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Moons of our Solar System
Moons of our Solar System

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2.9 Small bodies can have moons

Many other objects in the Solar System, not just planets, have moons orbiting them. Sometimes, with smaller primary bodies, the moon in orbit around it may be almost the same size.

An example of this phenomenon is the dwarf planet Pluto; its moon, Charon, is so large in relation to Pluto that the centre of mass about which it orbits is not within the body of its primary, but outside, between the two bodies. Pluto and Charon each orbit their common centre of mass every six days in a synchronous rotation, in a form of binary or dual-body system.

Many objects even smaller than Pluto are known to have their own moons. The first moon of an asteroid to be discovered was Dactyl (Figure 32), found to be orbiting the asteroid known as 243 Ida by the Galileo spacecraft that flew by on its way to Jupiter in 1993.

This is an image of Asteroid 243 Ida and its moon Dactyl, imaged in 1993.
Figure 32 Asteroid 243 Ida and its moon Dactyl, imaged in 1993 by the Galileo spacecraft. Ida is 54 km in length, but Dactyl is much smaller at less than 2 km across