So far, the battle between life on Earth and asteroids has been completely one-sided. But not for long. Soon, we’ll have the capability to deter asteroids from undesirable encounters with Earth. And while conventional thinking has said that the further away the better when it comes to intercepting one, we can’t assume we’ll always have enough advance warning.
A new study says we might be able to safely destroy potentially dangerous rocky interlopers, even when they get closer to Earth than we’d like.
Humanity faces a dilemma regarding asteroids. We’ve identified many of the potentially dangerous ones, but not all of them, especially smaller ones. We know there must be undetected small asteroids out there, and they can still cause a lot of damage. An asteroid’s potential damage is due not just to its size, but also its angle of impact, its velocity, and its density. (Check out Purdue University’s asteroid impact simulator.) As a general rule of thumb, an asteroid the size of a football field could wipe out a city like New York.
NASA and other space agencies are concerned when it comes to Near-Earth Objects (NEOs) and Potentially Hazardous Objects (PHOs). The US Congress gave NASA a mandate to identify and catalogue 90% of NEOs 140 meters (460 ft) in diameter or larger. Sometime in 2026, NASA plans to launch the NEO Surveyor mission to find more asteroids in our neighbourhood. But it’s doubtful we’ll ever have a complete picture of all the asteroids that could do us harm. The Universe is full of surprises.
The preferred method of dealing with an asteroid headed for Earth is to deflect it as one chunk using a non-explosive kinetic impactor. But we need advance warning of the asteroid’s approach to do that. If we know decades ahead of time that an asteroid is on an Earth-impacting trajectory, then we need only launch a low-mass impactor. But what if an asteroid is heading straight for Earth and we don’t have enough lead-up time? What if we have less than one year until impact?
We’ll have to blow the thing up best we can and hope that the fragments don’t strike Earth.
But blowing an asteroid up as it’s approaching Earth is a risky maneuver. The asteroid could split into a dangerous swarm of fragments. There’s also a host of technical risks. Attaching an explosive nuclear device to a rocket and launching it into space is not without risks.
A team of researchers has published a study that delves into the issue. It’s titled “Late-time small body disruptions for planetary defence” and it’s published in the journal Acta Astronautica. The lead author is Patrick King from Johns Hopkins University Applied Physics Laboratory. In the study, the term “late-time” refers to less than one year from impact.
Blowing up an asteroid might not be that difficult, in some ways. In this study, the authors wanted to focus on what happens after one is blown up. What happens to all the fragments? “Our focus is on following to a high degree of accuracy the orbits of the fragments following the disruption of a hazardous body on an Earth-impact trajectory, and if they result in any Earth impacts, estimate the scale of the consequences,” they write. This is particularly important since so many asteroids are of the “rubble-pile” type, and only loosely held together.
The study simulated a 100-meter asteroid approaching Earth and then being disrupted with a one-megaton explosive device. The explosive device wouldn’t actually strike the asteroid, it would be detonated a few meters above the surface. A detonation like that doesn’t make the asteroid disappear; it just breaks it into smaller pieces, which should pose less of a threat if all goes well.
They also simulated five different impact scenarios based on five real-world asteroids. They chose Apophis and Bennu because both of those asteroids are so well-studied. The team chose 343158 Marsyas because it has a retrograde orbit and has a very high relative velocity, and also passed within 0.5 AU of Earth, making it an interesting and extreme scenario. They chose 5496 1973 NA (a minor planet) because of its highly inclined orbit, and because it passed within 0.08 AU in 1973. They also used PDC 2019, the hypothetical asteroid used in the 2019 Planetary Defence Conference.
The simulation’s outcome is promising.
“One of the challenges in assessing disruption is that you need to model all of the fragment orbits, which is generally far more complicated than modelling a simple deflection,” lead author King said in a press release. “Nevertheless, we need to try to tackle these challenges if we want to assess disruption as a possible strategy.”
The team simulated the 100-meter asteroid in five different scenarios. In all five scenarios, the disrupting device was detonated when the asteroid was two months away from Earth. Also in all five, 99.9% of the asteroid’s mass missed Earth.
This study doesn’t change the fact that kinetic impacts are preferred when it comes to asteroid deflection. The further away an asteroid is when it’s detected, the better. A smaller kinetic impactor is enough. But this study is aimed at late-time small bodies. There isn’t enough time to launch a kinetic impactor with enough mass if we don’t have enough lead time.
But it does show that nuclear devices can be part of humanity’s arsenal in the struggle against asteroids.
“We focused on studying ‘late’ disruptions, meaning that the impacting body is broken apart shortly before it impacts,” he said. “When you have plenty of time — typically decade-long timescales — it is generally preferred that kinetic impactors are used to deflect the impacting body.”
But this study is focused on late disruptions, situations where we don’t have enough time to send an impactor. It’s important that we understand the ramifications of shattering an approaching asteroid into pieces. To help them understand all this, study co-author Michael Owen wrote a piece of software called Spheral. Spheral was able to model not only the nuclear disruption of the asteroid but also all of the resulting fragments.
After the explosion, the fragments were subject to each others’ gravity and the gravity of the Earth, Moon, and Sun. The team found that the fragments formed a coherent stream in space.
“If we spotted a hazardous object destined to strike the Earth too late to safely divert it, our best remaining option would be to break it up so thoroughly the resulting fragments would largely miss the Earth,” Owen said. “This is a complicated orbital question though — if you break up an asteroid into pieces, the resulting cloud of fragments will each pursue their own path around the sun, interacting with each other and the planets gravitationally. That cloud will tend to stretch out into a curved stream of fragments around the original path the asteroid was on. How quickly those pieces spread out (combined with how long until the cloud crosses Earth’s path) tells us how many will strike the Earth.”
NASA is about test the kinetic impactor method of asteroid protection with its Double Asteroid Redirection Test (DART) mission. Its launch is scheduled for November 24th, 2021.
The “Double” in the name refers to the double asteroid Didymos. Didymos is about 780 meters in diameter, but it has a little companion named Didymoon. Didymoon is DART’s target, and it’s only about 160 meters in diameter. Most of the asteroids that pose a threat are a similar size to Didymoon. The DART spacecraft itself is the kinetic impactor, and it’ll be sent to collide with Didymoon while sensitive telescopes watch and see what happens.
But there’s no real way to test a nuclear detonator on an asteroid. Maybe in the future, we could test one on an asteroid approaching another planet. The sci-fi-minded can easily envision that. But real-world testing on an asteroid anywhere in Earth’s vicinity is a bad idea. Too many things could go wrong. People would never accept it.
For now, we have to rely on simulations like the ones in this study, and future studies that build on this one and refine it.
“Our group continues to refine our modelling approaches for nuclear deflection and disruption, including ongoing improvements to X-ray energy deposition modelling, which sets the initial blowoff and shock conditions for a nuclear disruption problem,” said co-author Bruck Syal. “This latest paper is an important step in demonstrating how our modern multiphysics tools can be used to simulate this problem over multiple relevant physics regimes and timescales.”