The Alternate View
Defending Against Killer Asteroids
by John G. Cramer
On Thursday, July 13, 2023, a sizable undetected asteroid (subsequently named “2023 NT1”) was headed for the Earth from the direction of the Sun and just missed a collision. It passed within ~100,000 kilometers, about three times further out than the orbits of geosynchronous satellites. Because the Sun is an astronomical blind spot, this very close encounter with near-Earth asteroid 2023 NT1 wasn’t discovered and tracked until two days after the near miss.
The asteroid is estimated to have a diameter of ~34 meters, a mass of ~52 kilotonnes, and a speed of 11.27 km/s. (Note: a metric tonne is 1,000 kg or 2,204.62 lb.) If asteroid 2023 NT1 had impacted the Earth’s surface, it would have liberated energy equivalent to ~1.5 megatonnes of TNT, about 100 times the energy as released by the Hiroshima bomb, and it could have caused very significant local damage.
About a decade ago, a similar space object actually did hit the Earth. At 9:20 a.m. local time on February 15, 2013, a massive object exploded over Chelyabinsk Oblast in the southern Ural region in Russia. The object had an estimated diameter of ~18 m, a mass of ~9.1 kilotonnes, and a speed of 19.16 km/s. The fireball light was briefly brighter than the Sun, was visible 100 km away, and produced a dangerous atmospheric shock wave. The object exploded at an altitude of about 29.7 km and liberated energy equivalent to ~450 kilotonnes of TNT, or 30 times the Hiroshima bomb. Over 1,491 people were injured, mainly by the flying glass of broken windows from the atmospheric shock wave, 7,200 buildings were damaged, and there was about $33 million in property damage.
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These are only the recent asteroid near-misses and impacts. Roughly, the impact energy and damage that an asteroid strike produces scales as the 3rd power of the asteroid’s diameter. About 65 million years ago, the Chicxulub asteroid, about 10,000 m in diameter, stuck what is now the Yucatan peninsula, liberating an energy equivalent to ~72 teratonnes of TNT, filling the atmosphere with sunlight-blocking dust, and triggering the mass extinction of 75% of plant and animal species on Earth, including all non-avian dinosaurs. The Chicxulub strike and its aftermath vacated a vast number of ecological niches, making room in the ecology of Earth for the evolutionary rise of mammals and of humanity.
Earth bears the scars of at least 190 known impact craters from such events. Perhaps the impact crater most familiar to residents of the USA is Meteor Crater in Arizona, which is 1.2 km in diameter and was created by the impact of an iron-nickel asteroid about 40 m in diameter that impacted the Arizona desert about 49,000 years ago. The most recent impact craters are the Wabar craters in the Rub’ al Khali desert of Saudi Arabia, 100 m in diameter, created by impacts that probably occurred in the 1800s, and were only discovered some years after the initial impact.
Our Solar System is a busy place, with some 32,957 known near-Earth asteroids, at least 2,366 of which are large enough (over 10 m in diameter) and are projected to pass close enough to Earth to be classified as potentially hazardous. The potential danger of future asteroid impacts raises interesting questions: (1) Are there ways to accurately predict future asteroid impacts? (2) If an impact is predicted, are there ways of eliminating or reducing the damage? (3) How long in advance would we need to know that an impact was on the way to effectively prevent it? and (4) how big can an asteroid be before it cannot be effectively pulverized or deflected?
In 2005, the U.S. Congress passed a bill requiring NASA to find and track at least 90% of all near-Earth objects (NEOs) with diameters 140 m or larger by 2020 (but neglecting to provide any funding for this new mandate). This funding omission has been slowly corrected, and the 2024 NASA budget provides about $100 million for Near-Earth Object Observations. A project that is related to this mandate is NASA’s recent DART mission, launched from Vandenburg on November 24, 2022 with a SpaceX Falcon 9 rocket. DART traveled for over ten months before intentionally colliding with Dimorphos, a moonlet 160 m in diameter that orbits Didymos, a larger 780 m diameter asteroid as a binary system. The mission demonstrated (as expected from Newton’s laws) that the momentum transfer from a spacecraft-asteroid collision can significantly modify the orbit of the asteroid. It also showed that the transferred momentum is significantly boosted by the rocks and boulders ejected by the crash. However, it represents only a small step toward answering question (2) above.
A rule of thumb is that the earlier we know about the dangerous trajectory of a potential impact asteroid, the smaller the trajectory modification, perhaps by spacecraft impact, that would be required to divert it. If we know years in advance, the momentum transfer from something like the DART mission might solve the problem by deflecting a dangerous asteroid orbit away from an Earth collision. However, what if we detect a killer asteroid only a few weeks, days, or even hours before the predicted impact? There may be a solution.
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Until 2023, I served as member and ultimately chairman of the NIAC External Council of NASA’s NIAC (NASA Innovative Advanced Concepts) Program. During my time with the NEC, I learned about the more direct approach to answering the above questions, as proposed by Professor Philip Lubin and his group at the University of California at Santa Barbara.
The group has devised a method called “PI” (Pulverize It) that envisions a set of hypervelocity penetrator interceptors, 100–500 kg slugs of tungsten, to intercept an asteroid with a dangerous impact trajectory and convert it to an expanding cloud of well separated fragments. The fragments burn up in the atmosphere, forming fireballs with incoherent shock waves that dissipate their energy more broadly and harmlessly before impact. In pursuit of this program, Lubin’s group received NIAC Phase I ($175k) and Phase II ($600k) awards in 2022 and 2023, as well as funding from other sources. They have now submitted a major publication (Ref 1 below) showing their simulation results.
They simulate the situation in which an asteroid similar to 2023 NT1 is assumed to be on an Earth-collision trajectory, and PI penetrator interceptors are launched to intercept the asteroid at a collision speed of 20 km/s. The objective is to break up the asteroid into smaller fragments that spread out and dissipate its effects.
Here are three cases that were simulated: Case 1 is an asteroid with a 20 m diameter (about 2/3 the diameter of 2023 NT1) intercepted by a single 100 kg penetrator striking the asteroid. Case 2 is an asteroid with a 50 m diameter (about 5/3 the diameter of 2023 NT1) intercepted by single 500 kg penetrator. Case 3 is a 50 m asteroid intercepted by a cluster of five 100 kg perpetrators.
The simulations indicate that in Case 1 the 20 m asteroid would be broken into 282 fragments with an average diameter of about 1 m and a maximum diameter of about 4.3 m. Note that a fragment with a diameter less than 10 m should burn up in the atmosphere.
In Case 2 the 50 m asteroid would be broken into 1,509 fragments about 1 m in diameter. In Case 3 the 50 m asteroid would be broken into 1,010 fragments about 1.2 m in diameter by the cluster of five penetrators. In all cases, the fragment diameter distribution follows a power law, they have masses around 30 tonnes, and they would burn up in the atmosphere.
Overall, the group did 103 simulations of asteroid intercepts and 17 simulations of non-intercepted asteroid strikes. They used diameters ranging from 26 m to 60 m, fragment numbers ranging from 1,000 to 6,000, and intercept times ranging from 12 hours to 10 days.
What would such a massive asteroid intercept look like from the ground? There would be an intense light flash and a strong sonic-boom-like atmospheric shock wave. The sizes of these would depend on the size and number of fragments and the time between intercept and atmospheric entry. The simulations show that, for all but the most extreme cases considered, the light energy would be less than 200 kJ/m2 (too small to damage eyes or start local fires) and the shock wave overpressure would be less than 3 kPa (too small to break windows and roughly 10 times that of a jet plane’s sonic boom).
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For comparison, NASA’s DART mission collided with a ~580 kg spacecraft travelling at ~6.1 km/s with asteroid Dimorphos, which had a diameter of ~168 m and had a mass of ~6,300 kilotonnes. Unlike the simulated scenarios, the impact was not sufficient to give most of the resulting fragments escape velocity and break up the asteroid. However, some of the asteroid’s mass was apparently ejected during the collision, providing extra momentum, because the momentum transfer to Dimorphos was observed to be about 3.6 times larger than would be expected from a simple inelastic collision. Analysis suggests that ~40 boulders of diameter ~4 m were ejected, and that limited ejection rather than complete fragmentation was the dominant mechanism.
In summary, the simulations suggest that any asteroid roughly similar to 2023 NT1 on an Earth-collision trajectory could be fragmented to dissipate its effects with a single 100 kg penetrator closing at 20 km/sec and arriving one day before the predicted impact. However, to achieve an intercept one day in advance of impact, the penetrator payload would have to be launched several days before that. Thus, the initial threat detection would have to occur a week or more before the predicted impact (which was not the case for 2023 NT1.)
What would be required to implement planetary protection based on the PI proposal? It would probably require something like one of the ICMB nuclear missile sites that were constructed during the Cold War, except that the booster awaiting launch, perhaps a SpaceX Falcon 9, would not need to be housed in underground missile silos. But like the ICBMs, the penetrator launcher would have to be ready for launch on very short notice.
It should also be pointed out that there are a class of asteroid-Earth collision scenarios, those involving late detection or very large asteroids like the Chicxulub object, in which there is no possibility of significantly reducing the impending destruction. Fortunately, Earth collision likelihood scales with the inverse of asteroid size, so with present technology, the Earth could be protected from almost all collisions with asteroids on dangerous trajectories.
So, it looks as if NASA and/or our recently created U. S. Space Force may have an important new mission: fragmenting Earth-bound asteroids.
Brin K. Bailey, et al., “Asteroid 2923 NT1: A Cautionary Tale,” arXiv:2310.13112 [astro-ph.EP].
Philip Lubin, “PI—Terminal Planetary Defense,” arXiv:2110.07559 [astro-ph.EP].
John G. Cramer’s 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake—Entanglement, Nonlocality, and Transactions, (Springer, January 2016) is available online as a hardcover or eBook at: https://link.springer.com/book/10.1007/978-3-319-24642-0 or https://www.amazon.com/dp/3319246402. Editions of John’s hard SF novels Twistor and Einstein’s Bridge are available online at: https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/Einsteins-Bridge-John-Cramer/dp/0380788314. Electronic reprints of 207 or more “The Alternate View” columns written by John G. Cramer and previously published in Analog are currently available online at: https://www.npl.washington.edu/av.
Copyright © 2024 John G. Cramer