What is a potentially
hazardous asteroid?

Two criteria must both be met for an asteroid to be classified as potentially hazardous. First, its estimated diameter must be 140 metres or greater. Second, its minimum orbit intersection distance (MOID) - the closest that the asteroid's orbital path and Earth's orbital path come to each other, at their geometrically nearest points - must be 0.05 AU or less.

An astronomical unit (AU) is approximately 150 million km - the average distance from Earth to the Sun. So 0.05 AU is roughly 7.5 million km. The MOID is a geometric calculation about orbital paths, not a prediction of where the asteroid will actually be at any given time. Two orbital paths can intersect closely on paper even when the asteroid and Earth are on opposite sides of the Sun.

Both conditions must be satisfied simultaneously. A 200-metre asteroid whose orbit never approaches Earth's within 0.1 AU is not a PHA. A 50-metre asteroid whose MOID is 0.001 AU is not a PHA either. The classification only applies when size and proximity criteria are both met.

Below 140 metres, Earth's atmosphere does a significant amount of protective work. Smaller objects lose much of their energy to ablation and deceleration during entry. The result at the surface - if the object reaches it at all - is a localised event: damaging within a limited radius, but not capable of causing regional disruption. Objects below this threshold still warrant tracking but are not categorised as PHAs.

Above 140 metres, a surface impact releases energy comparable to a large nuclear weapon or more. The Tunguska event of 1908 - which flattened an estimated 2,000 square kilometres of Siberian forest - was caused by an object roughly 50 to 80 metres across that did not even reach the ground. A 140-metre object would deliver significantly more energy. At 1 kilometre, the effects become regional. At several kilometres, the comparison shifts to the K-Pg impact 66 million years ago.

The 140-metre threshold is a practical line for resource allocation. Objects above it justify sustained survey and monitoring effort. Those below it receive attention proportional to their approach distance and individual characteristics.

An asteroid whose orbital path never comes within 0.05 AU of Earth's orbit cannot, through ordinary gravitational interactions over human timescales, arrive at Earth's location. The 0.05 AU boundary - approximately 7.5 million km, or roughly 19.5 lunar distances (LD; 1 LD = 384,400 km) - defines the outer edge of the zone where Earth encounter scenarios become physically realistic.

Objects with MOIDs just outside this boundary can still drift closer over centuries as their orbits evolve under gravitational influences. The 0.05 AU threshold is not a hard physical barrier but a practical cut-off point: below it, monitoring is warranted. Above it, the probability of an encounter within any relevant timeframe is negligible.

Being classified as a PHA does not mean an asteroid is heading for Earth. It means the asteroid is large enough to matter, and its orbital path is close enough to Earth's that future encounters cannot be dismissed without careful monitoring. The classification is fixed by orbital geometry - not by impact probability.

All known PHAs are tracked continuously. The Torino Scale - a separate rating from 0 to 10 - communicates current impact probability. Nearly every catalogued PHA sits at Torino Scale 0. No known asteroid is on a collision course with Earth.