The results of one of the most important experiments carried out to date in space have just been published in the journal Nature: NASA’s DART mission successfully deflected an asteroid 160 meters in diameter called Dimorphos, satellite of an asteroid of 760 meters listed as Didymos. That DART impact with Dimorphos occurred on September 27, 2022 at 0:14am CET and marked a pivotal moment.
The implications are of such magnitude that they open a new era of active planetary defense. We have a defense plan 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 take matters into its own hands and use the growing knowledge about asteroids to test the efficiency of the kinetic impact method against an asteroid. This technique seeks to transfer the kinetic momentum of a kamikaze probe to the asteroid, without using an explosive charge.
we could think first 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 created multiple filaments of particles that could be followed with telescopes from the ground and even from space.
The milestone achieved by the DART kinetic impactor
The good news of the results that are now coming to light is the great efficiency demonstrated 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 that the contribution to the moment of that recoil by the particle ejection far exceeded the incident impulse from DART. 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, let’s not forget that the objective 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 from Northern Arizona University. It describes the observations made to quantify that orbital period 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 that are detached after the impact and the time that they remain around them.
These results encourage planetary defense to be developed efficiently to take action against any asteroid detected in a future direct collision path against our planet. 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 such as DART, as well as describe in full detail the discoveries made about the nature of Dimorphos and the impact site.
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 can discover hundreds every year and, no less important, follow and pinpoint the movements of those already known.
31,361 known asteroids and 119 comets in near-Earth space
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 on a probable future collision path against Earth. Land. 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.
We currently know of more than 10,400 potentially dangerous asteroids as large or larger than Dimorphs, and we must add a significant percentage of small asteroids that remain undiscovered.
The main threats we face are smaller asteroids, around 150 meters, about 60% of which are still unknown, and also certain extinct comets such as 2015 TB145, a 650-meter-diameter rocky object known as the “Halloween asteroid”.
That skull-shaped object put us on alert when it was discovered just three weeks before its passage on October 31, 2015 at a little more than the distance of the Moon, due to being very reflective and following a very eccentric, extended orbit. practically to the orbit 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 makes it possible 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 to which we are sadly becoming accustomed 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 speak 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, particularly in the transport of water and the evolution of life itself.
Currently, the flow of interplanetary matter is not negligible: each 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 on June 30, 1908 and its association with an asteroid that, despite being less than 50 meters in diameter, devastated 2,200 km² of Siberian taiga, should make us reconsider.
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In this context and with the healthy desire to continue learning, DART shows us the way: space exploration and a determined approach to the problems facing humanity, using our scientific-technological capabilities, will be the key to our survival.
This article was originally published on The Conversation. Read the original.
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Familiarity with the articles recently published in Nature and arXiv, which discuss the results of the DART mission, reveals the following. Firstly, the key data interpretations and the validity of the conclusions are not convincing without taking into account the results of direct ground-based observations of the collision process and the subsequent ejection of the comparatively large asteroid fragments. Among the published results of such direct observations, the most informative data is contained in a video animation compiled from a large series of photographs obtained using the telescope of the University of Hawaii in South Africa. Particularly, see the second part of the video, around the 50th second: https://www.youtube.com/watch?v=bfqVqOl9S9w
An analysis of this 500-fold time-compressed video animation provides evidence that the decrease in the asteroid’s orbital period declared by the authors of the above articles, which follows from photometric observations of mutual occultations-eclipses and radar data, may have an alternative explanation. Specifically, it could be a consequence of geometric-photometric distortions caused by the essential asymmetric increase of the observed Dimorphos’s size, which remained unchanged, while the small-sized component of the wide fan-shaped ejecta continued to move away. Since the brightness and scale of the background star images did not change, the metamorphosis of the asteroid’s image (see the attached enlarged photo fragments of the animation along with the observed asteroid dimensions before and after the impact) cannot be attributed to inaccuracies or errors. This phenomenon is the appearance of an asymmetric and optically dense “cloud” of mini-satellites in orbits around Dimorphos, into which relatively larger fragments ejected at lower velocities turned.
Observations of only two successive occultations-eclipses during Dimorphos’s orbital semi-period (Thomas et al., 2023) are able to create the illusion of a shortening of its orbital period due to the displacement of the photometric center of the distorting asteroid image relative to its center of mass. Estimates of the orbital period of the “cloud” of mini-satellites located at heights of several tens of meters above the surface of Dimorphos (according to the video animation) lead to its values being several times larger than the orbital period of the asteroid itself. Therefore, during these events, occurring about 6 hours apart, the “cloud” of mini-satellites will be located on opposite sides of the asteroid most of the time, which is moving in opposite visible directions. Finally, according to estimates, this should manifest as an opposite temporal shift in the positions of the brightness minima. The summarized shift will be close to the declared decrease in the orbital period of Dimorphos. Conversely, when using observational data of the same type occultations-eclipses, occurring only once during its orbital period, such a relative shift is unlikely, despite the presence of a distorting asymmetry in the visible image of the asteroid.
Additionally, the assertion that the ejection was much more efficient in transferring the pushing impulse compared to the actual impact also raises doubts. According to Li et al. (2023) and the above video animation, the mean initial speed of the wide fan-shaped ejection was around several meters per second. Therefore, with an estimated total ejecta mass of up to 1 million kg, its momentum was comparable to the impactor’s. Moreover, it is clear that only a small area near the impact direction (i.e., close to the axis of the ejection cone) can effectively act on an asteroid. At the same time, the rest of the ejecta regions significantly compensated for each other’s impulse transfer abilities, which is problematic in itself for a totally inelastic collision with a loose rubble pile asteroid.
In summary, at this point, the success of this generally complex and beautiful space experiment can be considered questionable in terms of its main stated goals. The interpretation of the photometric and radar observations in the published articles lacks the consideration of the direct ground-based observations of the collision process and the subsequent ejection of asteroid fragments, which are crucial for understanding the observed effects. The evidence provided by the video animation suggests an alternative explanation for the decrease in the asteroid’s orbital period. The assertion that the ejection was much more efficient in transferring the pushing impulse than the actual impact also raises doubts. These issues highlight the need for further investigation and the integration of all available data for a comprehensive understanding of the DART mission’s results.