An asteroid moving in a similar orbit to that of the Earth. Such asteroids have an orbital period of one year and hence are said to be in 1 : 1 mean motion resonance with the Earth. Most of the objects in this category probably come from the population of near-Earth asteroids, but some may be ejecta from past impacts on the Moon and a few may be man-made space debris. The first Earth coorbital asteroid to be discovered, 3753 Cruithne in 1986, diameter 2 km, has a semimajor axis of 0.998 au, period 364 days, perihelion 0.48 au, aphelion 1.51 au, and inclination 19°.8.