Let's take it from the beginning:
What exactly are NEOs? Are they worse than having your debit card refused due to lack of funds?
Where do they come from? Are the large crashing specimens perhaps related to me?
Do these random tumbling blocks pose any real threat to Earth? Is it safer to go to the corner store for smokes at 3 am?
If these things land in my backyard may I become rich and supply my town with power for 3000 year? These questions are now under investigation by planetary scientists. Thanks...but I'm not sure I feel better after hearing that!

Meteoroids, Asteroids, Comets
NEOs are collectively comprised of meteoroids, asteroids and comets. Meteoroids are rocky bodies about 50 meters (164 feet) across or smaller whose impact with Earth is likely to deliver meteorite samples to the surface, but produce no impact crater. The smallest meteoroids, about the size of grains of sand, produce nothing more than the flash across the sky, the result of the meteors encounter with Earth's atmosphere.

Asteroids are rocky bodies larger than 50 meters across. The largest of these known within near-Earth space is 1036 Ganymed, which has an estimated diameter of 41 kilometers (about 25 miles). The second largest is 433 Eros. This 23-kilometer (14-mile) asteroid will be the next target for the Near-Earth Asteroid Rendezvous (NEAR) mission launched in early 1996.

We can distinguish between a comet and an asteroid solely on the basis of the object's appearance through a telescope. If a newly discovered object appears fuzzy, then it is a comet. The "outgassing" that gives a comet its distinctive appearance is the result of ices vaporizing from the object's surface. It is likely that, for many comets, the surface ices evaporate completely after dozens of orbits around the Sun. If and when such extinct comets are found, their star-like appearance leads us to classify them as asteroids, even though they have a cometary origin.

Scientists categorize asteroids in near-Earth space as Atens, Apollos or Amors. Names for each group come from their best known members -- 2062 Aten, 1862 Apollo and 1221 Amor. At present, about 200 near-Earth asteroids have been discovered. There are probably about 10,000 of these objects larger than 0.5 kilometers (0.3 miles) in diameter, suggesting that we have so far detected only a small percentage of their total population. Scientists categorize objects displaying cometary properties according to their orbital period. At present we only know of about 200 short-period comets -- a small fraction of their total population. Although most have orbits that carry them well above and below the plane of Earth's path, approximately 30 short-period comets have low enough inclinations to make them accessible from Earth with modest propulsion systems. Objects in this category include Giacobini-Zinner, the first comet to be visited by a spacecraft -- the International Cometary Explorer (ICE) in 1985 -- and Comet Encke, perhaps the most carefully studied comet after Halley's Comet and Comet Hale-Bopp. The nuclei of these comets, which are about 10 kilometers (6.2 miles) across, may store virtually unaltered material from the early protoplanetary nebula. And one of these materials -- water -- may prove to be a valuable resource for future human space explorers.


Comets are categorized by their orbital periods. One of the more famous short-period comets is Halley's, shown above.
Maintaining the Supply of
Asteroids and Comets
Regardless of their asteroidal or cometary genesis, NEOs are efficiently removed from the inner solar system by collisions or gravitational interactions with the terrestrial planets and the Sun. Because this removal time is short compared with the age of the solar system, the NEO population we see today must be continually resupplied. (Any bodies that remained in the inner solar system immediately after the formation of the planets would have been depleted long ago.) Thus, one or more processes must add new objects to the inner solar system at about the same rate with which they are removed by planetary encounters.

Comets appear to be supplied from both the Oort cloud and the Kuiper belt. The Oort cloud is a hypothesized reservoir of primordial solar system material encircling the Sun more than 1,000 astronomical units (AU, the distance between Earth and the Sun) away. The Kuiper belt is a disk of icy planetesimals residing between 30 and 1,000 AU away. To date, scientists have discovered about two dozen members of this belt.

According to one hypothesis, the fresh supply of asteroids comes from the asteroid belt, the region between Mars and Jupiter where nearly 20,000 objects have known orbits. A process called chaotic dynamics may remove these objects from the asteroid belt and place them on trajectories that bring them near Earth. Suspect supply areas include the famous Kirkwood gaps, highly depicted regions within the main belt that American astronomer Daniel Kirkwood first noted in 1867. The locations of these gaps correspond exactly to positions of mean motion resonances with Jupiter. At these locations, the orbital period of an asteroid is an exact integer ratio of Jupiter's. For example, at the 3 to I resonance, an asteroid completes exactly three revolutions in the time it takes Jupiter to complete one.

Danger from the Skies?
The hazard posed by NEOs is a matter of intense interest, especially since scientists now realize that the mass extinction (which included dinosaurs) at the end of the Cretaceous Period was likely caused by the impact of a huge asteroid or comet. But one of the largest challenges in assessing the severity of the hazard is not knowing how many near-Earth objects there are. By knowing the statistics for the population, we can gauge how frequently a devastating global impact occurs. Perhaps just as important will be an understanding of how frequently smaller collisions occur, such as the explosion that occurred over the Tunguska River of Siberia in 1908.

Surveys sponsored in part by The Planetary Society are just the beginning to achieve a census of the near-Earth population. Larger surveys, such as the Spaceguard Survey, can reveal the full nature of the near-Earth environment. Using the information these surveys collect, we can make careful plans to protect Earth from future cosmic impacts.

Richard P Binzel is the editor of The NEO News and an associate professor of planetary science at the Massachusetts Institute of Technology.

Additional Notes: the orbits shown in the applet are color coded. The planets are white lines, and the asteroid/comet is a blue line. The bright white line indicates the portion of the orbit that is above the ecliptic plane, and the darker portion is below the ecliptic plane. Likewise for the asteroid/comet orbit, the light blue indicates the portion above the ecliptic plane, and the dark blue the portion below the ecliptic plane.

Orbit Viewer applet originally written and kindly provided by Osamu Ajiki (AstroArts),and further modified by Ron Baalke (JPL).