Some near-earth objects (NEO) are more dangerous than others. A potentially hazardous object has to be big enough and close enough to be of concern if it impacts the earth.
• Absolute magnitude is a measure of intrinsic brightness of a celestial object. For an asteroid it is the apparent magnitude that the object would have if it were 1 AU from both the sun and the observer.
• H correlates with size. An asteroid with an H >= 22.0 has a diameter >=500 feet (>= 150 m).
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