Want to stay on top of all the space news? Follow @universetoday on TwitterThe asteroid Cruithne, also known as 1986 TO, is locked in a 1:1 orbit with the Earth. The asteroid is sometimes referred to as the Earth’s second moon. It is not a true moon because the Earth’s gravity does not effect it nor does its effect the Earth. Cruithne’s nearest pass to Earth is .1 AU (40 moon lengths), although right now it never comes closer than .3 AU. The asteroid sort of runs like a corkscrew around the Earth while both are revolving around the Sun.
The asteroid Cruithne is 5 kilometers wide. Its orbital path and Earth’s do not cross, and its orbital plane is currently tilted to that of the Earth by 19.8°. If it were to suddenly break loose from its orbit or come close enough to Earth to be pulled in by its gravity, it would cause a very small impact event. It is large enough to survive the atmosphere, but most of it would burn up. Another possibility is that it will break into several pieces and all of them would them burn up before hitting the Earth.
The asteroid Cruithne is in a normal elliptic orbit around the Sun. Its 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. However, it does not orbit the Earth and is not a moon.
In 2058, Cruithne will come within 0.09 AU of Mars. Cruithne’s distance from the Sun and orbital speed vary a lot more than the Earth’s, so from the Earth’s point of view Cruithne actually follows a kidney bean-shaped horseshoe orbit ahead of the Earth, taking slightly less than one year to complete a circuit. Because it takes slightly less than a year, the Earth “falls behind” the bean a little more each year, and so from that point of view, the circuit is not quite closed, but rather like a spiral loop that moves slowly away from the Earth.
A true moon or not the asteroid Cruithne is worth note and study for the beginning astronomer.
Here is an interesting site that shows some mock-ups of the asteroid Cruithne’s orbit in relation to the Earth’s. There are several related articles here on Universe Today. These two are the best pieces to read to gain plenty of information on Earth’s second moon.