UNIVERSE: The Solar System – Asteroids and Comets

Asteroids and Comets

Asteroids and comets are probably remnants from the time the solar system formed. Consequently, they are fascinating to science, providing clues to the early life of the solar system.


Asteroids are irregularly shaped boulders, usually less than 60 miles (100 km) in diameter. They orbit the sun, generally in slightly elliptical orbits. A few can stray through the solar system.

Most asteroids are found in the region of the solar system called the asteroid belt, which is a ring of celestial objects of varying size, orbiting the sun between the planets Mars and Jupiter. Approximately 200 asteroids in this belt have diameters larger than 60 miles (100 km).

Asteroids are remnants of material from the beginning of the solar system, not fragments of planets as previously believed. Jupiter’s gravitational force prevents the asteroids in the asteroid belt from agglomerating into planets. Sometimes the asteroids’ orbits are disturbed by collisions or gravitational forces, causing them to career off through space and possibly impact with planets or their moons.

Sunlight reflected by asteroids can be analyzed by spectroscopic examination, which reveals the chemical composition of their surface areas. Fragments from asteroids sometimes fall to Earth as meteorites, which can be studied. Asteroids have also been examined by in-
terplanetary space probes.

Around 75 percent of asteroids have dark sur- faces containing carbon (graphite); the rest have bright surfaces and consist of silicates or iron and nickel. Occasionally, asteroids come close to Earth or cross its orbital path-some have even passed within the moon’s orbit. As the effects of a large asteroid colliding with Earth could be catastrophic, they are systematically monitored from various positions around the globe.

Although they may pose a threat now, it is a possibility that the asteroids and comet collisions with the Earth first supplied the planet with water and the chemical building blocks necessary for the later development of living organisms. Other names for asteroids are planetoids or minor planets. The first celestial object of many to be discovered in the asteroid belt was Ceres in 1801, by the Italian astronomer Giuseppi Piazzi.

Today, Ceres is the largest known asteroid with a diameter of 580 miles (933 km) and, like Pluto, is considered a dwarf planet.


The risk posed by an asteroid or meteorite increases with the probability of a hit and possible damage that could be inflicted The magnitude of damage expected depends on the speed and composition of the object (ice, porous or solid rock, metal).

The impact of a 325-foot (100-m) rock could devastate an entire region, and an asteroid several miles across could trigger a global, environmental catastrophe.