Asteroids

Asteroid 253 Mathilde, a Near-Earth Asteroid photographed by NASA's NEAR (Near Earth Asteroid Rendezvous) mission in June 1997. Mathilde is about 60 km in diameter and orbits in the asteroid belt between Mars and Jupiter.

Asteroids are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. None of the asteroids have atmospheres.

Asteroids are also known as planetoids or minor planets.

THE ASTEROID BELT

The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370 million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroid belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years.

The strong gravitational force of the planet Jupiter shepherds the asteroid belt, pulling the asteroids away from the Sun, keeping them from careening into the inner planets.

THE KIRKWOOD GAPS

The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled towards Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that orbit. There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866.

HOW MANY ASTEROIDS ARE THERE?

Gaspra, Asteroid #951.

There are about 40,000 known asteroids that are over 0.5 miles (1 km) in diameter in the asteroid belt. About 3,000 asteroids have been cataloged. There are many more smaller asteroids. The first one discovered (and the biggest) is named Ceres; it was discovered in 1801.

THE SIZES OF ASTEROIDS

Asteroid 4 Vesta, the brightest asteroid and the fourth largest. Vesta is the only asteroid that can be seen without a telescope (it is sixth magnitude).

Asteroids range in size from tiny pebbles to about 578 miles (930 kilometers) in diameter (Ceres). Sixteen of the 3,000 known asteroids are over 150 miles (240 km) in diameter. Some asteroids even have orbiting moons.

CERES: THE LARGEST ASTEROID

Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by the Italian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is so huge in comparison with the other asteroids that its mass is equal to over one-third of the 2.3 x 10²¹ kg estimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) in diameter. Ceres is now considered to be a dwarf planet.

ASTEROIDS BECOMING MOONS

The asteroid 243 Ida and its tiny asteroid moon, Dactyl. This is the first asteroid ever found with an orbiting moon. Ida's dimensions are about 56 x 24 x 21 kilometers (35 x 15 x 13 miles). Dactyl is only about 1.2 x 1.4 x 1.6 km (0.75 x 0.87 x 1 mile) across.

Asteroids can be pulled out of their solar orbit by the gravitational pull of a planet. They would then orbit that planet instead of orbiting the Sun.

Astronomers theorize that the two moons of Mars, Phobos and Deimos, are captured asteroids.

ORIGIN OF THE ASTEROID BELT

The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of all the asteroids is about 2.3 x 10²¹ kg ); our moon's mass is 7.35 x 10²² kg; the asteroids' mass combined is about 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet.

TROJAN ASTEROIDS

Trojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planet's orbit, either trailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planet balance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the first Trojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; those following Jupiter in its orbit were named for Trojan heroes.

NEAR-EARTH ASTEROIDS

 

Eros, Asteroid #433, is an elongated Near-Earth Asteroid which is 21 by 8 by 8 miles (33 by 13 by 13 kilometers). The density of Eros is 2.4 grams per cubic centimeter, roughly the same as the density of Earth's crust.

Asteroids whose orbits bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are called Near-Earth Asteroids (NEA) or Earth-Approaching asteroids. These asteroids probably came from the main asteroid belt, but were jolted from the belt by collisions or by interactions with other objects' gravitational fields (primarily Jupiter).

About 250 NEAs have been found so far, but many, many more exist. The largest known NEA is 1036 Ganymede, with a diameter of 25.5 miles (41 kilometers). According to astronomers there are at least 1,000 NEA's whose diameter is greater than 0.6 miles (1 kilometer) and which could do catastrophic damage to the Earth. Even smaller NEA's could cause substantial destruction if they were to collide with the Earth.

There are three types of NEA's:

• Amors (named for asteroid Amor, #1221): Asteroids which orbit between the orbits of Earth and Mars. Eros (#433) is an Amor.
• Apollos (named for asteroid Apollo, #1862): Asteroids which cross Earth's orbit and have a period longer than 1 year. They have semimajor axes greater than 1 astronomical unit (AU), and perihelion distances less than 1.017 AU. Geographos is an Apollo.
• Atens (named for asteroid Aten, #2062): Atens are asteroids that are always closer to the Sun than the Earth is; they have a period shorter than 1 year (the semi-major axis is smaller than Earth's). Ra-Shalom is an Aten.

Cruithne is an asteroid about 3 miles (5 kilometers) in diameter that is co-orbital with the Earth, which means that it shares roughly the same orbit as the Earth. It is a Near-Earth asteroid (NEA 3753). From the Earth, it appears to have a horseshoe-shaped orbit; it gets close to the Earth, then it moves away again. Its orbit is highly inclined to Earth's orbit. At its closest approach (which happens every 100,000 years), Cruithne comes to within 10 million miles (15 million km) of Earth (40 times the distance from the Earth to the Moon). Cruithne was named for the first Celtic tribal group that settled in the British Isles. Cruithne was discovered on October 10, 1986 by D. Waldron at Siding Spring Observatory, Coonabarabran, Australia. 

Dinosaurs and Asteroids

ASTEROIDS HITTING THE EARTH (METEORITES)

There are some asteroids that have orbits outside the asteroid belt. Some have orbits that cross the orbit of the Earth, and some have hit the Earth. A large asteroid could survive the trip trough Earth's atmosphere (called a meteor during its trip through the atmosphere) and form an impact crater.

An asteroid impact with the Earth may have caused the extinction of the dinosaurs. The Alvarez Asteroid Theory explains the huge K-T mass extinction 65 million years ago by a large asteroid hitting the Earth off the Mexican Yucatan peninsula. This impact would have caused severe climactic changes leading to the demise of many groups of organisms, including non-avian dinosaurs.

THE K-T MASS EXTINCTION
About 65 million years ago, at the end of the Cretaceous period, a large fraction of plant and animal families suddenly went extinct. In this Cretaceous-Tertiary or K-T mass extinction (K is for Kreide, meaning chalk in German, which describes the chalky sediment layer from that time; T is for Tertiary, the next geologic period), all land animals over about 55 pounds went extinct, as did many smaller organisms.

The K-T mass extinction obliterated the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, some families of birds and marsupial mammals, over half the plankton groups, many families of teleost (bony) fishes, bivalves, snails, sponges, sea urchins and others.

This catastrophe eventually led to the Age of Mammals.

THE ALVAREZ ASTEROID IMPACT THEORY
There are a lot of theories about why the K-T (Cretaceous-Tertiary) extinction occurred, but a widely accepted theory (proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist), is that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth about 65 million years ago. The impact would have penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires, volcanic activity , tsunamis, and severe storms with high windsand highly acidic rain . The impact could have caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path.

The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally.

Those organisms that could not adapt to the temperature and light changes would die out. Since plants' energy is derived from the sun, they would likely be the first to be affected by changes in climate. Many families of phytoplankton and plants would die out, and the Earth's oxygen levels may well have dramatically decreased, both on land and in the oceans, suffocating those organisms which were unable to cope with the lower oxygen levels.

Major changes in the food chain would result from all of these these environmental upheavals. The herbivores (plant eaters) who ate those plants would starve soon after the plants died. Then, at the top of the food chain, the carnivores (meat eaters), having lost their prey, would have to eat each other, and eventually die out. Their large carcasses must have provided smaller animals with food for quite a while.

LOCATION OF THE IMPACT CRATER
There are many impact craters on Earth. A 120-mile-wide (180 km), 1-mile-deep (1600 m) impact crater, Chicxulub, is found at the tip of the Yucatán Peninsula, in the Gulf of Mexico. This crater dates back to 65 million years ago, and is probably the site of the K-T meteorite impact. Evidence of K-T period tsunamis have been found all around the Gulf of Mexico.

CHEMICAL EVIDENCE FOR THE THEORY
In the clay layer from the Cretaceous-Tertiary (K-T) boundary, scientists have found chemical evidence that supports the Alvarez impact theory. The K-T layer consists of the sedimentary deposits that accrued from the end of the Cretaceous period to the beginning of the Tertiary period. It is divided into two layers, the Magic Layer (3 mm thick) and the Ejecta Layer (2 cm thick).

• Siderophiles - The Rare Earth Elements Os, Au, Pt, Ni, Co, Pd, and Ir, are Siderophile Elements. Their abundance in the lower K-T layer is indicative of an asteroid impact. Iridium (Ir) has been found in the K-T layer around the world. The discovery of a 100,000-years-thick layer of iridium in the K-T boundary in New Zealand, Denmark, and Italy. Iridium is rare on Earth except near the Earth's center, but relatively abundant in chondritic meteors (stony meteors with chondrules, spherical blobs of silicates which pre-date planetary formation). A meteoritic origin of this iridium layer seems likely. This layer became known as the iridium anomaly.
• Tektites - Tektites are quartz grains which are vaporized under intense heat and pressure, and cool into glass beads with no crystalline structure. Tektites were probably formed during a meteorite or comet collision. Tektites are abundant in the K-T layer.
• Shocked quartz - When quartz is put under extremely high pressure, it can cleave in parallel planes. Shocked quartz is found at nuclear bomb sites and known meteorite impact areas. Shocked quartz is abundant in the K-T layer.
• Stishovite (Silicon Dioxide) - a form of quartz created under conditions of high heat and pressure. It is used as an indicator of meteor impact. It has been found in abnormally high abundance in the K-T layer. Most likely formed during a massive collision.
• Glass beads - Kenneth Miller has discovered a two-inch layer of glass beads in the K-T layer near the Bass River in New Jersey, USA, supporting Alvarez' theory. 

List of Some Asteroids

 

Asteroid Name and Number*	Diameter (km)	Mass (kg)	Mean Distance from the Sun (km)	Orbital Period	Discoverer, Date of Discovery
1. Ceres	960 x 932	8.7 x 1020	4.139 x 108 km	4.60 years	Piazzi, 1801
2. Pallas	570 x 525 x 482 km	3.18 x 1020	4.145 x 108 km	4.61 years	Heinrich Olbers, 1802
3. Juno	240 km	2.0 x 1019	2.7 AU	4.36 years	K. Harding, 1804
4. Vesta	530 km	3.0 x 1020	3.534 x 108 km	3.63 years	H. Olbers, 1807
5. Astraea	.	.	3.89 x 108 km (2.58 AU)	4.13 years	Hencke, 1845
10. Hygiea	430 km	.	4.703 x 108 km	xx years	De Gasparis, 1849
15. Eunomia	272 km	.	3.955 x 108 km	xx years	De Gasparis, 1851
433. Eros (NEA)	34.7 x 17.4 x 14 km	7 x 1015 kg	2.25 x 108 km (1.5 AU)	1.76 years	Gustav Witt and Auguste H.P. Charlois, 1893
951. Gaspra	34 x 20 km	10 x 1015 kg	2.05 x 108 km	3.29 years	Grigoriy N. Neujamin, 1916
1221. Amor (NEA)	.	.	1.45 x 108 km (0.97 AU)	2.66 years	E. Delporte, 1932
1862. Apollo (NEA)	1.6 km	2 x 1012 kg	2.20 x 108 km (1.47 AU)	1.81 years	K. Reinmuth,1932
2062. Aten (NEA)	.	.	2.20 x 108 km (1.92 AU)	0.95 years	Helin, 1976

Composition of Asteroids

COMPOSITION OF ASTEROIDS

Type	Composition	Percentage of Asteroids	Albedo (reflectivity)
Carbon (C-type)	Carbon	over 75 percent	0.03-0.09 (Very dark)
Silicate (S-type)	Metallic iron mixed with iron-silicates and magnesium-silicates	17 percent	0.10 -0.22 (Relatively bright)
Metallic (M-type)	Iron/ nickel	less than 7 percent	0.10-0.18 (Relatively bright)
Dark (D-type)	Water ice/frozen carbon monoxide mixed with rock	less than 1 percent	0.05 (Relatively dark and reddish)

The asteroids are varied in their composition. Most are made of rock, but some are composed of metals and other materials.

Most of what we know about the composition of asteroids is from studying the asteroids that have fallen to Earth; thay are called meteorites at this point. Although some meteorites come from the moon and from comets, most are asteroids.