DART mission: Humanity already has a planetary defense plan

They just saw the light in a magazine Nature the results of one of the most important experiments done to date in space: NASA’s DART mission successfully deflected a 160 meter diameter asteroid called Dimorphos, a satellite of the 760 meter diameter asteroid cataloged as Didymos. This DART collision with Dimorphos occurred on September 27, 2022 at 0:14 am CET and was a turning point.

The consequences are so great that they open a new era of active planetary defense. We have a plan for protection thanks to the many missions to study these bodies, which in recent decades have expanded our understanding of near-Earth asteroids, grouped into different groups according to their orbits. And almost unintentionally, this field shows that the investments made in space in recent decades provide scientific milestones that mark our future.

The probability of an asteroid colliding with the Earth is not zero

The chance of hitting an asteroid a few hundred meters away is low, but not zero, although it seems to be the case in science fiction novels and movies. This hidden danger, like many others associated with our unbridled use of the resources of planet Earth, threatens our existence.

The scientific community, led by NASA and Johns Hopkins University, decided to take matters into their own hands and use the growing knowledge about asteroids to test the effectiveness of the kinetic asteroid impact method. This method aims to transfer the kinetic momentum of a kamikaze probe to an asteroid without the use of an explosive charge.

we might think a priori which is a simple applied physics experiment like the one we do on the pool table. Nothing is further from the truth.

DART reached Dimorphos at 6.14 km/s. When we collide with an asteroid at hypervelocity, some of the impact is elastically transferred, but as the crater is excavated, additional momentum is created, caused by the ejection of materials in the opposite direction of the projectile. This “recoil” component is involved in the supply of an impulse to the asteroid and very effectively contributes to its deviation from the trajectory. In fact, the materials ejected after the impact created numerous strands of particles that could be followed with telescopes from the ground and even from space.

Milestone achieved by DART kinetic impactor

The good news about the results that are now emerging is the greater efficiency shown in deflecting the asteroid Dimorphos. In a paper led by Andrew F. Cheng of the Johns Hopkins University Applied Physics Laboratory, we quantified the so-called beta factor associated with this inelastic component, which causes recoil and plays to enhance the effects of the kinetic impactor.

In fact, the experiment far exceeded expectations, since the multiplication factor of the transferred angular momentum, associated with the inelastic component of the deflection, reached a value of 3.6. This means that the contribution at the moment of this recoil from the ejection of particles was much greater than the incident momentum from the DART. This parameter is vital and most important for quantifying an asteroid with debris pile characteristics, as the images show.

As a result of the rejection, let’s not forget that the goal was to shorten the orbital period of Dimorphos around Didymos by just over a minute, but it was shortened by 33 minutes, as detailed in an article led by Christina A. Thomas of North University Arizona. It describes observations made to quantify this orbital period from photometric observations of the binary using the largest available telescopes.

Another work led by Jian-Yang Li of the Planetary Science Institute in Tucson, Arizona, studied the evolution of filaments populated by impact excavated particles that developed over months under the pressure of solar radiation. The results are of great importance in understanding what happens to materials that are released after impact and the time they stay around them.

These results prompt an effective planetary defense design to take action against any asteroid found in the path of a direct collision with our planet in the future. It is in an article led by Terik Daley, also of the Johns Hopkins University Applied Physics Laboratory, that we describe the scope of the scientific milestone that is to hit Dimorphos with a robotic and autonomous probe such as DART, as well as detail the discoveries made about the nature of Dimorphos and the site of the fall.

However, 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 this is not an easy task, thanks to the revolution in digital CCD camera technology, we can discover hundreds every year and, just as importantly, track and accurately determine the movements of those already known.

31,361 known asteroids and 119 comets in near-Earth space

Currently, monitoring programs initially encouraged by NASA indicate that there are about 31,361 asteroids and 119 comets in near-Earth space, and that at some point one of them may be identified on a likely future Earth impact path. In fact, this has already happened six times, except that it happened with asteroids several meters in diameter, which more often hit our planet and cause meteorites to fall.

We currently know over 10,400 potentially dangerous asteroids as large or large as 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, as well as some extinct comets such as 2015 TB145, a rocky object 650 meters in diameter, known as the “Halloween asteroid”.

This skull-shaped object alerted us when it was discovered only three weeks before its passage on October 31, 2015 at a distance slightly greater than the distance from the Moon, due to the fact that it reflects very well and moves along a very eccentric, elongated orbit. into the orbit of Jupiter. Such objects, capable of colliding with our planet with much more energy than a normal asteroid, illustrate the variety and complexity of the problem we face.

Impossible to be catastrophic, as all efforts to locate and catalog these bodies allow better quantification of impact frequency and suggest that an event like Tunguska will occur every few centuries. They also suggest that, fortunately, kilometer-long asteroid impacts occur every few tens of millions of years. In any case, the Jet Propulsion Laboratory’s (JPL) Center for Small Object Science (CNEOS) Sentry program catalog ensures that none of the cataloged near-Earth asteroids poses a risk on a multi-century scale. Thus, the catastrophic news that we sadly get used to at each relatively close collision of an asteroid with the Earth is completely untenable.

The enriching role of the past marked by impacts

In the distant past, the Earth was born after countless asteroid impacts and even, in the final phase, they were with genuine planetary embryos, the size of the planet Mars itself. On a larger time scale of billions of years, scientific evidence shows that asteroid and comet impacts have played a key role in Earth’s history, especially in the transport of water and the evolution of life itself.

At present, the flow of interplanetary matter is not negligible: annually about 100,000 tons reach the Earth and, although most of it does not reach the earth’s surface, it still evaporates and becomes part of our atmosphere.

Perhaps due to the difficulty of correctly interpreting the cataclysms caused by outer space, a large part of the population continues to underestimate this danger hanging over humanity. Despite this, the realization of the Tunguska impact on June 30, 1908 and its connection with an asteroid that, despite a diameter of less than 50 meters, devastated 2200 km² of the Siberian taiga, should make us reconsider our views.

In this context and with a healthy desire to continue learning, DART shows us the way: space exploration and a decisive approach to the problems facing humanity, using our scientific and technological capabilities, will be the key to our survival.

Josep M. Trigo Rodriguez, Principal Investigator of the Meteorites, Small Bodies and Planets Group at the Institute of Space Sciences (ICE – CSIC)

This article was originally published on The Conversation. Read the original.

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