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The Main Asteroid Belt, the Kuiper Belt, and the Oort Cloud
The Main Asteroid Belt is located in the region between Mars and Jupiter. It is called the “Main Asteroid Belt” to distinguish it from other asteroid groupings within the solar system.
It is composed of hundreds of thousands of solid, irregularly-shaped, rocky bodies called asteroids, or minor planets. First theorized by the German astronomer Johannes Kepler in 1596, and based on the work of succeeding astronomers looking for a lost planet between Mars and Jupiter, the first Main Asteroid Belt object, Ceres, was discovered in 1801, followed by Pallas in 1802, Juno and Vesta in 1807, with countless others having been discovered in the past two centuries.
It was the British astronomer William Herschel who, in 1802, suggested the term “asteroids” for these newly discovered bodies, with the term finally being accepted by the astronomical community in the 1850s. The four largest asteroids – Ceres (now classified as a dwarf planet), Vesta, Pallas and Hygeia – constitute approximately half the mass of the entire asteroid belt.
Initially it was surmised that these asteroids were fragments of a number of planets which formed in this region (between what are now Mars and Jupiter) of the early solar system. But they collided, broke apart, and spread out, over millions of years, into their current disc formation.
It is now theorized, however, that these bodies are leftover remnants of the rocky debris present when the solar system was first forming billions of years ago. While the vast majority of these rocky conglomerates eventually formed the planets within our solar system, the rocky fragments between Mars and Jupiter were perturbed enough by the strong gravity of Jupiter that they couldn’t coalesce into a planet, thus remaining as we see them today.
The Kuiper Belt
In last week's column, I mentioned the Kuiper Belt with regards to Pluto’s position in the solar system, and its subsequent demotion from planet to dwarf planet. The Kuiper Belt is a circumstellar disc of trillions of ice bodies lying outside the orbit of Neptune, extending approximately 50 AU from the sun. Like the Main Asteroid Belt, the components of this belt are believed to be remnants of the early formation of our solar system.
First theorized by British astronomer Kenneth Edgeworth in 1943, it wasn't until 1951 that the Dutch astronomer Gerard Kuiper predicted the existence of an extensive belt of icy objects at the outer edge of the solar system. As a result, this belt is sometimes referred to as the Edgeworth-Kuiper Belt. Discovery of two objects in the region of the predicted belt (QB1 In 1992, and FW in 1993) by American astronomers Dave Jewitt and Jane Luu confirmed the existence of the Edgeworth-Kuiper Belt. The three largest KB objects found to date are Haumea (2004), Eris (2005) and Makemake (2015).
The Oort Cloud
The Oort Cloud – sometimes referred to as the Opik-Oort Cloud – is a theorized region in space, out beyond the orbit of Pluto, thought to contain trillions of comets. While there is no direct evidence for the existence of the Oort Cloud, it was first postulated by the Estonian astronomer Ernest Opik in 1932, and later developed by the Dutch astronomer Jan Hendrik Oort in1950 to explain the possible origin of long-period comets (as opposed to that of short-period comets). The cloud is thought to be composed of an inner, torus-shaped cloud (extending from approximately 2,000 to 20,000 AU from the sun), and an outer, spherical cloud (out to about 50,000 AU). The Oort Cloud is theorized to lie beyond the heliosphere (the bubble-like region of space surrounding the sun and our solar system), in interstellar space.
As the sun sets, and the sky begins to darken, Jupiter, Venus and Saturn all appear in the southwest sky. Jupiter is lowest to the horizon, with Venus to the upper left of Jupiter, and Saturn to the upper left of Venus. Jupiter will set a short while after sunset, but Venus and Saturn remain up until well past nightfall. The three planets vary greatly in their brightness, with Jupiter (mag. -1.7) nine times as bright as Saturn (mag. +0.9), and Venus (mag.-3.8) seven times brighter than Jupiter.
Early morning risers may be able to catch a glimpse of both Mars (mag. +1.7) and Mercury (mag. -0.5) in the eastern, predawn sky. If you look toward the east about two hours before sunrise, and have an unobstructed view of the eastern horizon, you should be able to spot Mercury low on the horizon, with a much fainter, reddish Mars to the upper right.
If you are having difficulty locating Mercury, look in the eastern, predawn sky for the first star, Spica (brightest star in the constellation of Virgo - the Maiden). Mars lies to the lower left of Spica; by drawing a straight line from Spica through Mars, and continuing to the horizon, you should spot Mercury just above the horizon. Both Mars and Mercury will fade from sight once the eastern sky brightens with the rising sun.
The First Quarter Moon on Dec. 4 is the farthest (403,929 km. from Earth) first quarter moon of the year, as it occurs close to the moon's apogee on Dec. 5.
Next week - Near Earth Objects (NEOs). Until then, clear skies.
Dec. 4 - First Quarter Moon
5 - Moon at apogee (farthest from Earth)
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at email@example.com.