The Apollo asteroids are a class of asteroids with Earth-crossing orbits. The first Apollo asteroid was discovered in 1918 by Max Wolf observing from Heidelberg, Germany. A table of large Apollo asteroids is contained in Sky & Telescope (March 1990). There are 240 known Apollos (Minor Planet Center), but it is believed that there are at least 2000 Earth-crossers with diameters of 1 km or larger, 100,000 larger than the Rose Bowl, and 70-80 million larger than a typical house (Ostro 1997). One of the largest Apollos is Geographos, which is km in size and was discovered in 1951 as part of the Palomar Observatory Sky Survey (sponsored by the National Geographic Society, hence the asteroid's name).

Earth-crossing asteroids pose a very real (if statistically unlikely) danger to the Earth. An impacting asteroid makes a crate about 10-20 times its size. An asteroid 1 km in size kicks so much dust into the atmosphere that sunlight would be blocked for several years, resulting in a global agricultural catastrophe. Such events are estimated to happen every 100,000 years or so. 10-km asteroids are so large that their impact could result in the destruction of most animal life on Earth (Ostro 1997). The Tunguska event was caused by an asteroid roughly 60 meters across (Ostro 1997). These events happen once every several centuries.

There has recently been a great deal of interest (and concern) in the dangers of an Earth-crossing asteroid hitting the Earth. While the probability of such an event is relatively small, the consequences could be globally catastrophic. After all, a Tunguska-like even occur in Los Angeles or New York would be enough to spoil the day for many millions of people.

For many years, however, there was a pronounced lack of interest in locating Earth-crossing asteroids. An observing program called spacewatch is now underway under the auspices of Drs. Tom Gehrels and Bob McMillan at the Lunar and Planetary Lab of the University of Arizona. At the time this survey was begun, no more than a dozen or so Earth-crossers were known. This number has since been pushed into the hundreds.

In 1986, Carolyn Shoemaker discovered the object 1986JK which passed a mere km from Earth. Asteroid 1989FC (4769 Castalia) passed within 700,000 km of Earth on March 22, 1989. This asteroid was probably between 200 and 5000 m in diameter (Sky & Telescope 1989; Hudson and Ostro 1994). An Earth-crossing asteroid designated 1990MF sped past Earth on July 10, 1990 at a comparatively tiny distance of 3 million miles, or 0.033 AU (IAU Circular number 5050). The asteroid was observed with radar by the Arecibo Ionospheric Observatory, leading to extremely precise estimates of its trajectory. The asteroid was estimated to be 300 to 1,000 feet in diameter. 1991BA, a body m across passed within 800,000 km of Earth (Sky and Telescope 1991). On Dec. 8, 1992, Asteroid 4179 Toutatis passed within 0.024 AU (9.4 lunar distances). It was imaged by Goldstone and Arecibo (Ostro et al. 1995, Hudson and Ostro 1995). It appears to have two lobes, with dimensions km. On Sep. 24, 2004, Toutatis will pass within a scant 0.010360 AU (4 lunar distances) of Earth (Hudson and Ostro 1995). On May 19, 1996, asteroid 1996JA1 passed only slightly outside the orbit of the Moon, and on May 25, 1996JG passed by at 8 lunar distances (Ostro 1997).