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Earth's orbital companion: Asteroid Cruithne

Kulvinder Singh, News reporter
Jul 29, 2015, 15:43 UTC

Sen—The asteroid Cruithne, around 5 km in size, was first spotted in 1983 by Giovanni de Sanctis and Richard M. West of the European Southern Observatory, Chile. It was given the inital designation 1983 UH. But they never saw the object again, so were unable to track its motion. It was in October 1986 that amateur astronomer Duncan Waldron, working with Robert McNaught, Malcolm Hartley and Michael Hawkins at the Siding Spring Observatory in Australia, spotted the object again. What do we know now about the asteroid Cruithne (named by Waldron and colleagues and officially accepted by the International Astronomical Union), a body that's even been dubbed 'Earth's second moon'?

Although first spotted by de Sanctis and West, Waldron, McNaught, Hartley and Hawkins have been credited as Cruithne's official discoverers. The reason is because they were the first who were able to track its motion and prove it was indeed an asteroid. This is the tradition in astronomy. Waldron was examining photographic plates from the UK Schmidt Telescope at Siding Spring when he spotted 1983 UH. Conrad Bardwell, who would later work at the Minor Planet Center, realised that this and the 1983 object were one and the same and calculated its initial orbital parameters. Waldron et al then used Bardwell's calculations to determine a more accurate result.

They found Cruithne's orbit around the Sun is highly elliptical, with the Sun 'off-centre'. Its closest approach to the Sun, known as perihelion, is 72,405,369 km—0.48 times that of Earth's mean distance from it. The furthest Cruithne gets, known as aphelion, is 226,042,383 km—1.51 AU, where 'AU' is one Astronomical Unit, theEarth-Sun distance of 150 million km, a standard unit of measurement used in astronomy.

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The orbits of Cruithne and Earth over the course of a year (from September 2007 to August 2008). Cruithne's location is indicated by the red box as it is too small to be seen at this distance. Earth is the white dot moving along the blue circle. The yellow circle in the centre is our Sun. Credit:From Wikimedia Commons, the free media repository

There is a large overlap between Cruithne's orbit and that of Earth's—such that if you were to trace a 'top-down' view of both orbits they would resemble a Venn diagram. Whereas Earth takes 364.25 days to complete one orbit around the Sun, Cruithne takes 363.99 days. It completes a full rotation about its axis in 27.31 hours. However, Cruithne's orbital plane is tilted from Earth's by 19.81°. Thus its orbital path never coincides with Earth's, so there is no danger it will collide with our planet.

From Earth, Cruithne appears to be ahead of our planet in its solar orbit. Because both bodies orbit the Sun across similar distances—with Cruithne's orbit being very elliptical—and because the asteroid's orbital period is slightly different from Earth's, it appears to trace out a bean-shaped orbit from our perspective, as illustrated by the graphic below.

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The red path represents the path that Cruithne takes as it revolves around the sun. Since the camera is rotating with Earth, Cruithne appears to move along a bean-shaped path. This illusion is known as a horseshoe orbit. Credit: From Wikimedia Commons, the free media repository

But Cruithne's orbit is stranger and more complex still—the true nature of which was only realised 11 years after Waldron et al's calculations. In 1997 Seppo Mikkola of Turku University, Finland, and Paul Weigert and Kimmo Innanen of York University, Toronto, Canada found that Cruithne's bean-shaped orbit slowly moves in a horseshoe-shaped pathalong Earth's solar orbit. So currently it is moving away from us. The closest Cruithne ever gets to Earth is 12,000,000 km—nearly 32 times the Earth-Moon distance. As it slowly moves away from Earth over the centuries it will end up right around the other side, before appearing to move away again. Cruithne completes one such horseshoe orbit every 770 to 780 years.

Although Cruithne has very unusual orbital characteristics, it does not actually orbit the Earth. Thus it is incorrect to labelit Earth's second moon. A better term is that Cruithne's a 'companion body' to Earth. Although our planet has captured one object into its orbit before to form a natural satellite—the three meter-wide 2006 RH120—it was only there for a year. Earth only has one permanent moon.