NASA image reveals complex evolution of two near-Earth asteroids

In the minutes before NASA’s DART spacecraft collided with the asteroid Dimorphos in a historic planetary defense test in 2022, it captured high-resolution photos of the little celestial object and its larger companion Didymos.


These photographs have allowed scientists to piece together the convoluted history of these two stony bodies near Earth, as well as gain insight into the development of binary asteroid systems, which consist of a core asteroid and a secondary moonlet that orbits it.


Didymos formed approximately 12.5 million years ago, according on an analysis of its craters and surface strength. A similar research revealed that Dimorphos evolved around 300,000 years ago. Didymos most likely formed in our solar system’s main asteroid belt, between Mars and Jupiter, before being thrown into the inner solar system, according to the researchers.

An investigation of the largest rocks on Didymos and Dimorphos revealed information about the asteroids’ origins.

“Both asteroids are aggregates of rocky fragments formed from the catastrophic destruction of a parent asteroid,” said astronomer Maurizio Pajola of the National Institute for Astrophysics (INAF) in Italy, lead author of one of the five studies published on Tuesday in the journal Nature Communications.

“These large boulders could not have formed from impacts on the surfaces of Didymos and Dimorphos themselves, as such impacts would have disintegrated these bodies,” Pajola observed.



Didymos, with a diameter of approximately half a mile (780 meters), is categorized as a near-Earth asteroid. Dimorphos is approximately 560 feet (170 meters) broad. Both are “rubble pile” asteroids, made up of stony debris that consolidated under gravity.

“Their surface is covered in boulders.” The largest on Dimorphos is the size of a school bus, while the largest on Didymos is the size of a soccer field,” said Olivier Barnouin, a planetary geologist and geophysicist at the Johns Hopkins University Applied Physics Laboratory in Maryland and the primary author of another study.


“Dimorphos has fractures on its surface and rocks, whereas Didymos may have finer-grained soils around the equator, though the photographs we have make this impossible to determine. “The surfaces of both asteroids are weak, much weaker than loose sand,” Barnouin explained.

The researchers found that Dimorphos is made up of material that flew off Didymos’ equatorial region because to its high spin rate.

“In the case of Didymos, it is thought that in the past, it rotated faster around its axis due to the YORP effect (spin acceleration driven by the effect of sunlight on its uneven surface), and thus ejected the boulders from its equatorial region, forming Dimorphos,” Pajola noted.

Didymos presently spins once every 2-1/4 hours.

The equatorial portion of Didymos included a few pebbles.

“Its equator is much smoother, while mid-latitudes up to the poles are much rougher, with big boulders sitting on the surface,” Pajola pointed out.

The US space agency’s DART (Double Asteroid Redirection Test) conducted a proof-of-principle mission, demonstrating that a spacecraft might use kinetic force to redirect the path of a space object that would otherwise collide with Earth. Didymos and Dimorphos do not represent a direct threat to Earth.

DART collided with Dimorphos on September 26, 2022, at a speed of around 14,000 miles per hour (22,530 kph) at a distance of approximately 6.8 million miles (11 million kilometers) from Earth, successfully modifying its direction slightly. The collision also somewhat altered Dimorphos’ form.

The DART data have advanced our understanding of binary asteroid systems.


“Binary asteroid systems represent about 10-15% of the total number of asteroids that are in near-Earth space,” Barnouin informed me. “In general, each new observation of an asteroid or asteroid system teaches us more about how asteroids develop and evolve. They are complex systems, but they share several fundamental characteristics, particularly when we consider asteroids smaller than a kilometer (0.62 mile).