Herculina’s phantom moon

How a 1978 stellar occultation produced the first “confirmed” asteroid moon — and why Hubble could not find it.

Published 2026-10-23

Today more than a few asteroids are known to have moons. In 1978, the idea was new, and Herculina briefly became the poster child.^[2]

Watching a star blink

An occultation happens when one object passes in front of another, hiding it.^[5] When an asteroid crosses in front of a star, observers along its path see the star vanish for a few seconds. Timing the blink from several sites traces the asteroid’s outline.

The 1978 event

That year Herculina occulted the star SAO 120774. Besides the main event, some observations showed extra, anomalous dimming. The interpretation: Herculina, about 216 km across, had a satellite roughly 45 km wide orbiting about 1,000 km away. It was considered the first asteroid “confirmed” to have a moon.^[2]

Hubble looks

In 1993, astronomers searched carefully with the Hubble Space Telescope, which can resolve fine detail without the blurring of Earth’s atmosphere.^[6] They found no secondary.^[2]

The real first

That same year the Galileo spacecraft passed asteroid 243 Ida and found a small moon, later named Dactyl — the first asteroid moon seen directly.^[4]

The lesson

Occultation timing is powerful, but a single brief dip can come from many causes. Today’s occultation campaigns use many observers and cameras so unexpected events can be cross-checked.^[5]

Numbered citations link to the sources list on the Herculina overview page.