Is Asteroid Cruithne Earth’s Second Moon?

Although now dismissed as just one of the Earth’s 18,000 so-called mini-moons, is asteroid 3753 Cruithne just one of the most popular candidates for Earth’s “second moon”?

By: Ringo Bones

Some astronomers claim that asteroid 2016 HO₃ that was discovered back in April 27, 2016 is the best candidate for Earth’s so-called “second moon” because it is possibly the most stable quasi-satellite of Earth, asteroid 3753 Cruithne still remained very popular of the other 18,000 candidate asteroids that qualified to be Earth’s second moon. Even though all of them never truly orbit around the Earth like the Moon, it seems though that at the moment, asteroid 3753 Cruithne is the most popular candidate for Earth’s second moon.

The name Cruithne is from Old Irish reference to the early Picts, the asteroid was discovered back in October 10, 1986 by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. Although an earlier discovery of the asteroid back in 1983, when it was still designated as 1983 UH, is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile. It was not until 1997 that asteroid 3753 Cruithne’s unusual orbit was determined by Paul Wiegert and Kimmo Innanen working at York University in Toronto and Seppo Mikkola working at the University of Turku in Finland.

As a Q-type Aten asteroid, 3753 Cruithne got the dubious designation of “Earth’s Second Moon” despite there are other 18,000 asteroids that are viable candidates for this category because it orbits around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. Seen from the Earth’s surface, 3753 Cruithne seem to orbit around the Sun in a bean-shaped orbit that effectively describes a horseshoe that can change into a quasi-satellite orbit. Cruithne does not orbit Earth and at times it is on the other side of the Sun, placing Cruithne well outside of Earth’s Hill sphere. Cruithne’s orbit takes it inside the orbit of Mercury and outside the orbit of Mars. Cruithne orbits the Sun in about 1 year but it takes 770 years for the series to complete a horseshoe-shaped movement around the Earth.

At about 5-kilometers, or 3-miles, in diameter, asteroid 3753 Cruithne is far too small in size to achieve hydrostatic equilibrium to achieve a spherical shape. Its closest approach to Earth is 12-million kilometers, or 7 and a half million miles, approximately three times the separation between Earth and the Moon. From 1994 through 2015, Cruithne made its annual approach to Earth every November but it is far too small to be seen on the Earth’s surface at night via the naked eye. While the Moon’s surface gravity is about one-sixth that of Earth, on the surface of asteroid 3753 Cruithne, the surface gravity is probably one-one hundred thousandth that of Earth akin to what the Philae spacecraft experienced when it landed on the Comet 67P / Comet Churyumov - Gerasimenko back in 2014.