The solar system is the first stage in the human exploration of space. Observation and the desire to learn more about the sun, moon and stars spanned the journey of human beings from prehistoric times to modern civilisation.

With the advent of the space age, humans emerged from the cradle of the earth and launched a series of ambitious explorations. The solar system as we know it today consists of the sun and many smaller celestial bodies. Based on physical properties such as mass, shape and orbital characteristics, these smaller celestial bodies are divided into planets, dwarf planets, small celestial bodies and the Oort Cloud (which defines the cosmographic boundary of the solar system). The Oort Cloud is where the icy objects that we see as a light trail arrive and return from. It is 0.03 to 3.2 light years away and is home to around 100 billion asteroids and comet-like objects. It envelops our solar system like a huge shell and its growth and evolution have been the subject of numerous studies over the years. However, no one had yet succeeded in analysing it in its entirety.

With the launch of NASA’s mission Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) – which took place on 8 September 2016 – attention was turned to asteroids. In this article we will look, in particular, at what asteroids are and why to explore them.

An asteroid is a small celestial body. In astronomy, the name asteroid is used to refer to units of the inner solar system (bounded by the orbit of Jupiter) that orbit the sun.

There is a large number of asteroids in the solar system, mainly distributed in the asteroid belt between the orbits of Mars and Jupiter and the Kuiper belt outside Neptune. Their size ranges from one metre to 800 kilometres. Astronomers classify asteroids into those of the main belt – the near-Earth asteroids, Trojan asteroids (minor bodies sharing a heliocentric orbit with Jupiter), Kuiper belt asteroids, centaurs (a class of icy planetoids), etc. – according to their orbital positions.

Compared to other celestial bodies in the solar system, asteroids have the characteristics of small size, large number and long origin. More than a million asteroids have been discovered and there are currently about 20 known asteroids with a diameter of more than 200 kilometres, while about 99% of asteroids have a diameter of less than 100 kilometres. In terms of numbers alone, they are certainly the most numerous in the solar system.

Most asteroids are located in an area between the orbits of Mars and Jupiter, known as the asteroid belt. The asteroid belt lies between 2.1 and 3.3 AUs from the sun. The astronomical unit (AU) represents the average distance between earth and the sun, i.e.149,597,870.707 kilometres.

The total mass of all the rocks in the asteroid belt, however, is still much less than the mass of the moon. It is estimated from existing observational data that their total mass may only be a small percentage of that of the moon.

Thousands of asteroids have also been discovered in Jupiter’s orbit, known as Trojan asteroids. They gather around Jupiter, forming an approximate triangle with Jupiter and the sun. In terms of celestial mechanics, this orbit can be kept stable between the gravitational forces of the sun and Jupiter.

As ever more objects are discovered, they are collectively referred to as Trojan asteroids. The number of Trojan asteroids is far lower than that of the main belt asteroids. In 2018, at its 30th General Assembly in Vienna, the International Astronomical Union changed this naming convention, allowing it to be named after Olympic athletes, as the number of known Jupiter Trojans, which are currently over ten thousand, far exceeds the number of the available names of the Trojan War heroes in Greek mythology.

Asteroids are currently the only ones among the various types of celestial bodies that can be named according to the wishes of the discoverers and are internationally recognised after being examined and approved by international organisations. Because of the seriousness, uniqueness and permanent immutability of asteroid naming, it has become a recognised honour worldwide to bear the name of an asteroid.

The asteroid name consists of two parts: the first is the permanent number and the second is a name – for example 1 Ceres discovered on 1 January 1801 in Palermo by Giuseppe Piazzi (1746-1826), etc.

In recent years, the detection of asteroids has become one of the main directions of development in the field of deep space exploration of the major countries in the race for space. Asteroids, comets, etc. are all “fragments” left over from the early days of the solar system’s formation, and the same holds true also for the “materials” that form planets and dwarf planets, which are generally believed to have formed before planets.

Asteroids preserve the original components from the early days of the solar system and may contain important clues to the origin of life and water on earth. They are important samples for studying the origin and evolutionary history of the solar system.

It has been speculated that the asteroid belt may be the remnant of a mysterious planet that was destroyed in a giant cosmic collision in ancient times.

As small bodies in the solar system that are less conspicuous in mass and volume, most asteroids revolve around the sun in elliptical orbits like the eight major planets (I say eight because on 24 August 2006, after 76 years of “statistical” presence, Pluto was demoted to a dwarf planet in the aforementioned Kuiper belt). The orbital pattern based on classic rules, however, is often broken and asteroids wander on their own, with their characteristic dangerousness. Most of the holes, of large and small craters on the Moon are, indeed, the “credit” of asteroids, which well records the history of the unexpected visits of these celestial bodies, which are small but not so small as to leave no trace.

While the moon’s impact craters tell of asteroid visits, to date 190 craters have been discovered on earth, with diameters ranging from a few hundred metres to tens of kilometres, and a few even over 100 kilometres, with ages ranging from 50 thousand to two billion years, distributed mainly in North America, Europe and Oceania.

In astronomy, the concept of near-earth asteroids is defined as those asteroids whose minimum distance from the earth is within 0.3 AUs, i.e. 44,879,361.2121 kilometres.

The asteroids with a diameter of more than 140 metres within the minimum orbital distance of 0.05 AUs (7,479,893.53535 kilometres, which is about 20 times the distance between earth and the moon), are referred to as near-earth asteroids (potentially dangerous asteroids) that pose a potential threat to the earth. When the distance between the asteroid and the earth is 7,479,893.53535 kilometres, it can be captured by the strong gravitational force of the earth, change its orbit and run towards the earth until it collides). This danger exists in at least one-tenth of the total number of asteroids.

Because of the existence of these asteroids, the earth is always in danger. The dangers of asteroids striking the earth are mainly earthquakes, tsunamis and environmental disasters caused by very high velocity impacts, as well as panic among people not only in the vicinity of any impacts. The size of damage depends on the mass and velocity remaining after passing through the atmosphere, and these two parameters are related to the asteroid’s initial mass, initial velocity, asteroid structure and angle of impact.

The asteroid enters the earth’s atmosphere at very high speed, forming an extremely strong shock wave at high temperature and high atmospheric pressure, which first causes ionisation of atmospheric molecules and emits light, and then explodes and disintegrates under the interaction of a high-speed superforce and aerodynamic heat.

Disintegrated fragments with a smaller diameter will be reduced to ash in the atmosphere, while disintegrated fragments with a larger diameter will hit the earth surface, quickly releasing the enormous kinetic energy they carry.

If the impact occurs on land, the rocks break, melt and even gasify forming craters, while the shock waves generated by the impact cause strong earthquakes and tsunamis, triggering forest fires. Various gases (such as sulphur dioxide, carbon dioxide), dust and burning ash produced by the surface rocks fill the entire atmosphere and block sunlight.

If the impact occurs in the oceans, huge waves of hundreds of metres and strong tsunamis and earthquakes are produced, and the area of thousands of kilometres along the coast will be extensively flooded. A large amount of seawater evaporates, a large amount of seabed sediments and rock dust are thrown into the stratosphere to remain for a long time, and a large number of living organisms in the ocean would die.

Throughout history, asteroids have frequently struck the earth. Sixty-five million years ago, an asteroid with a diameter of about 10-13 kilometres hit the Yucatán Peninsula in Mexico at a speed of about 20 km/s, forming a crater with a diameter of 198 kilometres, causing 50% to 60% of the earth’s biological extinction. This is considered the cause of dinosaurs’ extinction.

On 30 June 1908, an asteroid with a diameter of about 30-50 metres hit the earth at a speed of 30-40 km/s and exploded over the Tunguska River (near Vanavara, located in the then Enisejsk Governorate in Siberia). It was equivalent to between 10 and 15 megatons, i.e. to about a thousand Hiroshima bombs, burning 80 million trees over two thousand square kilometres.

Asteroid transits still occur frequently today. Astronomers have been keeping a close eye on near-earth asteroids. According to data from the Minor Planet Centre, 22,268 near-earth asteroids were discovered in February 2020 alone, of which 906 have a diameter of more than one kilometre and 2,073 pose potential hazards.

At present, earth-threatening asteroids are continuously discovered through sky-tracking observations to calculate changes in their orbits and give early warning.

Giancarlo Elia Valori