1685 Toro

1685 Toro, provisional designation 1948 OA, is an eccentric asteroid classified as near-Earth object, about 4 kilometers in diameter. It is also an Apollo asteroid, a subgroup of near-Earth asteroids that cross the orbit of Earth. It was discovered by American astronomer Carl A. Wirtanen at Lick Observatory on Mount Hamilton, California on 17 July 1948.

The asteroid orbits the Sun at a distance of 0.8–2.0 AU once every 1.60 years (584 days). Its orbit shows a very high eccentricity of 0.44 and is tilted by 9 degrees to the plane of the ecliptic. The stony S-type asteroid is reported to be composed of L chondrite with a high geometric albedo in the range of 0.24–0.34. It has an extremely well-measured rotation period of 10.2 hours.

Its orbit also shows a 5:8 resonance with Earth and in a near 5:13 resonance with Venus. It was the third Apollo asteroid to be discovered. The current resonance with Earth will last for only a few thousand years. Calculations show that Toro will leave it in 2960 AD, and that it will enter the region of 5:13 resonance with Venus in 3470. This is because the distance from Earth's orbit will become larger and that from Venus's orbit smaller. A study of long-term stability shows that the alternating resonances will possibly be broken roughly 3 million years from now because of close approaches between Toro and Mars.