And Hua Feng, Bai Feng, and Yun Meng learned that the largest asteroid so far in the 1990s was Ceres.

Asteroid data available prior to 1991 were made only through ground-based observations. In October 1991, the Galileo probe visited 951 GASP

asteroid, thus obtaining the first high-resolution image of an asteroid. In August 1993, Galileo flew by asteroid 253, making it the second asteroid to be visited by a spacecraft. Gasp

Both the A and Ida asteroids are metal-rich and belong to the S-type asteroids.

Much of what we know about asteroids is through the analysis of space debris that has fallen to the Earth's surface. Those asteroids that collide with the Earth are called meteoroids. When a meteoroid breaks into our atmosphere at high speed, its surface vaporizes due to the friction with the air and emits a strong light, which is called a meteor. If a meteoroid does not burn up completely and falls to the ground, it is called a meteorite.

In 1999, Deep Space 1 visited 9969 B. Brell (B

aille）

2002 Stardust visited Anne Frank (A

ef

a

k）。

Some asteroids discovered in the Kuiper Belt since the 21st century are larger in diameter than Ceres, such as Va, which was discovered in 2000

u

a) with a diameter of 900 km, discovered in 2002 by Quaoa

) has a diameter of 1,280 kilometers, and the diameter of the Erquus discovered in 2004 may even reach 1,800 kilometers. The discovery of Sedna (asteroid 90377) in 2003 is located outside the Kuiper Belt and is about 1,500 kilometers in diameter.

In September 2005, the Japanese spacecraft Hayabusa arrived at 25143 to conduct a detailed survey and possibly bring back some samples back to Earth.

The next asteroid exploration program is the European Space Agency's Rosetta (launched in 2004), which is expected to detect 2867 Štei in 2008 and 2010

s and 21 Lutesia.

In 2007, NASA launched the Dawn spacecraft.

On April 19, 2017, an asteroid named 2014JO25 with a diameter of about 600 meters will "pass by" the Earth at "very close range", which was discovered by scientists in May 2014. On April 19, 2017, it will approach the Earth from the direction of the Sun, passing by the Earth at a distance of 4.6 times that of the Earth and the Moon. One or two nights can be observed with the help of small optical telescopes.

It is also the closest the asteroid has come to Earth in 400 years, and the next one will not be until 500 years later. This is the closest since 2004, 13 years ago, when the asteroid Thutatis, about 5 kilometers in diameter, passed by the Earth at four times the distance from the Earth and the Moon. The next similar event will not occur until 2027, when the 1999AN10 asteroid, with a diameter of about 800 meters, will fly past the Earth at a distance from the Earth-Moon.

As of December 24, 2017, 17,495 NEOs have been discovered around the Earth, including 17,389 asteroids, NASA spokesperson said.

NASA is advancing the development of a refrigerator-sized spacecraft that could prevent an asteroid from colliding with Earth, and plans to test it with an asteroid that poses no threat to Earth in 2024. This is the first-ever mission to demonstrate the technology to change the orbit of an asteroid. The "Double Asteroid Orbit Change Test" will use the so-called kinetic impact technology – striking an asteroid to change its orbit.

In May 2018, the European Southern Observatory announced that an international team of researchers had discovered a carbon-rich asteroid off Neptune, about 4 billion kilometers from Earth, using its Very Large Telescope in Chile. This is the first time that astronomers have discovered such objects in the marginal region of the solar system, which is expected to provide a basis for studying the early formation of the solar system.

Many people believe that asteroids are remnants of matter after the formation of the solar system.

There is a speculation that they may be the remains of a mysterious planet that was destroyed by a massive cosmic collision in ancient times. But judging by the characteristics of these asteroids, they don't look like they once came together. If all the asteroids were added together to form a single celestial body, it would be less than 1,500 kilometers in diameter – smaller than the radius of the Moon.

At first, astronomers thought that the asteroid was formed by the rupture of a planet between Mars and Jupiter, but the total mass of all the asteroids in the asteroid belt is smaller than the mass of the Moon. Astronomers believe that asteroids are remnants of material that did not form planets during the formation of the solar system. Jupiter's mass grows the fastest when it forms in the solar system, and it prevents the formation of another planet in the asteroid belt region.

The orbits of asteroids in the asteroid belt region are disturbed by Jupiter, and they are constantly colliding and shattering. Other matter was expelled from their orbits and collided with other planets. Large asteroids heat up after formation due to the decay of the radioisotope 26Al of aluminum (and possibly 60Fe, the radioisotope of iron).

Heavy elements such as nickel and iron sink into the interior of the asteroid in this case, while light elements such as silicon float up. This results in the separation of the material inside the asteroid. The composition of the new asteroids created after subsequent collisions and ruptures is therefore different. Some of these fragments later fell to Earth and became meteorites.

An asteroid is a celestial body in the solar system that orbits the sun like a planet, but is much smaller in size and mass than a planet. See the list of asteroids for details, while the largest asteroids are beginning to be reclassified and defined as dwarf planets.

There are about 16 asteroids with a diameter of more than 240 km. They are both located in space from the outer side of the Earth's orbit to the inner side of Saturn's orbit. The vast majority of asteroids are concentrated in the asteroid belt between the orbits of Mars and Jupiter. Some of these asteroids have orbits that intersect Earth's orbits, and some have collided with Earth.

According to the statistical analysis of the number of orbital roots, the number of asteroids with an orbital inclination of about 5 degrees and an eccentricity of about 0.17 is the largest. Kirkwood Crack is the most well-known distribution feature based on the mean heliocentric distance of an asteroid. There is a statistical relationship between the asteroid number N and the mean opposition magnitude m, logN=0.39m-3.3, and the statistical formula logd(km)=3.7-0.2g between the diameter d of the asteroid and the absolute magnitude g satisfies the statistical formula. The number of asteroids varies with diameter around 30 km in diameter.

About 90% of known asteroids have orbits in the asteroid belt. The asteroid belt is a fairly wide strip of land located between Mars and Jupiter. The first asteroids to be discovered, such as Ceres and Homo sapiens, are all asteroids within the asteroid belt.

Asteroids in Martian orbit generally divide into three groups:

Amor-type asteroid swarms: These asteroids travel through the orbit of Mars and come to the vicinity of the Earth's orbit. Its representative asteroid is the asteroid 433, discovered in 1898, which can reach a distance of 0.15 AU from Earth. When the asteroid 433 came near Earth in 1900 and 1931, astronomers used the opportunity to determine the size of the solar system.

The asteroid 719, discovered in 1911, went missing and was not rediscovered until 2000. The asteroid group named the star asteroid 1221 Amor orbits at a distance of 1.08 to 2.76 AU from the Sun, which is quite typical of one of the orbits of this group.

Apollo Asteroid Group: The asteroids of this asteroid group orbit between Mars and Earth. Some of the asteroids in this group have very high eccentricities in their orbits, and their perihelion all the way to the orbit of Venus. The typical asteroid orbit of this group is the asteroid Apollo 1862, discovered in 1932, which has an orbit between 0.65 and 2.29 AU. Asteroid 69230 flies past Earth at a distance of only 1.5 moons.

Ardennes asteroid group: The orbit of asteroids in this group is generally within the Earth's orbit. Its named star is the asteroid 2062 Ardennes, discovered in 1976. Some of the asteroids in this group have a high eccentricity, and they may intersect the Earth's orbit from within the Earth's orbit.

These asteroids are collectively known as near-Earth asteroids. The study of these asteroids is deepened because they are at least theoretically possible to collide with Earth. Some of the more successful projects include the Lincoln Near-Earth Asteroid Research Program (LINEAR), the Near-Earth Asteroid Tracking (NEAT) and the Lowwell Observatory Near-Earth Object Search Program (LONEOS).

Asteroids orbiting the Lagrangian point in orbit of other planets are called Trojan asteroids. The first Trojan asteroids to be discovered were asteroids in Jupiter's orbit, some of which were in front of Jupiter and some of which orbited behind Jupiter. Representative Jupiter Trojan asteroids are Asteroid 588 and Asteroid 1172. The first Martian Trojan asteroid, Asteroid 5261, was discovered in 1990, and four other Martian Trojan asteroids have been discovered since then.

The asteroids between Saturn and Uranus have a group of asteroids known as the Centaur Asteroid Group, all of which have considerable eccentricities. The first discovered asteroid in the Centaur asteroid group was Asteroid 2060. It is estimated that these asteroids fell into an unstable orbit from the Kuiper Belt due to gravitational interference from other large planets.

The full name is the Edgewos-Kuiper Belt

th-Kuipe

belt; EKB, generally referred to as the Kuiper belt, or translated as the Cooper belt, Cooper belt, etc.) The yellow dot ring is the Kuipe belt

Belt)

92.8% of the composition is silica (rock), 5.7% is iron and nickel, and the remainder is a mixture of these three substances. Meteorites with large stone content are called stony meteorites, accounting for 93.3% of the total number of meteorites, meteorites with large iron content are called meteorites, accounting for 5.4% of the total number of meteorites, and meteorites with a mixture of rock and iron-nickel alloy are called stony iron meteorites, accounting for 1.3% of the total number of meteorites. Because meteorites are very similar to Earth's rocks, they are more difficult to identify. It is estimated that the number of asteroids could be about 500,000. The largest asteroids are only about 1,000 kilometers in diameter, while miniature asteroids are the size of pebbles.