How astronomers categorize near-Earth asteroids by orbital geometry and quantify threats via the Potentially Hazardous Asteroid (PHA) designation.
1. The 4 Dynamical NEO Families
1. **Atiras**: Entirely inside Earth's orbit ($Q < 0.983 \text{ AU}$).
2. **Atens**: Earth-crossing with semi-major axes smaller than Earth's ($a < 1.0 \text{ AU}$, $Q > 0.983 \text{ AU}$).
3. **Apollos**: Earth-crossing with semi-major axes larger than Earth's ($a \ge 1.0 \text{ AU}$, $q \le 1.017 \text{ AU}$).
4. **Amors**: Exterior to Earth's orbit approaching near Mars ($1.017 < q < 1.3 \text{ AU}$).
2. Defining Potentially Hazardous Asteroids (PHAs)
Designated when meeting both strict criteria:
- **Earth Minimum Orbit Intersection Distance (MOID)** $\le 0.05 \text{ AU}$ (~7.48 million km / 19.5 Lunar Distances).
- **Absolute Magnitude** $H \le 22.0$ (roughly $\ge 140 \text{ meters}$ in diameter).
3. Ground & Orbital Early-Warning Surveys
Pan-STARRS, Catalina Sky Survey, NEOWISE, and upcoming infrared satellites (NASA NEO Surveyor) scan the skies to achieve 90% discovery completeness.