SKY · FIELD GUIDE
Near-Earth Asteroids — How They're Found, Tracked, and Why Close Passes Aren't Scary
Headlines love an asteroid 'skimming past Earth.' But what does a close approach actually mean, how do we know it'll miss, and should you ever worry? The reassuring answers come straight from the tracking data.
"Asteroid the size of a bus to skim past Earth" is a headline that runs almost every week, and it's almost always describing something completely routine and harmless. Here's what's really going on, and why a close approach is a reassuring piece of science rather than a reason to worry.
What a near-Earth asteroid is
Asteroids are rocky leftovers from the Solar System's formation, most of them orbiting in a broad belt between Mars and Jupiter. A near-Earth asteroid is simply one whose orbit brings it into Earth's neighbourhood. There are tens of thousands of known ones, ranging from a few metres across to several kilometres, and surveys find more every night. The vast majority will never come anywhere near us.
"Close" is farther than it sounds
Close approaches are measured in lunar distances (LD) — multiples of the average Earth–Moon gap, about 384,000 km. That's a useful yardstick because the Moon is the one distance in space everyone has a feel for. A pass at:
- 5 LD is five times farther away than the Moon — barely "close" at all.
- 1 LD is right at the Moon's distance — genuinely close in astronomical terms, still an enormous miss.
- under 0.5 LD is a notably close pass that gets astronomers' attention — and is still a confident, calculated miss.
When you see an object listed at "2.4 lunar distances," that's nearly a million kilometres of clear space.
How we know it'll miss
This is the part worth internalising: these passes aren't surprises, and the misses aren't luck. Survey telescopes discover an asteroid, many observations pin down its orbit, and that orbit is then projected decades into the future. NASA's Center for Near-Earth Object Studies (CNEOS) — the source behind our tracker — maintains this for every known object. If anything is found with a non-trivial chance of impact, it goes onto a public risk list and gets observed relentlessly until the orbit is precise. Historically, that extra data has always reduced the estimated risk, usually to zero. The objects on a close-approach list are, by definition, the ones already known to miss.
Why size is an estimate
You'll notice sizes are given as ranges, like "18–40 m." That's honest, not vague. We rarely measure an asteroid's diameter directly; instead we infer it from brightness — specifically its absolute magnitude (H). But brightness depends on both size and how reflective the surface is, and a dark rock reflects far less light than a shiny one. Two asteroids of the same brightness can differ in true size by about a factor of two, so a responsible figure is a band, flagged as an estimate, unless radar or a spacecraft has measured the object up close.
The real work: finding them early
None of this means asteroids are nothing to take seriously — they're taken very seriously, which is exactly why the surveys exist. The genuine concern isn't the cataloged rock passing at three lunar distances next Tuesday; it's an undiscovered object on a bad trajectory. So astronomers scan the sky every clear night, new observatories are coming online to find the smaller and fainter ones, and missions like NASA's DART have already shown we can nudge an asteroid's orbit if we ever need to. The close approaches you can browse here are the system working as intended: we see them coming, we know they'll miss, and we can tell you so with confidence.
Frequently asked questions
Is an asteroid going to hit Earth?
Not among the objects you'll see listed as upcoming close approaches — and not by luck, but by measurement. Those are catalogued asteroids on known orbits that have been calculated to pass Earth and miss. NASA's Center for Near-Earth Object Studies tracks every known near-Earth object and projects its path decades ahead. When something is found with any real impact chance, it goes on a public risk list and is observed intensively until the orbit is nailed down — which so far has always driven the risk to essentially zero.
What does 'close approach' actually mean?
A close approach is when an asteroid passes near Earth on a known trajectory that misses us. 'Near' in space terms is still vast: distances are measured in lunar distances (LD), where one LD is the average Earth–Moon gap of about 384,000 km. A pass at 5 LD is five times farther than the Moon. Even a genuinely close pass at well under 1 LD is a clean miss — it just sounds dramatic because the numbers are unfamiliar.
How are near-Earth asteroids discovered?
Automated survey telescopes photograph wide swaths of sky every clear night and software flags anything that moves against the fixed stars. New objects are reported to the Minor Planet Center, other observers confirm them, and their orbits are computed from the observations. Surveys like Catalina, Pan-STARRS and ATLAS find thousands of near-Earth objects, and upcoming observatories will find far more — the whole point is to discover them long before any could matter.
How do we know how big an asteroid is?
Usually we don't measure it directly — we estimate it from brightness. An asteroid's 'absolute magnitude' (H) tells us how much sunlight it reflects, and combined with an assumption about how reflective its surface is, that gives a size. The catch: a dark, sooty rock and a bright, icy one of the same brightness can differ in size by roughly a factor of two. That's why size figures are given as a range and clearly labelled as estimates, unless radar or a spacecraft has measured the object directly.
Why do some asteroids pass closer than the Moon?
Space near Earth is busy, and small asteroids are common. Objects a few metres to a few tens of metres across pass inside the Moon's orbit fairly regularly — most are never even noticed because they're tiny and faint. A small rock passing closer than the Moon is routine and harmless; the larger an object is, the rarer close passes become, and the more intensively it's tracked.
What would actually happen if a small asteroid did hit?
It depends entirely on size. Objects up to a few metres across burn up as bright fireballs or meteors and pose no danger — these hit the atmosphere regularly. It takes a much larger and far rarer object to cause damage on the ground. The objects on a close-approach list are, by definition, ones calculated to miss; the planetary-defense effort exists to find the rare hazardous ones early enough to do something about them.
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