Classification of near-Earth asteroid groups and documented close encounters (1988-2004)

Classification of near-Earth asteroid groups and documented close encounters (1988-2004). Amor asteroids have orbits exterior to Earth but interior to Mars and do not cross Earth's orbit Apollo asteroids are Earth-crossing with semi-major axis larger than 1 AU Atens (A10s) are Earth-crossing with semi-major axes smaller than Earth's

Near-Earth asteroids fall into four groups: Amor (between Earth and Mars), Apollo (Earth-crossing), Aten (Earth-crossing, smaller orbit), and Atira (within Earth's orbit). Scary close calls: 1989's Asclepius zipped within 500,000 miles undetected; 1998's ML14 crossed where Earth had been 18 hours earlier.

Key Facts

• Amor asteroids have orbits exterior to Earth but interior to Mars and do not cross Earth's orbit
• Apollo asteroids are Earth-crossing with semi-major axis larger than 1 AU
• Atens (A10s) are Earth-crossing with semi-major axes smaller than Earth's
• Atira asteroids have orbits entirely within Earth's orbit
• March 22, 1989: Asteroid 4581 Asclepius (half-mile diameter) passed within 500,000 miles, undetected until after closest approach
• August 25, 1989: Asteroid 4769 Castalia (4,600 ft diameter) passed within 2.5 million miles, 28,700 times more massive than Tunguska object
• May 1996: Asteroid 1996 JA1 (700 ft) passed within 279,000 miles, energy release 100x Tunguska (~1,500 megatons)
• August 10, 1998: Asteroid 1998 ML14 (~1 km diameter) crossed Earth's orbit at the exact point Earth had occupied 18 hours earlier
• Estimated 750-900 large asteroids have potential to cause devastation on impact

Source: THE ASTROID EPISODE: The Dangers and Opportunities by The Randall Carlson