Asteroid Alarm Averted
Former NASA Chief Scientist Ellen Stofan ’83, D.Sc. ’16, P ’10, P ’14 weighs in on the terrestrial object that had a historic chance of impacting Earth
March 19, 2025
By
Catherine Tyson ’20
Sixty-five million years ago, an asteroid struck the Yucatán Peninsula with enough force to trigger a mass extinction event. While nothing that catastrophic has entered Earth’s atmosphere since, the risk of a large asteroid impact still exists. As a safeguard against potential threats, observatories around the world scan the night sky, searching for tiny glimmers that might evidence an Earth-bound object.
On Dec. 27, 2024, scientists in Chile discovered such an object. Using a powerful telescope, part of the NASA-funded Asteroid Terrestrial-impact Last Alert System, they determined that a large asteroid had just passed Earth two days prior, coming within approximately 515,000 miles — about twice the average distance from the Earth to the moon. They also calculated that the asteroid would make two more traverses near the Earth in 2028 and 2032.
Named 2024 YR4, this asteroid quickly gained attention as calculations showed it had up to a 3.1% chance of hitting the Earth in 2032, the highest impact risk ever recorded for an asteroid of this size. However, in late February, that number was amended, and the risk fell to just 0.001%.
While no longer a hazard, 2024 YR4 raises important questions: What if an asteroid is discovered with a higher chance of hitting Earth? Would we be able to divert such a threat? W&M Alumni Magazine staff spoke with Ellen Stofan ’83, D.Sc. ’16, P ’10, P ’14, former chief scientist at NASA and current undersecretary for science and research at the Smithsonian Institution, to get her take on these questions and more.
Q: Why did this asteroid get so much media attention?
A: Honestly, I was surprised by how much attention it received. Observatories detect asteroids with a small, but nonzero, chance of hitting Earth every couple of years, and most don’t make headlines like this. But I think two factors contributed to the heightened interest. First, the percentage risk of impact was higher than we typically see. Second, was 2024 YR4’s size. Estimates place it between 40 and 90 meters in diameter. For context, the largest asteroid event in more than a century was in 2013, when an asteroid estimated at between 17 to 20 meters across exploded over Chelyabinsk, Russia, shattering windows and injuring over 1,000 people. So, you can imagine that an object like 2024 YR4 would cause a significant amount of damage.
Q: How do scientists determine the chance of an asteroid hitting Earth?
A: It starts with spotting it in the night sky. Observatories take long-exposure images, and since stars are light-years away, they stay fixed in place. But asteroids move. So, in the images, they show up as tiny streaks or smudges. That’s what the observatory in Chile saw in December.
The challenge is that a single observatory only captures one perspective. To really understand an asteroid’s orbit, we need observations from different locations around the world. By comparing those measurements, we can look at where the asteroid is headed in relation to the Earth’s orbit and see if those paths overlap. As we gather more data and refine the orbit, the risk can go up before it goes down, which is exactly what happened with 2024 YR4.
Q: Are we sure the risk of impact won’t go back up?
A: Yes. Once we’ve collected enough data about an asteroid’s orbit, we can predict with near certainty whether it will or won’t hit Earth. NASA and other space agencies will, of course, keep a close eye on 2024 YR4, but at this point, the odds will stay the same or drop even further.
Q: What if we discovered an asteroid with a much higher chance of impacting Earth? How would we prepare?
A: That really depends on how much time we have before the possible impact. Are we talking days? Weeks? The more time scientists can study the asteroid’s size, density and composition, the better.
Those data points help predict exactly how an asteroid will interact with the Earth’s atmosphere. Some asteroids are basically loose piles of rubble, which would break apart and burn up in Earth’s atmosphere high above the surface — so they don’t pose a huge threat. Other asteroids, especially those that are metal-rich, are more resistant to the atmosphere and can punch through it, hitting the ground with a tremendous amount of energy. Across the Earth’s history, asteroids are hypothesized to have caused craters in different places, including Canada, Russia and South Africa. One may have even created the Chesapeake Bay around 35 million years ago.
Q: Assuming we did have enough time to understand an asteroid’s size and composition, what tools do we have to try to prevent an impact?
A: Lots of different techniques have been posited, some even involve blowing up the asteroid by launching a nuclear weapon at it. A little less dramatic is the idea that we can collide an object with the asteroid and bump it off its course. In 2016, this was tested with NASA’s Double Asteroid Redirection Test, or DART mission. Basically, DART caused a spacecraft to crash into an asteroid, resulting in the shortening of its orbit by 37 minutes, which was a huge success and proved the theory could work.
Q: How worried should people be about an asteroid hitting Earth?
A: There is actually a whole group of asteroids called Earth-crossers that, at some point in their orbit around the sun, cross the Earth’s path. We’ve done a good job of identifying and understanding the orbit of the largest of these. And thankfully, the Earth is safely in another location when these asteroids cross its orbit. What we need to be worried about are potential Earth-crossers that are small enough to make their identification difficult, but large enough to cause significant damage if they impacted the Earth.
Q: In case there is a catastrophic event in the future, some people have raised the idea of settling Mars to preserve human life. What are your thoughts on that?
A: Mars is not a nice place to live, and the surface of the planet is certainly not habitable for human life. People there would have to live in specially constructed bubbles or underground. So, at this moment, creating a settlement there is pretty far-fetched. However, NASA is interested in sending scientists to explore the surface and look for signs of life, either past or present. NASA had proposed 2035 to put the first humans on Mars.
Speaking of Mars … William & Mary applied science research professor Joel Levine, assisted by some W&M students, has been closely involved in NASA’s mission to put scientists on Mars. Levine served as the co-chair of NASA’s Human Exploration of Mars Science Analysis Group, a panel tasked with coming up with the principal questions of investigation for the Mars mission. Other than looking into the possibility of life on Mars, one of the questions the panel proposed is to understand what happened to Mars to make it the way it is today.
“We know that Mars looked very similar to the Earth about 4 billion years ago. It was covered in water in the form of oceans and rivers. But something happened in Mars’ history that caused the planet to lose most of its atmosphere, evaporating the water and creating the conditions we observe today.”
One hypothesis is that a massive asteroid impact played a role in Mars’ transformation. The planet is home to Hellas Basin, one of the largest impact basins in the solar system, which may be evidence of such an event.
“If a massive asteroid was responsible for stripping away Mars’ atmosphere and transforming the planet, studying that event could offer crucial insights into how these impacts occur — and how we might protect the future of our planet.”