Asteroids, Comets, and Meteoroids

1. Asteroids, Comets, and Meteoroids (5)  Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to: (C)  contrast the characteristics of comets, asteroids, and meteoroids and their positions in the solar system, including the orbital regions of the terrestrial planets, the asteroid belt, gas giants, Kuiper Belt, and Oort Cloud;

2. Asteroids, Comets, and Meteoroids Himalia; one of Jupiter’s natural satellites…too small to be a moon? This is a picture of Asteroid 951 Gaspra. Gaspra has an irregular shape with dimensions of about 19 x 12 x 11 kilometers. (Courtesy USGS/NASA/JPL) In addition to the Sun and planets, our Solar System contains many small bodies known as asteroids, comets, and meteoroids. Although these bodies often have distinguishing characteristics, the classification of them can sometimes cause controversy. For example, some moons may actually be asteroids that were trapped by the planet's gravity. In order to classify celestial bodies, all their characteristics must be taken into consideration. These characteristics include their composition, location, orbit, and origin.

3. Bode’s Law In the eighteenth century, a German astronomer, Johann Bode, found a sequence representing the distances of the planets from the Sun. The sequence, known as Bode's Law, works by starting with 0, then 3, and doubling the number each time until you reach 192 (for Uranus) . Then, by adding four to each number and dividing the result by 10, the numbers give the approximate distance of each planet from the Sun in astronomical units (AU). Scientists noticed that the law worked flawlessly from Mercury to Uranus with one exception… between the orbits of Mars and Jupiter where there was no known planet. In 1801, Giuseppe Piazzi discovered an object between Mars and Jupiter and gave it the name Ceres. Although Ceres is the largest known asteroid it is still very small compared to the planets, with a diameter of only 1,003 kilometers.

4. Theories on Asteroid Formation Scientists were not satisfied with the discovery of Ceres, so they continued to search for a planet. They soon realized that countless asteroids orbited where they had once expected to find a planet. Let’s give them some perspective… It became obvious that there was no planet between Mars and Jupiter. Astronomers began to make theories explaining the origins of the many small bodies called asteroids. Some suggested that asteroids were the remnants of a planet that had been shattered by an explosion. The most commonly accepted theory, however, states that asteroids are pieces of matter that never coalesced to form a planet. The fact that the combined mass of all the asteroids in this area is far less than the mass of most planets led scientist to believe asteroids lack the gravity necessary to coalesce into a planet. Ceres Vesta Pallas Hygiea

5. Types of Asteroids The Asteroid Belt lies between the orbits of Mars and Jupiter and contains millions of asteroids. Over 6,000 asteroids have been named and 70,000 have been identified. Some asteroids even cross Earth's orbit. The unusually eccentric orbits of these asteroids are a result of collisions in the Asteroid Belt that sent asteroids on a new path. • Asteroids are commonly classified into types according to their spectra. • Type C asteroids are extremely dark carbonaceous objects and comprise more than 75% of all asteroids. • Type S asteroids are relatively light asteroids composed of metallic nickel-iron and make up 17% of all asteroids. • Type M, which are bright asteroids composed of pure nickel-iron. There are a few other rare types of asteroids.

6. Comets Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. As you can see, comets come in many forms…some are even tailless.

7. Comets Comets are large bodies rich in ices along with dust grains, gravel and small rocky bodies embedded in the ice. When the ices evaporate as they get closer to the Sun, they grow tails and we see them as 'comets'. They leave behind in their orbits, gravel and rocky debris that Earth may encounter from time to time as orbits cross. We see these encounters as 'meteor showers'. Because the orbit the Sun, we can see them come back from time to time. Halley's Comet appears about every 76 years, for example. We had a visit from this comet in 1986, and we won’t see it again until mid-2061

8. Where Do Comets Come From? Comets are in long elliptical orbits around the Sun from beyond Uranus. These objects are found in theKuiper Belt between Uranus and Neptune and theOort Cloud from way out beyond Pluto. Short period comets originate from the Kuiper Belt, while long-period comets come from the Oort Cloud. The name "comet" comes from the Greek meaning "long-haired" because of their tails. Comets throughout history have been thought to bring bad luck but these are just natural phenomena of the solar system.

9. Meteors • Meteorites are small rocky bodies, from micron-sized up to perhaps a meter across that impact Earth after a brilliant atmospheric display. • Cometary debris • Space junk (never got incorporated into cometary bodies, planets or asteroids) A meteor is a small body, usually the size of a pebble or sand grain, that enters the atmosphere and burns up, but some of the larger ones a foot or more across can reach the ground and are called meteorites when their fragments are recovered.

10. Meteor Showers As the meteor enters the Earth's atmosphere it becomes very hot due to friction. They become bright and seem to streak across the sky. When the Earth enters a meteoroid stream left by a comet it produces a meteor shower. These showers can be predicted and come every year or so. They are named after the constellation that the meteors appear to be coming from.

11. Terrestrial Planets The inner-most planets, composed of mainly rock and some metal and have high densities. They have solid surfaces and rotate slowly. The four terrestrial planets in our solar system are Mercury, Venus, Earth and Mars. Venus is the largest of the terrestrial planets.

12. Jovian Planets These are the giant planets, mainly composed of gas and not of any rock or other solid matter, examples of Jovian planet would be Jupiter, and Saturn, Uranus and Neptune.

13. Things to know… Frost Line: Also known as the snow line is and imaginary line that refers to a specific spot in the solar nebula where it is cold enough for hydrogen compounds to freeze. This line separates the terrestrial planets from the Jovian planets. Inside the frost line, these gases would not be frozen and therefore could not form planets. Accretion is the formation of a planet, small particles of gas and dust stick together, as they get larger they start to attract particles and their growth speeds up. Once they grow large enough gravity forces it into a sphere shape, and when the planet grows to a certain size it gravitationally dominates everything near it.