Earlier this year, on 13 July, an asteroid the size of an office block flew past Earth at about a quarter of the distance to the Moon. In astronomical terms, that’s a hair’s breadth, the closest of close shaves. A direct hit would have produced an explosion equivalent to 1.5 megatons of TNT. By comparison, the nuclear bomb dropped on Hiroshima produced just 20 kilotons.
Perhaps the most worrying aspect of this incident was that astronomers didn’t spot the body—called 2023 NT1—until 2 days after it had passed. They missed it because 2023 NT1 approached from the Sun. The same blind spot also hid the Chelyabinsk asteroid, which injured more than 1600 people and damaged 7000 buildings when it exploded in the atmosphere above Russia in 2013. The Chelyabinsk rock was half the diameter of 2023 NT1.
These incidents raise the prospect of potentially catastrophic scenarios and of what can be done to prevent them. If astronomers were to spot an asteroid heading towards us, what are our options if we have only a few days’ notice?
Now a team of astronomers and computer scientists have simulated this scenario, as if 2023 NT1 was discovered heading towards us and that the impact was just a few days’ away. “We investigate a variety of short warning time, terminal mitigation scenarios via fragmentation for a hypothetical impact of asteroid 2023 NT1,” say Brin Bailey, at the University of California, Santa Barbara, and colleagues.
They say the best option would be to blow the asteroid to smithereens and allow the fragments to hit Earth, an option known as the “Pulverize It” method. According to their simulations, the resulting shower would be significantly less damaging than a collision with the original body.
The team begin with what little is known about 2023 NT1. The asteroid was first spotted on 15 July by the Asteroid Terrestrial-impact Last Alert System (ATLAS), which, ironically, is designed to spot asteroids in the days or weeks before they hit Earth.
2023 NT1 turns out to be a Near Earth Object orbiting the Sun every 950 days. Its diameter is between 26 and 58 meters with astronomers estimating that if it were 34m across, of average density for an asteroid and hitting Earth at a speed of 16km/s, it would have an impact energy of 1.5 megatons.
Because most larger near-Earth asteroids are thought to have been catalogued, and because smaller objects are significantly more common, astronomers believe the most immediate threat to Earth is from objects about this size. One option in these scenarios is to launch a projectile towards the asteroid with enough kinetic energy to break it apart. The big question is whether this would work.
So Bailey and co decided to simulate the collision using state-of-the-art hydrodynamic modelling software. It turns out the Lawrence Livermore National Laboratory in California has developed code that simulates hypervelocity impacts, explosions and the resulting fragmentation for its work in nuclear stockpile stewardship.
These impacts are complex processes. Baily and co say that when the projectile hits the asteroid, its kinetic energy is converted to heat, vaporizing the projectile and the surrounding material and sending shockwaves through the asteroid that fracture it. The expanding vapor then pushes these fragments apart.
That’s the theory, anyway. An important question is how big the projectile should be in practice. This is a calculation fraught with uncertainty because the likely unknown structure and make up of the asteroid as well as its size.
But astronomers think that most asteroids are piles of rock bound together with a kind of interplanetary cement. So a key factor is the strength of this binding agent, whether it is as weak as dry earth, of medium strength like concrete or a highly robust material like steel.
To cover as much of these uncertainties as possible, Bailey and co simulated 100kg penetrator hitting a 20-meter asteroid and then a 500kg penetrator hitting a 50-meter asteroid. In each case, they modelled the asteroid as a pile of boulders of varying size bound together by a concrete-like substance.
The results suggest that these impacts turn both asteroids into fragments, many of which would hen rain down on Earth. As these rocks hit the atmosphere, they heat up, deform and break apart explosively in an airburst. This releases energy in the form of a bright flash that can be strong enough to blind or cause skin burns, and energy in the form of powerful shockwaves that can damage structures and break windows (as was the case with the Chelyabinsk asteroid).
The team determined the threshold levels below which both of these effects would be safe and then simulated the range of airbursts from the fragmented asteroids to see if they broke the thresholds.
The results suggest that by increasing the size or closing speed of the projectile, it should be possible to break the asteroid into fragments small enough to cause little or no damage on Earth.
“Threats like 2023 NT1 can be effectively mitigated with intercepts of one day (or less) prior to impact, yielding minimal to no ground damage, using modest resources and existing technologies,” conclude Bailey and co.
That provides some reason to hope that it will be possible to defend Earth from future asteroid impacts. There’s no question that objects the size of 2023 NT1 are headed our way, at the rate of one every 50 years or so. The Solar System is brimming with objects this size.
There are significant challenges ahead, however. One will be to improve our ability to observe near-Earth objects of this size before they hit. The good news is that the current early warning system is being upgraded and will continue to improve over time.
Next is the decision-making process—who has the authority and capability to decide to pulverize an incoming asteroid and using what criteria? For example, what is the threshold at which an asteroid should be allowed to hit Earth unmolested? And what is the legal status of such a decision? The danger for decision makers is that if they interfere, they will be blamed whatever happens.
Then there is the question of launching a suitable projectile. Bailey and co say a SpaceX Falcon 9 rocket could deliver a 2500kg payload towards an incoming asteroid. That would be more than enough for an object the size of 2023 NT1. But how could such a mission be launched with just a few days or hours’ notice?
These are all issues that will need to be explored in detail in the coming months and years by scientists, military organizations, diplomats and policy makers. There will also need to be informed public debate to help guide their decisions.
In the meantime, the rest of us will have to keep our fingers crossed that the next 50-meter asteroid with our name on it will hold off until we’re ready to pulverize it.
Ref: Asteroid 2023 NT1: A Cautionary Tale : arxiv.org/abs/2310.13112