Didymos orbits the Sun at a distance of 1.0–2.3 AU once every 2 years and 1 month. Its orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. Its approach to Earth in November 2003 was especially close, with a distance of 7.18 million km; it will not come that near until November 2123, with a distance of 5.9 million km. Didymos also occasionally passes very close to Mars: 4.69 million km in 2144.
Physical characteristics
In the SMASS classification, Didymos was classified as an Xk-type asteroid, which transitions from the X-type to the rare K-type asteroids. Subsequent visible and near-infrared spectroscopy showed it to be silicate in nature. It rotates rapidly, with a period of 2.26 hours and a brightness variation of 0.08 magnitude, which indicates that the body has a nearly spheroidal shape.
Satellite
Didymos is a binary asteroid with a satellite in its orbit. The minor-planet moon, named Dimorphos, moves in a mostly circular retrograde orbit with an orbital period of 11.9 hours. It measures approximately in diameter compared to for its primary. Prior to the official naming of Dimorphos, it was known by its provisional designation and has been informally known as "Didymoon" or "Didymos B".
Naming
This minor planet was named "Didymos", Greek for "twin", due to its binary nature. The name was suggested by the discoverer, University of Arizona Lunar and Planetary Lab astronomer Joseph Montani, who made the naming proposal to the International Astronomical Union after the binary nature of the object was detected. The approved naming citation was published on 13 July 2004. The asteroid moon's name comes from the word "Dimorphos", Greek for "having two forms". The meaning of the name represents how the form of Dimorphos's orbit will change after the DART spacecraft impacts the moon. Appropriately, Dimorphos serves dual roles as a both a test target and a part of a blueprint for future planetary protection. The name of the moon was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle University of Thessaloniki.
Proposed exploration
Didymos is the target of the proposed robotic Asteroid Impact & Deflection Assessment mission, a collaboration between ESA and NASA. While the European spacecraft remains in proposal stage, NASA announced it would proceed with the impactor portion of the mission, called Double Asteroid Redirection Test or DART. The NASA mission is intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth. DART will be the first spacecraft to intentionally target an asteroid known to have a minor-planet moon. Didymos is the most easily reachable asteroid of its size from Earth, requiring a delta-v of only 5.1 km/s for a spacecraft to rendezvous, compared to 6.0 km/s to reach the Moon. DART will be launched in July 2021 for an impact in October 2022. ESA's Hera mission was approved in November 2019 for a launch in 2024, to arrive at Didymos in January 2027. It will survey the dynamical effects of the DART impact and measure the characteristics of the crater made by DART.
