An asteroid passing near Earth is not the same as an asteroid heading for Earth. The difference is measured through observations, orbital calculations and uncertainty—not the size of a headline.
Reports of approaching asteroids can sound alarming, especially when they combine an enormous distance, a comparison with a famous building and the phrase ‘potentially hazardous’. The terminology describes a genuine field of scientific monitoring. It does not, by itself, announce an impending disaster.
To understand the risk, we need to distinguish an object’s classification, its predicted path and the consequences of a possible collision.
What does ‘near-Earth’ mean?
NASA’s Center for Near-Earth Object Studies (CNEOS) defines near-Earth objects by their orbits around the Sun. Their closest approach to the Sun is less than 1.3 astronomical units; one astronomical unit is approximately the average Earth–Sun distance.
The label includes many asteroids and some comets. It does not mean that an object is about to enter Earth’s atmosphere. Two objects can travel through the same general region of the Solar System without arriving at the same place at the same time.
‘Potentially hazardous’ is a screening category
CNEOS classifies an asteroid as potentially hazardous when its minimum orbital separation from Earth’s orbit is no more than 0.05 astronomical units—about 7.48 million kilometres—and its absolute magnitude is 22 or brighter. That brightness corresponds approximately to a diameter of 140 metres under an assumed reflectivity; it is not a direct size measurement.
This identifies objects whose orbits and estimated sizes merit attention. The minimum separation between two orbital paths is not the same as the distance of an actual encounter. Neither is it an impact probability.
The useful follow-up question is therefore not simply ‘Is it on the hazardous list?’ It is ‘What do the current calculations say about this object’s future encounters?’
How scientists work out an orbit
CNEOS computes orbital solutions and close approaches using observations of near-Earth objects. Its Sentry system assesses possible impacts over roughly the next century, while Scout examines newly reported objects that have not yet been confirmed as discoveries.
The Sentry FAQ explains that an orbit is fitted to the observed positions of an asteroid at different times. Additional observations improve that fit and confidence in its future position.
An early orbit is not a perfectly drawn railway track through space. It comes with a range of uncertainty. Knowing that range is part of the measurement, rather than an admission that the calculation is worthless.
Why the impact probability can rise
The European Space Agency’s explanation describes a corridor of possible future positions. Initially, this corridor can be broad. Further observations narrow it.
If Earth remains within that shrinking corridor, a greater proportion of the remaining possibilities can involve a collision. The calculated impact probability may consequently rise even while scientists understand the orbit better.
More observations can then place the corridor entirely away from Earth, causing the assessed risk to fall sharply. If an asteroid really is on a collision course, the probability would instead converge towards certainty.
The asteroid has not changed direction because a percentage changed. The percentage describes our improving knowledge of its motion. A changing estimate should be read alongside the observation history and the latest assessment.
A risk-list entry is not a collision forecast
The Sentry introduction says that newly discovered objects commonly leave its risk page as improved observations exclude impact trajectories. Removal is an expected part of the process, rather than evidence that the earlier uncertainty was invented.
A list can contain possible impacts because the available data cannot yet exclude them. Treating every entry as a prediction would confuse a possibility being investigated with an outcome judged likely.
Equally, ‘no identified threat’ should not be inflated into a guarantee that every object has been discovered. Monitoring known objects and searching for unknown ones are complementary tasks.
Probability and consequences belong together
The Torino Impact Hazard Scale combines collision probability with estimated impact energy on a scale from 0 to 10. It is designed to communicate the significance of an encounter, rather than rank objects by diameter alone.
Level 0 covers events with zero or effectively zero collision probability, as well as objects too small to cause meaningful ground damage. The higher levels distinguish encounters requiring attention from confirmed collisions with progressively greater consequences.
A percentage on its own is incomplete. Responsible coverage should identify the possible encounter date, estimated size or energy, uncertainty and current assessment. It should also date the information, because an old risk figure may have been superseded.
Can we change an asteroid’s path?
NASA’s DART results provide a practical demonstration. In September 2022, a spacecraft deliberately struck Dimorphos, the small companion of asteroid Didymos. The impact shortened Dimorphos’s orbital period around Didymos by about 33 minutes, according to the analysis NASA reported in 2023.
That was a change to the moonlet’s orbit around its companion, not a 33-minute change to its year around the Sun. NASA’s mission description makes clear that the test system was not threatening Earth.
DART demonstrated a kinetic impactor: transferring momentum through a collision. The recoil from material ejected off the asteroid contributed to the result. It established that this technique can alter an asteroid’s motion; it did not establish that any object could be diverted at short notice.
A real response would require an assessment of the particular object and the time available. Detection and continued observation remain essential parts of preparedness.
Concern without catastrophe
At the time of checking, NASA’s planetary defence overview states that no known asteroid larger than 140 metres has a significant chance of hitting Earth within the next 100 years. The qualifications—known objects, that size range and that time horizon—matter.
NASA also distinguishes routine encounters with small objects from much rarer damaging impacts. We should neither turn every flyby into an emergency nor dismiss the value of a monitoring programme because most of its investigations end reassuringly.
Our editorial view is that the strongest asteroid story is often the work behind the update: observatories sharing measurements, calculations narrowing uncertainty, and researchers checking one another’s results.
Planetary defence begins with finding objects and understanding their paths. A close approach is a reason to examine the evidence, not a reason to assume a collision.
Related reading
- Comets: Cosmic Visitors, Omens and What Science Reveals
- Solar Flares: What Space Weather Really Means for Earth
Sources and further reading
- NASA/JPL CNEOS — Near-Earth object groups and potentially hazardous asteroids
- NASA/JPL — About CNEOS, Sentry and Scout
- NASA/JPL — Sentry frequently asked questions
- NASA/JPL — Understanding Sentry impact monitoring
- NASA/JPL — Torino Impact Hazard Scale
- ESA — How asteroids go from threat to no sweat
- NASA — DART data validates kinetic impact as a planetary defence method
- NASA — Didymos and Dimorphos
- NASA — Planetary defence overview
Sources checked on 15 October 2026. Risk assessments change as observations improve; this article explains the assessment process rather than providing a live warning service.