On September 4, 2023, amateur astronomers were prepared when the small main-belt asteroid 5457 Queen's moved toward a 12.5-magnitude star in the constellation Pisces. Czech astronomer Jan Mánek, together with five colleagues from Poland and Switzerland, used telescopes and cameras to record the exact moment the star twinkled as seen from different locations. From this data, astronomers can work out the size and shape of the asteroid.

Asteroids often have moons, although we have only recently begun to record them in large numbers. This image shows a moonrise view of a moon rising from behind asteroid 152830 Dinkinesh, taken by NASA's Lucy mission. The satellite is now officially named Selam

NASA/Goddard/SwRI/JohnsHopkinsAPL/NOIRLab

The observations were good, even better than expected: Manek captured not only the 1.67-second eclipse of the asteroid as it passed in front of the star, but also another one that occurred just 0.87 seconds later. After just 0.2 seconds of eclipse, the star disappeared again - as if another object was following Queen.

No one else captured the second occultation that night. But on September 20, Serge Dramonis from Greece observed the same asteroid eclipsing another star. He also saw a "double" eclipse, consistent with Manek's observations. A second observation confirms what the first hinted at: Amateurs have discovered that Asteroid Queen has a moon!

Recorded light curve of the occultation of asteroid 5457 Queen's that occurred on September 4, 2023. Note the brief dimming caused by the satellite's shadow.

J.Mánek/IOTA-ES

Analysis by Christian Weber, European branch of the International Occultation Timing Association, shows the as-yet-unnamed satellite is less than 2.8 kilometers (1.7 miles) across. Queen's itself was discovered in Palomar Mountain in 1980 by Carolyn Shoemaker and according to the same analysis was only about 25x16 kilometers wide.

Nearly 500 asteroids are known to have satellites, most of which have only one, and some have multiple satellites. However, Manek's and Dramonis' moons are only the second main-belt asteroid moons discovered and confirmed through occultation methods. The other one was discovered by Australians David Gault and Peter Nosworthy in 2021 and belongs to 4227 Arecibo.

Other asteroid satellites discovered through occultation include the Neptune objects 5237642014WC510 and 10199 Chariklo Centauri, discovered in 2018 and 2014 respectively. Two other asteroid moons may have been discovered using stellar occultation - 15094 Polymele and 1723762002YE25 - but these discoveries are still awaiting independent confirmation.

This image shows the shadowed outlines of the asteroid and its small moon. Chord 8 represents the predicted centerline of the occultation; however, the actual path was offset by 10 kilometers to the south. Jan Mánek observed from this predicted centerline position and captured the occultation of the asteroid satellite.

C.Weber/D.Herald/OccultV4/IOTA-ES

Asteroid satellites: a brief history

In 1993, the Galileo spacecraft en route to Jupiter photographed the first known satellite of an asteroid, now named Dactyl, a satellite of the asteroid 243Ida. (Pluto was still considered a planet at the time, so Charon didn't count)

Four years later, astronomers at the La Silla Observatory in Chile discovered the first binary asteroid system through ground-based observations of the Amor-type near-Earth asteroid 3671 Dyonisus. In 2005, astronomers using the Very Large Telescope, also in Chile, discovered two moons orbiting the main-belt asteroid 87 Silvia. These came to be known as Romulus and Remus and were part of the first known multi-asteroid system.

Since then, all other binary and multi-asteroid systems have been discovered through light curve analysis, radar measurements, or spacecraft imaging. In fact, just a few weeks ago, the Lucy mission, currently en route to the Main Belt, discovered another moon, a contact binary orbiting 152830 Dinkinesh, now officially named Selam.

Progress in occultation research

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

Back in 1978, when James McMahon observed the asteroid 532 Herculina hiding a magnitude 5.9 star, observations were much more difficult. Nonetheless, he reported at least seven different "extinctions" on the star, which he attributed to moons, rings, or a combination of both. However, his observation apparently could not be confirmed.

However, when the Hubble Space Telescope surveyed the asteroid 5457 Queen's in 1993, it found no sign of a moon or rings. McMahon only had his eyes and a camera to observe the event, and fatigue or the thin cloud of reports that night could have easily deceived him.

Now, as recent discoveries show, stellar occultations have become increasingly important as a means of exploring the solar system. The technology has matured significantly with the advent of fast, sensitive cameras and precise GPS timing. The high-precision star position and asteroid orbit determinations provided by the Gaia mission also make occultation predictions more reliable than ever before. There is no doubt that more asteroid satellites, binary star systems, and possibly even ring-shaped celestial systems composed of multiple satellites will soon be discovered!