3753 Cruithne

Near-Earth Asteroid 3753 Cruithne

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• published: 08 Oct 2009
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Near-Earth Asteroid 3753 Cruithne Because the asteroid and the Earth do not go around in exactly the same amount of time (the asteroid currently goes around slightly faster than the Earth), the position of the kidney-shaped loop relative to the Earth varies over time. If the asteroid and the Earth were not in a special arrangement, the Earth would face potential danger as the asteroid drifted inexorably towards our planet. However, in their current relationship, the kidney's direction of drift is reversed every time it approaches the Earth. Through a remarkable interaction of gravity and the laws of motion, the Earth's effectively "repels" the asteroid at each close approach! Note: Gravity does NOT become repulsive. It is only the interplay between gravity and the physical laws of motion which creates this effect. Consider a comet on a highly elliptical orbit around the Sun. At its closest approach, it doesn't plunge into the Sun (unless it comes close enough to actually crash into it) but rather turns around and heads back away from the Sun. Did the Sun's gravity become repulsive? Of course not, it is just that the velocity the comet gained in falling towards the Sun is enough to carry it away again. The orbits of comets (and Cruithne and all other Solar System bodies) are regulated by this interplay between gravity and the laws of motion. This movie demonstrates the drift of the kidney relative to the Earth, and how it reverses direction upon close approaches. Note that the asteroid passes beneath the Earth at times. The drift rate of the asteroid has been increased by a factor of about 40 for ease of display: there are really about 40 kidney-loops for every one shown. More info here: http://www.astro.uwo.ca/~wiegert/3753/3753.html