Dimorphos

Dimorphos is a small asteroid and the secondary component of the near-Earth binary asteroid system Didymos–Dimorphos. It gained international attention as the target of NASA’s Double Asteroid Redirection Test (DART) mission in 2022, which was the first-ever attempt to deliberately alter the orbit of a celestial body for planetary defence purposes. The successful mission demonstrated the feasibility of deflecting potentially hazardous asteroids through kinetic impact.

Physical Characteristics

Dimorphos is a small, roughly spherical asteroid with an estimated mean diameter of about 160 metres. It orbits the larger primary asteroid Didymos, which is approximately 780 metres in diameter. The two objects form a binary system in which Dimorphos revolves around Didymos once every 11 hours and 55 minutes at an average distance of about 1.18 kilometres. The system belongs to the Apollo group of near-Earth asteroids and orbits the Sun at an average distance of about 1.64 astronomical units (AU).
The asteroid’s surface appears to be a loose collection of rocks and boulders, typical of a rubble-pile structure. This composition is common among smaller asteroids, which lack sufficient gravity to compact themselves into a solid body. The albedo (reflectivity) of Dimorphos is relatively low, suggesting a dark surface composed mainly of silicate materials and possibly metal-rich minerals.

Discovery and Naming

Dimorphos was discovered on 18 September 2003 by the University of Arizona’s Spacewatch Project at the Kitt Peak National Observatory. The name Dimorphos, which means “having two forms” in Greek, was officially approved by the International Astronomical Union (IAU) in 2020. The name symbolises the asteroid’s dual nature: before and after its orbit was changed by human intervention.
The larger body, Didymos (meaning “twin” in Greek), was discovered earlier in 1996. The two objects together form a system officially designated (65803) Didymos I Dimorphos.

Orbital and Dynamical Properties

Dimorphos’ orbit around Didymos is nearly circular and tidally locked, meaning the same face of Dimorphos always points toward its parent asteroid. The Didymos–Dimorphos system itself follows an elliptical orbit around the Sun, crossing the Earth’s orbit but not currently posing any impact threat.
Before the DART impact, the orbital period of Dimorphos around Didymos was 11 hours 55 minutes. After the collision, observations confirmed that the orbital period was shortened by about 33 minutes, a significant shift that validated the kinetic impactor method as a viable means of asteroid deflection.

The DART Mission and Impact Event

The Double Asteroid Redirection Test (DART) was a mission conducted by NASA’s Planetary Defense Coordination Office and managed by the Johns Hopkins University Applied Physics Laboratory (APL). It was launched on 24 November 2021 aboard a Falcon 9 rocket from Vandenberg Space Force Base in California.
The spacecraft, weighing about 610 kilograms, deliberately crashed into Dimorphos on 26 September 2022 at a relative speed of approximately 6.1 kilometres per second. The impact released a large plume of ejecta and significantly altered the orbit of the smaller asteroid.
The success of DART was confirmed through a combination of ground-based telescopic observations and radar data. These measurements showed that Dimorphos’ orbital period decreased from 11 hours 55 minutes to 11 hours 22 minutes, demonstrating a measurable change induced by the impact. This marked the first time humanity altered the motion of a celestial object in space.

Role of LICIACube

Accompanying the DART spacecraft was the LICIACube (Light Italian CubeSat for Imaging of Asteroids), developed by the Italian Space Agency (ASI). It was released from DART about two weeks before impact and was tasked with photographing the collision and its immediate aftermath. LICIACube successfully captured detailed images of the impact plume, ejecta trails, and the early evolution of the debris field, providing invaluable data for understanding the mechanics of kinetic impacts.

Observations and Analysis after Impact

Following the DART mission, extensive international observation campaigns were carried out using telescopes on Earth and in space, including the Hubble Space Telescope, James Webb Space Telescope (JWST), and various radar facilities. These studies revealed that the impact generated thousands of tonnes of debris and created a long tail of dust and fragments extending thousands of kilometres into space, similar in appearance to a cometary tail.
The momentum transfer caused by the escaping ejecta amplified the total deflection effect beyond the direct impact energy. This finding demonstrated that even a relatively small spacecraft could substantially alter the trajectory of a rubble-pile asteroid if the impact angle and composition are favourable.

Physical Effects and Morphological Changes

Preliminary analyses indicate that the impact reshaped the surface of Dimorphos, forming a crater and redistributing surface material across the asteroid. The ejected material’s behaviour helped scientists infer the asteroid’s internal structure, suggesting it consists mostly of loosely bound rock fragments rather than a solid monolith.
Post-impact studies revealed that the debris cloud gradually dispersed under the influence of solar radiation pressure, forming distinctive dust streams observable for several months. The behaviour of these particles helped refine models of asteroid material strength and cohesion.

Planetary Defence Implications

The DART mission’s success established a practical proof-of-concept for planetary defence through kinetic impact. It demonstrated that by striking an asteroid at high velocity, its trajectory could be predictably altered without the need for explosives or other complex technologies. Such a technique could be deployed in the future to protect Earth from potential asteroid impacts, provided there is sufficient warning time.
Dimorphos thus became the first celestial body in the Solar System to have its orbit intentionally modified by human action. The experiment confirmed theoretical predictions about momentum transfer and provided valuable data for refining future impact mitigation strategies.

Future Exploration: Hera Mission

The European Space Agency (ESA) is planning a follow-up mission called Hera, scheduled for launch in the mid-2020s. Hera will perform a detailed post-impact survey of the Didymos–Dimorphos system to assess the crater, measure Dimorphos’ mass and density, and study the binary system’s dynamics. This mission will complement DART’s results by providing high-resolution mapping and in situ data, thereby deepening understanding of asteroid structures and impact processes.

Scientific Significance

The Dimorphos experiment represents a turning point in space science and planetary defence. It validated kinetic impact as an effective asteroid deflection technique, contributed to the study of binary asteroid dynamics, and offered insights into the structure of small bodies in the Solar System. Dimorphos serves not only as a test case for planetary protection but also as a valuable natural laboratory for understanding the formation and evolution of near-Earth asteroids.