NASA's Asteroid Collision Mission Made More Than Just A Crater On Its Target

On 26 September 2022, NASA’s Double Asteroid Redirection Test (DART) mission successfully made an impact on a near-earth asteroid Dimorphos.

When the 570 kg DART impacted the asteroid, it ejected out 10,000 tonnes of material in the process – which is almost equivalent to the carrying capacity of 143 coal wagons of the Indian Railways, each with 70 tonnes capacity.

The impact was photographed by an onboard instrument called LUKE (LICIACube Unit Key Explorer). It captured images of the impact from a distance of 76 kilometres.

The images were taken between 29 and 320 s after impact revealing 'filamentary streams of ejecta and other complex patterns that expanded for several kilometres from the impact site.' Apart from these, there have also been earth-based observations from various observatories. All the data went into simulations.

Dimorphos is a moon, a natural satellite of another near-earth asteroid Didymos. This is a binary asteroid system. This system completes its revolution around the sun in 770 earth-days or two years and a month.

As of now the minimum distance between the orbit of earth and the orbit of this binary asteroid system is 6 million km. Astronomers have calculated that it will come closer to earth again in 2123 CE with a 5.86 million distance.

It was on this moon asteroid that the DART mission made an impact in 2022. The expectation has been to see if the impact alters the course of the asteroid. Naturally, an impact crater is also expected.

The initial target has been successfully achieved, surpassing NASA's intended outcome. Before the DART impact, the satellite asteroid Dimorphos orbited Didymos in 11 hours and 55 minutes. Following the impact, its orbital period has been reduced to 11 hours and 23 minutes. With a minimum success defined as a change of 73 seconds in orbiting time, the change of 32 minutes is indeed a phenomenal success.

But what did not seem to have happened was the impact crater.

According to a recently published paper in Nature, which is based on the ‘simulation that best matches the observations’, the impact scenario simulations show that, ‘the DART impact does not produce a conventional impact crater but instead causes global deformation of the target.’

In other words, the shape of the target asteroid was significantly changed or ‘globally deformed.’

Why did this happen? Initially, the scientists calculated the asteroid moonlet to have a bulk density of 2400 kg per m³. But the paper suggests that its density is much lesser than this estimate and Dimorphos‘ is probably more porous and, therefore, may have a rubble-pile structure throughout the whole body.’

The study further highlights the origin of Dimorphos as having emerged from accumulating material from Didymos. The paper says: ‘Dimorphos formed through rotationally or impact-induced mass shedding and subsequent reaccumulation from Didymos.’

According to the study, Dimorphos may most possibly be ‘a rubble pile that might have formed through rotational mass shedding and reaccumulation from Didymos.’

A rubble-pile asteroid is made up of accumulated small rocks which are held together by gravity in a loose manner. In 2018 and 2019, two near-earth asteroids Bennu and Ryugu observed from earth, to have diamond shapes, were photographed by unmanned spacecrafts. Though it was theorised that this peculiar shape could have been caused by rotational forces, simulations based on this approach showed a more flattened or asymmetric shape than the observed diamond shape.

Dr Tapan Sabuwala of Okinawa Institute of Science and Technology, Japan along with Prof. Pinaki Chakraborty of the same institute and Prof. Troy Shinbrot from Rutgers University published a paper in which he successfully simulated the diamond shape by applying ‘a simple granular physics model, normally used for the deposition of grains like sand or sugar, could predict the observed shape.’

It is like a pile of sand grains which are subjected to the forces of rotation. Another aspect of the rubble-pile asteroid is that it was not originally a spheroid structure that degenerated into a diamond shape. The deposition of debris material during its formation led to the creation of these diamond-shaped asteroids, establishing it as their fundamental shape from the outset.

The paper states that the mechanical properties of Dimorphos resemble those of Ryugu and Bennu. Dimorphos is, what is called a small s-type moonlet. The spectral classification s-type means that the asteroid is made of silicon material.

S-type asteroids form the majority of inner belt asteroids. The implication of the discovery of Dimorphos as a rubble-pile asteroid which should have gained material from the main body of the binary system Didymos, may give us greater insight into the way asteroids were formed in our solar system.

However a more detailed understanding of the nature of Dimorphos will be revealed in December 2026 and early 2027 when Hera mission by European Space Agency (ESA), presented as ‘humankind’s first probe to rendezvous with a binary asteroid system and Europe’s flagship Planetary Defender.’

Nature Paper can be downloaded here.