Introduction
Asteroids and Comets form part of holistic astronomical occurrences being characterized by key cords of compelling attraction. It is not a wonder that the predominance of bright comets in space is excelled only by that of the Sun and the Moon whose phenomenal glow changes unpredictably making them one of the most astounding astronomical occurrences. The real nature and the intricate intrinsic mechanisms in which comets operate accredits them of being held in such high astronomical esteem as the primary space objects that form a solid foundation in the inquiry on the origins of the solar system. Hence, this paves the way into demystifying the pattern of arrangement of galaxies in space. They are harnessed as rich resources for scientific probe on the environmental and chemical inter-relationships for different planets.
Comets and asteroids are also believed to have taken a central role in the initial stages of the emergence of life on Earth and as such, the occasional massive impacts highlight the never-ending process in which Comets and asteroids impact the Earth’s environment. This paper discusses comets and asteroids and their contribution to the understanding of the beginning of the Solar System. This study further explores the broad role played by Asteroids and Comets in the development of modern astronomy and their historical reputation in the public domain (Messenger et al. 2006, p. 1). The latter portion of which is of interesting significance to us is that comets have nothing mystical about them, for them to be dreaded or feared for they are just but ice balls under the toll of a very hot Sun.
Components of this study include An exposition on Asteroids and Comets, Origins of Comets and Asteroids and their constituents, the significance of Comets and Asteroids in the inquiry about the origins of the solar system and galaxy, comets as astrophysical laboratories, the role of comets in public and in the advancement of modern science.
Asteroids
Asteroids are astronomical objects with a size ranging from one to about one thousand kilometres in diameter and a structural appearance ranging from little granules to small planet prototypes. Astronomical inquiries boast of recording 26 asteroids with a diameter in excess of 200 km and claims to have observed 99% of those asteroids with a diameter ranging from 100 – 200 km. Nonetheless, the vast majority of smaller asteroids are yet to be clearly recorded by high precision appliances. Of essence still, is the fact that the mass of the moon is higher than the summative mass of all asteroids.
So far, astronomical records have it that the largest asteroid is 1 Ceres, which is 974 km across and bearing a mass constituting a quarter of the total mass of all asteroids. 2 Pallas, 4 Vesta and 10 Hygiea with a diameter ranging from 400 to 525 km follow each other respectively in the sequence of the biggest asteroids (ScienceDaily 2008, p.1).
Depending on their albedo and chemical makeup, asteroids have been categorized into the following common classes;
- C-type – this category comprises of three-quarters of all asteroids which are generally and entirely dark with a very little albedo value. They are best exemplified by carbonaceous chondrite meteorites. Their chemical makeup is virtually equivalent to that of the Sun in spite of the fact that they lack hydrogen, helium and some other volatiles.
- S-type – Comprise of around a fifth of all the asteroids, they have a shiny lustre for they are good reflectors of light and therefore their albedo value is relatively high. Their chemical composition comprises of metallic nickel-iron and at times some portions of magnesium – silicate.
- M-type – They are also good reflectors and are bright with a chemical makeup of nickel-iron.
The rest of the classes under this domain are not common.
Asteroids are not only categorized based on their chemical composition and surface lustre, but they are also grouped according to their relative positions in the solar system;
- Main Belt: This group of asteroids lies between Mars and Jupiter, an estimated distance of about two to four astronomical units from the Sun.
- Near-Earth Asteroids: this group of asteroids is within the proximity of the Earth and is subdivided further into three subset groups; the Athens, the Apollos and the Amors.
- Trojans: this group of asteroids is within the proximity of Jupiter’s Lagrange points (60 degrees are leading or trailing behind the orbit of Jupiter) (NASA 2011, p. 1).
Those asteroids which are within the outskirts of the solar system are referred to as ‘Centaurs’. For instance, 2060 Chiron revolves around an orbit which is sandwiched between planet Uranus and Saturn. This class of asteroids is unstable because their orbit crisscrosses the planetary orbit and therefore, with time, it is susceptible to untold disruptions. The chemical makeup of Centaurs is to a great extent in sync with that of comets or rather one of Kuiper Belt bodies than that of formal asteroids. The authenticity of this property of chemical composition of asteroids within the outer solar system resembling that of comets has led to the re-classification of the asteroid 2060 Chiron as a Comet.
Comets
It is generally acknowledged that the most classical and striking characteristics of a comet are its elongated tail and the diffuse spherical coma which changes unpredictably. Although many in the public domain are aware of these facts, it goes without saying that comets are rare to be seen and it is most probable that very few people if any have ever observed a comet. Comets are primarily small in size and dark in appearance having an approximate diameter of ten miles. Even though ten miles may appear to be observable in an ordinary view, the actual size of an observable comet is of approximate size to that of a dime. Such appearance clouds the nucleus of the comet from the observer, and most often it is virtually impossible to make precise observations of the comet nucleus. Nonetheless, the nucleus of comet Halley has been observed with some degree of precision.
Intrinsic Processes which obscure the nucleus of a comet
A greater percentage of comet nuclei constitute volatile compounds, easy to melt and vaporize. (The processes leading to their formation and existence in the solar system would be considered later in this study). The name of comets ‘Dirty Snowballs’ suggests that water constitutes the greatest percentage of the comet’s nuclei than any other volatile compound. As the comet gains continued proximity to the Sun, the volatile compounds gradually vaporize in a steadily increasing magnitude (NASA 2011, p. 1). A greater portion of this vaporization takes place over the surface of the nucleus of the comet with little, if any, from other active sections.
It is estimated that an average comet can lose five thousand gallons of water, sufficient to fill a medium fishing pond, every second. In addition to this water, the vaporization is coupled with a great host of all kinds of the other volatile compounds of which ammonia, carbon monoxide, cyanide, alcohol, soot and other hydrocarbons form part. Such happens to the extent of enabling the space within the proximity of the comet to portray a better resemblance of a speeding exhaust pipe of a motorcar whose oil is under combustion.
Though unconventional, the vaporization of these volatile compounds brightens the comet by making it appear luminous and hence enhances visibility which is a necessary ingredient for precise observations. The vaporized volatile compounds expand away from the comet, forming a pattern known as a coma, which is one of the three elements of what is universally perceived to be a comet. The clear visibility of the coma arises from the fact that the vaporized dust particles are good reflectors of sunlight coupled with the dazzling rainbow of the other gaseous compounds. The overall effect is the whitish glow which is commonly observed and registered by light-sensitive appliances (Christiansen, and Hamblin, 2007, p. 1). Even though some volatile compounds are easily done away with by the sunlight, others such as the atomic hydrogen do extend to more than ten million kilometres from the nucleus making the coma the most predominant feature of a comet.
The second part of the elements of a comet is the ion tail which is made up of two distinctions CO+ (blue in colour) and H2O+ (red in colour). This results from the outward movement of the gaseous ions from the nucleus upon which they are exposed to the ultraviolet solar radiation and thus ionizes some atoms and molecules of the gaseous compounds. The resultant attraction of unlike charges triggered by the ions produce charged solar particles known as the solar wind. It is this interaction of the solar wind and the ions that yields the fore mentioned coloured ions.
The third and last part of the elements of a comet is the debris of evaporation. Although much of the debris of evaporation from a comet is in the form of very small dust particles, bigger pieces are also produced. A portion of these large particles come from the surface of the comet while structural failure on the nucleus blows off some large debris of evaporation simultaneously with the vaporization of the underlying ice. Due to the manner in which a comet revolves around the Sun, the lines between the Sun and the comet and the direction of a comet’s orbit are parallel. The resultant effect is that the dust and ion extensions of the comet overlap (ScienceDaily 2008, p. 1).
Due to this continued recurrence, it is only understandable that a great populace in the public front hardly notice that a comet has two unique and distinct tails which have very different physical features. Equally important is the fact that the amount of debris in the dust tail is largely depended on the topography of the nucleus. On very rare occasions, the nucleus may break apart, and the whole comet may assume an elongation extending to the dust tail.
Origin of Comets
Since the nucleus of a comet is made up of ice, it is unlikely that it would be part of the inner solar system which comprises of the four terrestrial planets. The high temperatures in the inner solar system render it impossible for the survival of a comet within this region. It is thus predictable that comets must have their roots from, at and beyond the planet Jupiter. And as the truth would have it, the origin of comets can be traced from two distinct regions, the Oort cloud and the Kuiper Belt whose features, location and roots are intertwined with the origin of the solar system and the consequent evolution of the solar system.
The current theory of the solar system suggests that the formation (of the Sun and the planets) resulted from a massive cloud of interstellar gaseous dust. Due to substantial external bombardments, the gaseous cloud crumpled inwardly with its foci piling a huge amount of gas which would later become the Sun. The immediate neighbourhoods of the foci (Sun) formed a spinning and orbiting sets of material usually known as the protoplanetary disks. Such protoplanetary disks have also been observed in a manner similar to the adjacent young stellar bodies. In a long spell of time, bigger materials start accumulating gradually within the disk dust, and thus the resultant amalgam of dust and ice grow too big proportions. The small materials formed are referred to as planetesimals, and they are within the proximity of the Earth (ScienceDaily 2008, p. 1).
The high temperatures within this region of the solar system do not favour the formation of ice, and therefore, most of these planetesimals are of a pure rocky terrain. In contrast, within the outer solar system, which is characterized by low temperatures, there are phenomenal trappings of ice and gasses in the planetesimals. It is this latter category which is grouped into asteroids and comets.
The comets and asteroids are astronomically acclaimed as the key pillars and founding stones of the planets. As they gained weight from amalgamation within the dust disk, they simultaneously bombarded each other in unpredictable collisions and on some occasions being integrated to each other forming bigger elements. Consequently, the new protoplanets developed more to the extent that they formed their own dust disks which then combined with others of the same nature to form the well-known planets. Such accreted coalitions perturbed the orbits of the other planetesimals within their vicinity. The dynamically weak planetesimals were susceptible to successive accretion by the massive protoplanets, repulsion from the high temperature inner solar system or ejection out of the solar system into an orbit in space which is much more stable and strong than the former one.
The Oort cloud, the Asteroid belt and the Kuiper belt contain a very small percentage of the planetesimals, and they do act as the remains of the planetesimals in the present solar system. In the last few decades, it has been observed that the Kuiper belt bodies have stable orbits for the collisions within the novice protoplanets which bear no disruptive effect on them. They are also estimated to range in size from that of a small stone to that of planets (Messenger, et al. 2006, p. 1). On the other hand, the Oort cloud was synthesized by the materials and bodies which were ejected to the outer edges of the solar system by the protoplanets and planets. It is amazing to realize that the comets in the Oort cloud were relatively developed nearer to the Sun than their Kuiper belt counterparts. Although the aggregate statistical reflection of the number of comets in both regions is not known, it is taken to be great with approximately 700 billion in the Oort cloud and with the Kuiper belt bearing, inclusive of Pluto, some other additional billions to those of the Oort cloud.
One may at times wonder that even though the stability of the Oort cloud and the Kuiper belt is assured, it is virtually impossible to observe comets in the inner solar system. Such inquisitive inquiries have directed us to postulate that at the beginning of the formation of the solar system, the relative weight of the comets was comparatively higher than that of the planets. However, the merging of planetesimals leading to the formation of giant planets like Jupiter did disrupt the orbital course of the comets. Thus, the existence of some comets in the inner solar system derives its roots from the early history of the solar system. Before things settled down to the current stable condition in the solar system, it is projected that the Earth and other terrestrial planets were being bombarded by asteroids and comets at the rate of one bombardment per century (Brown, 2011, p. 1).
Therefore, the odds that such pelting by asteroids and comets would target the Earth is currently at its minimal level, making the Earth much calmer and secure than it was at the beginning of the solar system.
By virtue of their highly elliptical orbits, comets are usually far from the Sun leading to the conclusion that the most common comets must have had their orbits constricted by planet perturbation into much-reduced orbits whose period is also shortened. A good example is the comet Encke, whose period is only about 3.3 years.
Conclusion
The mysteries surrounding the formation of the solar system as exposed by the nebula theory are demystified by the study of asteroids and comets since they give very crucial clues into what transpired in making the final stitches on the solar system. Asteroids and comets act as astrophysical probes in that the processes associated with the solar system formation are similarly being observed on other parts of the Milky-way and other galaxies.
Comets and asteroids form a tool of unearthing the mysteries behind the Sun and the Solar wind in that the structural fluctuations of the coma and the ion tail provide useful electrostatic data whose analysis leads to a better understanding of the nature of the solar field. Comets and asteroids are postulated to be the progenitors of life on Earth in the sense that the very collisions made in the initial stages of the formation of the solar system produced a viable habitation of water and micro-organisms. In theory ‘Panspermia’, life is considered to have its roots from the comets and asteroids even before it was transferred to the Earth by bombardments. In as much as we may take the statistical odds for the occurrence of a planetary collision with a comet or an asteroid at its minimal level in modern times, comets pose a real danger to the existence of life on the planet (Earth).
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