Impact was imminent. Occasional gasps arose as the asteroid took shape and a jagged, rocky surface filled the view. Then the images abruptly stopped.
The mission control room at Johns Hopkins University Applied Physics Lab in Laurel, Md., erupted in cheers. “We have impact!” said the lead engineer, who gave a two-handed high five to a nearby colleague. Others waved their hands in the air in victory and slapped each other on the back.
This had been a test, and humanity had passed it, taking one crucial step closer to protecting Earth from an asteroid impact. The test was the culmination of NASA’s Double Asteroid Redirection Test (DART) mission, for which I was the coordination lead. On 26 September 2022, the DART spacecraft had successfully crashed into Dimorphos, a roughly 150-meter-diameter asteroid that was 11 million kilometers from Earth. The collision nudged the asteroid and modified its trajectory.
In 2022, NASA’s Double Asteroid Redirection Test slammed a golf-cart-size spacecraft, DART, into the near-Earth asteroid Dimorphos (1). DART—which first deployed a small observer craft, LICIACube, to observe the collision (2)—bumped Dimorphos’s trajectory (3) enough to alter its future course (4).GyGinfographics; Source: NASA
The celebrations in the control room were the culmination of years of effort to prove that the momentum from a golf-cart-size spacecraft can alter an asteroid’s future path. And DART’s collision with asteroid Dimorphos kicked off a new era in space exploration, in which technologies for planetary defense are now taking shape.
If one day an asteroid like Dimorphos is discovered to be headed toward Earth, an interceptor craft like DART could collide with the asteroid years in advance to avert disaster. Here’s how that might work.
Step 1: Find and Track Near-Earth Asteroids
The first step in averting an asteroid impact with Earth is just to know what near-Earth objects (NEOs) are out there.
The University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) station, in Chile plays a critical role in these observations of NEOs, which are asteroids orbiting near Earth’s orbit. In late December, it detected a previously unknown NEO during a routine sweep of the skies. The asteroid was given the name 2024 YR4, following the standard astronomical convention for new objects. “2024 Y” represents the 24th-half-month of the year 2024—that is, 16 to 31 December. The “R4” encodes the sequence of discovery—in this case, that it was the 117th object found during the year’s final couple of weeks.
Hera
Chris Philpot
This European Space Agency mission will rendezvous with the Didymos–Dimorphos asteroid system and study the aftereffects of NASA’s DART impact close up.
Launch:
2024
Rendezvous:
2026
Until that point in the year, more than 3,000 NEOs had already been discovered. Nothing about 2024 YR4 initially stood out as concerning. It was a seemingly run-of-the-mill asteroid. However, further observations soon suggested it wasn’t ordinary at all.
Throughout the first weeks of 2025, the probability of a 2024 YR4 collision with Earth kept growing. On 29 January, astronomers calculated its odds of eventual impact to be 1.3 percent. And in crossing the 1 percent threshold, 2024 YR4 triggered an alert from the International Asteroid Warning Network to the United Nations’ Office for Outer Space Affairs about the potential impact. Such alerts are posted publicly on the IAWN’s website. The 29 January notice assessed the regions of the planet at highest risk from 2024 YR4 (also known as its risk corridor), as well as the expected damage if the asteroid did crash into Earth.
On average, an object of 2024 YR4’s size—estimated at 60 meters across—slams into our planet once every thousand years. It’s considered a “city-killer” asteroid—not big enough to trigger a mass extinction, like the estimated 10-km one that likely killed the dinosaurs, but still big enough to be deadly up to roughly 50 km from the impact location. Fortunately, by 24 February, further observations by telescopes across the globe had refined the asteroid’s trajectory enough to rule out near-term Earth impact.
Yet when it comes to asteroids and Earth, there won’t always be such an uncomplicated, happy ending. Another asteroid that size or even larger will eventually be on a collision course with the planet [see chart below].
Among near-Earth object (NEO) asteroids, the most devastating and least widely catalogued categories today are the 50-meter and 140-meter classes—also known as the “city killers.”
The world’s space agencies track an estimated 95 percent of NEOs greater than 1 km in diameter. The International Asteroid Warning Network and a related Space Mission Planning Advisory Group (SMPAG) are global coordinating bodies that monitor these efforts. And thankfully, none of the giant NEOs tracked by the above pose an impact risk to Earth for at least the next hundred years. (Meanwhile, comet impacts with Earth are even rarer than those of asteroids.)
But you can only track the NEOs that are known. And plenty of city-killer asteroids remain lurking and undiscovered, potentially still posing a real risk to life on the planet. In the 50-meter range, a meager 7 percent of NEOs have been found. That’s not for lack of trying. It’s just more difficult to find small asteroids because smaller asteroids appear dimmer than larger ones.
New hardware is clearly needed. Sometime soon, the Vera C. Rubin Observatory, in Chile, is expected to see first light. The observatory will survey the entire visible sky every few nights, through a 3,200-megapixel camera on an 8.4-meter telescope. No Earth-based telescope in the history of the NEO hunt can match its capabilities. Adding to our NEO search will be NASA’s NEO Surveyor, an infrared space telescope scheduled to launch as soon as 2027. Together, the two new facilities are expected to discover thousands of new-to-us near-Earth asteroids. For objects 140 meters and larger, the two telescopes will locate an anticipated 90 percent of the entire population.
Once an NEO has been discovered, astronomers routinely track its orbit and extrapolate its trajectory over the coming century. So any NEO already on the books (for example, in NASA’s database or ESA’s database) is quite likely to come with decades of warning. Ideally, that should leave ample time to develop and deploy a spacecraft to learn more about it and redirect the wayward space rock if necessary.
Step 2: Send an NEO Reconnaissance Mission
Imagine that the probability of 2024 YR4 colliding with Earth rose instead of fell, with the estimated impact to take place sometime in 2032. Here’s why that would have been especially worrying.
Asteroid 2024 YR4’s elongated orbit made it unobservable from Earth after mid-May of this year. So we wouldn’t have been able to see it with even the most sensitive telescopes until its next swing through our region of the solar system—around June 2028.
In that alternate universe, we would’ve had to wait three years to launch a reconnaissance mission to study the object up close. Only then would we have known the next steps to take to redirect the asteroid away from Earth before its fated visit four years later.
As it happens, SMPAG held preliminary discussions about 2024 YR4 in late January and early February. However, because the asteroid’s risk of collision with Earth soon dwindled to zero, the group didn’t develop specific recommendations.
Hayabusa2#
Chris Philpot
The Japan Aerospace Exploration Agency has extended a previous mission (Hayabusa2) to encounter two more near-Earth asteroids over the next six years.
Flyby:
2026
Rendezvous
2031
DART would have provided a foundation for a 2028 reconnaissance mission, as would NASA’s Lucy mission, which flew past the asteroid Dinkinesh in 2023. Reconnaissance flybys provide as little as a few precious seconds to capture the needed data about the target asteroid. Of course, inserting a reconnaissance craft into orbit around the asteroid would allow more detailed measurements. However, few NEO trajectories offer the opportunity for any maneuver other than a flyby—especially when time is of the essence.
Whatever the trajectory, the most important question for a reconnaissance mission would be whether the asteroid was in fact on a collision course with Earth in 2032. If so, where on the planet would it hit? That future impact location could potentially be narrowed down to within a hundred kilometers.
The mission might also uncover some complications. For starters, we might discover that the asteroid is actually plural. Some 15 percent of NEOs are believed to have secondary objects orbiting them—they’re asteroids with moons. And some asteroids are essentially a flying jumble of rocks.
Another wrinkle comes in determining the asteroid’s mass. We need to know the mass to calculate the damage it could cause on impact, as well as the oomph required to divert it.
Unfortunately, the technology to measure the mass of a city-killer asteroid doesn’t exist. The mass of a larger, kilometer-size asteroid is measured by determining the gravitational pull on the reconnaissance spacecraft, but that trick doesn’t work for smaller asteroids. Right now, the best we can do is estimate the mass by measuring the asteroid’s physical size from closeup imaging during a flyby and then inferring the composition.
These challenges will need to be mastered in time for the reconnaissance mission, as the spacecraft—traveling at up to 90,000 kilometers per hour—flies past the potentially irregularly shaped object or objects half-shrouded in darkness. So it probably makes sense to tackle those challenges now rather than waiting until an actual threat emerges.
Step 3: Change NEO’s Course With Interceptor
If the reconnaissance mission does conclude that a killer asteroid is on the way and narrows down the date of impact, then what? Returning to 2024 YR4, that might make 22 December 2032 a very bad day for one city-size region of the planet. Even if it fell in the ocean, we’d need to look at geological and oceanic computer models to forecast the tsunami risk. If that risk is small, then world leaders and NEO advisors might opt to let the asteroid proceed.
On the other hand, if the asteroid is on course to strike a highly populated area, then launching a spacecraft to deflect the asteroid and prevent impact might be warranted.
NEO Surveyor
Chris Philpot
NASA’s infrared space telescope has been designed to detect and track near-Earth object (NEO) asteroids that are potentially hazardous to Earth.
Launch:
as early as 2027
Here, lessons from DART are instructive. For one thing, a spacecraft impact can pack only so much punch. It’s unclear whether a deflection spacecraft the size of the DART would be able to nudge a 2024 YR4–like asteroid with enough force to avoid Earth. It’s also possible the impactor’s nudge could inadvertently cause it to land in an even worse spot, inflicting more damage. And if the asteroid is only weakly held together, a DART-like collision might break it into multiple, smaller rubble piles—one or more of which could still reach Earth. So any kind of deflection mission has to be carefully considered.
Other asteroid defense technologies are also worth considering. These other options are still untested, but we might as well get started, when nothing’s yet at stake.
If you have decades of lead time, for instance, a rendezvous spacecraft could be dispatched to orbit the killer asteroid and slowly and continually act on it. Researchers have suggested using such a spacecraft’s gravity to tug the asteroid off its path or ion-beam engines to gradually push it. The spacecraft could use one or both techniques over the span of years or decades to cause a large enough change in the asteroid’s trajectory to prevent Earth impact.
But if time is short, there are far fewer options. If the situation is dire enough, with a monster asteroid likely heading for a populated area, then using a nuclear explosive to break up or divert the asteroid could be on the table. That’s the premise of the 1998 blockbuster Armageddon (as well as the 2021 Netflix satire Don’t Look Up). Absurd, yes, but worth considering if you’re otherwise out of options.
Of course, the whole idea of planetary defense is to have options and to do as much advance preparation as possible. A number of countries have planetary-defense missions currently in space or planned in the next few years.
The ESA’s Hera mission launched last year and is on its way to rendezvous late next year with the asteroid system that DART struck, to investigate the aftermath of DART’s 2022 deflection test. The Japanese Aerospace Exploration Agency’s Hayabusa2 is set to fly by an NEO in 2026 and rendezvous with a different asteroid in 2031. It’s the next chapter to JAXA’s original Hayabusa2 mission, which brought back samples of the asteroid Ryugu in 2020. China plans to perform a kinetic impactor demonstration similar to DART, with an observer spacecraft to watch, scheduled to launch in 2027.
And in 2029, a 340-meter asteroid called Apophis—after the Egyptian god of chaos and darkness—will pass within 32,000 km of Earth, which is closer than some geosynchronous satellites. This will happen on 13 April 2029—Friday the 13th, that is. Apophis won’t hit Earth, but its close pass has prompted the U.N. to designate 2029 the International Year of Asteroid Awareness and Planetary Defense. The asteroid will be bright enough to be seen by the naked eye across parts of Europe, Asia, and Africa. And NASA has redirected its OSIRIS-REx spacecraft (which returned samples of the asteroid Bennu to Earth in 2023) to rendezvous with Apophis. The renamed OSIRIS-APEX mission will give astronomers an important opportunity to further refine how we measure and characterize NEO asteroids.
While NEO researchers will continue to collect new data and develop new insights and perspectives, leading toward, we hope, better and stronger planetary defense, one perennial will hold as true in the future as it does today: In this very high-stakes game, you never get to pick the asteroid. The asteroid always picks you.
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