by Ankita Das
With the launch of the Double Asteroid Redirection Test (DART) mission, the age of active planetary defense has formally begun. The DART mission is the first interplanetary spacecraft testing an asteroid redirection method to better prepare humankind for a potential mass extinction event due to the impact from a planetary body or asteroid fragment [1]. The spacecraft, launched in November 2021, is intended to crash into Dimorphos, a moonlet of asteroid Didymos, in September 2022, to see how much the speed and path of the moonlet can be altered.
Although this is the first dedicated spacecraft to be sent to an asteroid to study planetary defense techniques, ideas of such a mission have been around for decades. In 1977, at NASA Ames Summer Study on Space Settlements, Dr. Brian O׳Leary, a former NASA astronaut candidate, proposed using mass drivers to move Earth-approaching Apollo and Amor asteroids to Earth’s vicinity during opportunities when the required velocity change to redirect them was low [2]. A critical development in this area occurred when a 2010 NASA study proposed the Asteroid Redirect Robotic Mission (ARRM) to use high-power solar electric propulsion technology to capture, and return an entire, very small (~10,000 kg), near Earth asteroid to the International Space Station [3]. In this article we reflect on how active planetary defense missions can safeguard us from a catastrophic impact events and if it is worthwhile to invest in a defense procedure.
The idea of protecting the planet from asteroid or cometary impacts emerged when researchers gained more knowledge about the small bodies of the solar system and investigated the impact history of the Earth. Upon investigation, scientists found multiple impacts from the Earth’s past, which have now been masked by erosion, geologic activities, and vegetation. The early discussions on planetary defense started once it was found that asteroid impacts most likely led to the mass extinction event that wiped out the dinosaurs. In the meantime, we also gained more knowledge about the small bodies of the solar system, informing scientists about the likelihood and frequency of potentially catastrophic impacts on the Earth. These studies also helped identify how past events from the Earth’s history could be linked to impacts from outer bodies [4].
For example, the infamous Tunguska event of 1908 involved an explosion equivalent 12 megatons of TNT, attributed to a meteor air burst, where a stony meteoroid of more than 50 m in diameter entered the Earth’s atmosphere at a speed of about 27 km/s and disintegrated near the Tunguska river in a sparsely populated region of Siberia in Russia. It was estimated that about 80 million trees over an area of 2150 sq. km. perished due to the impact [5]. More recently, an event that occurred in the city of Chelyabinsk, Russia in 2013 drew attention from the scientific community where a small asteroid - about the size of a six-story building - broke up over the city of Chelyabinsk. The asteroid, about 17 m in diameter and weighing approximately 10,000 metric tons, hit the Earth’s atmosphere at about 18 km/s. The energy of the resulting explosion exceeded 470 kilotons of TNT. The blast was so strong that it triggered detections from monitoring stations as far away as Antarctica [6].
Assessing Potential Threats
The Earth Impact Database [7], maintained by the University of New Brunswick, currently identifies as many as 190 confirmed impact structures on Earth’s surface. They range from small (tens to hundreds of meters in diameter) impact craters to large ones like Vredefort in South Africa measuring 160 km in diameter, dating back to 2023 million years. It is also true that not all impacts from outer bodies would result in terrestrial craters (i.e., they could explode in the atmosphere causing only air burst like the one at Tunguska), not all impact structures on the Earth’s surface have been identified [8]. Therefore, we may consider that these events are common and frequent on geologic timescales and the fact that our awareness of the population of potential impactors in the Solar System has been improving, how much of a threat do asteroids and comets really pose?
Imagine the possibility of an asteroid with a diameter of more than 300 m heading towards a critical infrastructure like a nuclear plant. While the probabilities of such an event may be extremely low, it is essential that we develop our understanding of the risk associated with the entry of planetary bodies, considering the potential damage even a smaller asteroid (with diameter of less than 300 m) may cause to our civilization. Potentially hazardous asteroids and comets are categorized by NASA [9] and researchers [10]. The threat and potential of an impact primarily depend on the size and composition of the object, the surface being impacted, and the angle of impact. As of today, more than 28 000 near-Earth asteroids (NEAs) have been identified with majority of them having diameters in the range 30 – 100 m [11]. Smaller objects burn up in the atmosphere harmlessly as they approached the Earth. Larger objects, even if they burn up before hitting the ground, cause air burst or explosion, leading to severe damage.
To assess the potential damage and probability of impact, first, we need to detect these objects, and then we need to monitor their orbits. This is done from ground-based and space telescopes. By applying Newton’s laws and N-body simulations (i.e., modeling equations of motions for N objects interacting gravitationally), the orbits of most of these objects are predictable for at least 100 years into the future. Only the asteroids whose orbits cross that of the Earth are potentially dangerous. However, as mentioned earlier, not all asteroids are of the same size, and the larger the object, the higher is the threat. At the same time larger objects are rarer. Events like the Tunguska and Chelyabinsk were caused by smaller bodies compared to the events that caused the mass extinction approximately 66 million years ago due to an impact from as asteroid of diameter of about 10 km [12].
Thus, although the Solar System is populated with small bodies like asteroids and comets, only a fraction of these objects are of sizes that can be damaging and can potentially go on a trajectory that will coincide with the Earth to cause an impact. Asteroids that follow orbital trajectories within Earth's "neighborhood" (i.e., within 7.5 million km of Earth's orbit) around the Sun and that are more than 140 m in diameter are potential hazard to Earth and identified as Potential Hazardous Asteroids (PHAs) [13]. Ongoing research has enabled us to come up with a special classification of asteroids and objects which are closer to Earth’s orbit [14]. Not all near-Earth objects (NEOs) will impact the Earth at some point, but it is more likely that if an impact does happen, it will be an NEO.
Our Options for Defense
So, what are our options for defense as a species if an asteroid were to head our way? There are a few popular ideas. The first one is sending a spacecraft to the asteroid that can fragment the asteroid into smaller pieces. This idea sounds great but is not practical since the smaller fragments can also cause harm to the planet. Think of it as breaking down a big problem into 100 tiny chunks and having to deal with these 100 tiny chunks of the same problem. An alternate and favored line of action currently being investigated by the scientific community is deflecting the asteroid into a new orbit so that it misses the Earth completely. This can be achieved in a few ways. One would be to crash a spacecraft into the asteroid itself to gently nudge the asteroid into a newer orbit. This is what the DART mission is set to test on the asteroid Dimorphos, a small (160 m diameter) asteroid in orbit around a larger orbit of the asteroid Didymos (780 m diameter). DART’s LICIACube spacecraft will crash on Dimorphos to create a kinetic impact that will change the orbit of Dimorphos around Didymos.
Image: Schematic of LICIACube attempting to change the orbit of Dimorphos
Image Source: Johns Hopkins Applied Physics Laboratory /NASA
Although Didymos is not a threat to Earth, this will be a demonstration of how effective the kinetic impactor method is when it comes to changing the course of an asteroid, called redirection.
Another idea involves the gravity tractor method that exploits the gravitational attraction of a spacecraft with the asteroid to cause continuous minuscule changes in the orbit, which would, cumulatively, over time result in a more visible change of the trajectory of the asteroid. The only downside to this method is it is a long process that involve several years [15], and hence we will need to know about the asteroid well in advance from the date of potential impact. This brings us to the question, what happens if we discover an asteroid heading our way and we do not have sufficient time to send a spacecraft to the asteroid to deflect it? Such a scenario will call for a damage control strategy where the trajectory of the asteroid is monitored, the potential places on the Earth where the asteroid is expected to impact is calculated, and measures are taken to minimize the damage that could be caused to life or infrastructure.
To conclude, planetary defense is an exciting field of study which is necessary for the safekeeping of the planet. In 2016 NASA established the Planetary Defense Coordination Office (PDCO) to manage its ongoing mission of planetary defense. NEOs and NEAs need to be monitored constantly, in addition to the continued identification and discovery of additional potential impactors capable of significant damage so that we can prepare for a potentially catastrophic impact event. The DART mission is a critical mission that will be our first step in equipping ourselves better in the event of a hazardous asteroid coming Earth’s way.
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Sources:
[1] https://www.nasa.gov/specials/pdco/index.html
[2] Mazanek, D. D., Merrill, R. G., Brophy, J. R., & Mueller, R. P. (2015). Asteroid redirect mission concept: a bold approach for utilizing space resources. Acta Astronautica, 117, 163-171.
[3] https://authors.library.caltech.edu/86061/1/Asteroid_Redirect_Robotic_Mission.pdf
[4] Sleep, N. H., Zahnle, K. J., Kasting, J. F., & Morowitz, H. J. (1989). Annihilation of ecosystems by large asteroid impacts on the early Earth. Nature, 342(6246), 139-142.
[5] https://www.sciencedirect.com/science/article/abs/pii/S0019103518305104?via%3Dihub
[6] https://www.space.com/33623-chelyabinsk-meteor-wake-up-call-for-earth.html
[7] http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/World.html
[8] https://www.boulder.swri.edu/~cchapman/crcepsl.pdf
[9] https://cneos.jpl.nasa.gov/about/neo_groups.html
[10] http://www.boundarycondition.com/NEOwp_Chapman-Durda-Gold.pdf
[11] https://cneos.jpl.nasa.gov/stats/size.html
[12] https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JE01743
[13] https://solarstory.net/asteroids/near-earth-asteroids
[14] https://space.nss.org/national-space-society-planetary-defense-library/
[15] https://iaaspace.org/wp-content/uploads/iaa/Scientific%20Activity/conf/pdc2015/IAA-PDC-15-04-11.pdf
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