Perhaps, no other category of celestial bodies is surrounded by as many different myths and misconceptions as asteroids. So, today we would like to talk about them. The first part of our material will be dedicated to the history of discovering these bodies and the challenges associated with their classification.

The celestial police are searching

In 1766, the German mathematician Johannes Titius drew attention to a rather curious pattern, formulated as follows: take the number 0.4. Then, add 0.3 to it. Now, add 0.3 multiplied by 2 to the result. Then, add 0.3 multiplied by 4. If we keep doubling the multiplier for 0.3, we get the following series of numbers:

  • 0,4
  • 0,7
  • 1
  • 1,6
  • 2,8
  • 5,2
  • 10
  • 19,6

These numbers correlate accurately with the average distance from the Sun to all known planets in the XVIII century (if we count it in astronomical units).

At first, the strange finding of Titius did not cause much scientific excitement, but it caught on with another German scientist, the astronomer Johann Bode, who expounded on it in his 1772 book. This later earned the name of the Titius-Bode rule.

In 1781, William Herschel discovered Uranus. Its distance to the Sun is 19.2 astronomical units (2.87 billion kilometers), close to the 8th place in Titius’ row. Before that, most astronomers did not pay much attention to the Titius-Bode rule, considering it only a bizarre coincidence. However, after the discovery of Uranus, they started talking about it seriously.

It is easy to see that the sequence above has eight digits, while Uranus is the 7th planet from the Sun. So, if the Titius-Bode rule is correct, then there must be another planet in the solar system with an orbit running between the orbits of Mars and Jupiter. We just need to find it.

The German scientist Franz Xaver von Zach decided to work on this task. For this purpose, in 1800, he created the so-called “celestial police.” Franz Zach assembled a group of 25 astronomers to search for the missing planet. They divided the sky into sectors and prepared for years of searching, and then received unexpected news from Italy. On the first night of 1801, the astronomer Giuseppe Piazzi spotted an object shifting relative to the stars through his telescope. At first, he thought it was a comet, but then, from a number of circumstantial indications, he assumed it was an unknown planet.

The dwarf planet Ceres
The dwarf planet Ceres

Subsequently, other astronomers, including Franz Zach, could find the object Piazzi discovered and calculate that it orbited at an average distance of 2.8 a.u. from the Sun – just as far as the Titius-Bode rule predicted. The newly found body was named Ceres. Ironically, Giuseppe Piazzi was also sent an invitation to join the celestial police, but by the time Ceres was discovered, he simply had not yet received it.

Planets become asteroids

The discovery of Ceres seemed a brilliant confirmation of the Titius-Bode rule. However, strange things soon began to happen. As early as the following year, 1802, the celestial police found another object whose orbit passed at a distance of about 2.8 a. u. from the Sun. It was named Pallada. In the next few years, two more such bodies were found – Juno and Vesta.

Not surprisingly, the question arose within the scientific community as to what to call the newly found four. Since even with the most powerful telescopes of their time, astronomers could not see their disks, and they still looked like stars, William Herschel proposed the term asteroids (i.e., star-like) to refer to them. Still, a significant part of scientists of that era perceived Ceres, Pallas, Juno, and Vesta as planets. In this capacity, they were mentioned in different sources of that epoch. That is, in the first decades of the 19th century, there were formally as many as 11 planets in the solar system.

After the discovery of Vesta, the celestial police searched for new bodies between Mars and Jupiter for several more years, but they ended inconclusively. Therefore, most astronomers decided that there was nothing else there and stopped further research.

However, not everyone thought this was right. One such dissenter was the German amateur astronomer Karl Henke. In 1830, he began his own search. 15 years later, he finally achieved success. Henke discovered a new object called Astrea between Mars and Jupiter.

The discovery shook up the scientific community, causing a surge of interest in the issue. More discoveries soon followed. It finally became clear that we are not a group of a few small planets but an entire belt of many celestial bodies between Mars and Jupiter’s orbits. After that, the term asteroids was finally assigned to them.

size comparison of asteroids

Quite quickly, a new problem was discovered – the number of newly discovered asteroids became so large that the traditional Roman-Greek mythology simply was no longer sufficient for them. Therefore, a rule was introduced, according to which the founder had the right to give the asteroid a name at his own discretion. Albeit with a number of additions and restrictions, this principle still works.

Interestingly, the discovery of Astraea almost coincided with the discovery of Neptune, whose orbit falls completely out of the Titius-Bode sequence. After that, scientists began to see it as an unusual coincidence rather than a strict law.

Originally, astronomers searched for asteroids with their own eyes. The revolution came in 1891 when Max Wolf used the method of astrophotography to detect them, in which asteroids left short, bright lines on images with a long exposure period. While before that, scientists had found an average of a dozen asteroids annually, they started to discover hundreds and thousands of them after the revolution.

The next revolution in the search for asteroids occurred at the turn of the millennium. It was associated with the emergence of automated surveys of the sky and the introduction of specialized software that allowed detecting asteroids on a stream. In 2000, astronomers knew about 100 thousand asteroids. Later this number exceeded one million. Still, it is only the visible part of the iceberg. According to some estimates, there may be several hundred trillion asteroids in the solar system.

Difficulties with classifying asteroids

Although the term “asteroid” is more than 200 years old, it still has no formal definition by the International Astronomical Union (IAU) or any other scientific organization. Previously, the phrase “minor planets” was used as a synonym for the word asteroid. However, when the IAU first defined the term “planet” in 2006, it was dropped from official circulation. It was replaced by the category “minor solar system bodies,” which includes all bodies of irregular shape which orbit the Sun and are not planets, dwarf planets, or their satellites.

At the 2006 IAU Assembly mentioned before, there was another important castoff. Ceres was reclassified as a dwarf planet. Thus, from a formal point of view, the laurels of the most giant asteroid in the solar system went to Vesta.

Although the word “asteroids” still does not have a legal definition, in astronomy, they are usually seen as small bodies in the solar system that are not comets – that is, they do not exhibit cometary activity in the form of coma and tail formation. However, there is a vital nuance – many comets lost their volatile components long ago, so it’s quite difficult to distinguish them from asteroids. Often, such “burned-out comets” give out more elongated orbits, but this is not always the case.

There are several different classifications of asteroids. Depending on their orbital characteristics, they are combined into groups and families. Usually, a group is named after the first asteroid detected in a given orbit. Groups are relatively loose formations, while families are denser formations resulting from the destruction of large asteroids due to collisions with other objects.

Another approach to classifying asteroids is based on their chemical composition and reflective characteristics of their surface (albedo). In the most simplified form, we can distinguish three main types of asteroids: carbon (they have a very dark surface), silicic (stony), and metallic. The first two categories account for about 75% and 17% of all known asteroids.

A visual comparison of some asteroids sizes

More recently, asteroids have also had their own holiday. In 2016, the UN General Assembly proclaimed June 30 as International Asteroid Day. The date was not chosen by chance – it was June 30, 1908, when a celestial body exploded over Siberia, later called the Tunguska meteorite (although it is fair to admit that scientists are still debating about its nature). Nowadays, events are held worldwide to inform the public about asteroid danger and possible methods to prevent asteroid collisions with the Earth.

Where do asteroids live?

The vast majority of all known asteroids are located between the orbits of Mars and Jupiter, known as the Main Belt. Back in the 19th century, it was hypothesized that they might be debris of some dead planet that used to be here. Despite the seductiveness of such an assumption, we now know this is not the case. There could never have been a planet in the asteroid belt because of Jupiter. Its powerful gravity simply didn’t allow any large body to form there. So at their core, asteroids are peculiar remnants of the “building material” that remained unused during the solar system’s formation.

Given the huge number of asteroids, it might seem that a visit to the Main Belt would offer us a spectacular picture in the Star Wars style with a bunch of rocks constantly colliding with each other, but it’s all different. The multiplicity of asteroids is crushed by the sheer volume of space in which they are located. The average distance between asteroids in the Main Belt is about a million kilometers. This is 2.5 times the distance between the Earth and the Moon. A space traveler standing on the surface of an asteroid will, with very few exceptions, simply not see any other asteroids.

We should add that the total mass of all asteroids is relatively small and amounts to only 4% of the mass of the Moon. However, this was not always this way. It is believed that the Main Belt was much more massive in the distant past. However, due to the various gravitational perturbations that accompanied the process of changing the orbits of the giant planets, a significant part of its population was displaced from its original positions. Some asteroids were ejected outside the solar system, and some collapsed onto the planets.

Traces of this bombardment can be easily seen even with ordinary binoculars simply by looking at the Moon. Of course, our planet did not escape the same fate. Undoubtedly, it had a huge impact on its future history. On the one hand, asteroids could deliver water and organic molecules to Earth, which became the building blocks providing the emergence of living beings. On the other hand, these same asteroids nearly wiped out all life on our planet several times. The most famous mass extinction, which wiped out the dinosaurs 66 million years ago, was caused precisely by a ten-kilometer rock impacting the Yucatan Peninsula.

all known asteroids as of January 2018
Known asteroids for 2018

As we have said, most asteroids are located between the orbits of Mars and Jupiter. However, their range is not limited to the Main Belt. To date, astronomers know of a number of asteroids that move in elongated orbits and, at perihelion, approach the Sun closer than Mercury. There is also a whole group of asteroids that occasionally approach our planet at relatively close distances. Such objects are monitored particularly closely because, in the future, they may well change their orbit and collide with the Earth.

Lagrange points (places in space where the gravity of the Sun and a planet effectively compensates each other) can also serve as storage for asteroids. The celestial bodies caught there can remain in a stable state for millions of years. Jupiter has the largest population of such objects, usually called Trojan asteroids. Trojan asteroids have also been found near Mars, Neptune, Uranus, and even Earth.

We should also mention the centaurs. This is the name given to the group of asteroids located between Jupiter and Neptune. They move along strongly elongated and often unstable orbits, which is explained by the gravitational impact of the giant planets. Astronomers estimate that the centaurs have an average life span of 1-10 million years. After that, they are ejected by gravity into new orbits. Later, some may collide with the Sun or other planets, while others fly away into interstellar space.

Given the prevalence of asteroids, it is not surprising that they have been visited by quite a large number of spacecraft. Curiously, the first such encounter occurred not so long ago, in 1990, after humans had landed on the Moon and sent probes to all the planets of the solar system. In our next article, we will discuss how humanity studies asteroids and plans missions to them.