On 27 September 2022, NASA's DART (Double Asteroid Redirection Test) mission collided with its target, the asteroid Dimorphos, changing its orbit. This was the first planetary defense test mission designed to alter the course of an asteroid. Its success was followed by an intensive analysis of the collision, which includes the study of the tons of asteroidal rocks that were displaced and thrown into space. The results of these analyses are published today in four articles in Nature, that include the participation of researchers from the Institute of Astrophysics of Andalusia (IAA-CSIC) and the Institute of Space Sciences (ICE-CSIC).
The DART mission sought to prove the usefulness of the kinetic impact method to deflect potentially dangerous asteroids without using explosive charges. Its target, located 11 million kilometres from Earth, was the Dimorphos satellite (with a 160-m diameter) that orbits the asteroid Didymos (with a 780-m diameter), forming a binary system. The impact of the probe, which was travelling at about six kilometres per second, deflected Dimorphos’ orbit and shortened its orbital period by more than half an hour regarding Didymos, which was a success for the project.
"However, many other aspects remained to be studied, in particular, the characterisation of the material ejected after the collision," says Fernando Moreno, a researcher at the IAA-CSIC who participates in one of the articles. Thus, from the moment of the impact and up to several months later, the Hubble Space Telescope (HST) has taken images of this material and characterised its evolution. The CSIC researcher clarifies: "Although part of the material consists of particles ejected at a high speed, at several hundred metres per second, and that quickly disappears from the field of view of the cameras, we have been able to observe the low-speed component."
This article presents a fundamentally morphological study of the evolution of this material, which allowed us to determine the complex interaction between the asteroid system and the dust under the action of the solar radiation pressure.
“When DART excavates the impact crater, the surface and subsurface structure of the asteroid play a role. Large rocks are thrown but, to a large extent, we have seen that many have been weakened by space processing on the surface of the asteroid and, therefore, were preferentially shredded by the impact and immediately thrown into space in the opposite direction of the projectile as centimetre to micrometre particles, then being subjected to the solar radiation pressure”, points out Josep Maria Trigo, a researcher at the ICE-CSIC and member of the Institute of Space Studies in Catalonia (IEEC) and co-author of the study.
“This radiation pressure pushes micrometre particles away to distances of several thousand kilometres in a couple of days, while the larger particles, ejected at speeds close to the escape velocity of the system (about forty centimetres per second) show spiral movements around the system and a complicated evolution over the days. We see, for example, the appearance of a double tail, which could be related to the re-impact of a portion of the largest emitted particles or ‘boulders’ on the surface of Didymos, either because of the disintegration of those same boulders due to high speed of rotation or to the effect of mutual collisions”, indicates Moreno.
Asteroid activation is a phenomenon that occurs naturally in the Solar System and produces an increase in brightness of the object and the deployment of a tail of dust similar to that of comets. The DART experiment will help characterise active asteroids in which collisions with other asteroids act as a trigger mechanism.
Great effiency in deflecting asteroids
On the other hand, Trigo, ICE-CSIC and IEEC researcher, has studied and interpreted the Dimorphos images obtained by the Draco camera onboard DART and from the Italian LICIACube probe, as well as the effects produced in the environment of the binary system of some of the largest telescopes on Earth and in space before and after the impact. The specialisation of the ICE-CSIC team in the chondritic meteorites that make up these asteroids has made it possible to improve the interpretation of the processes that occurred in them. The CSIC researcher has also contributed to quantifying the impulse factor produced by the DART crash, the so-called beta factor, participating in three of the four articles published by Nature.
“We have verified through these images the effects caused by the impact of DART. For several weeks, Dimorphos' period of revolution measurements were hampered by the enormous amount of dust emitted by the DART crater. We cannot forget that Dimorphos is enormously fractured by colossal impacts and seems to have a fragile rubble pile structure, with which the density and porosity of the material are key factors when it comes to quantifying the beta factor”, says Trigo.
Scientists have found that a probe like DART, based on a technique known as a kinetic impactor to deflect asteroids, has great potential to be effective. “Humanity now has a plan in case an asteroid on a direct collision path with the Earth is discovered. In fact, we could say that DART has ushered in a new era of active planetary defense against the danger of asteroid impact,” he concludes.
The DART mission observations will produce more results shortly. “We will characterise the ejected material with the application of Monte Carlo dynamic codes, which allow us to study the dynamic evolution of the particles and build synthetic images, which in turn reveal the properties of the dust: size distribution, velocities and total ejected mass. This is very important with a view to determining the so-called beta factor on the efficiency of the transmission of linear momentum in the collision, apart from the knowledge that it transmits about the natural collision processes in the asteroid belt”, specifies Fernando Moreno.
“We will soon obtain a better understanding of the structure, composition and porosity of both asteroids thanks to the arrival in this binary system of the Hera mission of the European Space Agency (ESA), which will allow us to delve even deeper into the dynamic origin and collisional evolution of these bodies, representative of those that could jeopardise life on Earth”, says Trigo.
The Applied Physics Laboratory (APL) at John Hopkins University (USA) built and operated the DART spacecraft and manages the DART mission for NASA's Planetary Defense Coordination Office as a project of the Office of Missions Program Planetary of the American agency. LICIACube (Light Italian Cubesat for Imaging of Asteroids) is a mission of the Italian Space Agency (ASI) that is part of the DART mission carried out by Argotec.
The properties of the crater generated on the surface of Dimorphos, as well as the evolution of the system's dynamics, will be studied by the ESA's Hera mission, which will be launched in 2024 and will begin the study of the system in 2026.