Asteroid belt: facts and training



Scattered in orbits around the sun, pieces of rock left by the dawn of the solar system. Most of these objects, called planetoids or asteroids – meaning “star-like” – orbit between Mars and Jupiter in a group known as the main asteroid belt.

The main asteroid belt is more than two and a half times farther than Earth from the sun. It contains millions of asteroids, according to NASA. Most of them are relatively small, from the size of rocks to a few thousand feet in diameter. But some are significantly bigger.


At the start of the solar system’s life, the dust and rocks surrounding the sun were gathered by gravity into planets. But not all the ingredients created new worlds. A region between Mars and Jupiter has become the asteroid belt.

Sometimes people wonder if the belt was made up of the remains of a destroyed planet or a world that hasn’t quite started. However, according to NASA, the total mass of the belt is less than that of the moon, far too small to weigh like a planet. Instead, the debris is guided by Jupiter, which has prevented it from merging on other growing planets.

Observations of other planets help scientists better understand the solar system. According to a developing theory known as the Grand Tack, during the first 5 million years of the solar system, Jupiter and Saturn are believed to have moved inward toward the sun before changing direction and returning. to the external solar system. Along the way, they would have scattered the original asteroid belt in front of them, then sent back material to fill it.

“In the Grand Tack model, the asteroid belt was purged at a very early stage and the surviving members are sampling a much larger region of the solar nebula,” John Chambers of the Carnegie Institution for Science wrote in an article. Perspectives ”published online in the journal Science.

Our solar system is not the only one with an asteroid belt. A cloud of dust around a star known as zeta Leporis looks a lot like a young belt. “Zeta Leporis is a relatively young star – roughly the age of our sun when Earth was forming,” Michael Jura said in a statement. “The system we observed around Zeta Leporis is similar to what we believe to have taken place in the early years of our own solar system when planets and asteroids were created.” A professor at the University of California at Los Angeles, Jura has since passed away.

Other stars also contain signs of asteroid belts, suggesting that this may be common.

At the same time, studies of white dwarfs, sun-like stars at the end of their life, show signatures of rocky material falling to their surface that suggest such belts are common around dying systems.

Asteroids, like Itokawa, pictured here are believed to be more like piles of rubble stuck together than solid chunks of rock. (Image credit: ISAS / JAXA)


Most of the main belt asteroids are made of rock and stone, but a small portion of them contain iron and nickel. The remaining asteroids are made up of a mixture of these, as well as carbon-rich materials. Some of the more distant asteroids tend to contain more ice. Although they are not large enough to hold an atmosphere, there is some evidence that some asteroids contain water.

Built and operated by the Applied Physics Laboratory at Johns Hopkins University in Laurel, Md., NEAR was the first spacecraft launched as part of NASA’s Discovery Program of Small-Scale, Low-Scale Planetary Missions cost. (Image credit: NASA / JPL / JHUAPL)

Some asteroids are large, solid bodies – there are over 16 in the belt with diameters greater than 150 miles (240 km). The largest asteroids, Vesta, Pallas and Hygiea, are 400 km long and more. The region also contains the dwarf planet Ceres. With a diameter of 590 miles (950 km), or about a quarter of the size of our moon, Ceres is round but is considered too small to be a full-fledged planet. However, it makes up about a third of the mass of the asteroid belt. [Gallery: Asteroid Pictures]

Other asteroids are clusters of rubble held together by gravity. Most asteroids are not massive enough to achieve a spherical shape and are rather irregular, often resembling a lumpy potato. Asteroid 216 Kleopatra looks like a dog bone.

Asteroids are classified into several types based on their chemical composition and reflectivity, or albedo.

  • type C asteroids represent over 75 percent of known asteroids. The “C” stands for carbon, and the surfaces of these extremely dark asteroids are almost carbon black. Carbonaceous chondrite meteorites on Earth have a similar composition and are considered to be broken pieces of the larger asteroids. While C-type asteroids dominate the belt, according to the European Space Agency, they only make up about 40% of the asteroids closer to the sun. These include B-type, F-type, and G-type subgroups.
  • S-type asteroids are the second most common type, accounting for about 17 percent of known asteroids. They dominate the inner asteroid belt, becoming rarer further away. They are brighter and contain metallic nickel-iron mixed with iron and magnesium silicates. The “S” stands for siliceous.
  • M-type asteroids (“M” for metallic) are the last major type. These asteroids are quite bright and most of them are made of pure nickel iron. They tend to be found in the middle region of the asteroid belt.
  • The rare remaining types of asteroids are type A, type D, type E, type P, type Q, and type R.

In 2007, NASA launched a mission, Dawn, to visit Ceres and Vesta. Dawn reached Vesta in 2011 and stayed there for over a year before traveling to reach Ceres in 2015. It will remain in orbit around the dwarf planet until the end of its mission. [Related: Asteroid Vesta and NASA’s Dawn Spacecraft]

While most of the asteroid belt is made up of rocky objects, Ceres is an icy body. Clues of organic matter spotted by Dawn suggest it may have formed further into the Solar System before landing in the Belt. Although organic matter was only seen on the surface, that doesn’t mean more matter could be on the dwarf planet.

“We cannot rule out that there are other places rich in organic matter not sampled by the survey, or below the detection limit,” said Maria Cristina De Sanctis, of the Institute of Space Astrophysics and Space Planetology in Rome, to by e-mail.

Build a belt

The main belt lies between Mars and Jupiter, about two to four times the Earth-Sun distance, and spans an area about 140 million kilometers in diameter. The objects in the belt are divided into eight subgroups named after the major asteroids in each group. These groups are the Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas.

Although Hollywood often flaunts ships making close stops across asteroid belts, the trip is generally uneventful. A number of spacecraft have traveled safely through the asteroid belt without incident, including NASA’s New Horizons mission to Pluto.

“Fortunately, the asteroid belt is so huge that, despite its large population of small bodies, the chance of meeting one is almost slim – well less than one in a billion,” wrote Alan Stern, senior researcher at New Horizons. “If you want to get close enough to an asteroid to do detailed studies, you have to aim for one.”

Within the asteroid belt are relatively empty regions known as the Kirkwood Gaps. These gaps correspond to orbital resonances with Jupiter. The gas giant’s gravitational pull keeps these regions much more empty than the rest of the belt. In other resonances, asteroids may be more concentrated.

Discovery of the asteroid belt

Johann Titius, an 18th-century German astronomer, noted a mathematical pattern in the arrangement of planets and used it to predict the existence of a pattern between Mars and Jupiter. Astronomers have scoured the skies in search of this missing body. In 1800, 25 astronomers formed a group known as the Celestial Police, each looking 15 degrees from the zodiac for the missing planet. But the discovery of the first body in this region came from a non-member, the Italian astronomer Giuseppe Piazzi: he named it Ceres. A second body, Pallas, was found just over a year later.

For a while, these two objects were called planets. But the rate of discovery of these objects increased, and by the beginning of the 19th century, more than 100 had been found. Scientists quickly realized that they were too small to be considered planets, and they began to call them asteroids.

Editor’s Note: This article has been updated to reflect a correction on November 2, 2018. The original article stated that there could be billions or even billions of asteroids in the main asteroid belt.

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