According to the spectral type, asteroids can be divided into C, s, m and so on. Among them, M-type asteroids are mainly composed of metals. It is estimated that about 5% of the asteroids in the solar system are M-type. These asteroids are rich in all kinds of rare metals on earth. For example, the No. 16 asteroid "spirit star" has a diameter of about 250 kilometers. Through observation, scientists estimate that it contains 500 million tons of iron, 5000 trillion tons of nickel and hundreds of millions of tons of other rare metals.
In the past, "mining" on asteroids was just an illusion, but with the continuous strengthening of deep space exploration capability, mankind will have the ability to mine asteroids. Both the United States and Japan have launched probes to carry out unmanned close-up exploration of asteroids. The United States is planning to carry out manned landing exploration of near Earth Asteroids in 2025. It is believed that with the enhancement of human deep space exploration ability, the development and utilization of the rich resources on asteroids will no longer be an illusion.
At present, unmanned spacecraft that have been launched or plan to launch mainly detect asteroids by flying over, flying around, attaching, sampling and returning, etc. Overflight detection refers to the detection that the detector passes over an asteroid at a high speed (usually greater than 1 km / s) and at a short distance (thousands of meters to thousands of kilometers) and carries out optical imaging on it. Flying around detection refers to the form that the detector runs around the asteroid to detect the asteroid.
For some small asteroids with small mass, if there is not enough gravity to make the detector fly around them, they will be detected in the form of accompanying flight. Attachment detection means that the detector lands on the surface of an asteroid to detect it at a short distance. Because asteroids usually have small mass and very small gravity, the process of the probe landing on their surface can only be called "attachment" rather than "landing". Sampling return refers to the detection form in which the probe collects material samples from the asteroid surface and sends them back to the earth.
As of 2011, humans have launched eight probes to detect asteroids in the form of overflight, flying around, attachment, sampling and return. Five of them succeeded and three were still on the way. The early mission is mainly overflight, and the later stage gradually transitions to companion flight, attachment and sampling return.