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On October 7, 8, and 9, 1970, Charles Kowal, in search of fast-moving asteroids, exposed three photographic plates of an area of the sky near the ecliptic with the 122 cm Schmidt reflector of the Hale Observatories. Several weeks later Eleanor Helin of the California Institute of Technology examined the plates under a blink microscope and discovered a fast-moving object of about 15 mag. During the 15 min exposures the object had left a slightly bumpy trail indicating light variations with a period of a few minutes.
Four positions of the object were reported to the author, who derived the heliocentric orbit defined in table I. From table II it is seen that the residuals of that orbit are not very satisfactory. Note that the third and the fourth position represent the end points of the same trail. There is a clear indication that after October 9, the predicted heliocentric positions would soon diverge from the actual positions. This circumstance, and the fact that the orbit is so similar to that of Earth, suggested that the object could actually be moving in an Earth-centered orbit.