Small Solar System Bodies The term used to describe some types of interplanetary material  a small solar system body Definition: a category of celestial objects orbiting the Sun that are not classified as planets, dwarf planets, or moons; this includes objects known as asteroids, meteors, comets, and some trans-Neptunian objects Asteroids An area called the asteroid belt is filled with a debris field of rocky asteroids that also orbit the Sun between a distance of 2.2 and 3.3 AU from the center of the solar system. Definition: a region in the solar system located between the orbits of Mars and Jupiter where there are a high number of asteroids Definition: an object orbiting the Sun that is smaller than a planet and has no atmosphere

Not all asteroids lie within the asteroid belt – some are located inside the orbit of the Earth around the Sun (these are called Aten asteroids). Some asteroids are classified by their ability to cross the Earth’s orbit (these are known as Apollo asteroids). –These are a concern to astronomers because they have the potential to strike the Earth’s surface. –Astronomers estimate that an asteroid strikes the Earth once every 2,000 years.

Meteoroids Smaller chunks of rock that are located in the solar system are called meteoroids, meteors, and meteorites. Definition: small chunks of rock, no larger than a few feet in diameter, that travel through space Definition: a small chunk of rock, no larger than a few feet in diameter, that is traveling through space and enters the Earth’s atmosphere; commonly known as a shooting star Definition: a meteor that does not burn up in the atmosphere, and strikes the Earth’s surface –Meteorites are grouped into three categories based on their composition: Stony meteorites are composed of silicate rock material. Iron meteorites are composed of an iron-nickel alloy and are very dense.

Stony-iron meteorites are composed of a mixture of silicate rock and iron. During certain times of the year, many meteors enter the Earth’s atmosphere at once in an event called a meteor shower. Definition: an event that describes a group of meteors entering and burning up in the Earth’s atmosphere Comets Once referred to as dirty snowballs, comets are unique in their appearance. Definition: a mixture of frozen compounds and rock that orbits the Sun and which has a distinct tail composed of vaporized gas and dust that always points away from the Sun –A comet consists of three parts: nucleus, coma, and tail

–A comet’s nucleus: the frozen compounds and rock that form the core of a comet –A comet’s coma: the dust and gas surrounding the nucleus that was once held within the nucleus –A comet’s tail: the vaporized gas and dust that trail away from the nucleus of a comet Most comets come from a region at the edge of the solar system called the Oort cloud. Definition: a hypothetical area that is located approximately 100,000 astronomical units from the orbit of the planet Pluto, where comets are believed to originate –There are two main types of comets, which are classified based on their period of revolution around the Sun. Long-period comets take an extremely long time to orbit the Sun, such as 30 million years. Short-period comets orbit the Sun in periods less than 200 years.

Chapter Six The Solar System and Its Place in the Universe Section One – The Solar System Section Two – The Earth’s Place in the Universe

The Earth’s Place in the Universe When viewed with very powerful telescopes, galaxies are revealed to be some of the most beautiful objects in space – the French astronomer Charles Messier was the first scientist to begin to catalog these amazing objects. –Definition: a large grouping of stars There are three types of galaxies: spirals, elliptical, and irregular. –SPIRAL: appears much like a rotating pinwheel At the center of a spiral galaxy is the nucleus, which is surrounded by many spiral arms. The solar system is located is a spiral galaxy called the Milky Way. –Definition: the name for the galaxy of stars in which our solar system is located

–ELLIPTICAL: have a nucleus and a halo of stars, but no spiral arm. Smaller elliptical galaxies are known as dwarf elliptical. Astronomers believe that elliptical galaxies are the most common type of galaxy found in the universe. –IRREGULAR: have no well-defined shape Quasars Besides galaxies, another structure is found out on the edges of the universe – this is called a quasar. Definition: a very high energy celestial object believed to be a type of galaxy that is rapidly moving away from the center of the universe –Some quasars are moving away from the Earth at incredible speeds of almost 90% of the speed of light – these are the fastest objects yet discovered in the universe.