NASA's DART mission captures high-resolution image of the binary asteroid system Didymos

An international team of astronomers, with participation from the Instituto de Ciencias del Espacio (ICE-CSIC) and the Instituto de Astrofísica de Andalucía (IAA-CSIC), both from the Consejo Superior de Investigaciones Científicas (CSIC), and the Instituto de Estudios Espaciales de Cataluña (IEEC), presents the first high-resolution images of Didymos, a near-Earth binary asteroid system. The images obtained with the DART and LICIACube probes have allowed astronomers to infer key information about the geology and evolution of the asteroid system. The study, with the participation of CSIC, an agency of the Ministry of Science, Innovation and Universities (MICIU), is published today in five papers in a dedicated issue of Nature Communications.

Observations of near-Earth asteroids (NEAs) have shown that many of them exist as binary systems. Astronomers have theorized that these binary systems form when the rotational speed of a single rubble pile asteroid increases. It ejects debris, which in turn forms a new companion. This process depends on the geophysical properties of the asteroid. Therefore, studying the shape and morphology of the surface of asteroids is key to interpreting their evolution.

Thanks to close-up observations of the Didymos system - collected by NASA's DART mission when it collided with the asteroid system on 27 September 2022, as well as images collected by the Italian Space Agency's (ASI) LICIACube Unit Key Explorer (LUKE) just after the impact - the team has had a unique opportunity to observe closely at the geological level an NEA binary system from which we can infer its geophysical properties and broaden our understanding of its formation.

Image of the Didymos binary system obtained by the DRACO camera aboard the DART probe Credits: NASA.

“Through this analysis, we have been able to infer their geophysical properties and expand our understanding of the physical nature of these asteroids,” explains Juan Luis Rizos, researcher at IAA-CSIC and member of the Proximity Imaging Working Group of the DART mission, whose main role has been the analysis and photometric modeling of the images captured by the DRACO camera of the probe.

ICE-CSIC and IEEC researcher Josep M. Trigo-Rodríguez has participated in the analysis of the higher resolution images obtained by the DRACO camera on board the DART spacecraft. The specialization of the ICE-CSIC team in chondritic meteorites that form these asteroids has allowed to improve the interpretation of the processes that occurred in them.

These images reveal that Didymos is flattened and shows signs of undulation along its equatorial perimeter. Its polar regions are rough and contain large rocks and craters, while near its equator the surface of Didymos is smooth, with few large rocks and craters. Dimorphos, on the other hand, has a surface covered with rocks, cracks and some craters. The surfaces of both asteroids include large boulders suggesting that both are of the debris pile type.

Astrophysicist Josep M. Trigo states, “By studying the high-resolution images provided by the DART camera we identified the complexity of Dimorphos' surface. Its surface is covered by an infinity of sharp and fragmented rocks due to its exposure to space, also subject to long cracks and lineaments that indicate the deformation imposed by the gravitational action of its massive companion. These processes are burying impact craters and changing the structure of Dimorph."

Asteroid Dimorphos was captured by NASA's DART mission before the spacecraft hit its surface. Credits: NASA/Johns Hopkins APL

The study also reveals that these potentially hazardous asteroids share a “surprisingly fragile” nature, exhibiting surface cohesions of less than 1 pascal and an interior cohesion of around 10 pascals. “This is a surprising and challenging result, exemplifying the complexity of exploring these bodies,” notes the ICE-CSIC researcher. “We can exemplify such values by thinking of those expected for a giant pile of sharp rocks collected in the Pyrenees. Take them all into space in a big box, and then remove it. Exactly: you get a huge, crumbly pile of rocks. Not unlike a flying castle of wet sand,” he adds.

Looking at craters on the surface of an asteroid can reveal its age and history. As they travel through space, asteroids collide with other asteroids. Small collisions produce craters on the surface, while a larger collision can break it into pieces. When an asteroid breaks apart, a new surface is exposed.

Observations identified 16 and 22 plausible craters at Didymos and Dimorphos, respectively. In the case of Didymos, most of the craters are at higher latitudes and the largest craters are more than 160 meters in diameter and resemble wide circular depressions found on other asteroids. Didymos and Dimorphos are heavily fractured bodies sculpted by large impacts.

Analyses of surface strength and the number of craters allowed the team to conclude that the surface of Didymos is between 40 and 130 times older than its satellite Dimorphos, with probable absolute surface ages of about 12.5 million years for Didymos and 300,000 years for Dimorphos.

“We think that Dimorphos originated from Didymos due to an increase in its rotation caused by interaction with solar radiation,” explains Rizos. “The low cohesion of the materials allowed the object to deform as its rotation accelerated, until, upon reaching a certain speed, part of the parent body's material broke off and formed the moon Dimorphos. This phenomenon is known as rotational fission,” concludes the researcher.

An interesting implication of the study is that the estimated surface age inferred for Didymos is substantially younger than the age determined for its most plausible asteroid family, the Baptistina family of asteroids, which has an estimated age range ten times larger and a much higher surface cohesion. Rather, Didymos may represent the last of multiple generations of asteroids derived from the original progenitor.

These observations are crucial to understanding the nature of potentially hazardous asteroids. The DART mission has demonstrated the ability to deflect asteroids, and the fact that most of them are highly fractured is, according to the researchers, excellent news for planetary defense and allows kinetic impact missions to be really efficient in deflecting them.

In addition, ESA's upcoming Hera mission, in which CSIC is also participating through ICE-CSIC and IAA-CSIC, will be launched in October 2024 and will perform a six-month exploration of the Didymos-Dimorphos binary system in 2026. The longer duration of the mission compared to DART will provide data for a better understanding of the origin and evolution of the fascinating Didymos system.

Artist's conception of the Hera mission.Credit: ESA