Dimorphos

The primary asteroid Didymos was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona. The satellite Dimorphos was discovered on 20 November 2003, in photometric observations by Petr Pravec and colleagues at the Ondřejov Observatory in the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutual eclipses and occultations. With his collaborators, he confirmed from the Arecibo radar delay-Doppler images that Didymos is a binary system.

The Working Group for Small Bodies Nomenclature of the International Astronomical Union (IAU) gave the satellite its official name on 23 June 2020. The name Dimorphos is derived from a Greek word (Δίμορφος) meaning 'having two forms'. The justification for the new name reads: "As the target of the DART and Hera space missions, it will become the first celestial body in cosmic history whose form was substantially changed as a result of human intervention (the DART impact)". The name was suggested by Kleomenis Tsiganis, a planetary scientist at the Aristotle University of Thessaloniki and a member of both the DART and Hera teams. Prior to the IAU naming, the nickname Didymoon was used in official communications.

Dimorphos has a surface age of less than 0.3 million years, which is small in comparison to its parent object Didymos's surface age of ~12.5 million years. Madeira et al. (2023) proposed that Dimorphos formed because of the YORP effect, speeding Didymos's spin rate until it ejected matter from its equator to make a ring, which was about 25% the mass of Didymos. That ring would then become moonlets; a fraction of the mass of the ring would become Dimorphos and a set of objects outside of the Roche limit. After that, most of the ring's mass would come back to Didymos.

On 24 November 2021, NASA and the Applied Physics Laboratory launched an impactor spacecraft towards Dimorphos as part of their Double Asteroid Redirection Test (DART). DART was the first experiment conducted in space to test asteroid deflection as a method of defending Earth from potentially hazardous asteroids. Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around 24,000 kilometers per hour (15,000 miles per hour). The collision successfully decreased Dimorphos's orbital period around Didymos by 32±2 minutes. Fifteen days prior to its collision, the impactor released LICIACube, an Italian Space Agency CubeSat that photographed the impact and the resulting dust plume as it performed a close flyby of the Didymos system. Spacecraft and observatories such as Hubble, James Webb, Lucy, SAAO and ATLAS also captured the dust plume trailing the Didymos system in the days following the impact. As part of its Hera mission, ESA launched three spacecraft to the Didymos system in 2024 to reach this asteroid system in December 2026 to further study the aftermath of the impact.

The DART impact on the center of Dimorphos decreased the orbital period, previously 11.92 hours, by 33±1 minutes. This large change indicates the recoil from material excavated from the asteroid and ejected into space by the impact (known as ejecta) contributed significant momentum change to the asteroid, beyond that of the DART spacecraft itself. Researchers found the impact caused an instantaneous slowing in Dimorphos's speed along its orbit of about 2.7 millimeters per second — again indicating the recoil from ejecta played a major role in amplifying the momentum change directly imparted to the asteroid by the spacecraft. That momentum change was amplified by a factor of 2.2 to 4.9 (depending on the mass of Dimorphos), indicating the momentum change transferred because of ejecta production significantly exceeded the momentum change from the DART spacecraft alone. While the orbital change was small, the change is in the velocity and over the course of years will accumulate to a large change in position. For a hypothetical Earth-threatening body, even such a tiny change could be sufficient to mitigate or prevent an impact, if applied early enough. …

Dimorphos is approximately 170 meters (560 ft) in diameter, compared to Didymos at 780 meters (2,560 ft). Dimorphos does not have a confirmed mass, but it is estimated to be about 5×10 kg (5.5 million tons), or about the same mass and size as the Great Pyramid of Giza, when assuming a density of 2.17 g/cm similar to Didymos. It is one of the smallest celestial objects given a formal name by the IAU, after 367943 Duende and 469219 Kamoʻoalewa.

The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has a rubble pile structure.