What defines a potentially hazardous asteroid (PHA) and why are these celestial bodies scientifically important?

Potentially hazardous asteroids (PHAs) are defined based on their potential to make threatening close approaches to Earth. Specifically, asteroids with a minimum orbit intersection distance of 0.05 astronomical units (approximately 7.5 million kilometers) or less are classified as PHAs. An astronomical unit equals about 150 million kilometers, roughly the distance from Earth to the Sun, according to NASA's Jet Propulsion Laboratory. The scientific interest in these objects stems from their status as relatively unchanged remnant debris from the solar system's formation process. They provide valuable insights into the early development of our planetary system, serving as cosmic time capsules that have remained largely unaltered since the solar system's birth.