An asteroidal companion to the Earth


Near-Earth asteroids range in size from a few metres to more than 30 km: in addition to playing an important role in past and present impact rates on the Earth, they might one day be exploited as bases for space exploration or as mineral resources. Many near-Earth asteroids move on orbits crossing that of the Earth, but none has hitherto been identified as a dynamical companion to the Earth. Here we show that the orbit of asteroid 3753 (1986 TO), when viewed in the reference frame centred on the Sun but orbiting with the Earth, has a distinctive shape characteristic of ‘horseshoe’ orbits. Although horseshoe orbits are a well-known feature of the gravitational three-body problem1, the only other examples of objects moving on such orbits are the saturnian satellites Janus and Epimetheus2—and their behaviour is much less intricate than that of 3753. Moreover, the fact that 3753 exhibits such a dynamical relationship with the Earth shows that, although it is not a satellite of our planet per se, it is, apart from the Moon, the only known natural companion of the Earth.

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Figure 1: A view of the inner Solar System projected onto the ecliptic plane in a frame which co-rotates with the Earth; in this frame, the Earth appears stationary and is located at the symbol .
Figure 2: The heliocentric semimajor axis a of asteroid 3753 (1986 TO) over the next 2,000 years.
Figure 3: The orbits of the inner planets Mercury, Venus, Earth and Mars, along with that of asteroid 3753 when seen from the direction of the vernal equinox.


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We thank M. Holman for the use of his integration codes. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada.

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Correspondence to Paul A. Wiegert.

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