Eros, a small asteroid (minor planet) discovered in 1898. It was the first asteroid known to come closer to the earth than the planet Mars. Eros passes within 14,000,000 miles (22,500,000 km) of the earth's orbit every 1.8 years. Usually, however, the earth is in a distant part of its orbit when this occurs. Calculations of Eros's distance from the earth made during relatively close approaches in 1901 and 1931 enabled astronomers to determine the size of the earth's orbit more accurately than had previously been possible.