What Objects Are within One Light Year of the Earth?
Spectral Classifications of Asteroids
- Seventy-five percent of known near-Earth asteroids are carbonaceous or C-type asteroids. These asteroids are very dark. Seventeen percent of near-Earth asteroids are siliceous or S-type asteroids. These asteroids are brighter than C-type and are composed of iron and other rocky material. The remainder of asteroids are metallic or M-type. These asteroids are primarily composed of metallic iron and appear very bright in the sky.
Orbital Classifications of Asteroids
- Near-Earth asteroids have a distance from the sun of 1.3 AU. An AU stands for "astronomical unit," Earth's distance from the sun (93 million miles). Near-Earth asteroids are classified into three groups: Amors, Apollos and Atens. Amors are asteroids that cross Mars's orbit but do not cross into Earth's orbit. There are currently more than 1,500 Amors known. There are more than 1,700 Apollos asteroids that cross Earth's orbit and have an orbital period greater than one year. Atens are asteroids that cross Earth's orbit within a period less than a year. Currently, there are around 300 known Atens.
Trojans and Centaurs
- There are several hundred Trojans known to exist. Trojans are located within 60 degrees ahead and behind Jupiter's orbit. Some Trojans are thought to be comets, and object 617 Patroclus is definitely icy. Centaurs are Kupier belt objects also known as comets. These objects orbit in the outer solar system, far away from the sun. Currently, there are around 50 Centaurs known to exist.
- Aside from the numerous man-made satellites that orbit Earth, there are a number of man-made objects that are not satellites but are classified as near-Earth objects. Sometimes these objects are thought to be asteroids but are later discovered to be man-made, like object J002E3, which was discovered to be an upper stage section from a Saturn rocket that was launched for the Apollo 12 mission.