Thousands of asteroids travel through the region of space around Earth, and astronomers continuously calculate their orbits to determine whether any could become an impact threat. Close approaches can sound alarming, but passing relatively near Earth and being on a collision course are very different things.
The reassuring part is important: NASA says there is currently no known asteroid larger than 140 meters with a significant chance of hitting Earth during the next 100 years.
That does not mean asteroid impacts are impossible. Scientists continue searching because many near-Earth objects remain undiscovered, particularly smaller objects and asteroids that are difficult for ground-based telescopes to see.
The story of planetary defense is therefore less about a list of asteroids destined to hit Earth and more about how astronomers discover potential hazards, refine uncertain predictions and prepare technologies that could one day prevent an impact.
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What is a near-Earth object?
A near-Earth object, or NEO, is an asteroid or comet whose orbit brings it into Earth’s broader orbital neighborhood. Most known NEOs are asteroids.
NASA uses a more specific definition for potentially hazardous objects. Size and how closely an object’s orbit can approach Earth’s orbit are among the factors used to determine that classification.
Crucially, the words “potentially hazardous” do not mean an asteroid is predicted to hit Earth. The classification identifies objects whose size and orbital characteristics make them particularly important to monitor.
Apophis will make an extraordinary close approach in 2029
Few asteroids illustrate the difference between an alarming early calculation and a refined scientific prediction better than 99942 Apophis.
Discovered in 2004, Apophis initially attracted worldwide attention because early observations raised the possibility of a future Earth impact. Years of additional observations transformed scientists’ understanding of its orbit.
NASA now says Apophis poses no impact risk to Earth for at least the next 100 years.
On April 13, 2029, however, the asteroid will make a remarkably close – but safe – passage. NASA says it will pass roughly 20,000 miles (32,000 kilometres) above Earth’s surface.
That encounter is scientifically valuable precisely because a large asteroid will pass unusually close without threatening the planet. Earth’s gravity is expected to alter Apophis’ orbit and affect the asteroid during the encounter, giving researchers an unusual opportunity to study how a near-Earth asteroid responds to a close planetary flyby.
Why an asteroid can appear dangerous and later be declared safe
Asteroid risk assessments can change dramatically after discovery, and that does not necessarily mean scientists made a mistake.
When an asteroid is first detected, astronomers may have only a short observational record from which to calculate its orbit. That uncertainty creates a range of possible future trajectories. In some cases, Earth initially lies within that range.
As telescopes collect additional observations, scientists can refine the orbit and shrink the uncertainty. Frequently, the refined trajectory rules out an impact.
Asteroid 2006 QV89 provides a useful example. It once carried a very small possibility of an impact during its September 2019 approach. NASA’s Center for Near Earth Object Studies later removed that potential impact after additional analysis and observations showed the asteroid would pass safely.
A similar process occurred with 2021 QM1. The asteroid once appeared on risk lists because calculations indicated a possible impact in 2052. Additional observations obtained with the European Southern Observatory’s Very Large Telescope allowed ESA specialists to refine its orbit and rule out that impact scenario.
This process explains why newly discovered asteroids sometimes generate alarming headlines before eventually disappearing from risk lists. An initial probability is not the same as a prediction that an impact will occur.
2023 DZ2 demonstrated how close approaches are monitored
Asteroid 2023 DZ2 generated considerable attention before its March 25, 2023 flyby.
ESA estimated the object at roughly 42 to 94 metres across and calculated that it would pass approximately 168,000 kilometres above Earth’s surface. Its estimated impact probability for that encounter was zero.
Close encounters like this still have scientific value. They allow astronomers to improve measurements of an object’s orbit and learn more about how Earth’s gravity alters the trajectory of an asteroid passing through the planet’s neighborhood.
Some famous “upcoming” asteroid encounters have already happened
Online videos and older articles about asteroid threats can become misleading simply because time passes.
For example, asteroid 2001 FO32 made its widely reported close approach on March 21, 2021. NASA says it passed approximately 1.25 million miles, or 2 million kilometres, from Earth.
Asteroid 1994 PC1 likewise had a notable close approach in January 2022, rather than January 2023 as some older summaries have stated.
Dates matter in planetary-defense reporting. A close approach that was “upcoming” when a video script was written may be years in the past when that material is encountered again.
How scientists watch for hazardous asteroids
Planetary defense begins with detection.
Ground-based surveys repeatedly scan the sky, while observations are combined to calculate asteroid orbits. NASA’s Center for Near Earth Object Studies at the Jet Propulsion Laboratory uses these observations to compute trajectories and assess possible future encounters.
Its Sentry system performs long-term impact monitoring, while Scout examines newly reported objects for possible short-term impact risks.
Pan-STARRS and the Catalina Sky Survey are among the important asteroid-discovery efforts contributing observations.
NASA’s NEOWISE spacecraft also played an important role in asteroid science, but its mission has ended. NASA retired NEOWISE in 2024 after more than 14 years in space.
NEO Surveyor is designed to find asteroids that are difficult to detect
The next major step in NASA’s asteroid-search capabilities is NEO Surveyor, an infrared space telescope designed specifically for planetary defense.
NASA says the spacecraft is undergoing integration and testing and is scheduled for launch no earlier than September 2027.
Unlike telescopes that depend primarily on reflected visible light, NEO Surveyor will detect infrared radiation from objects warmed by the Sun. That capability should help it identify dark asteroids and objects approaching from directions that are difficult for conventional ground-based observations.
The mission is designed to find at least two-thirds of near-Earth objects larger than 140 metres during its five-year baseline survey.
Finding such objects early is fundamental to planetary defense: a deflection strategy becomes far more practical when scientists have substantial warning time.
DART proved an asteroid’s motion can be deliberately changed
Detection is only part of the problem. Scientists also want to know what humanity could do if an asteroid were genuinely found on a collision course with Earth.
NASA’s Double Asteroid Redirection Test, better known as DART, supplied the first full-scale demonstration of one possible answer.
On September 26, 2022, DART intentionally collided with Dimorphos, the small moon of asteroid Didymos. Neither object threatened Earth; the binary asteroid system served as a test target.
Follow-up observations confirmed that the collision changed Dimorphos’ orbit around Didymos. NASA reported that the orbital period was shortened by about 33 minutes.
The experiment demonstrated that a kinetic impactor — deliberately striking an asteroid with a spacecraft — can alter an asteroid’s motion.
That does not mean every possible asteroid could automatically be redirected in the same way. The effectiveness of a future mission would depend on factors including the object’s size, structure, composition, orbit and, critically, how much warning time was available.
Why scientists keep searching when no major known threat is imminent
The absence of a known major threat is not a reason to stop looking. It is a reason to improve detection.
NASA has estimated that many near-Earth objects at least 140 metres across remain undiscovered. Objects in this size range are particularly important because an impact could cause severe regional damage.
Small asteroids enter Earth’s atmosphere much more frequently and often disintegrate before reaching the surface. Very large impacts are far rarer, but their consequences make early detection valuable.
Planetary defense therefore operates on a simple principle: discover potentially hazardous objects as early as possible, calculate their trajectories accurately and maintain the ability to respond if a genuine threat is ever identified.
Should people be worried about Apophis or other known asteroids?
Based on current NASA assessments, there is no known asteroid larger than 140 metres with a significant probability of striking Earth during the next century.
Apophis is perhaps the clearest example of why continued observations matter. It went from being one of the world’s most discussed potential asteroid threats to an object whose impact risk has been ruled out for at least 100 years.
The scientific challenge has not disappeared, however. Astronomers still need to discover objects that have not yet been catalogued, improve orbit calculations for newly discovered asteroids and continue developing planetary-defense technologies.
The bigger story is preparedness, not panic
Asteroids are a genuine natural hazard on astronomical timescales, but planetary-defense science has advanced considerably.
Modern surveys can discover and track near-Earth objects. Automated systems continually calculate potential future encounters. DART demonstrated that humanity can deliberately alter an asteroid’s motion, while NEO Surveyor is being built to improve the search for difficult-to-detect objects.
The next time an asteroid makes headlines for a close approach, the most useful questions are not simply how large it is or how close it will come. Ask whether an impact is actually predicted, what uncertainty remains in its orbit, and what NASA, ESA or another planetary-defense authority says about the risk.
That distinction turns an alarming asteroid headline into what it usually represents: scientists doing the work required to understand our cosmic neighborhood.