SOLAR SYSTEM FAMILY-FACTS ABOUT THE ASTEROID BELT
The space between Mars and Jupiter is occupied by numerous irregularly shaped objects that are hundreds of magnitudes smaller than the smallest planet, Mercury. These objects form a torus shaped belt, called the ''Asteroid Belt'' or specifically, the ''Main Asteroid Belt''. In short, the asteroid belt is a region of rocky bodies(also called planetoids) which never got the chance to fully develop into planets and thereby settled as relics of the violent collisions that created all the major objects of our Solar System.
Discovery & Early Observations: The asteroid belt is invisible to the unaided eye, and it's existence can only be determined from careful observations of the orbital motion of Mars and Jupiter. During the 16th century, the early astronomers knew of only six planets viz., Mercury, Venus, Earth, Mars, Jupiter and Saturn. But detailed observation revealed that something was not right with the orbits of Mars and Jupiter and there was a huge gap in between them. Further observations made it clear that there is indeed some object(probably a planet) to account for the discrepancies in their orbital motions.
Among the prominent astronomers, Johannes Kepler in 1596, while analyzing Tycho Brahe's observational data, found that the empty region between Mars and Jupiter were too large, and he predicted that a planet must exist between the two. Then came a law(at present it is a discarded law) called the Titus-Bode Law named after their proposers Johann Daniel Titus and Johann Elert Bode during the mid-eighteenth century. The Titius-Bode Law, which is a numerical sequencing of the position of planets from the Sun suggested that each planet should be twice as far away from the Sun as the one before. This provided an approximation to the orbital radii of the planets known at that time and also predicted that there must be a planet in between Mars and Jupiter. With the discovery of Uranus in 1871 by William Herschel, and based upon the predictions of the Titius-Bode law, astronomers finally became sure that a planet was indeed missing.
The search for the missing planet reached such a height that a group of astronomers in Germany, who tirelessly scanned the night sky started calling themselves the ''celestial police''. The planet was ultimately discovered by an astronomer in Sicily, called Giuseppe Piazzi, on January 1, 1801. At first he believed the unknown object to be a comet, but its lack of coma made him conclude that he had discovered a new planet. The missing planet was named ''Ceres'', after the Roman Goddess of harvest. Fifteen months later, another astronomer Heinrich Olbers discovered a second object, called ''Pallas'' in the same region. And by 1845 two more i.e., ''Juno'' and ''Vesta'' were discovered. With subsequent discoveries, these objects under the suggestions of William Herschel were classified as ''Asteroids''-a name derived from the Greek word ''Asteroides'' meaning ''Star-Like''.
Starting from the mid-nineteenth century with the discovery of the asteroid ''Hygiea'' by Annibale de Gasparis, up to modern times, countless more have joined the list. The current estimates for the number of asteroids in the main belt stand at more than a million pieces, ranging from small lumps of loose rock and gravel to minor planets.
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| The white torus is the asteroid belt/Image Credits: Wikimedia Commons |
Origin & Formation Mechanism: The study of asteroids is of profound importance if we want to have a complete idea about the formation of our solar system and its subsequent evolution. Apart from that, it is also speculated that asteroid impacts on Earth might be the reason for the abundance of water on our home planet as asteroids contain a lot of ice.
After the discovery of Pallas, in addition to Ceres, Olbers suggested that both of them are fragments of a larger planet that once existed between Mars and Jupiter. Another astronomer, K.N. Savchenko gave the idea that the four little planets(later to be designated as dwarf planets) viz., Ceres, Juno, Pallas and Vesta were perhaps escaped moons. These theories were later discarded because the massive amount of energy required to destroy a planet didn't quite match with the low density of the asteroid belt. Further, a significant difference of chemical composition also suggested that the belt was not a by-product of some exploded planet but was rather a collection of ancient planetesimals which never got the chance to develop into planets.
Based upon thorough examination and computer simulations it can be ascertained that the asteroid belt is definitely a product of planet formation. The Nebular Hypothesis states that the Sun started its journey from the gravitational collapse of a molecular gas cloud(or a stellar nebula) and in turn formed an accretion disc around its periphery. Within the dense accretion disc, molecules of gas and dirt collided amongst themselves and condensed to form small clumps of solid matter. These clumps transformed into planetesimals, which under the action of gravity developed into planets. There were some planetesimals which could have condensed into a planet, but they were perturbed by the strong gravitational influence of Jupiter when the latter migrated a few astronomical units towards the young Sun(Jupiter might have formed much farther than its present distance from the Sun). This inward migration exerted too much gravitational forces and the planetesimals instead of coalescing into a single planet started orbiting the Sun as a bunch of irregular rocks.
General Characteristics: The asteroid belt is a toroidal region(like a doughnut) located at a distance of 2.2 and 3.2 AU(Astronomical Units, where 1 AU is the average distance of 150 million km between the Sun and Earth) or 329-478.7 million km from the Sun. The belt possesses a thickness of 149.6 million km and its mass is 2.39✕10²¹ kg(3% of Moon's mass). The temperature of the belt ranges from 200 K at the inner region of 2.2 AU to 165 K at the outer region of 3.2 AU.
The asteroid belt, contrary to popular imager, is quite empty as the relative distance between two asteroids amounts to somewhat 966,000 km. There are more than a million asteroids ranging in size from a few meters to a few hundred kilometers. It has also been found that certain asteroids, which are comparatively bigger, have even got moon(s) of their own. The belt is divided into three distinct regions, based upon calculation of orbital resonances with Jupiter and formation of Kirkwood Gaps. They are the inner region, the middle region and the outer region
The four largest objects are the dwarf planets-Ceres(mean diameter=939 km), 4 Vesta(mean diameter=525 km), 2 Pallas(mean diameter=513 km) and 10 Hygeia(mean diameter=434 km). Apart from these, other notable asteroids include the first asteroid to be imaged by a space-craft i.e., 951 Gaspra(mean diameter=12.2 km), 704 Interamnia(mean diameter=330 km), 3 Juno(mean diameter=247 km), and 16 Psyche(mean diameter=225 km).
Composition: The asteroids are not relics from the early era of our solar system. Although some are 4.5 billion yeas old, most of them have undergone drastic changes in structure and composition within this vast time frame. Scientists estimate that 99% of the primordial debris have been thrown out because of gravitational perturbations, while the remaining have been affected by mutual collisions, space weathering, internal heating, volcanism, and micro-meteorite impacts. Based on composition, there are three distinct classes of asteroids in the main belt. They are as follows:
- C-type or Carbonaceous Asteroids: These are carbon-rich asteroids that occupy nearly 75% of the main belt. They are located in the outer regions and are the only group of asteroids whose compositions match with the composition of the early solar system.
- S-type or Silicate Asteroids: These are silicate-rich(a compound made up of Silicon and Oxygen) asteroids that occupy nearly 17% of the main belt and are found in the inner regions. Spectral analysis reveals the presence of silicates, some metals and an absence of carbonaceous compounds suggest that their composition has been altered over time.
- M-type or Metallic Asteroids: These are metal-rich asteroids, occupying nearly 10% of the asteroid populations in the main belt and are supposedly the remnants of early protoplanets.
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| 951 Gaspra/Image Credits: NASA, Public domain, via Wikimedia Commons |
Families or Groups of Asteroids: Based upon certain similar orbital eccentricity, orbital configurations, and spectral signatures, the asteroids can be grouped into different families. The major groups are as follows:
- Flora- The Flora family is one of the largest families, comprising more than 13,000 S-type asteroids from the main belt. This group is thought to be around 200 million years old based upon studies of the asteroid 951 Gaspra. The largest member among them is the 8 Flora, having a mean diameter of 140 km.
- Eunoma- The Eunoma family is named after their largest member, 15 Eunomia(spans for 300 km across its longest axis) which is also the largest S-type asteroid found in the main belt. This family comprises 6000 members.
- Koronis: The Koronis Family, also known as the Lacrimosa Family is another large group of S-type asteroids which might have formed from the collision of two larger bodies 2 billion years ago. It is also divided into two sub-families, the Karin Family with 541 members and the Koronis (2) Family with 246 members.
- Themis: The Themis family, made up of 4700 members, is a group of C-type asteroids. Two popular members are the 24 Themis(mean diameter of 198 km) and 90 Antiope(mean diameter of 116 km).
- Eos: The Eos Family, made up of 4400 members is another major group of asteroids in the main belt. Although they resemble S-type asteroids but because of some differences they have been assigned a separate group of K-type asteroids.
- Others: These include the Hungaria Family-located near the inner edge of the main belt is a group of some minor planets, the Phocaea Family, the Cybele Group, the Veritas Family, the Nysa Family, and the Datura Cluster.
Collisions and other activity: The asteroid belt is a very active region with frequent(in astronomical timescales) events of collisions. Asteroids that break up into bits and pieces from impacts, and in addition to a hazy cloud of tiny pieces of dust and micrometeoroids produce a phenomenon called the Zodiacal Light. This zodiacal light can be seen at night in a direction away from the Sun towards the plane of the ecliptic.
The asteroid belt is also home to a number of active asteroids, formerly main belt comets, as they show cometary characteristics like possession of a dust tail, and a luminous head called coma. But they are not proper comets and their orbits remain within the orbit of Jupiter.
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| Zodiacal Light/Image Credits: Steven Keys, CC BY 4.0, via Wikimedia Commons |
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