What Is Lunar Distance?

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). 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.

What does lunar distance mean?

Lunar distance is a unit of length equal to the mean separation between the centers of Earth and the Moon. Because the Moon's orbit is an ellipse, the real separation varies between roughly 356,500 km at perigee and 406,700 km at apogee; the 384,400 km figure is the average, and that average is what the unit fixes. It is a convenience unit, not an SI one: astronomers use it precisely because almost everybody already has an intuition for how far away the Moon is.

How many kilometers is 1 lunar distance?

One lunar distance is 384,400 kilometers — about 238,900 miles, or roughly 30 Earth diameters stacked end to end. Light covers it in about 1.28 seconds. Here is the conversion table most useful when reading asteroid close approaches:

Why asteroid trackers use lunar distances

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. That is why every NASA asteroid tracker view, including the list of near-Earth objects today, reports LD alongside kilometers.

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.

Lunar distance vs astronomical unit

The astronomical unit (AU) is the other distance yardstick you will see in NASA's data. One AU is the average Earth–Sun distance, about 149,600,000 km — equivalent to roughly 389 lunar distances. AU is the right unit for interplanetary scales: Mars sits at about 1.5 AU from the Sun, Jupiter at 5.2 AU. For flybys of Earth, AU values collapse into unreadable decimals — 0.0026 AU means nothing at a glance, while its equivalent, 1 LD, means a great deal. Trackers therefore use AU for orbits and LD for close approaches.

Examples of asteroid close approaches in lunar distances

• 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, and common for small metre-scale rocks.
• 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.

To see real numbers rather than illustrative ones, open asteroids passing Earth today or asteroids passing Earth this week. Every row lists the miss distance in both kilometers and LD.

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. For the terminology behind the event itself, see what a close approach means, and for how the measurements are produced, how NASA tracks asteroids.

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.