DACTYL

The asteroid moon

Mass: Unknown Diameter: 1.4km (0.87 miles) Parent planet: 243 Ida Discovered: 1993, Galileo space probe

243 Ida’s moon is tiny, just 1.6 kilometres (0.99 miles) on its longest axis. Thanks to the larger asteroid’s weak gravity, Dactyl is unlikely to be an object captured into orbit, but the alternative – that Ida and Dactyl formed alongside each other – raises as many questions as it answers.

Ida is a major member of the Koronis family of over 300 asteroids, all of which share similar orbits. The family is thought to have formed 1 or 2 billion years ago during an asteroid collision. Dactyl could be a smaller fragment of debris from the collision that ended up in orbit around Ida, but there is a problem – computer models suggest Dactyl would almost certainly be destroyed by an impact from another asteroid. So how can it be over a billion years old?

One theory is that the Koronis family is younger than it appears, and Ida’s heavy cratering is due to a storm of impacts triggered in the original break-up. Another theory is that Dactyl has suffered a disrupting impact, but has pulled itself back together in its orbit – which might explain its surprisingly spherical shape.

Dactyl’s orbit is still unknown – Galileo approached the asteroid in Dactyl’s orbital plane, so its images provided limited data