Asteroid

V

Asteroids in the Asteroid Belt

The asteroid belt between the orbits of Mars and Jupiter contains about 98 percent of the known asteroids in the solar system. The belt also includes dust and other small debris caused by collisions among asteroids. A few icy comet-like objects orbit within the asteroid belt as well. These so-called main-belt comets are chemically different from the comets that originate in the outer solar system. The asteroid belt is also home to at least one dwarf planet.

The dwarf planet 1 Ceres is the largest object in the asteroid belt, with a diameter of about 950 km (about 590 mi). Long classified as an asteroid, 1 Ceres contains over a third of the mass of the entire asteroid belt. Unlike an asteroid, however, 1 Ceres has settled into a rounded shape and is thought to have a differentiated internal structure, with a rocky core surrounded by a mantle of material containing water ice.

The next largest objects in the asteroid belt are the asteroids 2 Pallas and 4 Vesta, with diameters of about 530 km (about 329 mi). Other large asteroids in the belt include 10 Hygiea, with a diameter of 408 km (253 mi); 511 Davida, with a diameter of 326 km (202 mi); and 3 Juno, with a diameter of 235 km (150 mi).

VI

Asteroids Outside the Asteroid Belt

In addition to the many thousands that make up the asteroid belt, asteroids are found in other parts of the solar system. In some cases, such nonbelt asteroids may have orbits that date back to the early solar system. However, asteroids can also move out of the asteroid belt, disturbed by collisions or by effects of Jupiter’s gravitation. Light energy from the Sun may also warm asteroids unevenly, making them drift slowly away from their original orbits as they radiate heat back into space.

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The two small moons of Mars, Phobos and Deimos, likely are asteroids captured by that planet’s gravity. Gravitational capture is thought to be a complex process, but the Martian moons have circular, counterclockwise, and equatorial orbits similar to those of regular moons that formed in place around other planets. Astronomers are not certain how Phobos and Deimos ended up with such normal-looking orbits if they are captured asteroids. Some of the small outer moons of Jupiter and Saturn may also be captured asteroids. However, these irregular moons of the giant planets often have elliptical, clockwise (retrograde), and inclined orbits.

The so-called Trojan asteroids lie in two clouds, one moving 60° ahead of Jupiter in its orbit and the other 60° behind. A gravitational balance between the Sun and Jupiter holds these clusters of asteroids in place at spots called Lagrangian points, named after the 18^th century French mathematician Joseph Louis Lagrange. Lagrange predicted such orbits could exist, but astronomers did not discover the first Trojan asteroid until 1906. A few Trojan-type asteroids have been detected at similar Lagrangian points along the orbit of Mars. Astronomers have recently found groups of Trojan asteroids that share Neptune’s orbit, possibly representing a population several times larger than the Jupiter Trojans.

Astronomers recognize a number of groups of Sun-circling asteroids that follow similar orbits within the inner solar system. Asteroids that intersect the orbit of Mars are called Amors; asteroids that intersect the orbit of Earth are known as Apollos; and asteroids that have orbits smaller than Earth’s orbit are called Atens. One of the largest inner asteroids is 433 Eros, an elongated body measuring 13 by 33 km (8 by 21 mi). The peculiar Apollo asteroid 3200 Phaethon, about 5 km (about 3 mi) wide, approaches the Sun more closely, at 20.9 million km (13.9 million mi), than any other known asteroid. It is also associated with the yearly return of the Geminid stream of meteors (see Geminids).

VII

Study and Exploration of Asteroids

The first asteroid was discovered in 1801 by the Italian astronomer Giuseppe Piazzi. He originally thought the object might be a comet, but soon decided it was a planet predicted to exist in the gap between the orbits of Mars and Jupiter according to Bode's Law. Piazzi named the new planet Ceres. When more such objects were discovered in the zone between Mars and Jupiter, they were also classified as planets. However, British astronomer Sir William Herschel coined the term asteroid (meaning “starlike”) in 1802 because the small objects looked more like stars than planets when viewed through a telescope. Astronomers adopted Herschel’s term asteroid a few decades later when they decided such bodies were too small to qualify as planets.

The naming of asteroids and other solar system bodies is now governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name.

Although astronomers once gave mainly classical Greek and Latin names to asteroids, they now often give names that honor famous people in science, history, or the arts. Almost any source for a name is permitted, however. Among the recent asteroid names is 100,000 Astronautica. The name was given in 2007 to honor the 50^th anniversary of the space age begun by Sputnik in 1957 and alludes to the official definition of space as 100,000 meters (100 km) above Earth. Asteroids are usually referred to by both a number and a name in the form 4 Vesta, 2001 Einstein, 4487 Pocahontas, or 8749 Beatles. The new asteroid is also added to the official catalog of minor planets, which lists other asteroids as well as Kuiper Belt Objects (KBOs), centaurs, and dwarf planets. Comets are recorded in a separate catalog of their own.

Astronomers can study asteroids from Earth using telescopes and radar. Detailed information about such small objects, however, requires close-up encounters using space probes. Several Earth-approaching asteroids are relatively easy targets for space missions. In 1991 the United States Galileo space probe, on its way to Jupiter, took the first close-up pictures of an asteroid. The images showed that the small, lopsided body, 951 Gaspra, is pockmarked with craters, and revealed evidence of a blanket of loose, fragmental material, or regolith, covering the asteroid’s surface. Galileo also visited an asteroid named 243 Ida and found that Ida has its own moon, a smaller asteroid subsequently named Dactyl. (Dactyl’s official designation is 243 Ida I, because it is a satellite of Ida.)

In 1996 the National Aeronautics and Space Administration (NASA) launched the Near-Earth Asteroid Rendezvous (NEAR) spacecraft. NEAR was later renamed NEAR Shoemaker in honor of American scientist Eugene M. Shoemaker. NEAR Shoemaker’s goal was to go into orbit around the asteroid Eros. On its way to Eros, the spacecraft visited the asteroid 253 Mathilde in June 1997. At 60 km (37 mi) in diameter, Mathilde is larger than either of the asteroids that Galileo visited. In February 2000, NEAR Shoemaker reached Eros, moved into orbit around the asteroid, and began making observations. The spacecraft orbited the asteroid for a year, gathering data to provide astronomers with a better idea of the origin, composition, and structure of large asteroids. After NEAR Shoemaker’s original mission ended, NASA decided to attempt a “controlled crash” on the surface of Eros. NEAR Shoemaker set down safely on Eros in February 2001—the first spacecraft ever to land on an asteroid.

In 1999 Deep Space 1, a probe NASA designed to test new space technologies, flew by the tiny asteroid 9969 Braille. Measurements taken by Deep Space 1 revealed that the composition of Braille is very similar to that of 4 Vesta. Scientists believe that Braille may be a broken piece of Vesta or that the two asteroids may have formed under similar conditions. NASA’s Stardust spacecraft photographed the 6 km- (3 mi-) wide asteroid 5535 Annefrank as the probe passed through the asteroid belt in 2002 on its way to study comet Wild 2.

The Japanese Hayabusa space probe reached the near-Earth asteroid 25143 Itokawa in September 2005 and orbited the 540-m (1,171-ft)-long body for three months, taking detailed images and studying the asteroid’s mass and surface environment. The images and data indicate that 25143 Itokawa is a loose pile of rubble rather than a solid body and consists of material that ranges in size from sandlike grains up to rocks 50 m (170 ft) wide. The probe also descended to the surface twice to attempt to collect samples of the asteroid. When Hayabusa returns to fly by Earth in 2010, it is scheduled to eject a canister containing the samples. If all goes as planned, the canister will enter the Earth’s atmosphere and land by parachute in Australia, providing the first direct samples of an asteroid.

In September 2007 NASA launched the Dawn spacecraft on the first mission to provide a detailed study of some of the largest objects in the asteroid belt. Dawn uses an ion-propulsion engine powered by two giant solar panels. The spacecraft is expected to orbit 4 Vesta in 2011 then move on to orbit the dwarf planet 1 Ceres in 2015. The two bodies are thought to represent different evolutionary stages in the formation of rocky planets from primitive materials in the early solar system.

The European Space Agency (ESA) has proposed a mission called Don Quijote to study how the path of an asteroid might be changed to avoid striking Earth. The spacecraft would orbit a near-Earth asteroid to collect data while another craft smashes into the object.

A possible manned mission to a near-Earth asteroid has been proposed as the part of the Constellation manned space program under development by NASA. Because the gravity on asteroids is so low, astronauts may need a special tether to keep from floating off the surface. Some scientists have suggested that future space explorers could use asteroids as resources to obtain water, oxygen, fuel, and minerals.

VIII

Asteroids and Earth

Astronomers have found more than 4,000 asteroids with orbits that approach Earth’s orbit. Some scientists project that tens of thousands of these near-Earth asteroids may exist and that as many as 1,000 could be large enough to cause a global catastrophe if they collided with Earth. Still, the chances of such a massive collision average out to only one collision about every 300,000 years.

Many scientists believe that a collision with an asteroid or a comet may have been responsible for at least one mass extinction of life on Earth over the planet’s history. A giant crater at Chicxulub on the Yucatán Peninsula in Mexico marks the spot where an asteroid struck Earth at the end of the Cretaceous Period, about 65 million years ago. This is about the same time as the disappearance of the last of the dinosaurs.

In 1998 scientists reported finding a meteorite fragment in mud from the Pacific Ocean that likely came from the Chicxulub impact, indicating the body was a carbonaceous chondrite asteroid. In 2007 another team of scientists linked the object that struck Earth with a particular family of asteroids in the asteroid belt. Calculations showed that a collision between two very large asteroids about 160 million years ago created the 298 Baptistina asteroid family. A 10 km- (6 mi-) wide chunk of debris from this ancient smashup apparently left the asteroid belt and hit Earth 65 million years ago. Another chunk from the same asteroid family may have blasted the Moon 110 million years ago, forming the Tycho crater.

A collision with an asteroid large enough to cause the Yucatán crater 65 million years ago would have sent so much dust and gas into the atmosphere that sunlight would have been dimmed for months or years. Reactions of gases from the impact with clouds in the atmosphere would have caused massive amounts of acid rain. The acid rain and the lack of sunlight would have killed off plant life and the animals in the food chain that were dependent on plants for survival.

The most recent major encounter between Earth and what may have been an asteroid was a 1908 explosion in the atmosphere above the Tunguska region of Siberia. The force of the Tunguska blast flattened more than 200,000 hectares (500,000 acres) of pine forest and killed thousands of reindeer. The number of human casualties is unknown. The first scientific expedition went to the region two decades later. This expedition and several detailed studies following it found no evidence of an impact crater. This led scientists to believe that the heat generated by friction with the atmosphere as the object plunged toward Earth was great enough to make the object explode before it hit the ground.

If the Tunguska object had exploded in a less remote area, the loss of human life and property could have been astounding. Military satellites—in orbit around Earth watching for explosions that could signal violations of weapons testing treaties—have detected dozens of smaller asteroid explosions in the atmosphere each year. In 1995 NASA, the Jet Propulsion Laboratory, and the U.S. Air Force began a project called Near-Earth Asteroid Tracking (NEAT). NEAT uses an observatory in Hawaii to search for asteroids with orbits that might pose a threat to Earth. By tracking these asteroids, scientists can calculate the asteroids’ precise orbits and project these orbits into the future to determine whether the asteroids will come close to Earth.

Astronomers believe that tracking programs such as NEAT would probably give the world decades or centuries of warning time for any possible asteroid collision. Scientists have suggested several strategies for deflecting asteroids from a collision course with Earth. If the asteroid is very far away, a nuclear warhead could be used to blow it up without much danger of pieces of the asteroid causing significant damage to Earth. Another suggested strategy would be to attach a rocket engine to the asteroid and direct the asteroid off course without breaking it up. Both of these methods require that the asteroid be far from Earth, and assume that the asteroid is a solid body and not a pile of rubble only loosely held together by gravity. If an asteroid exploded close to Earth, chunks of it would probably cause damage. Any effort to push an asteroid off course would also require years to work. Asteroids are much too large for a rocket to push quickly. If astronomers were to discover an asteroid less than ten years away from collision with Earth, new strategies for deflecting the asteroid would probably be needed.

A study published in 2007 reviewed the different methods proposed for changing the course of an asteroid. The top choice was a swarm of mirrors that would hover near the asteroid and focus sunlight on a spot on its surface. Intense heat would vaporize material into gas, creating a small amount of thrust that would change the direction of the asteroid. The number of mirrors needed and the length of time they focused sunlight would depend on the size of the asteroid. Other researchers argue that ramming a small asteroid with a spacecraft would be a more feasible way to change the object’s course. Very small adjustments to the path of the asteroid could then be made by using another spacecraft as a “gravity tug.” The tiny gravitational pull of the spacecraft passing by the asteroid would slightly alter the asteroid’s motion.

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