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Space Topics: Asteroids and Comets

Asteroid Facts

Asteroid 433 Eros
Asteroid 433 Eros
Eros' northern hemisphere as seen by the NEAR spacecraft on February 29, 2000. The view is a mosaic of six images. Credit: NASA / JHU APL

Asteroids, or minor planets, are small, rocky (or sometimes metallic) bodies with no atmospheres.  There are hundreds of thousands of minor planets; nearly 100,000 have been numbered, and more than 12,000 have been named. 

Only a few minor planets have been visited by spacecraft: Gaspra, Ida and its satellite Dactyl, Mathilde, and Eros.  Therefore, most of our understanding of minor planets comes from Earth-based optical and radio telescope observations.  Minor planets are divided into classes based upon the sizes and shapes of their orbits and also upon their spectral characteristics (essentially, their brightness and color).  In addition, a few meteorites found on Earth have been identified as pieces of specific minor planets.

Spectral Classification of Asteroids

Minor planets are classified into broad categories such as C-type, S-type, or M-type according to their brightness and color (technically, their spectra).  Brightness, or albedo, is a measurement of how well the object reflects light.  Albedo ranges from 0 (perfectly black) to 1 (perfectly reflecting).  Most minor planets are relatively dark objects with albedos ranging from a carbon-black 0.03 to a dark 0.22.  The composition of asteroids can be determined by breaking down the light they reflect into a spectrum, a graph of how reflective the object is at each wavelength of light.  Astronomers can often identify the abundances of silicate minerals in spectra.  Based on those identifications, different classes of meteorites found on Earth have been linked to the different classes of asteroids.

Carbonaceous or C-type Asteroids: More than 75 percent of asteroids fall into this category. With albedos of 0.03 to 0.09, these objects are very dark.  Their compositions are similar to carbonaceous chondrite meteorites. These asteroids have approximately the same chemical composition as the Sun, but lack hydrogen, helium, and other volatiles. C-type asteroids are most commonly found in the outer regions of the main asteroid belt.

Silicaceous or S-type Asteroids: Approximately 17 percent of known asteroids are S-type.  They have fairly bright albedos ranging from 0.10 to 0.22. Moving in orbits within the inner regions of the main asteroid belt, S-type asteroids are likely equivalent to the ordinary chondrite meteorites, composed of rocky materials plus a small amount of metallic iron.

Metallic or M-type Asteroids: These mid-region main belt objects are relatively bright, with albedos ranging from 0.10 to 0.18.  M-type asteroids are composed of metallic iron, like the iron meteorites.

Orbital Classification of Asteroids

Asteroids are also categorized by their locations in the solar system.  Earth lies at a distance of 1 astronomical unit (AU) -- the size of Earth’s orbit, 150 million kilometers or 93 million miles -- from the Sun.  Asteroids that orbit near this distance are of particular interest to Earthlings.

Main Belt: Most minor planets orbit the Sun in the main asteroid belt, a vast ring of hundreds of thousands of rocky objects circling between the orbits of Mars and Jupiter, from 2 to 4 AU from the Sun.  These objects are believed to be leftover fragments from the formation of the solar system about 4.6 billion years ago. Scientists suspect that these fragments were prevented from accreting into larger objects because of Jupiter's massive gravitational pull. As Jupiter orbited outside the asteroid belt, its gravity would tug on the small bodies, disturbing their orbits and preventing them from coalescing into a single body.

In 2006, scientists reported that at least three bodies in the outer part of the main belt demonstrate classical cometary behavior, and defined a new class of bodies called the main belt comets. This followed the 2005 discovery that the largest main belt asteroid, Ceres, must contain a lot of water ice. All of these formed in the same places as the asteroids but have retained liquid water over the age of the solar system. Many main belt asteroids may in fact contain a lot of ice, especially in the outer part of the belt; the line separating asteroids and comets is growing increasingly fuzzy.

Near-Earth Asteroids (NEAs): Asteroids with orbits that bring them within 1.3 AU of the Sun.  Scientists believe that many NEAs are objects nudged from the main asteroid belt.  The nudging can result from collisions between objects, the gravitational tug of the giant planets, or a subtle force known as the Yarkovsky effect. The Yarkovsky effect occurs when a rotating asteroid’s sun-warmed surface re-radiates its heat on its afternoon side.  The photons departing the surface of the asteroid create a tiny change in the asteroid’s momentum over time. Scientists also speculate that some NEAs may be the nuclei of dead, short-period comets that have burned off all their gases. NEAs, together with the comets whose orbits take them through near-Earth space, form the category of near-Earth objects, or NEOs.  The Planetary Society supports many programs and projects to identify and track NEOs and mitigate the risk they pose to Earth.

NEAs are grouped into three main categories, named for each category’s most famous member: 1221 Amor, 1862 Apollo, and 2062 Aten.

  • Amors cross Mars' orbit but do not quite cross Earth's orbit. Eros, which was visited by the Near Earth Asteroid Rendezvous (NEAR) spacecraft, is an Amor. More than 1,500 Amors are known.
  • Apollos are Earth orbit-crossing asteroids with orbit periods that are longer than one year. Asteroid Geographos is an Apollo.  More than 1,700 Apollos are known.
  • Atens cross Earth's orbit with an orbital period less than 1 year. Roughly 300 Atens are known.

More than 3,400 NEAs have been found to date, a fraction of their suspected total population. 1036 Ganymed, with an approximate diameter of 41 kilometers (25 miles), is the largest NEA. It is currently estimated that there are about 1,100 NEAs with diameters larger than 1 kilometer; they are large enough to produce global disaster if they were to hit Earth.  A goal has been set by NASA’s Near Earth Object Program to discover 90% of the kilometer-and-larger NEAs by 2010.

Trojans: These objects are located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known, while thousands may actually exist. There are also some asteroids known to orbit in the Lagrange points for Mars and Neptune. These may well have the compositions of comets; a rare binary Trojan, 617 Patroclus, is definitely icy.

Centaurs: Possibly better classified among the comets or Kuiper belt objects, these objects orbit far from the Sun in the outer solar system.  For example, 2060 Chiron orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars to beyond Uranus; and 5145 Pholus orbits from Saturn to past Neptune. There are probably many Centaurs; more than 50 are currently named and tracked.  Their planet orbit-crossing paths are unstable and are frequently perturbed by planets and other objects. The composition of these objects is probably more like that of comets or Kuiper Belt objects, rather than that of ordinary asteroids.

Man-made objects: Increasingly, human space exploration is leaving objects in asteroid-like orbits in the solar system.  Some are functioning spacecraft, but most are spent rocket stages or dead spacecraft.  Sometimes, objects are provisionally identified as asteroids, and are later found to be artificial objects, like object J002E3 (a Saturn rocket upper stage from Apollo 12).  About 200 such objects are known to exist.