IntroductionAsteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.

1. Most of these objects, called planetoids or asteroids — meaning 'star-like' — orbit between Mars and Jupiter in a grouping known as the Main Asteroid Belt. The Main Asteroid Belt lies more than.
2. Jul 01, 2020 First asteroid found within Venus’s orbit could be a clue to missing ‘mantle’ asteroids. By Nola Redd Jul. 1, 2020, 12:20 PM. Earlier this year, astronomers discovered an oddball asteroid.

The Trojans – these are asteroids that share an orbit with a planet, circling around the sun in front or behind the planet. The Apollos, Atens, and Amors are better known as near Earth asteroids.There were approximately 8000 known near Earth asteroids. Most of the time they are a very small threat to Earth and are observed by astronomers.

The current known asteroid count is: .

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.

Many Shapes and Sizes

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.

Composition

Composition

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.

Jupiter's massive gravity and occasional close encounters with Mars or another object change the asteroids' orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other 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, 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.

Asteroid Classifications

Asteroid Classifications

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.

How Asteroids Get Their Names

How Asteroids Get Their Names

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, beloved teachers such as Florida’s Cynthia L. Reyes, 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.

Additional Resources

Additional Resources

http://www.minorplanetcenter.net/iau/lists/MPNames.html

• ◊Solar System Formation
• ◊Giant Planets
• ◊Moons
• ◊Kuiper Belt
  Objects
• ◊Space Junk
• ◊Missions

SUMMARY: Asteroids are small, rocky bodies that orbit the Sun much like planets, but are much smaller than planets. In fact, while comets are the size of a typical city, asteroids are about the size of a typical US state. Most asteroids are found in the asteroid belt.

General Characteristics of Asteroids

Asteroids are planetesimals left over from the birth of our solar system and have changed very little since the collapse of the solar nebula 4.6 billion years ago. The largest asteroid, Ceres, is about 1,000 km in diameter and has recently been promoted to dwarf planet status. Smaller asteroids are much more common.

Orbit Of Asteroids

All asteroids orbit the Sun in the same direction as the planets. The vast majority of asteroids that have been catalogued are located in the asteroid belt between the orbits of Mars and Jupiter; however, not all asteroids are located in the asteroid belt. Two sets of asteroids, called Trojan asteroids, share Jupiter's 12-year orbit around the Sun. One clump of Trojan asteroids stays 60 degrees ahead of Jupiter and the other clump stays 60 degrees behind. There are also many renegade asteroids that have highly elongated or unusual orbits, some coming very close to Earth and other planets.

Measuring Asteroid Properties

Orbit Of Asteroid Bennu

Direct observations make it easy for scientists to research asteroids; however, spectroscopic analysis has given us many clues about the composition and properties of asteroids. The light we see shining from asteroids is actually reflected sunlight. The brightness of an asteroid in our sky depends on its size, distance and reflectivity.

We calculate physical size from the asteroid's angular size and distance. We determine an asteroid's distance from its position in its orbit. We determine reflectivity by comparing the brightness of light in the visible spectrum to the brightness of light in the infrared spectrum.

The light astronomers see shining from asteroids is reflected sunlight. Some of the sunlight is reflected and some is absorbed. When an asteroid absorbs light from the sun, it emits this absorbed light back into space in the infrared portion of the spectrum, in other words, as heat. If we compare the amount of visible light to the amount of infrared light, we can establish a ratio of the amount of incoming sunlight an asteroid reflects to the amount it absorbs.

We can also determine the surface compositions of asteroids through spectroscopy.
ASTEROID PROPERTIES IN REGIONS OF ASTEROID BELT:

• OUTER REGIONS: dark, show absorption bands from carbon-rich materials
• INNER REGIONS: more reflective, show absorption bands characteristic of rocky materials
• A small number of asteroids have heavier metals, such as iron

Some asteroids actually have other asteroids as their own orbiting satellites. This has been particularly helpful in measuring asteroid properties. By measuring the orbital period and orbital distance of the satellite, we can use Newton's version of Kepler's 3rd law to calculate the mass of the asteroid. From this, we can calculate the asteroid's density, and with this information we get an important clue of what the asteroid is made of. Check out a list of asteroids with satellites at Johnston's Archive.

Origin and Evolution of Asteroids

Asteroids are concentrated in the asteroid belt because of orbital resonances between asteroids and Jupiter. Asteroids with orbital periods that are a simple ratio of Jupiter's 12-year orbital period experience the same gravitational nudge on a regular basis. For example, asteroids with an orbital period of 6 years (½ Jupiter's period) will feel gravitational tugging from Jupiter when the asteroid and Jupiter line up in their orbits. These asteroids will eventually be pushed out of their orbits altogether. The same is true for asteroids with other simple ratios of Jupiter's orbit, such as 4 years (1/3 Jupiter's orbit) and 3 years (¼ Jupiter's orbit).

This graph shows the number of asteroids with various orbital periods, which correspond to different average distances from the Sun (labeled along the top). Notice the lack of asteroids with periods exactly ½, 1/3 or ¼ of Jupiter's — Jupiter has cleared gaps in the asteroid belt, knocking out asteroids that used to live there.

Once pushed out of the asteroid belt, these asteroids either crashed into a planet or moon or were flung out of the solar system altogether. Asteroids with unusual orbits are the result of Jupiter's gravity, tugging the asteroid out of its original orbit into a new one.

The constant tugging Jupiter's intense gravity has on the asteroid belt explains why no planet ever formed here. Essentially, the asteroids are pieces of a planet that never came to be — a failed planet. Orbital disruptions of planetesimals in this region prevented them from accreting into a full-fledged planet.

MIssions to Asteroids

The NEAR (Near-Earth Asteroid Rendezvous) spacecraft was launched in 1996. It was the first spacecraft to orbit an asteroid and eventually landed successfully on the asteroid Eros in 2001. The Johns Hopkins University Applied Physics Laboratory managed the mission for NASA.

The most recent mission to visit the asteroid belt was launched on September 27, 2007. The Dawn spacecraft will fly by Mars in 2009 on its journey to two of the largest asteroids in the asteroid belt, Vesta and Ceres. Dawn is scheduled to arrive at Vesta in late 2011, and scheduled to arrive at Ceres in early 2015.

RELATED LINKS
Want to land on an asteroid? (video)
Where is Eros now?

Do Asteroids Orbit The Sun

Orbit Shape Of Asteroids

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