The Rev Dodgson said:
There was a thing on QI the other night saying we have 1000’s of moons.
On the other hand Brian Cox still says we have just one.
So I guess it depends how you define it.
QI is wrong. At the end of that segment of QI, Stephen Fry said words to the effect that NASA calls them moonlets rather than moons.
The five co-orbitals I’ve known about are “(3753) Cruithne, (3362) Khufu, 1989 VA, 1993 WD and 1994 TF2.” See http://arxiv.org/pdf/astro-ph/9904016.pdf for instance, from the year 1999.
“Lagrange demonstrated the existence of five equilibrium positions in the three-body problem. Those that form an equilateral triangle with the Sun and the planet, the leading L4 point and trailing L5 point, are stable to small displacements. However, it was not until the subsequent discovery of the first Trojan asteroid, (588) Achilles, near the L 4 point in the Sun-Jupiter system that a real example of such motion was first observed. There are now known to be at least 400 Trojan asteroids in the orbit of Jupiter, and at least one, (5261) Eureka, near Mars’s L5 point; there are also three examples of Trojan moons in the Saturnian system. In fact, all the known Trojan objects are librating about L4 or L5 points and their orbits are often referred to as tadpoles (or T orbits) because of their shape with respect to the equilibrium position, as viewed in the frame rotating with
the mean angular velocity (mean motion) of the planet.”
“When the angular amplitudes of the leading and trailing tadpoles are sufficiently large, the orbits merge near the unstable L3 point located 180◦ from the planet; the resulting paths in the rotating frame are referred to as horseshoes (or H orbits). A variation on such a structure is seen in the orbits of Janus and Epimetheus, the coorbital satellites of Saturn. In this particular case, a good theoretical understanding of the peculiar orbital
dynamics is possible because of the low eccentricities (e < 0.01) and inclinations (I < 0.4◦) of both objects. In contrast, it has recently been discovered that asteroid (3753) Cruithne, previously designated 1986TO, performs a temporary horseshoe-like orbit with respect to the Earth. This is a more extreme example that has been difficult to incorporate in an analytical theory because of Cruithne’s large eccentricity (e = 0.515) and inclination (I = 19.8◦).”
I have yet to see the “1000’s” paper that QI was referring to. In fact, I strongly suspect that no such paper ever existed, and that QI was misquoting a stray comment by one of the many NASA employees.