Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-earth objects.
Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.
Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Vesta - the largest at about 329 miles (530 kilometers) in diameter - to bodies that are less than 33 feet (10 meters) across. . The total mass of all the asteroids combined is less than that of Earth's Moon.
Editor's note: Even with more than one-half million asteroids known (and there are probably many more), they are still much more widely separated than sometimes seen in Hollywood movies: on average, their separation is in excess of 1-3 million km (depending on how one calculates it).
Most asteroids are irregularly shaped, though a few are nearly spherical, and they 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. More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.
The three broad composition classes of asteroids are C-, S-, and M-types. The C-type (chondrite) asteroids are most common, probably consist of 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. Only one such asteroid, Vesta, survives to this day.
Jupiter's massive gravity and occasional close encounters with Mars or another object changed the asteroids' orbits, knocking them out of the main belt and hurling them into space in both directions towards or away from the sun, across the orbits of the planets. Stray asteroids and asteroid fragments have 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, and near-Earth asteroids that approach Earth's orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid's orbit, rotation, size, shape, and metal concentration.
Several 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-Shoemaker) mission studied asteroids Mathilde and Eros; and the Rosetta mission encountered Steins in 2008 and Lutetia in 2010. 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. On June 3, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.
NASA's Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year. Once it left in September 2012, it headed towards dwarf planet Ceres, with a planned arrival of 2015. Vesta and Ceres are two of the largest surviving protoplanet bodies that almost became planets. By studying them with the same complement of instruments on board the same spacecraft, scientists will be able to compare and contrast the different evolutionary path each object took to help understand the early solar system overall.
Main asteroid belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.
Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan's tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.
Near-Earth asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids - those that could pose a threat to Earth.
The International Astronomical Union's Committee on Small Body Nomenclature is a little less strict when it comes to naming asteroids than other IAU naming committees. So out there orbiting the sun we have giant space rocks named for Mr. Spock (a cat named for the character of "Star Trek" fame), rock musician Frank Zappa, regular guys like Phil Davis, and more somber tributes such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Asteroids are also named for places and a variety of other things. (The IAU discourages naming asteroids for pets, so Mr. Spock stands alone).
Asteroids are also given a number, for example (99942) Apophis. The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.
Queen of the Asteroids
With a diameter of about 600 miles (966 kilometers), Ceres is the undisputed ruler of the asteroid belt. The dwarf planet accounts for a quarter of all the mass of all the known asteroids in the main asteroid belt.
While studying asteroid 243 Ida in 1993, Jupiter-bound Galileo spotted a surprise - a tiny moon. Scientists named their discovery - the first known moon orbiting an asteroid - Dactyl.
Cosmic Small Change
If you could lump together the more than 500,0000 asteroids in orbit between Jupiter and Saturn, their total mass wouldn't even equal that of Earth's moon.
Space in Space
The asteroid belt is not as tightly packed as we see in the movies. Asteroids are actually far apart - on aver more than 1.2 million miles (2 million kilometers).
In 2010, astronomers using the Hubble Space Telescope spotted what appears to be the first collision between two asteroids. Such a smash-up had never been seen before.
More than 150 of the asteroids are known to have a small companion moon. Some even have two moons.
About once a year an car-sized asteroid enters Earth's atmosphere, creates an impressive fireball and disintegrates before ever reaching the surface.
After completing it's mission, NASA's NEAR-Shoemaker spacecraft descended to surface of asteroid Eros - the first soft landing on an asteroid.
It is possible that a gigantic asteroid impacted the Earth and led the extinction of the dinosaurs.
About 20,000 to 50,000 years ago, a small asteroid about 80 feet (24 meters) in diameter smacked into what is now Arizona and formed Meteor crater. Measuring about 4,000 feet in diameter (1.2 kilometers), Barringer Crater is the best preserved impact crater on Earth.
Two Times One Equals ... Binary
We've found double systems of stars, planets and even asteroids. Binary asteroids are two rocky bodies of roughly equal size orbiting each other. We've even found triple asteroid systems.
Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.
In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.
We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.
As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. Taking less than 200 years to orbit the sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is, 100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.
Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tails - a dust tail and an ion (gas) tail.
Most comets travel a safe distance from the sun - comet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.
Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley's nucleus. NASA's Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.
NASA's Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.
Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.
After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact's mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact's 2005 encounter.
Comet naming can be complicated. Comets are generally named for their discoverer -- either a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.
Sungrazer comets are comets that come so close to the sun that they vaporize and disappear -- never to be seen again. However, in December of 2011 one comet (Lovejoy) was the first spacecraft-viewed comet to graze the sun and survive to tell the tale.
If you weigh 45 kg (100 lb.) on Earth, you'd weigh only about 0.005 kg (0.01 lb.) on a comet. At that weight, you could easily jump right off into space (although we don't recommend it ). If you rode on a comet close to the sun, you'd probably get blown off into space on a jet of dust and gas.
A comet's nucleus is very dark -- as dark as coal or asphalt, which are some of the darkest materials on Earth. This makes a comet's nucleus one of the darkest things in our solar system.
Small Town / Big State
The size of a comet is no small matter: Did you know that the average size of a comet is as big as an entire town? Now, imagine a comet as an average town and the Kuiper Belt (where comets reside and come from) as a state, but this state would be gigantic. The Kuiper Belt expands from about 30 to 55 AU, which is about 3.7 billion km (2.3 billion miles) long. The circumference of the entire Earth is only about 40 thousand km (25 thousand miles).
Were the dinosaurs wiped out by a comet impact 65 million years ago? It's possible. In the earliest days of the solar system, comets regularly bombarded the planets.
One Tough Robot
No one expected Europe's Giotto spacecraft to survive the beating it took from dust and rocks when it made a close pass by comet Halley in 1986. Damaged, but far from dead, Giotto flew on to study a second comet.
When they are far from the sun, most comets are insignificant specks less than 10 km (6 miles) across. But when a comet gets close to the sun, the cloud of gases surrounding it can swell larger than the size of Jupiter -- more than 10 times the diameter of Earth. Comets also sprout beautiful tails that stretch for millions of kilometers away from the sun.
Thar She Blows
Comet Hale-Bopp spewed out about 250 tons of gas and dust per second -- more than 50 times what most other comets churn out. It made for a spectacular show as it passed through the inner solar system in 1997.
Don't Wait Up
Comet Hale-Bopp won't return to our skies for about 2,740 years -- and that's not even close to the longest orbit comets make around our sun. Some far travelers take millions of years to make one orbit. Others, such as comet Encke, run by the sun every few years.
Creating space fireworks just in time for the Fourth of July (2005), Deep Impact flew by and sent an impactor into the path of the nucleus of comet Tempel 1. Deep Impact was the first mission to collide with and eject material from a comet.
Little chunks of rock and debris in space are called meteoroids. They become meteors -- or shooting stars -- when they fall through a planet's atmosphere; leaving a bright trail as they are heated to incandescence by the friction of the atmosphere. Pieces that survive the journey and hit the ground are called meteorites.
Shooting stars, or meteors, are bits of interplanetary material falling through Earth's atmosphere and heated to incandescence by friction. These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they streak across the sky and create glowing trails.
Scientists estimate that 44 tonnes (44,000 kilograms, about 48.5 tons) of meteoritic material falls on the Earth each day. Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically - these events are termed meteor showers. Some occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet. Meteor showers are usually named after a star or constellation that is close to where the meteors appear in the sky. Perhaps the most famous are the Perseids, which peak around 12 August every year. Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years. Other meteor showers and their associated comets are the Leonids (Tempel-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke). Most comet dust in meteor showers burns up in the atmosphere before reaching the ground; some dust is captured by high-altitude aircraft and analyzed in NASA laboratories.
Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites. Most meteorites found on Earth are pebble to fist size, but some are larger than a building Early Earth experienced many large meteorite impacts that caused extensive destruction.
One of the most intact impact craters is the Barringer Meteorite Crater in Arizona, about 1 kilometer (0.6 mile) across, formed by the impact of a piece of iron-nickel metal approximately 50 meters (164 feet) in diameter. It is only 50,000 years old and so well preserved that it has been used to study impact processes. Since this feature was recognized as an impact crater in the 1920s, about 170 impact craters have been identified on Earth.
A very large asteroid impact 65 million years ago, which created the 300-kilometer-wide (180-mile-wide) Chicxulub crater on the Yucatan Peninsula, is thought to have contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs.
Well-documented stories of meteorite-caused injury or death are rare. In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 3.6-kilogram (8-pound) stony meteorite that crashed through her roof in November 1954.
Meteorites may resemble Earth rocks, but they usually have a burned exterior. This fusion crust is formed as the meteorite is melted by friction as it passes through the atmosphere. There are three major types of meteorites: the "irons," the "stones," and the stony-irons. Although the majority of meteorites that fall to Earth are stony, more of the meteorites that are discovered long after they fall are irons - these heavy objects are easier to distinguish from Earth rocks than stony meteorites. Meteorites also fall on other solar system bodies. Mars Exploration Rover Opportunity found the first meteorite of any type on another planet when it discovered an iron-nickel meteorite about the size of a basketball on Mars in 2005, and then found a much larger and heavier iron-nickel meteorite in 2009 in the same region. In all, Opportunity has discovered six meteorites during its travels on Mars.
More than 50,000 meteorites have been found on Earth. Of these, 99.8 percent come from asteroids. Evidence for an asteroid origin includes orbits calculated from photographic observations of meteorite falls projected back to the asteroid belt; spectra of several classes of meteorites match those of some asteroid classes; and they are very old, 4.5 to 4.6 billion years. However, we can only match one group of meteorites to a specific asteroid - the eucrite, diogenite, and howardite igneous meteorites come from the third-largest asteroid, Vesta. Asteroids and the meteorites that fall to Earth are not pieces of a planet that broke apart, but instead are the original diverse materials from which the planets formed. The study of meteorites tells us much about the earliest conditions and processes during the formation and earliest history of the solar system, such as the age and composition of solids, the nature of the organic matter, the temperatures achieved at the surface and interiors of asteroids, and the degree to which materials were shocked by impacts.
The remaining 0.2 percent of meteorites is split roughly equally between meteorites from Mars and the moon. The over 60 known martian meteorites were blasted off Mars by meteoroid impacts. All are igneous rocks crystallized from magma. The rocks are very much like Earth rocks with some distinctive com-positions that indicate martian origin. The nearly 80 lunar meteorites are similar in mineralogy and composition to Apollo mission moon rocks, but distinct enough to show that they have come from other parts of the moon. Studies of lunar and martian meteorites complement studies of Apollo Moon rocks and the robotic exploration of Mars.
|Major Meteor Streams||Peak Night
(may vary by +/- 1 day)
|Time to Watch*
(24 hour clock)
|Parent Body (Asteroid or Comet)|
|Quadrantids||January 3-4||23:00 to dawn||60-200||(196256) 2003 EH1|
|Lyrids||April 21-22||21:30 to dawn||10-15 typical||Comet C/1861 G1|
|Eta Aquarids||May 5-6||01:30 to dawn||40-85||Comet 1P/Halley|
|Delta Aquarids||July 27-28||21:30 to dawn||15-20||Unknown sungrazing comet|
|Perseids||August 11-12||dusk to dawn||60-100||Comet 109P/Swift-Tuttle|
|Orionids||October 20-21||22:00 to dawn||25||Comet 1P/Halley|
|Leonids||November 17-18||23:30 to dawn||10-15||Comet 55P/Tempel-Tuttle|
|Geminids||December 13-14||19:00 to dawn||60-120||(3200) Phaethon|
|* For observers in the northern hemisphere.|
|** Under perfect conditions|
(Primary source material from: Dr. Harry B. Herzer, III, NASA Aerospace Education Services Project, Oklahoma State University, Stillwater, Oklahoma.)
The largest individual meteorite found is the Hoba meteorite in southwest Africa, which has a mass of about 54,000 kg and mostly consists of iron. (Picture is of a smaller iron meteorite.)
How is it that the same object can have three names? Through a change in appearance or attributes, that's how. Meteoroids become meteors -- or shooting stars -- when they interact with a planet's atmosphere and cause a streak of light in the sky. Debris that makes it to the surface of a planet from meteoroids are called meteorites.
Bits and Pieces
Scientists estimate that 1,000 tons to more than 10,000 tons of meteoritic material falls on the Earth each day. However, most of this material is very tiny -- in the form of micrometeoroids or dust-like grains a few micrometers in size. These grains pose no threat to life on Earth.
It Fell From the Sky
There are only two known instances of someone getting hurt from a meteorite. In one instance, a woman was severely bruised by an eight pound stony meteorite when it crashed through her roof in 1954.
Meteoroids, meteors and meteorites cannot support life on their own. However, these chunks of space debris may have brought the building blocks of life (amino acids) to Earth.