• NEOs (near Earth objects) are asteroids or comets with an orbit that brings them relatively close to Earth.
• Asteroids originate in the asteroid belt between Mars and Jupiter; comets are formed beyond the limits of our solar system.
• There are over a thousand NEOs with a diameter of 1 km or more and millions of smaller objects.
• Asteroids that reach Earth's surface are termed meteorites and are composed of materials similar to those of our planet's core, mantle, and crust.

Figure 24. The 1.2 km-wide Meteor (Barringer) Crater, near Winslow, Arizona (top), was formed 50,000 years ago by the impact of a meteorite with a diameter of approximately 50 meters. Ceres, the largest asteroid, is approximately 18,000 times larger.

he term near Earth object (NEO) is used to refer to objects such as asteroids or comets that approach Earth. Asteroids originate in the asteroid belt, a relatively dense jumble of cosmic debris that lies in orbit between Mars and Jupiter. The gravitational attraction of nearby Jupiter jostles asteroids from their consistent orbit causing them to crash into one another. These collisions can send small asteroids or crash debris looping through space toward the inner planets. These materials follow  eccentric orbits and can plunge into any of the terrestrial planets, leaving impact scars that can still be observed today (Fig. 24). An asteroid on a course to collide with Earth is termed a meteoroid. The actual object that strikes Earth's surface is termed a meteorite. Therefore, meteorites and asteroids are essentially the same thing, just in different locations. Asteroids range in size from little more than space dust to nearly 1,000 km in diameter (Fig. 24).

Comets originate beyond the margins of our solar system and approach the Sun on wide elliptical orbits (Fig. 25). Like the distant Jovian planets located far from the Sun's heat, much of a comet's mass is composed of ice, probably surrounding a rocky core. The ice evaporates as the comet approaches the interior of the solar system, forming a trailing tail that points away from the Sun in the direction of the solar wind. Although comets do not collide with Earth as frequently as asteroids the consequences of an impact would be just as catastrophic. A mysterious 1908 explosion in Tunguska, Siberia, has been attributed to the air blast of a comet that disintegrated in the atmosphere a few kilometers above the land surface. The blast left no crater but flattened forests over an area of 2,100 square kilometers (840 square miles) and would be sufficient to lay waste to the largest urban areas on Earth.

Figure 25. Meteorites that strike Earth's surface originate in the asteroid belt between Mars and Jupiter. Comets originate beyond our solar system. Comet "tails" are oriented away from the Sun and indicate the direction of the solar wind. Pluto's orbital path is inclined relative to the orbits of the other planets. Diagram not to scale.

A string of up to 20 separate parts of a comet known as Shoemaker-Levy smashed into Jupiter over the span of a week in 1994. This was the first time scientists were able to observe a collision between two bodies in our solar system.

An estimated 100 million kilograms of meteorites strike Earth's atmosphere each year with the bulk of this material in the form of small particles. Frictional heating of these objects as they fall through the atmosphere ensures that most are vaporized well before they can reach Earth's surface. Fortunately, the largest asteroids are not heading for Earth but the impact of an object of less than 100 meters diameter would be sufficient to destroy a large city. A 50-meter (165-foot)-wide meteorite gouged out Meteor Crater, a deep hole over a kilometer (0.6 miles) wide in the Arizona desert. (For more on impact features see Impact Hazards).

Astronomers have estimated that there are approximately a thousand asteroids, with diameters of over 1,000 meters and estimate that there are another million that are 50 meters in diameter or larger. Scientists are currently focusing their detection efforts on the largest NEOs that could cause catastrophic global or continental-scale consequences should they impact Earth (Fig. 26). There is no program to locate small asteroids of less than 1 km diameter because they are too small to detect easily and there are too many to find with current resources. (For more on detection of NEOs see Beware Flying Rocks.)

Figure 26. Proximity of NEOs to Earth and Moon. At least five NEOs have approached Earth more closely than the distance to the Moon in the last decade. The closest approach was by meteorite XM1 that came within 112,000 km (70,000 miles) of Earth in December 1994. All of the NEOs shown are several kilometers in diameter. One astronomical unit is the distance from Earth to the Sun.

Meteorites found on Earth's surface are composed of rocks or metals or some combination of both. Stony meteorites, composed of rocks similar to those found in Earth's crust or mantle, account for over 90% of known meteorites. Six percent of meteorites are made up of a mix of iron and nickel and are known as irons. These metals are thought to form Earth's core. The contrasting composition of meteorites is interpreted to reflect the fact that asteroids are composed of the same materials as the terrestrial planets. A small number of meteorites are composed of rocks similar to those found on the Moon or Mars. These meteorites are thought to have been knocked into orbit by an earlier collision of asteroids with the lunar or martian surfaces.

Figure 27. Animation of Eros tumbling through space. Image courtesy of NASA.

The five-year Near Earth Asteroid Rendezvous mission (NEAR) placed a spacecraft in close orbit with the asteroid known as 433 Eros. The goal of the mission was to learn more about the geology and physical properties of NEOs. Future efforts to destroy or deflect incoming asteroids will require an understanding of the composition and rotation sequence. Some bodies may have larger proportions of metals, others may be little more than rubble piles. Understanding the makeup of asteroids will help scientists better determine how to protect against their collision with Earth. Eros is a large peanut-shaped asteroid, 33 km long (21 miles) and 13 km (8 miles) around. Its surface is pockmarked with craters, some up to 6 km (4 miles) across. Eros is made up of solid rock with density similar to Earth's crust in contrast with the asteroid Mathilde which was visited earlier in the mission and discovered to be little more than a pile of debris. The NEAR mission ended dramatically with the spacecraft landing on the surface of Eros in February 2001. Such maneuvers may be necessary in the future if scientists need to place explosives in key locations on an asteroid on a collision path with Earth.

Think about it . . .

Use the Venn diagram found here to compare and contrast the similarities and differences between planets and asteroids.