A large number of asteroids are present in the universe so far. During the process of skygazing guide, we come across hundreds of thousands of asteroids that are viewable through the telescope today. Although thousands of new asteroids come into existence every year, many of them are not quite visible. The reason lies in the fact that the these celestial objects are so small to that we cannot see them from the Earth.
Skygazing Guide: Known Asteroids
There are 26 known asteroids. They have a diameter of 200 kilometers. About 99% of the discovered asteroids are larger than 100 kilometers in diameter.
Out of 100 kilometers, the diameters of 10 to 100 kilometers include the classified asteroids. There are only a few cataloged asteroids that are smaller in diameter. In the range of 1 kilometer, there are more than a million of them.
However, they are so small that the individual mass of the moon is more than the entire group of all the asteroids collectively.
There are 11 comets and asteroids which are explored by the spacecraft. Some of them are as per their categorisation and along with the spacecrafts as follows:
- Multiple flyby missions to Comet Halley
- Galileo flybys of Gaspra and Ida (and Ida satellite Dactyl)
- Giotto (retarget) to Comet Grigg-Skellerup
- ICE flyby of Comet Giacobini-Zinner
- DS-1 flybys of asteroid Braille and Comet Borrelly
- NEAR-Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros
- Stardust flyby of asteroid Anne frank
- Recent sample collection from Comet Wild 2.
Skygazing guide Tip II: Asteroids Future Discovery
In addition to the Explored Asteroids, there are others that are yet to be discovered. Some of these celestial objects that the astronomers classify in the list of the future asteroids include the following:
- Deep Impact to Comet Tempel 1
- Hayabusa (MUSES-C) to asteroid Itokawa
- Dawn to orbit asteroids Vesta and Ceres
- Rosetta to Comet Churyumov-Gerasmenko
Skygazing guide Tip III: Types of Asteroids
These are classified in several types depending upon their albedo, spectra, and chemical composition. These include:
- C-type Asteroids
- M-type Asteroids
- S-type Asteroids
The C-type includes exceptionally dark (albedo 0.03) asteroids. These are more than 75% known asteroids, more similar to carbonaceous chondrite meteorites. Their composition is identical to that of the Sun. However, we exclude hydrogen, helium, and other volatile gas from it.
Contrary to the C-type Asteroids, the M-type asteroids are brighter (albedo 10-18) as compared to the others. They are composed of pure nickel-iron. There are 58% known M-type asteroids in total.
There are 17% known S-type asteroids. Their composition includes magnesium silicates and iron in a mixture with metallic nickel-iron. The S-type asteroids are relatively bright (albedo 10-22).
The classification of Asteroids in accordance with their position in the solar system is as follows.
- Main Belt Asteroids
- Near-Earth Asteroids (NEAs)
- Trojans Asteroids
The Main Belt Asteroids
The main-belt asteroids are located between Mars and Jupiter and are 2-4 AU away from the Sun. The main-belt asteroids further classify into subgroups:
Phocaea, Eos, Hildas, Hungarians, Koronis, Floras, Cybeles, and Themis.
Near-Earth Asteroids (NEAs)
As compared to the main belt asteroids, these asteroids have a closer approach towards the earth. These include Aten, Amors, and Apollos
In Aten asteroid, their semimajor axes are less than 1.0 AU, and as for an aphelion, the distance Is greater than 0.983 AU.
The Amors asteroids have a perihelion distance (near point to the Sun) is between 1,017 and 1.3 AU.
The semimajor axis of apollos asteroid is more significant than 1.0 AU, and as for the perihelion distance (near point to the Sun), it is less than 1.017 AU.
Trojans – located near Jupiter’s Lagrange points, are 60 degrees behind and ahead Jupiter in its orbit. Hundreds of such types of asteroids are known today, and as the estimate shows, there may be a thousand or more altogether of such entities present in the outer space.
According to the estimate, the leading Lagrange point (L4) asteroids are more than in the trailing one (L5). In the Lagrange points of Venus and the Earth, there are also a few smaller ones. The Earth’s second moon is also sometimes known as Trojans.
A few of them are also present in the outer solar system between Saturn and Uranus. An asteroid Pholus orbits from Uranus to past Neptune. Damocles orbits range from Mars to Uranus. The composition of these is similar to that of the comets and other objects in the Kuiper belt and a little different from ordinary asteroids.
Interesting Facts about Asteroids
Some facts about the asteroids are as follows.
The Composition of C-type Asteroids
Silicate rocks and clay make up most of the C-type asteroids. The sunlight does not alter these asteroids because they are present in the outer belt. Moreover, their orbits are at the farthest points away from the Sun; the C-type asteroids are the most common types in the classification of asteroids.
The irregular shape of asteroids
Since the small gravitational field and smaller size of asteroids, they are of irregular shape. Objects having a large mass attain a large gravitational field. The gravitational force pulls the large items inwards (planets and moons), forcing them in a regular spherical shape because of the immense gravitational field.
As the asteroids have a less gravitational field, they lack this property and thus are irregular shaped. They have enough gravity to hold the materials but not sufficient to force them in a round shape.
Skygazing Tip IV: How to View Asteroids with Telescopes?
The chances of finding out the Asteroids are greater if you hunt near the ecliptic. Although most of the asteroids exhibit a little increase in their brightness near the point that is opposite to the Sun, the success is still inevitable. If we deviate and move away from the ecliptic, the chances of finding the asteroids drastically drop to zero. However, those that live in temperate latitudes can find the ecliptic low in the Sun.