Asteroids

Asteroids are minor bodies of various sizes in orbit around the Sun, primarily but not exclusively located between the orbits of Mars and Jupiter. They provide important clues regarding the solar system's early history, including the effect of their collisions on the surfaces of planets or their satellites. A class popularly referred to as near-Earth asteroids threatens to impact Earth.

Overview

Although the discovery of the first asteroid was accidental, it was no surprise to the astronomical community of the day. In 1766, German astronomer Johann Titius (1729-1796) observed that the positions of the planets could be approximated very closely by a simple empirical rule. Adding 4 to each number in the sequence {0, 3, 6, 12, 24, 48…} and dividing the sum by ten yields the mean planetary distances from the Sun in astronomical units (the distance from the Earth to the Sun is one astronomical unit or one AU). The exception to this rule is the fifth element in that purely mathematical sequence, where an apparent gap occurs at 2.8 AU. However, this is just an empirical observation with no known physical basis. Johann Bode (1747-1826) publicized this rule and led to a search for a missing planet in the gap between Mars at 1.5 AU and Jupiter at 5.2 AU.

On January 1, 1801, the Sicilian astronomer-monk Giuseppe Piazzi (1746-1826) accidentally discovered a moving object during a routine star survey. He named it Ceres for the patron goddess of Sicily. Soon, its orbit was calculated by Carl Friedrich Gauss (1777-1855). At 2.77 AU, Ceres was found, coincidentally, to conform closely to the Titius-Bode rule. However, the search continued since Ceres seemed too small to be classified as a planet.

In March 1802, German astronomer Heinrich Olbers (1758-1840) found a second minor body at the same predicted distance. He named it Pallas. In 1803, Olbers proposed that meteorites come from an exploded planet near 2.8 AU. This possibility led to a continued search, resulting in the discovery of Juno in 1804 and Vesta in 1807. The latter discovery again was made by Olbers. It took quite some time for a fifth small body to be discovered (in 1845), but by 1890, the total had reached three hundred. These bodies came to be called “asteroids” for their faint, starlike images. In 1891, German astronomer Max Wolf (1863-1932) began using a long-exposure camera to detect asteroids. Since then, thousands of asteroids have been registered in the official catalog of the Institute of Theoretical Astronomy in Leningrad.

Asteroids are usually referred to officially by both a number and a name, such as 3 Juno or 1,000 Piazzi. About one hundred newly numbered asteroids are cataloged each year. Sky surveys indicate as many as 500,000 asteroids large enough to appear in telescopic photographs. Most asteroids are within the central belt, extending from 2.1 to 3.4 AU, and about half are between 2.75 and 2.85 AU. Asteroids revolve around the Sun in the same direction as the planets but tend to have more elliptical orbits. Their orbits are inclined up to 30° to the ecliptic plane but far less eccentric than comet orbits. The most minor asteroids are a few kilometers wide; the largest, 1 Ceres (now considered a dwarf planet), is about 1,000 kilometers wide. In 1867, American astronomer Daniel Kirkwood (1814-1895) discovered gaps in the asteroid belt where relatively few asteroids are found. These so-called Kirkwood gaps occur where asteroids have orbital periods that are simple fractions of the twelve-year revolution period of the giant planet Jupiter about the Sun, resulting in periodic gravitational influences called resonances. Such depletions occur, for example, at about 3.3 AU (where the periods have a six-year, 1:2 resonance with Jupiter) and 2.5 AU (a four-year, 1:3 resonance); other resonances, however, act to stabilize certain asteroids, such as the Hilda group at 4 AU (2:3 resonance), which is named for 153 Hilda.

Some asteroids have orbits departing greatly from the main belt. French mathematician Joseph Lagrange (1736-1813) showed that points in Jupiter’s orbit 60° ahead of and behind the planet are gravitationally stable (1:1 resonance). In 1906, Max Wolf discovered the first so-called Trojan asteroid asteroid, 588 Achilles, at the Lagrangian point 60° ahead of Jupiter. Subsequent discoveries have revealed several hundred Trojan asteroids. Those ahead of Jupiter are named for Greek heroes, and those behind are named for Trojan heroes; there is one Greek spy (617 Patroclus asteroid) in the Trojan group and one Trojan spy (624 Hektor) in the Greek group. Hektor is the largest known Trojan asteroid, at about 150 by 300 kilometers, and is the most elongated of the more massive asteroids. At least two objects have orbits that extend beyond Jupiter: 944 Hidalgo, which may be a burned-out cometary nucleus, and 2060 Chiron, whose orbit extends beyond Saturn.

Some asteroids depart from the main belt over only part of their orbit. Mars-crossing Amor group bodies have elongated orbits that carry them inside Mars’s orbit but keep them well outside Earth’s orbit. The Martian satellites Phobos and Deimos have long been suspected by many to be captured asteroids, perhaps from this group. Apollo group members come into Earth’s orbit. (The groups were named for their first examples, discovered in 1932.) Estimates indicate about thirteen hundred Apollos ranging from 0.4 to 10 kilometers across, with an estimated average Earth-collision rate of about one in 250,000 years. The closest known approaches were Hermes, in 1937, at about 780,000 kilometers, and 1566 Icarus, in 1968, at about six million kilometers. Smaller Apollos may be an essential source of meteorites and 100-meter objects capable of making a one-kilometer crater strike Earth about every two thousand years. Aten-type asteroids are Earth-crossers with elliptical orbits smaller than Earth’s. Some asteroids appear to be grouped in families that may be the fragments resulting from an earlier collision between asteroids.

Asteroids' chemical and physical characteristics are mostly determined by remote sensing techniques that study electromagnetic radiation reflected off their surfaces. Remote sensing and radar astronomical techniques have studied more than five hundred asteroids. These studies have indicated asteroidal compositions similar to those of meteorites. Comparison with reflected light from meteorites suggests several classes. Rare E-type asteroids possess the highest albedo (23 to 45 percent reflection). They appear to be related to enstatite (a magnesium silicate mineral) chondrites and are concentrated near the inner edge of the main belt. About 17 percent of asteroids are S-type, which have relatively high albedos (7 to 23 percent) and appear reddish. They likely are related to stony chondrites, are found in the inner to central regions of the main belt, and generally range in size from 100 to 200 kilometers. The largest S-type is 3 Juno, at about 250 kilometers in diameter. More minor Apollo asteroids are also in this category. A few asteroids in the middle belt are classified as M-type since their reflected light (7 to 20 percent) reveals evidence of large amounts of nickel-iron metals on their surface, similar to iron or stony-iron meteorites.

About three-quarters of all asteroids are C-type, having relatively low albedos (2 to 7 percent) and grayish colors similar to Earth’s Moon. They are found in the outer belt and among the Trojans. They resemble carbonaceous chondrite meteorites, containing water-bearing silicate-based and carbon-based minerals and some organic compounds (about 1 percent). The largest of all the asteroids, 1 Ceres (now considered a dwarf planet), is in this category. Some evidence supports the claim that Ceres has a mixture of ice and carbonaceous minerals on its surface. Dark, reddish, D-type asteroids have similar albedos in the same regions.

About 10 percent of asteroids remain unclassified and are designated as U-type. In general, asteroids with low-temperature volatile materials lie farther from the Sun. In contrast, those in the inner part of the main belt are richer in high-temperature minerals, displaying little evidence of volatile water and carbon compounds.

Many asteroids exhibit periodic variations in brightness that suggest irregular shapes and rotation. Their measured rotational periods range from about three to thirty hours. There is some evidence that S-type asteroids rotate faster than C-type asteroids but more slowly than M-type asteroids. Large asteroids (greater than 120 kilometers) rotate more slowly with increasing size, but small asteroids rotate more slowly with decreasing size, suggesting that large asteroids may be primordial bodies, while smaller ones may be fragments produced by collisions. Calculations show that rotation rates longer than two hours produce centripetal forces weaker than gravity, which indicates that loose debris can exist on the surface of even the fastest known rotating asteroid, the Apollo object 1566 Icarus, which has a 2.25-hour rotation rate. Polarization studies of light reflected from asteroids indicate that many have dusty surfaces.

Named after the Greek god of love, Eros is an S-type asteroid belonging to the Amor group. As the second-largest of the near-Earth asteroids, it is larger than the asteroid generally accepted to have been responsible for the extinction of the dinosaurs that impacted Earth near the Yucatán peninsula sixty-five million years ago. Eros is thirteen by thirteen by thirty-three kilometers in size. It was the first asteroid recognized to approach inside the orbit of Mars, and in 1975, it became the first asteroid to be studied with Earth-based radars.

Computer models suggest the possibility that larger asteroids have a deep layer of dust and rock fragments (or regolith) similar to that on the surface of the Moon. Asteroids with diameters larger than 100 kilometers are believed to have undergone a process of differentiation in which heavier metals sank to the core, leaving a stony surface of lighter materials later pulverized by collisions to form a layer of dust.

Asteroid elongations can be estimated from the change in brightness, which can vary by a factor of three or more. For example, radar evidence indicates the unusual Trojan asteroid 624 Hektor (150 by 300 kilometers) may be a dumbbell-shaped double asteroid. Kilometer-scale asteroids have been observed with lengths up to six times greater than their width. Main-belt asteroids tend to be less elongated than Mars-crossers of the same size, perhaps because of more significant erosion from collisions in the belt. Asteroids larger than 400 kilometers tend to be more spherical since their gravitational attractions exceed the strength of their rocky materials, causing deformation and plastic flow into a more symmetric shape.

Sometimes, an asteroid’s size can be accurately determined by timing its passage in front of a star in a stellar occultation. In a few cases, celestial light has been occulted more than once in a single passage, indicating that asteroids may possess satellites. Radar-based studies have confirmed this theory. Also, as the Galileo spacecraft flew through the main belt on its way to enter orbit in the Jupiter system, it imaged a satellite revolving around an asteroid. The irregularly shaped asteroid Ida was discovered to have a small satellite later named Dactyl. Ida is a member of the Koronis family and of S-type, which is fifty-six by twenty-four by twenty-one kilometers in size and rotates once around its own axis every 278 minutes. Dactyl is only 1.2 by 1.4 by 1.6 kilometers and is also of S-type. This strongly suggests that it was created when a larger asteroid smashed into Ida. Previously, the Galileo spacecraft had also provided the first close-up images of an asteroid when it passed within five thousand kilometers of the nineteen by twelve by eleven kilometer-sized S-type asteroid Gaspra on October 29, 1991. Gaspra has an irregular shape, one resembling a potato.

The distribution of asteroid sizes and masses supports the idea that many have undergone a process of fragmentation. Typical relative velocities of encounter, about five kilometers per second in the main belt, are adequate to fragment most asteroids. Ceres contains nearly half the mass of all the asteroids, but it is more than three times smaller than the Moon and about fifty times less massive. About 80 percent of the total mass of all asteroids is in the four largest ones, and only about ten are larger than 300 kilometers. Studies suggest that the main belt was several times more massive, but the smallest dust particles were removed by radiation pressure from the Sun in the fragmentation process.

Interest in asteroids increased when strong evidence was advanced to solve the mystery of the demise of the dinosaurs sixty-five million years ago. Physicist Luis Alvarez and his geologist son Walter sampled the worldwide clay layer that marks the end of the Cretaceous period and the start of the Tertiary period (the so-called K-T boundary, which essentially marks the demarcation between the age of dinosaurs and the rise of mammals within the fossil record). This thin layer of clay is enriched in the rare elements of iridium and osmium, having levels more akin to asteroids than Earthly materials. Thus, the impact theory for killing off the dinosaurs was proposed and largely accepted except by certain portions of the paleontology community. That is until a crater dated to sixty-five million years was discovered off the coast of the Yucatán peninsula. Some still insist that more than an asteroid impact was necessary to account for the observed diminishment of dinosaur species leading up to the extinction event sixty-five million years ago. However, most of the scientific community has come to accept the asteroid impact theory, at least as the principal cause of the sudden mass extinction at the end of the Cretaceous period. This event marks the boundary between the Cretaceous and Tertiary periods, often called the K-T event.

This spurred interest in asteroid and comet impacts, causing extreme environmental damage to the Earth at other times in the past, along with a desire to search for near-Earth asteroids that might represent a threat in the future. Twenty-five years after the proposal that an asteroid impact killed the dinosaurs received initial lukewarm acceptance by paleontologists, some researchers proposed that an even more giant asteroid (or comet) impact was responsible for the so-called Great Dying, the mass extinction at the end of the Permian period that closed out the Paleozoic era. At the end of the Permian 248 million years ago, more than 95 percent of all species died off rather suddenly; life nearly did not make it into the Mesozoic era, during which the dinosaurs eventually arose to dominance.

Researchers point to a large crater in the Antarctic (1.5 kilometers under the ice pack that dates to the time of the Permian mass-extinction event) as well as heavily jumbled areas in Siberia (known as the Siberian Traps) that might have received tremendous seismic energy after impact energy would have undergone antipodal focusing off Earth’s core. The Siberian Traps were also areas of tremendous volcanic activity at the end of the Permian period. Was this coincidental or the result of an impact with antipodal focusing of seismic energy? In 2008, this theory remained highly speculative rather than enjoying the widespread acceptance of the K-T event that killed the dinosaurs. However, if the theory is correct, such an event underscores the danger posed by asteroid and comet impacts on Earth.

The impact of even a small asteroid could pose a tremendous threat to human civilization. Throughout the 1990s and the early twenty-first century, several newly discovered near-Earth asteroids were thought to have a significant chance of hitting Earth. However, in each of those cases, additional observations refined the asteroid’s orbit to the point where it was clear it would not hit Earth after all. There remained one major exception, however. Discovery of the asteroid 99942 Apophis, a member of the Aten group, led to major concern beginning in late 2004 that this 350-meter-across rock had a relatively worrisome potential to impact Earth in April 2029. Precise observations of Apophis’s orbit, ranging from 0.746 AU to 1.099 AU, dramatically lowered the probability that it would strike the Earth. However, Apophis would indeed pass within the altitude of geosynchronous satellites, less than 36,000 kilometers from Earth’s surface. If Apophis passed within a special corridor only 400 meters across, gravitational influences could cause it to return and strike the Earth on Friday, April 13, 2036.

The Torino scale assesses the relative impact hazard of an asteroid impact. For a time after its discovery, Apophis rated a level 4 on the Torino scale, the highest threat level. Further orbit refinements lowered the threat assessment to a level 0 threat, but after realizing the possible return in 2036, it was raised to Level 1. Although Apophis will come very close to Earth in 2029, refinement of available orbital data has since determined that the chances of Apophis hitting the Earth in 2036 are more comforting: less than 1 in 45,000. The 2036 encounter will set up another close encounter the following year, but the chances that this would result in an Earth impact are calculated to be less than 1 in 12.3 million. Nevertheless, Apophis points out the critical necessity for close monitoring of asteroids, particularly those near Earth, and the development of asteroid deflection or destruction to preserve Earth’s biosphere and save human civilization. This sort of natural megadisaster is one of the few that humans have the potential to mitigate or prevent if scientists take action once they identify a threat.

Hollywood has even taken notice of the asteroid or comet impact threat to Earth. Several scientifically incorrect action movies were produced, some popularly received. Many of these movies, such as Armageddon (1998), portray the use of some type of nuclear device as the only viable way to avert an asteroid impact. In many real cases, nuclear explosions detonated within, on the surface of, or close to asteroids either would be insufficient or could merely fragment it so badly that an even worse situation, a swarm of impacting bodies, might ensue. Acknowledging this threat, NASA launched its first test of the agency's planetary defense technology, called the Double Asteroid Redirection Test (DART) Mission, in 2022. The mission targeted the asteroid Didymos and its moonlet Dimorphos in an attempt to alter its path. Though this asteroid and its Moon were not a threat to Earth based on its 2022 trajectory, the data collected from the mission's test has applications for a future crisis.

Methods of Study

Studies of asteroids hold the potential for expanding our understanding of the formation of bodies of sizes between the smallest objects and full planets and could lead to the development of technology to prevent an impact that might devastate life on Earth and even wipe out civilization.

The Galileo spacecraft passed near enough to two asteroids to photograph them directly. The NEAR spacecraft orbited Eros and later landed on its surface, providing close-up photographs of an asteroidal surface. For the most part, however, indirect methods of remote sensing must be used to determine asteroidal properties by studying the reflected electromagnetic radiation that comes from their surfaces. These methods include photometry, infrared radiometry, colorimetry, spectroscopy, polarimetry, and radar detection. They can be augmented by comparative studies with meteorites, whose composition and structure can be analyzed by direct methods in the laboratory. Such methods include chemical, spectroscopic, and microscopic analysis, and processes of fragmentation can be studied by producing high-speed collisions between comparable materials in the laboratory. Such comparative studies must recognize various differences between meteorites and asteroids. The masses of only the three largest asteroids have been determined from their gravitational effects on other bodies; their densities are between 2.3 and 3.3 grams per cubic centimeter.

Photometry is the study of how various surfaces scatter light. Photoelectric observations can measure the brightness of reflected sunlight from asteroids to determine their rotation periods and approximate shapes. One test of this method was made in 1931 when the Amor asteroid 433 Eros came close enough (twenty-three million kilometers) for scientists to observe the tumbling motion of this elongated object (seven by nineteen by thirty kilometers) and to confirm its 5.3-hour rotation. The asteroid’s size can be estimated from its brightness, distance, orbital position, and albedo. The albedo is vital since a bright, small object may reflect as much light as a dark, large object. Since a dark object absorbs more heat than a light object, albedos can be determined by comparing reflected light with thermal radiation measured by infrared radiometry. Photometric measurements also give information on surface textures. Colorimetry involves measuring the range of wavelengths in the reflected light to determine surface colors. Most asteroids are either fairly bright, reddish objects (with albedos of up to 23 percent) mainly composed of silicate-type materials or grayish objects, at least as dark as the Moon (11 percent albedo), consisting of carbonaceous materials.

Spectroscopy is the spectral analysis of light and can be used to infer the composition of many asteroids. Optical and infrared reflectance spectra exhibit absorption bands at characteristic frequencies for given materials. An asteroid’s surface composition is determined by comparing its spectrum with the spectra of light reflected from meteorites of known composition. Examples of this method applied to U-type (unclassified) asteroids include the identification of the silicate mineral pyroxene in the infrared spectrum of Apollo asteroid 1685 Toro and the matching of the surface of Vesta with a basaltic achondrite that resembles lava. Most asteroids appear to have unmelted surfaces with little evidence of lava eruptions. About two-thirds of the Trojans are D-type asteroids with no known meteorite counterparts because of their distance from Earth. Their spectra have matched those of coal-tar residues, suggesting possible organic compounds.

Polarimetry uses measurements of the alignment of electric field vibrations of the reflected sunlight and its variation with direction to estimate albedos. Polarization measurements have also been interpreted as evidence for dust-covered surfaces, but they leave uncertainty about the depth of the dust layer. Radar observations of Eros during a close approach to Earth in 1975 were made at a wavelength of 3.8 centimeters and indicated that the surface must be rough on a scale of centimeters. Since optical polarimetry suggests that Eros is dusty, the radar results imply that the dust must be too thin to smooth rock outcrops of more than a few centimeters. Radar measurements also provided independent estimates of the size of Eros, confirming photometric estimates of its dimensions. The NEAR spacecraft confirmed these observations.

As spacecraft results such as this demonstrate, the best method to study asteroids is using a space probe. When Pioneers 10 and 11 passed through the asteroid belt, scientists found that it had no more dust than any other part of the solar system. The Galileo probe encountered Gaspra in 1991 and Ida in 1993, both S-type asteroids. The probe determined the masses, sizes, and shapes. On its way toward orbit around Saturn, the Cassini spacecraft flew through the asteroid belt and passed asteroid 2685 Masursky at a distance of 1.6 million kilometers. Named after the famed planetary scientist Hal Masursky, this body was a little-understood fifteen-by-twenty-kilometer asteroid before Cassini’s encounter.

Before the Pioneer 10 and 11 passages, there were serious concerns that spacecraft might not be able to pass safely through the main asteroid belt. Much has been learned about the density of material in the belt since the space age began. Thus far, no spacecraft sent into the belt has experienced serious damage from an impact with asteroidal material or an actual asteroid body. Minor hits on dust detectors have been recorded, however. Robotic spacecraft investigations have provided much information about the nature of the various types of asteroids, as has analysis of meteorites found on Earth that are believed to have come from certain asteroids.

The NEAR spacecraft was launched on February 17, 1996, and was directed toward a rendezvous with the asteroid Eros three years later. Eighteen months out from Earth, NEAR flew by the asteroid Mathilde. It successfully reached Eros, and for over a year, NEAR orbited Eros at varying altitudes, providing high-resolution images of the surface of this S-type asteroid. After completing its primary mission, NEAR gently touched on Eros on February 12, 2001. A total of sixty-nine high-resolution images of the asteroid’s surface were taken on the way down during a soft-contact landing. The final picture was taken at 130 meters and covered an area of six meters by six meters. Within that final frame was a portion of a four-meter-wide boulder and evidence of a dusty surface pocked with small rocks and tiny craters. Much to the surprise of the Johns Hopkins UniversityApplied Physics Laboratory research team controlling the spacecraft, NEAR survived its landing and transmitted data back to Earth for two weeks before falling silent. The team was lucky that the spacecraft’s antenna pointed toward Earth and the solar arrays faced partially toward the Sun after impact.

The next step in spacecraft-based investigations of asteroids is the Dawn mission, a robotic probe designed to orbit two different bodies. Dawn’s mission is to visit the two largest asteroids, Ceres and Vesta. By comparison with Ceres, the asteroid upon which NEAR settled was a tiny speck. Ceres is a spherical body with a diameter of 960 kilometers. Indeed, under an official review of classification for solar-system objects, Ceres is now officially designated a dwarf planet—a characterization it shares (much to the displeasure of many in the scientific community) with Pluto, which was demoted from full planet status to that of a dwarf. To accomplish its mission on a minimum of propellant, Dawn is outfitted with ion propulsion similar to that demonstrated by the Deep Space 1 spacecraft. To achieve its science goals, Dawn is outfitted with a framing camera, a mapping spectrometer, and a gamma-ray and neutron spectrometer. The goal is to image the surface of these two large asteroids and to determine their composition.

Dawn launched on September 27, 2007, and was set up for a gravity assist from Mars in early 2009. Arrival at Vesta was planned for September 2011. The ion propulsion system would then break Dawn out of Vesta orbit in April 2012 and send the spacecraft toward a rendezvous with Ceres in February 2015. Assuming the spacecraft remains healthy and propellant is available when the primary mission ended in July 2015, Dawn could be redirected to other asteroids within reach.

Samples of rocks believed to come from various portions of the asteroid belt fall on Earth regularly and have been subjected to intense study. The next step in asteroid investigation would be to return pristine samples of asteroids so the asteroid samples are not altered on their outer layers by passage through Earth’s atmosphere. A robotic mission to collect and then return samples from an asteroid is possible with contemporary technology.

The greatest potential for insight into the nature of asteroids is a human expedition to such a body. Shortly after adopting the Vision for Space Exploration in 2004, the National Aeronautics and Space Administration (NASA) entertained the possibility of sending a crewed Orion Crew Exploration vehicle into deep space for a rendezvous with an asteroid. Regarding propulsion requirements, sending a piloted spacecraft to a near-Earth asteroid is slightly less intensive than to the Moon. Such a mission could take at least six months to reach a target and up to another year to return to Earth. It could return large amounts of carefully selected asteroid samples to Earth for detailed analysis. The potential for gathering information that might be used someday to divert an asteroid threatening to impact Earth would be tremendous.

Context

Asteroids usually cannot be seen with the unaided eye, but they provide important clues for understanding planet formation. They can have significant effects on the Earth and have had such an impact during its history. It was once assumed that the breakup of a planet between Mars and Jupiter formed the asteroid belt. However, the combined mass in the belt is much less than that of any planet (only 0.04 percent of Earth’s mass), and the observed differences in the composition of asteroids at different locations in the belt make it unlikely that they all came from the same planet-sized object. It now appears that asteroids are original debris left over after planet formation and have undergone complex processes such as collisions, fragmentation, and heating. Intense tidal forces caused by Jupiter’s enormous mass prevented small bodies between it and Mars from combining to form a single planet in their region.

Asteroids are among the oldest objects in the solar system, left over from the time immediately before planet formation concluded. Studies of these objects should provide clues to the structure and composition of the primitive solar nebulae. Different types of asteroids found in other regions of the solar system support the theory of planetesimal origin through a sequence of condensation from a nebular disk around the Sun. Asteroids farther from the Sun, beyond the main belt, may have contained more ice; those that formed closer, within the belt, may have been primarily stony or stony-iron materials. Some of these planetesimal precursors of asteroids were probably perturbed during close passes by neighboring planets into elongated Apollo-like orbits that cross Earth’s orbit. Other objects on similar orbits may have been comets that remained in the inner solar system long enough to lose their volatile ices by evaporation. Processes of collision and fragmentation among these objects provide direct evidence about the earliest forms of matter.

Special interest in Apollo asteroids arises from their potential for Earth collisions. Objects as small as 100 meters hit Earth about once every two thousand years, and the 30 percent that fall on land can produce craters a kilometer in diameter. Such impacts would devastate much wider areas with their shock waves, and dust thrown into the upper atmosphere could have marked effects on climate. Growing evidence suggests that asteroid collisions in the past might have contributed to major extinctions of species, such as the dinosaurs, and perhaps even caused reversals of Earth’s magnetism. Thiridium layersium, often found in meteorites, have been identified in Earth’s crust at layers corresponding to such extinctions. Satellite photography has revealed about one hundred apparent impact craters on Earth with diameters up to 140 kilometers. Many more likely succumbed to processes of erosion. Knowledge of Apollo orbits might make it possible to avoid such collisions.

Asteroids also offer the possibility of recovering resources with great economic potential. Some contain great quantities of nickel-iron alloys and other scarce elements; others may yield water, hydrogen, and other materials useful for space-based construction. Estimates of the economic value of a kilometer-sized asteroid reach as high as several trillion dollars. A well-designed approach to space mining might someday help to take pressure off Earth’s ecosystem by providing an alternative to dwindling resources, and space-borne manufacturing centers might alleviate pollution on Earth.

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