What are potentially hazardous asteroids (PHAs) and why are they significant for astronomical discovery efforts?

Potentially hazardous asteroids (PHAs) are dangerous celestial objects that can appear unexpectedly anywhere in the night sky, making their detection both challenging and urgent. These asteroids pose significant threats to Earth, with objects as small as one kilometer in diameter having the potential to cause civilization-ending extinction events. While astronomers have successfully identified over 95% of large asteroids, the ongoing challenge lies in detecting smaller but still dangerous objects that continue to emerge, requiring constant vigilance from the global astronomical community to track and calculate their orbits for planetary defense purposes.