Solar systems orbit the centre of their galaxies. Planets orbit stars. Moons orbit planets. Does anything orbit moons? If not, why not?
鈥� In principle Earth鈥檚 moon could have natural satellites, but in practice it doesn鈥檛 鈥� or they don鈥檛 stay in orbit for long. It does have one artificial satellite: NASA鈥檚 Lunar Reconnaissance Orbiter has been orbiting since June 2009.
The region of space around an object where a satellite could orbit indefinitely is called the Hill sphere, roughly defined as where the object鈥檚 gravitational field is stronger than that exerted by another object outside the sphere.
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Though a more massive body has a bigger gravitational sphere of influence, it is reduced by the proximity of other large objects. That鈥檚 why Neptune has the biggest Hill sphere in the solar system despite being less massive than Jupiter: it is further away from the sun. Further out in the solar system there are tiny asteroids being orbited by even smaller moons.
The moon鈥檚 Hill sphere has a radius of 60,000 kilometres, about one-sixth of the distance between it and Earth.
However, the Hill sphere isn鈥檛 an exclusion zone, as every body in the universe tugs on every other body. For example, Earth raises tides on the moon and vice versa, and the planets beyond Uranus were discovered by the gravitational perturbations they exerted on the seven planets that had already been discovered.
To complicate things further, the gravitational field strength at the moon鈥檚 surface isn鈥檛 uniform. There are localised increases caused by mass concentrations, or mascons, associated with the impact basins Imbrium, Crisium, Serenitatis and Orientale.
These cause anomalies in the field strength that extend into the Hill sphere and perturb the orbits of any satellites. For example, when the PFS-2 was released from Apollo 16 in 1972 it was expected to stay in orbit for 18 months, but these perturbations caused it to crash onto the lunar surface after only 34 days.
鈥淭he gravitational field at the moon鈥檚 surface isn鈥檛 uniform. There are localised increases鈥�
Even though the lunar mascons were discovered in 1968 by Paul Muller and William Sjogren at NASA鈥檚 Jet Propulsion Laboratory, they weren鈥檛 fully mapped until 2001. This revealed 鈥渇rozen orbits鈥� where a lunar satellite can remain in low orbit indefinitely, inclined at 27掳, 50掳, 76掳 and 86掳 with respect to the moon鈥檚 equatorial plane.
NASA has considered capturing an asteroid and placing it in orbit around the moon, close enough for us to extract resources without risking a collision with Earth. However, it would still need the occasional orbital boost to prevent tidal forces bringing it crashing onto the lunar surface.
Mike Follows, Sutton Coldfield, West Midlands, UK
鈥� It is possible for natural objects to orbit moons, though the host moon must have sufficient mass that its Hill sphere is large enough to keep the orbiter within its grasp despite the draw of the host planet.
Saturn鈥檚 moon Rhea is a candidate for hosting a thin ring system, though evidence is patchy. The Pluto system has moonlets with orbits that focus on the point that the Pluto/Charon pair both orbit. Because Charon is a moon of Pluto, we could say these moonlets orbit a moon.
Some moons have fascinating habits. For example, Janus and Epimetheus, a pair of moons that orbit Saturn, swap orbits every four years.
Paul Hunter, East Ilsley, Berkshire, UK
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