In late 2024, at an observatory in the Chilean Andes, a robotic telescope spotted a faint dot moving against the night sky. That dot turned out to be a new asteroid, later named 2024 YR4. It briefly set off alarms: initial calculations suggested a small but worrisome chance it could strike Earth in 2032. For a few tense days, scientists around the world raced to gather more observations. As new data poured in, the asteroid’s predicted path shifted away from Earth, eliminating the danger. The close call showcased how NASA and its partners use open data and rapid collaboration to identify asteroid threats. This teamwork helps ensure these wandering space rocks don’t catch us by surprise.
Spotting Asteroids That Could Hit Earth
Every night, astronomers scan the skies for moving points of light. Near-Earth objects (NEOs) are asteroids or comets that come within about 120 million miles of the Sun. These space rocks travel through Earth’s cosmic neighborhood, making them especially important to track. Discovering these objects is a global effort. NASA supports sky surveys like ATLAS, which operates telescopes in Hawaii and Chile, and Pan-STARRS in Hawaii. Other observatories, such as the Catalina Sky Survey in Arizona, also contribute to the search. These automated telescopes capture wide-field images of the night sky. They look for the telltale streak of an asteroid drifting against the background stars.
When astronomers discover a new asteroid, they report it to the Minor Planet Center (MPC). Operated by the Smithsonian Astrophysical Observatory on behalf of the International Astronomical Union, the MPC serves as the world’s clearinghouse for asteroid observations. It collects and verifies thousands of position measurements from observatories across the globe. These include observations from giant professional telescopes as well as from the backyard rigs of dedicated amateur astronomers. All this information goes into NASA’s open databases, where anyone – professional or not – can inspect and use it.
If a newly spotted object looks likely to be a NEO, the Minor Planet Center posts it on a public “NEO Confirmation” page. This alerts the worldwide astronomy community that follow-up is needed. Dozens of telescopes might then turn toward the object over subsequent nights. Each new observation helps refine what we know about its orbit. With enough data, astronomers can pin down the asteroid’s path around the Sun and see whether it might cross Earth’s orbit in the future.
Calculating the Odds of an Impact
Finding an asteroid is only step one. Next comes figuring out if it might hit us. This task falls to NASA’s Center for Near-Earth Object Studies (CNEOS), based at the Jet Propulsion Laboratory in California. CNEOS acts as the planetary defense data nerve center. It takes the observations compiled by the Minor Planet Center and computes the asteroid’s most likely orbit.
Because early observations can be sparse, there’s usually some uncertainty in an object’s predicted path. An asteroid could be anywhere within a broad region of space when it makes a close approach years or decades in the future. To assess risk, CNEOS runs computer models to see if that uncertainty region intersects Earth. If there is even a remote chance of a future impact, CNEOS will flag the object for special attention.
CNEOS also maintains an automated monitoring system called Sentry. This program continuously checks the orbits of all known NEOs and flags any object with even a tiny probability of collision. In most cases, further observations erase the risk. But if an asteroid’s chances of impact stay uncomfortably high, CNEOS will sound the alarm to NASA’s Planetary Defense Coordination Office (PDCO). NASA established the PDCO in 2016 to oversee these planetary defense efforts. The office coordinates with other government agencies and international partners to decide what to do next when a threat arises.
For context, NASA considers an impact probability of about 1% or more (within the next 50 years) as a threshold for issuing formal warnings. Fortunately, nearly all asteroids ever flagged as risks turn out to pose no real threat. Once astronomers gather additional observations, the supposed danger usually disappears. The process highlights the self-correcting nature of science. Each new observation tweaks the orbit calculation, often shrinking the asteroid’s projected path uncertainty. With enough data points collected from around the world, the potential impact solutions typically vanish or shift away from Earth.
This was exactly the case with asteroid 2024 YR4. Initially, its calculated trajectory had a very slim possibility of striking Earth. As additional observations came in, scientists updated their models and watched the odds plummet. Within about a week, the collision probability fell effectively to zero. Thanks to the flood of data, NASA’s scientists officially declared YR4 no longer a threat and struck it off the agency’s asteroid risk list.
Asteroid 2024 YR4: A Global Test of Planetary Defense
When ATLAS first detected 2024 YR4 on December 27, 2024, it seemed unremarkable at first – one more small asteroid among thousands discovered each year. But early in 2025, as the asteroid’s projected path was analyzed, researchers realized this space rock might come perilously close to Earth in late 2032. On February 18, 2025, NASA’s risk assessment models showed about a 3% chance that YR4 could collide with Earth on December 22, 2032. That probability – roughly a 1 in 32 chance – was the highest ever recorded for an asteroid of that size.
At roughly 40 to 90 meters (130 to 300 feet) across, 2024 YR4 is comparable in scale to the object that caused the Tunguska airburst in 1908, which flattened a forest in Siberia, or the 2013 Chelyabinsk meteor that exploded in the sky over Russia and shattered windows across a city. News of YR4’s potential impact quickly spread, raising public concern about what damage such an asteroid could do.
Scientists swung into action immediately. Around the world, telescopes large and small trained on 2024 YR4 to nail down its orbit. Many of these observatories were already part of the International Asteroid Warning Network (IAWN) – a coalition of astronomy institutions and space agencies that share asteroid tracking information. Within days, new data arrived from observatories in North America, Europe, and beyond. Even NASA’s space-based assets got involved; the James Webb Space Telescope, for instance, planned special observations of YR4. As fresh measurements were fed into CNEOS’s models, the picture changed dramatically. The cluster of possible future paths for the asteroid shifted away from Earth. By late February, refined calculations showed no realistic chance of an Earth impact in 2032 or beyond.
While Earth was in the clear, 2024 YR4 wasn’t entirely off the hook. The updated trajectory indicated a very slight possibility – on the order of a few percent – that the asteroid could hit our Moon in 2032. Even in that unlikely event, the outcome would be harmless to us, aside from creating a new crater on the lunar surface. (Scientists noted that if YR4 did strike the Moon, the flash might even be visible from Earth with the naked eye.)
But importantly, the Earth was safe. The rapid turnabout for 2024 YR4 – from headline-grabbing threat to cosmic footnote – demonstrated how well the planetary defense system worked. It was a live-fire exercise for NASA’s data systems and the global network of asteroid watchers. Open access to the observation data meant that independent experts everywhere could verify the findings, cross-check calculations, and be confident in the all-clear verdict.
Interestingly, the intense scrutiny of 2024 YR4 yielded some scientific surprises as well. Follow-up studies using large telescopes like Gemini South in Chile revealed that this asteroid has an unusual shape, more like a flattened disk than a typical rocky chunk. Clues from its reflected light suggest it likely came from a collision in the main asteroid belt ages ago. Such details, while tangential to the impact risk, show how planetary defense observations can double as pure science investigations. In studying what first looked like a threat, astronomers ended up learning about the asteroid’s composition and origin.
Defending the Planet: From Data to Action
NASA’s strategy for protecting Earth hinges on early detection and open data. The goal is to find threatening asteroids as early as possible and share information widely. That open approach ensures the whole international community can pool its expertise and resources. No single nation can survey the entire sky; it truly takes a planet to defend the planet. The United States, through NASA’s PDCO, works closely with partners like the European Space Agency and observatories worldwide to keep watch. Together, they’ve cataloged nearly 40,000 near-Earth asteroids to date. New ones are being found at a rate of dozens per week. Most pose no danger, but it only takes one, so the search continues unabated.
If astronomers ever spot an asteroid truly on a collision course with Earth, planetary defenders would quickly pivot from observation to action. Scientists and engineers have spent years preparing emergency playbooks for such a scenario. Depending on the lead time and the asteroid’s size, options would range widely. If warning time was short or the impactor was small, civil defense — like evacuating the impact zone — might be the only recourse. If the space rock were large and found well in advance, scientists could attempt to deflect it off course with a spacecraft. The key is having enough warning to attempt a deflection mission.
In 2022, NASA demonstrated that deflection is more than science fiction. The Double Asteroid Redirection Test (DART) mission launched a refrigerator-sized spacecraft that deliberately collided with a small asteroid moon called Dimorphos. This test was a resounding success. The impact slightly changed Dimorphos’s orbit around its larger companion asteroid, proving that a kinetic impactor can nudge an asteroid off course. Though Dimorphos was never a threat to Earth, DART’s outcome gave researchers confidence that, if need be, we could use a similar technique to push a hazardous asteroid onto a safer path. It was humanity’s first full-scale experiment in planetary defense, and it won’t be the last.
To increase the chances of discovering any worrisome space rocks well in advance, NASA is building a new space-based observatory dedicated to planetary defense. The NEO Surveyor mission is slated for launch in 2027. It will orbit the Sun and hunt for near-Earth objects from a vantage point beyond Earth’s orbit. Using infrared sensors, it will pick up the heat signatures of asteroids. These are objects too dark or too distant to notice with current telescopes. Crucially, NEO Surveyor’s data will be open to scientists and the public through NASA’s archives. Once operational, this spacecraft is expected to rapidly accelerate asteroid discoveries. It will be particularly adept at finding mid-sized asteroids a few hundred meters across that current surveys might miss. Finding these objects early is essential. The more time we have before a potential impact, the more options we have to prevent it.
NASA’s Planetary Defense Coordination Office also continues to strengthen ties with international programs. Exercises and workshops are held with agencies around the world to practice responding to a hypothetical asteroid threat. In these exercises, scientists from dozens of countries role-play the discovery of an imminent impactor. They coordinate how to share information with governments and the public. These dry runs help identify any communication gaps. They ensure that if a real threat ever emerges, the global response will be swift and coordinated.
Open Data and Vigilance: A Winning Strategy
Asteroids have been hitting Earth for billions of years, and they will continue to do so. But for the first time in our planet’s history, a species has the knowledge and tools to see the next big impact coming – and maybe even stop it.
Key to this progress is the principle of open science. By freely sharing asteroid data, NASA empowers researchers everywhere to work together as one planetary defense team. A small observatory in rural Arizona, a university telescope in Italy, or a passionate amateur in Australia – all can contribute observations that might make the difference in catching an incoming space rock.
Open data also builds public trust. When the numbers show that an asteroid like 2024 YR4 won’t hit Earth after all, anyone can double-check those calculations. Transparency keeps everyone informed and helps squash unfounded rumors or panic.
The case of 2024 YR4 ended as both a scientific triumph and a public reassurance. No impact will happen in 2032, and our vigilant watchfulness paid off. It won’t be the last false alarm – and that’s actually a sign that the system is working. Each time astronomers resolve an asteroid scare with new observations, it means our telescopes and data-crunching pipelines are doing their job. Meanwhile, NASA and its partners will keep improving the planetary defense toolkit. Future generations may never witness a city-destroying asteroid strike. That’s not because such rocks aren’t out there, but because we’ll see them coming and know how to react.
