What is an asteroid? Asteroids are small stony celestial bodies that revolve around the sun, usually between 10 meters and 1,000 kilometers in diameter, and when they are less than 10 meters, they are called meteoroids. Generally speaking, asteroids only refer to small celestial bodies in the inner solar system (within the orbit of Jupiter).
Asteroids evolved from small planets that were not incorporated into the planets of the solar system. They formed in different regions of the solar system and recorded the physical and chemical characteristics of the environment in which they were formed. Scientists know the chemical composition of the solar system and the formation age of the earth and solar system through rocks (meteorites) from asteroids.
Water and life are likely to be transported to the earth through asteroids. Asteroid impacts have also played an important role in the evolution of the earth system. There have been many asteroid impact events on the earth. So far, the impact of some near-Earth asteroids (NEO) on the Earth is like the sword of Damocles hanging above humans. An asteroid with a diameter of about 1.5 kilometers is enough to destroy human civilization. Therefore, more and more asteroid research teams in the world are focusing on the disaster warning research of near-Earth asteroids. These work are not only for those asteroids that are known to have potential threats to the Earth, but more importantly, to discover. Those unknown near-Earth asteroids that are potentially threatening to the Earth.
With the continuous development of aerospace science and technology, mankind has begun to look for resources to asteroids outside the earth. Different asteroids may be rich in water, organic matter, precious metals, etc. These will be future moon bases, Mars bases and even space. Important resources required by the base.
More than 800,000 asteroids have been confirmed so far. This may be only a small part of them, and a large number of asteroids are discovered every year. The number of asteroids increases exponentially as the mass decreases.
The largest asteroid in the asteroid belt is Ceres, also known as a dwarf planet, with a diameter of 939 kilometers. The second largest asteroid is Vesta, with a diameter of 525 kilometers, and is considered to be the parent asteroid of the HED meteorite.
Spectral classification: The most important way for humans to study asteroids is through various ground-based observations. One of the commonly used methods is to measure the asteroid’s visible-near infrared spectrum. According to the different spectra of small stars, they are mainly divided into five major spectral types: C-type, S-type, M-type, E-type and V-type.
C-type asteroids are usually darker in color because they are rich in carbon, and the mineral particles they make up are usually very small. These two reasons make their surface albedo very low, only 0.05. C-type asteroids are huge in number, accounting for about 75% of all asteroids, and are considered to be the source of carbonaceous chondrites. The Ryugu asteroid is of type C. Currently, the Hayabusa 2 probe of the Japan Aerospace Exploration Agency (JAXA) is carrying samples from the surface of Ryugu to return to Earth, and it is expected to return to Earth before December 2020.
S-type asteroids are the second most numerous type of asteroids. They are usually distributed inside the asteroid belt and have a high albedo, usually between 0.15 and 0.25. The near-Earth asteroid Itokawa belongs to the S-type asteroid. Japan’s JAXA Hayabusa-1 brought about 1,500 particles from the surface of the Itokawa asteroid back to Earth on June 13, 2010. Studies have shown that its material composition is consistent with the LL group of ordinary chondrites. This also directly proves that it is scientifically feasible to link the remote sensing spectra of asteroids with meteorites.
M-type asteroids are the third most common type of asteroids. The albedo of this type of asteroid is lower than that of S-type asteroids, and may be mainly composed of iron-nickel metals. The meteorite types corresponding to this type of asteroid are iron meteorites and enstatite chondrites.
The number of E-type asteroids is small, mainly composed of enstatite, so the color is very light, so it has a very high albedo (generally greater than 0.4). The meteorite corresponding to this type of asteroid is enstatite achondrite. The Norton County meteorite that landed in Kansas, USA in 1948 is the most well-known enstatite achondrite.
V-type asteroids are relatively rare, and the surface of this type of asteroid contains a lot of pyroxene, and its spectrum has significant absorption peaks at 0.8 microns and 2 microns, respectively. Vesta (VESTA) is a V-type asteroid.
Orbital classification: According to their orbital position, asteroids are classified as main belt asteroids, Trojan asteroids and near-Earth asteroids.
The orbits of the main belt asteroids are located in a wide area between Mars and Jupiter. They usually have a low orbital eccentricity. There are about 1-2 million small asteroids larger than 1 km in diameter and millions of small ones under 1 km in this area planet. Ceres, Vesta, Juno, Zhiren and Miriam all belong to the main belt asteroids.
Trojan asteroids share an orbit with Jupiter, and they are distributed at 60 degrees in front of and behind Jupiter’s orbit. Asteroids with similar distributions in other planetary orbits are also called Trojan asteroids. At present, it has been discovered that Venus, Earth, Mars, Uranus and Neptune all have Trojan asteroids in the orbits, but Jupiter has a huge number of Trojan asteroids, with thousands of them. The Jupiter Trojan asteroid has been identified.
Near-Earth asteroids are those asteroids that orbit close to the Earth. According to their relationship with the Earth’s orbit, they are divided into Amor, Apollo and Aten. ). Amor-type asteroids cross the orbit of Mars and are close to the orbit of the Earth. Eros is a representative of this type of asteroid. The asteroid’s closest distance to the Earth is 0.15 AU.
The orbits of Apollo-type asteroids are located between Mars and the Earth. Some of these asteroids have very high orbital eccentricities, and their perihelion can reach within the orbit of Venus. The Apollo asteroid is a representative of this type of asteroid, with an orbit between 0.65 and 2.29 AU. The orbits of Arden-type asteroids are usually within the orbit of the Earth. Such asteroids are named after the Ardennes discovered in 1976. Such asteroids have a relatively high orbital eccentricity and sometimes intersect with the Earth’s orbit.
The history of asteroid exploration
So far, human beings have successfully carried out vehicle exploration to 15 asteroids, most of which were completed by NASA.
Galileo (Galileo) was launched by NASA in 1989 with the main goal of detecting Jupiter and its satellites. On the way to Jupiter, the Galileo probe detected two asteroids incidentally in 1991 and 1993 respectively. In 1991, the Galileo probe realized the first approach of human beings to an asteroid and obtained images of the S-type main belt asteroid Gaspra. The probe approached the S-type near-Earth asteroid Ida in 1993 and discovered Ida’s satellite Dactyl.
The main goal of NASA’s launch in China is to spend more than a year orbiting to detect the Amor-type near-Earth S-type asteroid Eros, and finally the vehicle descends to the southern part of Eros under control. On the way to Eros, the Rendezvous-Shoemaker flew to the main belt C-type asteroid Mathilde on June 27, 1997.
Deep Space 1 was launched by NASA in 1998. After completing its flight to the main belt asteroid Braille, the aircraft flew to the comet Borrelly.
Stardust was launched by NASA in 1999. Its main goal is to collect dust particles of comet Wild 2 and return to Earth. In the end, the probe successfully collected samples of the comet and returned to Earth safely in 2006. During the mission, the aircraft flew over the main belt S-type asteroid Annefrank on November 2, 2002. Image analysis showed that the asteroid was twice the expected size.
Hayabusa was launched by Japan’s JAXA in 2003. After 7 years of travel in the solar system, the spacecraft finally overcame many difficulties and brought more than 1,500 precious asteroid dust samples back to Earth. The Itokawa asteroid detected and sampled is an S-type near-Earth asteroid with a diameter of about 500 meters, which is considered to be a potential threat to the safety of the earth. For the first time, Hayabusa made a breakthrough for humans to sample and return from the surface of an asteroid.
Rosetta is a probe launched by the European Space Agency (ESA) in 2004. It consists of the Rosetta probe and the Philae lander. Its purpose is to land the Philae lander on comet 67P. (Churyumov–Gerasimenko). On the way to comet 67P, the spacecraft flew to the main belt M-type (or C-type) asteroid Lutetia and the main belt E-type asteroid respectively? teins.
New Horizons (New Horizons) is a probe designed by NASA to study the dwarf planet Pluto and its satellites. It was launched in 2006 and is still performing missions to this day. On June 13, 2006, New Horizons flew over the main belt S-type asteroid APL, and the observation of the asteroid’s arrival was not planned. In January 2019, New Horizons flew to the Kuiper belt asteroid Arrokoth, which is peanut-shaped in shape. This is the first time the probe has flew to a small Kuiper belt celestial body.
The Dawn probe was launched by NASA in 2007 to detect the two largest asteroids in the asteroid belt of the solar system, Cere and Vesta. The spacecraft took four years to reach Vesta. , After about 14 months of exploration of Vesta, it flew to Ceres in September 2012. On March 6, 2015, the Dawn probe arrived in the orbit of Ceres. Currently, the spacecraft is still in the orbit near Ceres.
Chang’e 2 (Change 2) is the backup satellite of the first phase of my country’s lunar exploration project, which has undergone technical improvements. It took off in 2010. After completing its lunar exploration mission, Chang’e-2 passed the Toutatis asteroid at the closest distance of 3.2 kilometers on December 13, 2012, and obtained a series of parameters related to the Toutatis asteroid. The Toutatis asteroid is an S-type near-Earth asteroid with a potential safety threat to humans, with a diameter of about 2.45 kilometers.
Hayabusa 2 was launched by JAXA in Japan on December 3, 2014. The main goal is to sample from the surface of the C-type near-Earth asteroid Ryugu and return to Earth. Since the surface of the Dragon Palace is more rugged and complex than expected, the execution of the sampling task had to be postponed in order to ensure the smooth completion of the sampling. Finally, the surface samples of the Ronggong asteroid were collected on February 21, 2019, and the subsurface samples of the Ronggong asteroid were collected on June 5, 2019. Hayabusa 2 is currently carrying samples on its way back to Earth, and is expected to arrive on Earth by the end of 2020.
OSIRIS-Rex is a NASA-led asteroid research and sampling return mission. The probe was launched on September 8, 2016, and arrived at the C-type near-Earth asteroid Bennu on December 3, 2018. track. Due to the large amount of gravel on the surface of Bennu and the relative lack of topsoil, this has brought great challenges to the safety of sampling. Therefore, the sampling task has been postponed. collection.
With the rapid development of space technology, mankind will devote more and more energy to the field of space exploration. Asteroid disaster warning and response strategy research, sampling return research, resource development and utilization are the main development directions in the future. Let us look forward to the arrival of a prosperous era of asteroid exploration.