Explainer

How close do asteroids
pass Earth?

Most asteroid flybys happen at distances measured in lunar distances - several times the Moon's distance or more. A handful come much closer. Here is how scientists measure these passes and what the numbers mean.

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Where the close approaches happen

Most recorded passes land between the Moon's orbit and the PHA limit. The record holders sit far to the left, closer than our own satellites.

The distance units

Asteroid miss distances are reported in three units, depending on scale. Lunar distances (LD) are the most common for near-Earth passes. One LD is 384,400 km - the average gap between Earth and the Moon. An asteroid at 5 LD passes at five times the Moon's distance.

For passes under 10,000 km, kilometres make more intuitive sense - just as you would measure the gap between two cars in metres rather than miles. Astronomical units (AU) appear in orbital data: one AU is the average Earth-Sun distance, about 150 million km. The boundary for an official "close approach" is 0.05 AU from Earth's orbit.

Distance scale reference

Low Earth orbit (ISS)

Satellites and space stations

~400 km

~0.001 LD

Geostationary orbit

TV and weather satellites

~35,800 km

~0.093 LD

Moon

Reference point for LD unit

384,400 km

1 LD

PHA inner boundary

0.05 AU - monitoring threshold

7,480,000 km

~19.5 LD

Close-approach limit

Outer edge of NeoWs catalogue

7,480,000 km

~19.5 LD

Some of the closest passes on record

The closest confirmed non-impacting pass belongs to asteroid 2023 BU, which flew about 3,600 km above Earth's surface in January 2023. That is closer than many operational satellites. At roughly 4-8 metres across, it was too small to survive entry through the atmosphere even if it had been on a collision course.

A more watched event is coming: Apophis, a 370-metre asteroid, is predicted to pass about 31,000 km from the surface in April 2029 - inside the ring of geostationary satellites at around 35,800 km. That pass will be visible to the naked eye over parts of Europe and Africa. Astronomers rule out an impact on that pass with high confidence.

Asteroid	Date	Distance	Size	Note
2023 BU	January 2023	~0.009 LD ~3,600 km above surface	4–8 m	Closest confirmed non-impacting pass
Apophis	April 2029 (predicted)	~0.08 LD ~31,000 km from surface	~370 m	Will pass inside geostationary orbit
2004 FH	March 2004	~0.11 LD ~43,000 km	~30 m	Well below geostationary orbit
2011 CQ1	February 2011	~0.014 LD ~5,480 km	~1 m	Tiny object, pass closer than most satellites

When does a close pass trigger concern?

Miss distance alone does not determine whether an asteroid is dangerous. An object can pass at 0.5 LD on this orbit and have essentially zero impact probability across the next century - because its orbit never quite intersects Earth's. What matters is the combination of size, orbital geometry, and the range of future encounters.

NASA classifies an asteroid as a potentially hazardous asteroid (PHA) when it is 140 metres or larger and its orbit comes within 0.05 AU of Earth's orbit. That threshold was chosen because objects in that zone can be gravitationally perturbed into Earth-crossing trajectories over long timescales.

For any PHA with a non-trivial impact probability, NASA's CNEOS publishes updated risk tables. The Torino Scale and Palermo Technical Impact Hazard Scale give standardised ways to communicate impact risk. Most known PHAs sit at Torino Scale 0, meaning the likelihood of a collision causing localised destruction is no different from the background rate of random objects falling from the sky.

What is a close approach?

The formal definition and how NASA monitors these events.

What is a near-Earth object?

NEOs, PHAs, and how the classifications work.

Next close approach

Live countdown to the next upcoming flyby.

Asteroids passing Earth in 2026

Full live table of approaches for the current year.

Common questions

What units are used to measure asteroid miss distances?

Three units are common. Lunar distances (LD) are the standard for near-Earth flybys - one LD is 384,400 km, the average Earth-Moon gap. Kilometres are used for close passes and mission planning. Astronomical units (AU) - the average Earth-Sun distance of 150 million km - appear when comparing asteroid orbits at larger scales.

What is the closest an asteroid has ever passed Earth?

The record for a confirmed non-impacting pass belongs to 2023 BU, which flew roughly 3,600 km above Earth's surface in January 2023 - closer than many operational satellites. The object measured an estimated 4-8 metres across. For a larger asteroid, Apophis (about 370 metres wide) is predicted to pass approximately 31,000 km from the surface in April 2029, putting it inside the ring of geostationary satellites.

What miss distance triggers a potentially hazardous classification?

NASA classifies an asteroid as potentially hazardous (PHA) if it is 140 metres or larger and its orbit passes within 0.05 AU of Earth's orbit - roughly 7.5 million km, or 19.5 lunar distances. That boundary is not a danger threshold; it is a monitoring threshold. Objects inside 0.05 AU are tracked more closely because their long-term orbital evolution could, in theory, bring them into a collision path.

What is the difference between miss distance and impact risk?

Miss distance tells you how close the asteroid comes during one specific pass. Impact risk is a probability calculation that looks at hundreds of future encounters over decades. An asteroid can fly past at 2 lunar distances and still have essentially zero long-term impact risk - because its orbit never intersects Earth's. Conversely, an asteroid passing much further away could have a marginally higher long-term probability if its orbit is well-aligned with Earth's.

Can NASA's predicted miss distance change as an asteroid approaches?

Yes, and it usually narrows rather than shifts dramatically. Early predictions carry large uncertainties because only a few observations exist. As more data come in - both new telescope sightings and archived survey images - the orbit is refined. The predicted distance generally stays within the original uncertainty range. If a refinement shifts the predicted path significantly closer to Earth, NASA would announce this publicly through its planetary defence channels.