Defining Potentially Hazardous Asteroids

The International Astronomical Union has created a category for asteroids that pose a potential threat of an Earth impact that would inflict serious damage. They are called Potentially Hazardous Asteroids (PHAs). PHAs must meet these requirements:

• Their orbits must come within 0.05 au of the orbit of Earth.
• The objects must be at least 140 m in diameter.

If an asteroid comes within 0.05 au of Earth, it is considered close enough that over short time scales (within about one hundred years) its orbit could be altered by the gravitational attraction of a planet so that the asteroid could hit Earth. Most often, Jupiter, because of its large mass, is the planet responsible for altering asteroid orbits.

It’s important to note that, in the PHA definition, the distance of 0.05 au is the distance between the orbits of Earth and the asteroid at their closest approach, not the actual distance between Earth and the asteroid. The closest distance between the two orbits is known as the Minimum Orbit Intersection Distance (MOID).