What is an asteroid close approach?

A close approach is the moment an asteroid's orbit brings it to its minimum distance from Earth on a given pass. It is a predicted event computed from a known orbit.

How close does an asteroid have to be to count?

There is no single threshold. NASA's close approach tables commonly list passes within 0.05 astronomical units, about 19.5 lunar distances, and trackers highlight passes inside a few lunar distances.

Do close approaches happen often?

Constantly. Dozens of catalogued close approaches occur in a typical week, almost all of them small objects passing far beyond the Moon's orbit.

Asteroid Close Approaches

A close approach is the predicted low point of an asteroid's pass by Earth — a scheduled event, measured precisely, and almost always routine.

When an asteroid's orbit and Earth's orbit bring the two bodies to their minimum separation on a particular pass, astronomers call it a close approach. The word "close" is doing a lot of work: on solar system scales, a pass at five times the Earth–Moon distance is close, while on any human scale it is nearly two million kilometres of empty space.

How a close approach is measured

Four quantities describe any approach, and the Orbitfall tracker shows all of them:

Miss distance — the minimum separation, reported in kilometres, in astronomical units, and in lunar distances.
Approach date and time — in UTC, precise to minutes for well-observed objects.
Relative velocity — how fast the object moves relative to Earth, typically 5 to 30 km/s.
Estimated diameter — a range inferred from brightness, since most of these objects are never resolved as discs.

Why the distance is a prediction, not a measurement

Close approach figures come from propagating a fitted orbit forward in time. An object observed over several oppositions has an orbit known so precisely that its approach distance decades out is accurate to a fraction of a percent. An object discovered three nights ago has a much looser solution, and its predicted miss distance can move as new observations arrive. This is ordinary orbital mechanics, not a sign of danger — see how NASA tracks asteroids.

Which approaches are worth attention

Distance alone is a poor filter. A ten-metre rock passing at 0.3 lunar distances is invisible to almost everyone and harmless; a 400-metre object at 15 lunar distances is a far more significant piece of the near-Earth population. The Orbitfall Index exists to reconcile the two: it weighs proximity, size, velocity, and NASA's potentially hazardous classification into one 0–100 number. This score ranks how notable the asteroid approach is. It is not an impact prediction. The full formula is on the methodology page.

See current close approaches

Live lists are available for asteroids passing Earth today, asteroids passing Earth this week, and near-Earth objects today. For background on where the figures originate, see the NASA asteroid tracker page, and for the terminology, the close approach explainer and glossary.

Frequently asked questions

What is an asteroid close approach?: A close approach is the moment an asteroid's orbit brings it to its minimum distance from Earth on a given pass. It is a predicted event computed from a known orbit.
How close does an asteroid have to be to count?: There is no single threshold. NASA's close approach tables commonly list passes within 0.05 astronomical units, about 19.5 lunar distances, and trackers highlight passes inside a few lunar distances.
Do close approaches happen often?: Constantly. Dozens of catalogued close approaches occur in a typical week, almost all of them small objects passing far beyond the Moon's orbit.

Asteroid guide