Close approaches explained: AU → km → lunar distances

What a ‘close approach’ really means, how to interpret AU, and why lunar distance is a useful shortcut.

When an asteroid’s orbit brings it near Earth’s orbit, catalogs may list close approaches. This does not automatically mean danger — it’s just geometry + time.

Units you will see

Astronomical Unit (AU)

1 AU ≈ 149,597,870 km

Lunar Distance (LD)

1 LD ≈ 384,400 km (average Earth–Moon distance)

We convert AU → km (and sometimes to LD) to make the numbers intuitive.

Relative velocity

Close approach tables often include a relative speed (e.g., km/s). We may also show km/h for readability.

NEO vs PHA (quick definitions)

NEO: a Near-Earth Object (orbit comes relatively close)
PHA: Potentially Hazardous Asteroid (technical threshold based on proximity + estimated size)

These are labels, not predictions.

Keyword focus: “what is lunar distance”, “asteroid close approach meaning”, “NEO vs PHA explained”.