Quasi-satellite of Earth - Asteroid 3753 Cruithne

A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.

A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity (usually greater). When viewed from the perspective of the planet, the quasi-satellite will appear to travel in an oblong retrograde loop around the planet.

In contrast to true satellites, quasi-satellite orbits lie outside the planet's Hill sphere, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighbourhood, then possibly later move back to a quasi-satellite orbit, etc.

3753 Cruithne is an asteroid in orbit around the Sun in approximate 1:1 orbital resonance with the Earth. It is a near-earth asteroid orbiting the Sun in an apparent horseshoe orbit. It has been incorrectly called "Earth's second moon".

Discovery

Cruithne was discovered on October 10, 1986, by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Australia. It was not until 1997 that its 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.

Naming

The asteroid is named after the Cruithne or Cruthin, a people of early medieval Ireland mentioned in the Annals of Ulster.

Stats

Diameter: ~5 km

Semi-major axis: 0.998 AU (same as Earth)

Rotation: 27.44 hours

Minor planet category: Aten asteroid

Orbit

Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".

Cruithne's closest approach to Earth is approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November.

Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time.

There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross.

Cruithne, having a maximum near-Earth magnitude of +15.8, is fainter than Pluto and would require at least a 12.5-inch reflecting telescope to be seen.

Others Quasi-satellite of Earth

As of 2011, Earth has four other known quasi-satellites: 2002 AA29, 2003 YN107, 2004 GU9 and 2010 SO16.