3671 Dionysus is a potentially hazardous asteroid (PHA) because its minimum orbit intersection distance (MOID) is less than 0.05 AU and its diameter is greater than 150 meters. The Earth-MOID is 0.01989 AU (2,976,000 km; 1,849,000 mi).[2] Its orbit is well-determined for the next several hundred years.
Dionysus makes modestly close approaches to Earth. On 19 June 1984 Dionysus passed 0.0305 AU (4,560,000 km; 2,840,000 mi) from Earth.[2] On 18 June 2085 it will pass 0.028 AU (4,200,000 km; 2,600,000 mi) from Earth.[2]
Moon
In 1997, a team of astronomers at the European Southern Observatory announced that lightcurve observations indicate the presence of a small moon orbiting Dionysus. Its provisional designation is S/1997 (3671) 1. This moon measures 300 meters in diameter, and orbits 3.6 km from Dionysus with an eccentricity of 0.07 and an orbital period of 27.72 hours.[5] From the surface of Dionysus, S/1997 (3671) 1 would have an apparent diameter of roughly 3.02 degrees.[a] For comparison, the Sun appears to be 0.5° from Earth.