It’s a subject that has enthralled fans of both science and science fiction for generations. An asteroid or comet is on a collision course with the Earth, threatening to wreak massive devastation and potentially wipe out humanity.
We received an unpleasant reminder of the non-fictional reality of this scenario recently when we learned that an asteroid named 2021 SG that was roughly one half the size of the Great Pyramid in Egypt had zipped past the Earth inside the orbit of the moon in September and NASA didn’t even see it until it was going by.
Is there a real-world solution to this terrifying possibility? Unfortunately, knowing about an incoming Near-Earth Object (NEO) in advance doesn’t do us much good unless we can actually do something about it. If you look to Deep Impact or Armageddon for solutions, you might be tempted to send a crew of space cowboys out into the void with some huge nuclear weapons to take care of the problem. But in the past, when such discussions have come up, scientists have assured us that nuking an asteroid wouldn’t work because you would simply risk generating multiple impact events from the wrecked asteroid.
That viewpoint appears to be changing, however. A recent study by a team of scientists that was published at Acta Astronautica indicates that under the right conditions, we could indeed use a nuclear blast to save the Earth from an incoming NEO. The report is titled “Late-time small body disruptions for planetary defense,” and as the name suggests, a significant “disruption” (meaning a nuclear blast) could produce a successful outcome for a modest-sized asteroid that was discovered too late to have its course significantly altered by other methods that have been previously discussed.
THE THREAT IS REAL, BUT PROBABLY NOT IMMINENT
How real is the danger from a NEO and if we found one incoming, how would this nuclear disruptive intervention theory work? The Debrief reached out to Dr. Megan Bruck Syal, a physicist working in planetary science and planetary defense at Lawrence Livermore National Lab and one of the authors of the study. The first thing we wanted to determine is how many of these space rocks that are large enough to pose a serious threat to humanity are out there and what are the odds one of them might actually hit us.
“There are about 130,000 NEOs that are 100 m or larger,” Dr. Bruck Syal said, “and only approximately 20% have been discovered and had their orbits well-characterized. Each of these could wreak regional devastation. While more than 95% of asteroids 1 km or larger in diameter have been discovered and are known to not be an imminent threat to Earth, there are hundreds of thousands of possible city-flattening asteroids out there, and we have no idea if or when they may be a threat to Earth.”
“Something as small as 50 m, such as the Meteor Crater impactor, which was metallic, can take out a city,” she continued. “That crater is 1 km across and its impact released about 10 Megatons of TNT equivalent, which is roughly 1000 times more energy than the Hiroshima nuclear explosion.”
WHY IS A NUCLEAR DISRUPTION SEEN AS VIABLE WHEN IT WASN’T PREVIOUSLY?
As previously noted, earlier theories about impact avoidance suggested that blowing up an asteroid was impractical. One common phrase that has frequently been invoked was the risk of “turning one falling object into many.” But others have suggested that multiple smaller objects might cause less cumulative damage. The Debrief asked Dr. Bruck Syal how her team’s research led to the conclusions that they published in terms of this question.
“If the asteroid is broken up too late, yes, more than one impact could occur,” she said. “This is why our research is so critical: it tells us, for a given warning time, how much mitigation can be achieved, including whether any of the fragments are likely to hit Earth. Some of the pieces would be small enough to burn up in the atmosphere, but some may not be. As long as those larger fragments are given a sizable change in velocity at an early enough time, they will miss the Earth. This is what the modeling shows. ”
“Still,” she continued, “reducing a 100 m object impact, which would deliver tens of Megatons of TNT energy, more than 1000 times the Hiroshima nuclear explosion, to something that is approximately 20 m, which is near Chelyabinsk-size, would be a dramatic improvement in outcome for Earth. It is the difference between completely decimating an entire metro region and having some injuries from broken windows.”
So the end goal of this process is to minimize destruction on the planet. And according to this team’s models, a sufficiently large blast delivered in a timely fashion could make the majority of the NEO miss the Earth entirely. It wouldn’t have to be a matter of altering the object’s trajectory. If you slow it down sufficiently, the Earth would be safely out of the way before it reaches our orbit. The ultimate objective is to not have both objects at the same spot in our orbit at the same time.
THIS ISN’T A MISSION THAT COMES TOGETHER OVERNIGHT
The conclusions in the linked study suggest that a different method of avoiding an impact would still generally be preferable if scientists identify and plot the course of an incoming asteroid sufficiently in advance. One such method would be a kinetic impactor, where we would literally slam a very massive object into the asteroid at a tremendous rate of speed, seeking to push it into a slightly different orbit. The orbital shift required wouldn’t be very great if done far enough in advance.
But we may not have that much notice, as asteroid 2021 SG just demonstrated. So how much time would we need to pull off a mission using a nuclear disruptor and how much of a safety margin would that obtain for us? The team’s research concluded that this type of disruption event, when applied to a “small body” asteroid (roughly 100 meters in diameter or less) could take place as little as two months before impact and still massively reduce the amount of damage Earth would sustain, if not eliminate it entirely. But it would still require a long time to put the plan in motion and execute it.
“We need enough time to get a launch vehicle and spacecraft ready,” Dr. Bruck Syal said, “which probably is a minimum of 18 months, though this is debatable, then cruise time taking months to approximately a year depending on the NEO’s orbit. Then comes executing the disruption at least a few months before the impact date. In our example, 2 months was enough time for 99.9% of the mass to miss the Earth.”
Given how much warning we are currently getting in advance of most of the significantly large NEOs being discovered (often in the range of decades), it seems that we might still have plenty of time. But all of these plans will not be applicable for a truly massive NEO such as the one thought to have taken out the dinosaurs. And then there’s the largest comet ever discovered which is currently heading our way but not threatening to impact our planet.
MAXIMIZING OUR CHANCES OF SUCCESS
The Debrief asked Dr. Bruck Syal how well we are currently doing in terms of identifying these potential threats and preparing to respond. What sorts of resources and funding are being put into these studies and where are the areas we could be doing better? She explained that there are indeed many scientists taking on these tasks, but there is still a long way to go and additional assets will be required. We also asked if we can reduce the number of “surprise” arrivals such as asteroid 2021 SG.
“Yes, a lot of work is going into earlier detection and orbital characterization for near-Earth asteroids,” she explained. “We really need a space-based telescope fully dedicated to hunting down asteroids. The Near-Earth Object Surveillance Mission (NEOSM), if it becomes fully approved, will be a major contributor to filling out our understanding of the threat, down to smaller asteroid sizes. The Vera Rubin Observatory in Chile will be another key contributor when it comes fully online in a year or two. Asteroid 2021 SG is right in that sweet spot, of asteroids that are large enough to be potentially very damaging but small enough that existing detection methods had missed it until now. Luckily, Asteroid 2021 SG is NOT a threat to the Earth, but there may be other asteroids out there that could be.”
“We won’t know until observatories find and track them. If telescopes can find these asteroids with just a few years warning time before an Earth impact, nuclear disruption can help prevent a natural disaster. If there is even more warning time, typically a decade or more, a deflection can be staged, which keeps the bulk of the asteroid together while gently nudging it, causing it to miss Earth. This is why it is so critical to detect and characterize asteroids early.”
Follow and connect with author Jazz Shaw on Twitter:@JazzShaw