Minor Members of the Solar System 7: Touring Our Solar System

Meteoroids 3

A few meteoroids are believed to be fragments of the moon, or possibly Mars, that were ejected when an asteroid impacted these bodies. Some meteoroids are as large as asteroids. Most, however, are the size of sand grains. Consequently, they vaporize before reaching Earth's surface. Meteoroids that enter Earth's atmosphere and burn up are called meteors. The light that we see is caused by friction between the particle and the air, which produces heat.

Metorites

A meteoroid that actually reaches Earth's surface is called a meteorite. A few very large meteorites have blasted out craters on Earth's surface, similar to those on the moon. The most famous is Meteor Crater in Arizona. Prior to moon rocks brought back by astronauts, meteorites were the only extraterrestrial materials that could be directly examined.

How would you describe the structure of a comet?

As a comet approaches the sun, a glowing head called a coma is visible. Within the coma, a small, glowing nucleus is sometimes present. Most comets have long tails.

Comets 3

As a comet moves away from the sun, the gases forming the coma recondense, the tail disappears, and the comet returns to cold storage. Material that was blown from the coma to form the tail is lost from the comet forever. Therefore, it is believed that most comets cannot survive more than a few hundred close orbits of the sun. Once all the gases are expelled, the remaining material—a swarm of tiny metallic and stony particles—continues the orbit without a coma or a tail.

Asteroids

Asteroids are small rocky bodies that orbit the sun. The largest, Vesta, is about 500 kilometers in diameter, but more than a million are greater than 1 kilometer across. By definition, asteroids are larger than 10 meters in diameter. As you will see in the figure on the next screen, most asteroids lie in the asteroid belt between the orbits of Mars and Jupiter. They have orbital periods of three to six years. However, asteroids are found in several places in the inner solar system.

Kuiper Belt

Comets apparently originate in two regions of the outer solar system. Those with short orbital periods are thought to orbit beyond Neptune in a region called the Kuiper belt. The Kuiper belt is thought to be remnants of materials from the formation of the solar system. This material includes more ices than the asteroid belt due to the colder temperatures which allows for comets to exist in this region.

Comets

Comets are among the most interesting and unpredictable bodies in the solar system. Comets are pieces of rocky and metallic materials held together by frozen water, ammonia, methane, carbon dioxide, and carbon monoxide. Many comets travel in very elongated orbits that carry them far beyond Pluto. These comets take hundreds of thousands of years to complete a single orbit around the sun. However, a few have orbital periods of less than 200 years and make regular encounters with the inner solar system.

Meteorites and the Age of the Solar System

How did scientists determine the age of the solar system? They used evidence from meteorites, moon rocks, and Earth rocks. Radiometric dating of meteorites found on Earth shows that the oldest meteorites formed more than 4.57 billion years ago. These meteorites are the oldest-known materials in the solar system. Some are made mostly of iron. Others, called stony meteorites, contain silicates. Scientists think that the composition of meteorites is similar to the composition of other materials in the inner solar system during its formation.

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Types of asteroids on belts 2

In addition, Trojan asteroids exist in the orbit of Jupiter. At two points that are 60 degrees ahead of and behind Jupiter in its orbit are Lagrange points. Lagrange points are where gravitational forces are balanced. A group of asteroids has collected in these two points in Jupiter's orbit and orbit the Sun with Jupiter staying 60 degrees in front of or behind the planet. There are a few known asteroids in the outer solar system. These are known as Centaurs. The composition of these Centaurs is more similar to Kuiper belt objects that the asteroids of the inner solar system.

Where are most of the asteroids of the solar system?

In orbit around the sun, between the orbits of Mars and Jupiter

Kuiper Belt 2

Like the asteroids in the inner solar system, most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two Kuiper belt comets, or the gravitational influence of one of the Jovian planets, may occasionally alter the orbit of a comet enough to send it to the inner solar system and into our view from Earth. Comets and asteroids that enter Earth's proximity during their orbit are called Near-Earth Objects (NEOs).

Asteroids 2

Many asteroids have irregular shapes. Because of this, planetary geologists first speculated that they might be fragments of a broken planet that once orbited between Mars and Jupiter. However, the total mass of the asteroids is estimated to be only 1/1000 that of Earth, which itself is not a large planet. What happened to the remainder of the original planet? Others have hypothesized that several larger bodies once coexisted in close proximity, and their collisions produced numerous smaller ones. The existence of several families of asteroids has been used to support this explanation. However, no conclusive evidence has been found for either hypothesis.

Meteorites and the Age of the Solar System 2

Moon rocks from the lunar highlands have a composition similar to that of stony meteorites. These moon rocks date to about 4.5 billion years ago, almost as old as the oldest meteorites. From these facts, scientists infer that the moon must be just slightly younger than the formation of the solar system, which occurred 4.567 billion years ago. The ages of the oldest known Earth rocks are consistent with this conclusion. Scientists have dated rocks found in northwestern Canada at about 4 billion years old. These are the oldest rocks found on Earth so far. In addition, some tiny crystals of the mineral zircon found in sedimentary rocks in Australia are 4.4 billion years old.

Meteoroids 2

Most meteoroids originate from any one of the following three sources: (1) Interplanetary debris that was not gravitationally swept up by the planets during the formation of the solar system (2) Material from the asteroid belt (3) The solid remains of comets that once traveled near Earth's orbit

Meteoroids

Nearly everyone has seen a "shooting star." This streak of light occurs when a meteoroid enters Earth's atmosphere. A meteoroid is a small solid particle that travels through space.

Meteoroids 4

Occasionally, meteor sightings can reach 60 or more per hour. These displays, called meteor showers, result when Earth encounters a swarm of meteoroids traveling in the same direction and at nearly the same speed as Earth. As shown in the table, some meteor showers are closely associated with the orbits of some comets, strongly suggesting that they are material lost by these comets. The Perseid meteor shower, which occurs each year around August 12, maybe the remains of Comet 1862 III.

Major Meteor Showers

Shower Approximate Dates Each Year Associated Comet Quadrantids Jan.4-6 Lyrids Apr.20-23 Comet 1861 I Eta Aquarids May 3-5 Halley's Comet Delta Aquarids July 30 Perseids Aug. 12 Comet 1862 III Draconids Oct. 7-10 Comet Giacobini-Zinner Orionids Oct. 20 Halley's Comet Taurids Nov. 3-13 Comet Encke Andromedids Nov. 14 Comet Biela Leonids Nov. 18 Comet 1866 I Geminids Dec. 4-16

Types of Asteroids on Belts

Some asteroids have very elongated orbits and travel very near the sun, and a few larger ones regularly pass close to Earth and the moon. Main belt asteroids, the majority of all known asteroids, are those in the asteroid belt between Mars and Jupiter. Atens asteroids are those that orbit the Sun inside the orbit of Earth. Amors asteroids orbit between the orbits of Earth and Mars. Apollo asteroids have orbits that potentially cross the orbit of Earth. Many of the most recent impact craters on the moon and Earth were probably caused by collisions with Apollo asteroids. Inevitably, future Earth-asteroid collisions will occur.

Comets 2

The fact that the tail of a comet points away from the sun in a slightly curved manner led early astronomers to propose that the sun has a repulsive force that pushes the particles of the coma away, thus forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails are observed.

Halley's Comet

The most famous short-period comet is Halley's comet. Its orbital period averages 76 years. When it passed near Earth in 1910, Halley's comet had developed a tail nearly 1.6 million kilometers long and was visible during the daylight hours. In March 1986, the European probe Giotto approached to within 600 kilometers of the nucleus of Halley's comet and obtained the first images of this elusive structure. We now know that the nucleus is potato-shaped, 16 kilometers by 8 kilometers. The surface is irregular and full of craterlike pits. Gases and dust that vaporize from the nucleus to form the coma and tail appear to gush from parts of its surface as bright jets or streams.

How do many comets change as they approach the sun?

Their heads begin to glow and a long tail forms.

Oort Cloud

Unlike Kuiper belt comets, comets with long orbital periods aren't confined to the plane of the solar system. These comets appear to be distributed in all directions from the sun, forming a spherical shell around the solar system called the Oort cloud. The Oort cloud formed as the orbits of comets in the early solar system were influenced by the gravity of the giant planets. Comets were sometimes sent toward the Sun, where they may have collided with inner planets or the Sun. But many were also flung towards the edge of the solar system. Jupiter may have sent some comets out of the solar system, but many could not fully escape and collected in the region called the Oort Cloud. This region extends from just outside the solar system to perhaps as far as a third of the way to the nearest star - Alpha Centauri. The gravitational effect of another object in space is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the sun. However, only a tiny portion of the Oort cloud comets pass into the inner solar system.

Coma

When first observed, a comet appears very small. But as it approaches the sun, solar energy begins to vaporize the frozen gases. This produces a glowing head called the coma. A small glowing nucleus with a diameter of only a few kilometers can sometimes be detected within a coma. As comets approach the sun, some, but not all, develop a tail that extends for millions of kilometers.

Which objects of the solar system are made of rocky and metallic materials mixed with frozen gases, including water ice?

comets

Most meteoroids that enter Earth's atmosphere are _____.

particles the size of sand grains that come from many sources

Which is the least likely origin for a meteoroid that approaches Earth's atmosphere?

rock dislodged from the moon

What orbits the sun between the orbits of Mars and Jupiter?

the asteroids of the asteroid belt