strict. So, for example, according to the astronomers, their method favored pairs with small orbital periods of more than 10 hours.

The selected asteroids are similar in physical parameters to those discovered by other methods. They also cover a size spectrum, rotation periodicity, and distance that other methods do not cover. The authors of the study noted that large “curved” asteroids with a diameter of more than 100 kilometers may have been mistakenly included in the final list. Such objects can visibly “spin” while rotating around their axes.

“Finding binary asteroids is difficult because they are small and far away from us. We expect one in six asteroids to have a companion, whereas so far we have only found 500 binary objects among the millions of known asteroids. This discovery shows that there are many more asteroids with satellites waiting for us to find them,” explained Luana Liberato, lead author of the new paper from the Côte d’Azur Observatory (France).

The next version of the Gaia data is scheduled for release in mid-2026. In the meantime, scientists will continue to study the objects found. In particular, they want to compare Gaia’s astrometric observations with data obtained by other methods. It is also possible to develop other algorithms for scanning objects to perform deeper and more complex analysis of the physical properties of the candidates.

Binary asteroids are a “laboratory” for studying many fundamental processes in the Solar System: from the formation and evolution of bodies to collision dynamics and gravitational interaction.