When the small main-belt asteroid 5457 Queen’s was moving to eclipse a 12.5-magnitude star in the constellation Pisces on September 4, 2023, amateur astronomers came prepared. Czech astronomer Jan Mánek, together with five colleagues from Poland to Switzerland, had their telescopes and video cameras to time the exact moment the star winked out, as seen from various locations. From those data, astronomers could derive the size and shape of the asteroid.

The observations worked out well — even better than expected: Mánek managed to capture not only the 1.67-second occultation of the star as the asteroid passed in front of it, but also another eclipse that occurred just 0.87 seconds later. For a mere 0.2 seconds the star disappeared again — as if another object were following close behind Queen’s.

No one else captured that second occultation that night. But on September 20th, Serge Dramonis from Greece observed the same asteroid occult another star. He, too, saw a “double” eclipse, consistent with Mánek’s observation. The second observations confirmed what the first had hinted at: The amateurs had discovered that the asteroid Queen’s has a moon!

Analysis by Christian Weber (International Occultation Timing Association, European Section) revealed that the as-yet-unnamed satellite measures less than 2.8 kilometers (1.7 miles) in diameter. Queen’s itself, which was discovered in 1980 at Palomar Mountain by Carolyn Shoemaker, is only about 25×16 km wide, according to the same analysis.

Almost 500 asteroids are known to have satellites. Most are binary; some are even multiple systems. However, Mánek’s and Dramonis’s satellite is only the second main-belt asteroid moon found and confirmed with the occultation method. The other one, which Australians David Gault and Peter Nosworthy discovered in 2021, belongs to 4227 Arecibo.

Other asteroid moons discovered via occultation include the trans-neptunian object 523764 2014 WC₅₁₀ and the centaur 10199 Chariklo, discovered in 2018 and 2014, respectively. There are probably two more asteroid moons detected using stellar occultations — 15094 Polymele and 172376 2002 YE₂₅ — but these finds still await independent confirmation.

Asteroid Moons: A Brief History

The Galileo spacecraft was en route to Jupiter in 1993 when it imaged the first known satellite of a minor planet, now named Dactyl, a moon of asteroid 243 Ida. (Remember, Pluto was still considered a planet at this time, so Charon didn’t count!)

Four years later, astronomers at the La Silla Observatory in Chile found the first binary asteroid system via ground-based observations of 3671 Dyonisus, an Amor-type near-Earth asteroid. In 2005, astronomers using the the Very Large Telescope, also in Chile, found two moons orbiting the large main-belt asteroid 87 Sylvia. These were later dubbed Romulus and Remus, part of the first-known multiple asteroid system.

All other binary and multiple asteroidal systems have since been discovered either by light-curve analysis, radar measurements, or spacecraft imaging. In fact, just weeks ago, the Lucy mission, currently on its way to the main belt, has found another one, a contact-binary moon orbiting 152830 Dinkinesh.

Advances in Occultations

Asteroid occultations are not easy to predict or observe, but technological advances have improved the prospects.

Back in 1978, when James McMahon observed the asteroid 532 Herculina occult a 5.9-magnitude star, the observations were much more difficult. Nevertheless, he reported no less than seven different “extinctions” of the star, which he attributed to satellites, rings, or a combination of the two. However, his observation could never be confirmed.

However, when the Hubble Space Telescope investigated Herculina in 1993, no trace of satellites nor rings was found. McMahnon had only his eyes and a tape recorder to observe the event, and fatigue or the thin clouds reported that night might easily have fooled him.  

Now, as recent discoveries demonstrate, stellar occultations have gained importance as a means to explore the solar system. The technique has matured significantly with the availability of fast and sensitive video cameras and precise GPS timing. High-precision positions of stars and determinations of asteroid orbits, provided by the Gaia mission, have also made occultation predictions more reliable than ever before. There will no doubt soon be many more asteroid moons, binaries, and perhaps even ring systems discovered!