What Is a Lunar Distance (LD)?

Every asteroid close approach comes with a number: the miss distance. NASA reports it in kilometers, in astronomical units, and — most usefully for everyday reading — in lunar distances (LD).

One lunar distance is 384,400 kilometers: the average distance between the centers of Earth and the Moon. When an asteroid passes at 2 LD, it goes by at roughly twice the Earth–Moon gap. At 0.5 LD, it cuts inside the Moon's orbit.

Why astronomers reach for LD

Kilometers work fine for headlines, but they get unwieldy fast. Saying an asteroid missed by 4,987,000 kilometers conveys very little. Saying it missed by 13 lunar distances is instantly legible: it went by well outside the Moon's orbit, on a scale most people can already picture.

Lunar distances also line up naturally with the thresholds planetary-defense astronomers care about. The Potentially Hazardous Asteroid cutoff, for example, is 0.05 AU — which works out to roughly 19.5 LD.

A quick reference

• 0.0 LD — an impact. This has never happened for a catalogued asteroid larger than a few meters.
• ≤ 0.5 LD — inside the Moon's orbit. Rare and newsworthy for a sizeable asteroid.
• 1 LD — one Earth–Moon gap. Close, but still hundreds of thousands of kilometers of clearance.
• 5 LD — a comfortably wide miss; the typical "close approach" headline range.
• 19.5 LD — the 0.05 AU line used in the PHA classification.
• > 20 LD — routine flyby, well beyond any monitoring threshold.

Some perspective

Even at 1 LD, an asteroid is still passing at roughly 30 Earth diameters away. Close approaches at fractions of a lunar distance are worth writing about, but the sky is a very large place and most "close" objects are, in absolute terms, hundreds of thousands of kilometers off.

In the Orbitfall tracker, LD is the primary unit used to describe miss distance. See how the Orbitfall Index combines miss distance with size, velocity, and the hazard flag to rank how notable each approach is.