They just saw the light in the magazine Nature the results of one of the most important experiments carried out to date in space: the dart mission NASA successfully deflected a 160 meter diameter asteroid called Dimorphssatellite of another 760 meter cataloged as Didymos. The impact occurred on September 27, 2022 at 0:14am CET and marked a key moment.
The implications are of such magnitude that they open a new era of active planetary defense. This is thanks to multiple missions to study these bodies, which in recent decades have increased our understanding of near-Earth asteroids, grouped into various groups according to their orbits. And, almost unintentionally, this field exemplifies that the investment made in the last decades in space provides scientific milestones that mark our future.
The possibility of an impact of an asteroid with the Earth is not zero
The possibility of an impact with an asteroid of a few hundred meters is low, but not null, although it seems relegated to science fiction novels and movies. This latent danger, like so many others linked to our own unbridled use of planet Earth’s resources, threatens our existence.
The scientific community led by NASA and Johns Hopkins University has decided to put matters on the table and use the growing knowledge about asteroids to test the efficiency of the kinetic impact method against one of these bodies.
This technique seeks to transfer the kinetic momentum of a kamikaze probe to the asteroid, without using an explosive charge. we could think a priori which is a mere applied physics experiment, similar to the one we perform on a pool table. Nothing could be further from the truth.
DART reached Dimorphos at a speed of 6.14 km/s. When we hit an asteroid at hypervelocity, a part of the collision is transmitted elastically, but as a crater is excavated, an additional momentum is created caused by the emission of materials in the opposite direction of the projectile.
This “recoil” component participates in the impulse supplied to the asteroid and contributes very efficiently to divert it from its trajectory. In fact, the materials ejected after the impact gave rise to multiple filaments of particles that could be followed with telescopes from the ground and even from space.
The milestone achieved by the kinetic impactor of the DART mission
The good news of the results that are now coming to light is the great efficiency demonstrated by the DART mission in deflecting the Dimorphos asteroid. In the article led by Andrew F. Cheng, from the Johns Hopkins University Applied Physics Laboratory, we quantified the so-called Beta factor associated with this inelastic component that causes recoil and that plays in favor of increasing the effects of a kinetic impactor.
In fact, the experiment far exceeded expectations as that multiplying factor of the angular momentum transfer associated with the inelastic component of the deflection reached a value of 3.6.
That means the contribution to the momentum of that recoil from the particle ejection far exceeded the incident impulse from the DART mission. This parameter is of vital importance and just the most important to quantify in an asteroid of these characteristics, a pile of rubble as the images revealed.
As a consequence of the diversion, the goal was to shorten the orbital period of Dimorphos around Didymos by just over a minute. But it was reduced by 33 minutes, as detailed in the article led by Cristina A. Thomas of Northern Arizona University. It describes the observations made to quantify that orbital period. This is based on the photometric observations made of the binary system using the largest available telescopes.
In another work, led by Jian-Yang Li of the Planetary Science Institute in Tucson, Arizona, the evolution of the filaments populated by the particles thrown after the excavation of the impact and which evolved over the months subjected to the pressure of sunlight radiation. The results are of great relevance to understand what happens to the materials released after the impact. And the time they stay around.
Such results encourage that the planetary defense can be developed efficiently to take action against any asteroid on a collision course with the Earth. Precisely in the article led by Terik Daly, also from the Johns Hopkins University Applied Physics Laboratory, we describe the magnitude of the scientific milestone that is to hit Dimorphos with a robotic and autonomous probe like DART. Just as the discoveries made about the nature of this asteroid and the impact site are described in detail.
Even so, the key to our ability to deflect asteroids will be to continue investing in the early detection of all those bodies that pose a real danger. Although it is not an easy task, thanks to the revolution in CCD digital camera technology, we are able to discover hundreds every year. And no less important, follow up and specify the movements of those already known.
At present, monitoring programs, initially encouraged by NASA, show that there are some 31,361 asteroids and 119 comets in near-Earth space. And that, at some point, one could be identified in a probable path of future collision with Earth. In fact, this has already happened six times, but with the exception that it happened with asteroids a few meters in diameter that impact our planet more often and generate meteorite falls.
Currently, we know of more than 10,400 potentially dangerous asteroids as large or larger than Dimorphs. We must add a considerable percentage of similar bodies, but smaller, which remain undiscovered.
The main threats we face are asteroids of about 150 meters, about 60% of which we still don’t know. Also certain extinct comets like 2015 TB145a rocky object 650 meters in diameter.
That skull-shaped object put us on alert since it was discovered just three weeks before its passage on October 31, 2015, a little more than the distance from the Moon. This is due to being very reflective and following a very eccentric orbit, practically extended to that of Jupiter. Such objects, being able to strike our planet with much higher energy than a conventional asteroid, exemplify the diversity and complexity of the problem we face.
It is not possible to be catastrophic since all the effort to discover and catalog these bodies allows us to better quantify the impact frequency and suggests that an event like Tunguska would occur every several centuries. They also suggest that, fortunately, impacts by kilometer-sized asteroids occur every several tens of millions of years. In any case, the catalog of the Sentry Program of the Center for the Study of Minor Objects (CNEOS) of the Jet Propulsion Laboratory (JPL) ensures that, among the cataloged near-Earth asteroids, none is a source of risk on a scale of several centuries. Thus, those catastrophic news that we are getting used to with each relatively close encounter of an asteroid with the Earth are totally unfounded.
The enriching role of a past marked by impacts
In the remote past, the Earth was born after innumerable impacts with asteroids and even, in a final phase, they were with authentic planetary embryos, the dimensions of the planet Mars itself. If we talk on a larger time scale of billions of years, scientific evidence shows that the impacts of asteroids and comets have played a key role in the history of the Earth. In particular with the transport of water and the evolution of life itself.
At present the flow of interplanetary matter is not negligible. Every year about 100,000 tons reach Earth and, although most of it does not reach the Earth’s surface, it does evaporate and become part of our atmosphere.
Perhaps due to the challenge of correctly interpreting cataclysms caused from outer space, a large part of the population continues to underestimate this danger that hangs over humanity. Despite this, awareness of the Tunguska impact in 1908, which, despite being less than 50 meters in diameter, devastated 2,200 km² of Siberian taiga, should make us reconsider.
In this context, and with the healthy desire to continue learning, the DART mission shows us the way. Space exploration and decisive approach to the problems facing humanity. Especially, using our scientific-technological capabilities, which will be the key to our survival.