Asteroids are small, rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most of this ancient space rubble can be found orbiting the Sun between the planets Mars and Jupiter. Asteroids range in size from about 952 kilometers (592 miles) in diameter, to bodies that are less than 1 kilometer (0.6 mile) across.
The total mass of all the asteroids is less than that of the Moon. Early in the history of the solar system, the formation of Jupiter brought an end to the formation of planetary bodies in the gap between Mars and Jupiter and caused the small bodies that occupied this region to collide with one another, fragmenting them into the asteroids we observe today. This region, called the asteroid belt or simply the main belt, may contain millions of asteroids. Many of these worlds have remained the same since they first formed. Their relatively pristine state makes the comets, asteroids and dwarf planets wonderful storytellers with much to share about what conditions were like in the formation of the solar system.
Nearly all asteroids are irregularly shaped, though a few are nearly spherical, and are often pitted or cratered. As they revolve around the Sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go.
The three broad composition classes of asteroids are C-, S-, and M-types. The C-type asteroids are most common, probably consist of carbonaceous materials, clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system. The S-types ("stony") are made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). The asteroids' compositional differences are related to how far from the Sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. One such asteroid, Vesta, survives to this day.
Most scientists imagined asteroids as isolated mountains or rock piles tumbling alone through space. But while en route to Jupiter in 1993, NASA's Galileo spacecraft flew by a 19-mile-wide asteroid called Ida and discovered that it has its own little moon. Just one mile wide, the moon was named Dactyl. Ida and Dactyl were the first discovered binary -- or double -- asteroids.
More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.
It was six years before another asteroid-moon system was found, but a small avalanche of discoveries followed:
- In 1999, astronomers using Earth-based telescopes found that 135-mile-wide Eugenia has an eight-mile-diameter moon, which they dubbed Petit Prince.
- In 2000, 90-mile-wide Pulcova was discovered to have its own moon, about nine miles wide.
- In 2001, scientists found Linus orbiting Kalliope, and another moon around asteroid Sylvia.
Dozens of binary asteroids have been confirmed in the main asteroid belt and among Near-Earth Objects, including some in which the moon is much closer in size to the main asteroid. Some Trans-Neptunian Objects (TNO's) (beyond the orbit of Neptune) are also binaries.
Protection from Potential Impacts
While asteroid impacts are common, scientists believe they were more prevalent when the solar system was young. They believe stray objects or fragments from earlier collisions slammed into Earth in the past, playing a major role in the evolution of the entire solar system.
Further, Jupiter's massive gravity and occasional close encounters with Mars or another objects change the asteroids' orbits, knocking them out of the main belt and hurling them into space in both directions across the orbits of the planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth. Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth's orbit, including near-Earth asteroids that may pose an impact danger.
By immediately tracking potentially hazardous near-Earth objects, we have more time to study potentially threatening situations. NASA's Near Earth Object Program was established in 1998 to coordinate NASA-sponsored efforts to detect, track and characterize all 1,397 known potentially hazardous asteroids and comets that could approach the Earth.
Radar is a valuable tool in detecting and monitoring potential impact hazards. By bouncing transmitted signals off objects, images and information can be derived from the echoes. Scientists can learn a great deal about an asteroid's orbit, rotation, size, shape, and metal concentration. The U.S. is the only country that has an operating survey and detection program for discovering near-Earth objects. Missions to comets and asteroids provide valuable information about their composition and structure, helping scientists assess the best methods to deal with those in potentially hazardous orbits.
Several NASA space missions have flown by and observed asteroids.
The Galileo spacecraft flew by asteroids Gaspra in 1991 and Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR) mission studied asteroids Mathilde and Eros; and Deep Space 1 and Stardust both had close encounters with asteroids.
In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples.
NASA's Dawn mission, launched in September 2007 on a 3-billion-kilometer (1.7-billion-mile) journey to the asteroid belt, orbited the asteroids Vesta in (August 2011) and is currently en route to Ceres (February 2015). Vesta and Ceres are sometimes called "baby planets" - their growth was interrupted by the formation of Jupiter, and they followed different evolutionary paths.
Understanding the small worlds of our solar system is an ongoing, ever-changing process. We now have the technology to learn much more about our cosmic neighbors. It was less than 20 years ago that a spacecraft took the first close-up photo of an asteroid. Now a series of bold and exciting missions is taking us on a journey to explore the small worlds of our solar system up close, for the first time.
Discovery Program: NASA's Discovery Program gives scientists the opportunity to unlock the mysteries of the solar system, hidden in these small worlds. Since the program's inception in 1992, NASA launched seven missions to investigate asteroids, comets, and dwarf planets to seek answers to age-old questions: Where did we come from? Where are we going? Where and how could life evolve in the solar system? As keys to the past and possible resources for the future, the small worlds are ancient treasure chests, filled with clues that have remained hidden throughout history - until now.
More Information about Asteroids:
NASA's Asteroid and Comet Watch Program
NASA's Jet Propulsion Laboratory (JPL) Asteroid Watch Program