In 2012, astronaut Ron Garan did an AMA on Reddit. In between questions about aliens (he didn’t see any in space) and where his coffee came from (recycled urine), he responded to a question about why we should accept the risks of a future mission to Mars. Garan quoted a colleague: “If the dinosaurs had a space program, they’d still be here.”
Putting aside the unlikelihood of giant reptiles with brains the size of walnuts developing their version of Apollo 11, the point here is that the dinosaurs were almost certainly wiped out by a nearly 6-mile-wide asteroid that struck the Earth with the destructive power of billions of Hiroshima-scale nuclear bombs, causing an “impact winter” that cut off sunlight and led to drastic cooling far beyond what most dinosaurs could survive.
The dinosaurs, of course, could do nothing about the killer asteroid, other than presumably waving their tiny arms at the oncoming doom. But if they did have a space program — and yes, now I’m imagining a T. rex in a space suit, swaggering to a rocket like John Glenn in The Right Stuff — they might have been able to detect that incoming asteroid decades in advance, and done something to avert their doom.
Humans, though, are in a better place — as shown by the recent news over an asteroid called 2024 YR4 that briefly appeared to be threatening the Earth.
Killer asteroids, briefly explained
The Chicxulub asteroid that likely wiped out the dinosaurs wasn’t the first time a massive asteroid collided with the Earth. An asteroid 12 to 16 miles wide hit the planet more than 2 billion years ago, in what is now Vredefort, South Africa, while another 6 to 10 miles wide hit what is now Sudbury, Ontario 1.85 billion years ago. More recently, a 130-foot-wide space rock exploded 6 miles above Siberia in 1908, creating a blast strong enough to knock over 80 million trees.
The Earth exists in a cosmic shooting gallery, and while truly civilization-threatening strikes of the kind seen in movies like Deep Impact are incredibly rare, they do happen. And given enough time, they will happen again.
Until very recently, were a Chicxulub-sized asteroid to find itself on a collision course with Earth, we wouldn’t have been able to do much more than the dinosaurs did. The result would be global firestorms, massive earthquakes, and potentially megatsunamis, followed by an impact winter that would wipe out the global food supply. Very bad stuff.
But we’re not helpless anymore.
Like a lot of cool things, the field of asteroid defense began with a bunch of kids at MIT with brainpower to spare. In 1967, MIT professor Paul Sandorff asked his class to imagine that a real-life asteroid called Icarus, which astronomers had already identified, would hit the Earth in the near-future — and it was their job to devise a way to save the world. (In real life, the asteroid came within 4 million miles of the Earth — 15 times the distance between our planet and moon, but a close shave by cosmic standards.)
So was born “Project Icarus.” The students created a plan to launch six Saturn V rockets, each carrying a 100-megaton nuclear warhead, at the asteroid. The warheads would detonate near the asteroid and create enough force to alter its trajectory and miss the Earth.
For all its careful engineering, “Project Icarus” was largely science fiction; among other inconveniences, the largest nuclear bomb ever made only had a force of 50 megatons. Our space science was so rudimentary at the time that we had no way to reliably identify potentially dangerous asteroids very far in advance, and no real way to deflect them.
But Project Icarus put the idea of asteroid defense out into the public. The discovery of the actual Chicxulub crater in 1990, confirming the likely cause of dinosaurs’ demise, and the sight of the Shoemaker-Levy 9 comet walloping Jupiter in 1994, convinced Congress to take the threat of killer asteroids seriously. In 1998, Congress directed NASA to detect and catalog within 10 years at least 90 percent of what are called near-Earth objects (NEOs) that were more than a kilometer wide.
NASA and its partners hit that goal with time to spare, and so in 2005, Congress directed the agency to identify at least 90 percent of all NEOs 140 meters or wider — not big enough to end the world, but big enough to destroy a city. Though over 18,000 NEOs have been identified, about 40 every week, there may be a million or more out there. That mission continues.
The recent scare over the asteroid known as 2024 YR4 made this search for killer asteroids so we can knock them off course a bit less academic. (When NEOs are discovered, they are initially given a name that reflects the year of identification, followed by letters and numbers that indicate the order it was identified that year, starting with AA. But the discoverer does get to propose a formal name for it, provided it’s less than 16 characters and meets the approval of the International Astronomical Union, which is cool.)
2024 YR4 was discovered on December 27 of last year by the Asteroid Terrestrial-impact Last Alert System (ATLAS) — a NASA-funded asteroid detection program with telescopes around the world — at its station in Chile. With an estimated diameter of 130 to 300 feet, it wouldn’t be a world-ender, but it could cause severe local damage if it were to collide with the Earth. Which was worrying, because early calculations suggested it had as much as a 3.1 percent chance of striking our planet on December 22, 2032.
3.1 percent may not seem like much of a risk — it’s about the same chance as flipping a coin five times and getting all heads or all tails — but it was three times higher than that of any other large known asteroid. For skywatchers this was a big deal. So they swung into action, pulling in data from observatories run by NASA, the European Space Agency, and the Japanese Aerospace Exploration Agency.
Asteroids do offer us the opportunity to stave off at least one kind of planetary disaster because, like all objects in space, they follow a clear and largely predictable orbit. An asteroid impact happens when the orbits of the object and the Earth intersect, like two cars trying to merge onto the highway. Get enough data, do some math, and scientists can figure out with astounding precision whether the Earth will suffer a cosmic fender bender decades into the future.
Once the new measurements were taken and the math was done, the probability of YR4 hitting the Earth began to decline, eventually falling to just 0.004 percent. Crisis, such as it was, averted. But while YR4 won’t be obliterating any cities, it did provide an invaluable test for planetary defense science — one we passed.
Now, what would happen if a big asteroid was confirmed to be on a collision course impact path with Earth? While our asteroid detection systems are way ahead of our asteroid defense systems, there are some options, at least theoretically.
Project Icarus had already figured it out back in the 1960s: You don’t need to destroy an asteroid to protect the Earth — you just need to give it a slight nudge. Treat it like the eight ball on a pool table, and knock it away. The cue ball in this analogy would be something known as a “kinetic impactor” — a spacecraft that crashes into the asteroid with enough force to alter its orbit.
We know this can work. On September 26, 2022, NASA’s Double Asteroid Redirection Test (DART) collided with the tiny asteroid Dimorphos, more than 7 million miles from Earth. DART was a success, shortening Dimorphos’s orbit by 32 minutes.
DART wasn’t perfect. The collision also unleashed a swarm of boulders, demonstrating some of the unintended consequences of smashing something into a space rock at roughly 14,760 mph. As the science writer Robin Andrews pointed out on X, DART was proof of principle at best, and not yet something we could use on an asteroid like YR4 if we needed.
Of course, a much bigger asteroid that would actually threaten the whole planet would require far, far more force to deflect, and technology we don’t yet have. (No, we cannot yet send up oil drillers with a nuclear bomb, like Bruce Willis in Armageddon.)
But still. Thanks to brilliant space scientists, international collaboration, and yes, even an act of Congress, our species is closer to being able to permanently protect itself from a natural existential risk that has obliterated the dominant species in our planet’s past. If that’s not good news, I don’t know what is.
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