Before beginning this article, I think it prudent to first explain the difference between meteoroids, meteors, and meteorites.
Here’s an easy and simple way to differentiate between the three: a “meteoroid” is a rocky body that is traveling through space “outside’ our planet’s atmosphere (just think of the “o” in “oid” as meaning “outside” the atmosphere); a “meteor” is a rocky body traveling through Earth’s atmosphere (what we commonly refer to as “shooting stars”); while a “meteorite” is a rocky body that has, after plunging through the atmosphere, reached the Earth’s surface (just think of the “i” in “ite” as meaning it has crashed “into” our planet’s surface).
It is important to distinguish between the three terms. Most people, including many astronomers, would label any bright “shooting star” streaking across the night sky as a “meteor”.
This would be correct only insofar as the simple definition of a meteor (mine) above; it would not, however, be precisely correct or accurate with respect to defining the object if it survives its trip through the atmosphere and reaches the Earth’s surface.
What are Meteors?
We know that most “meteors” are debris, in the form of small dust particles and/or sand to pea-sized bits of rocky material, ejected from comets, as they make their way, in an elongated elliptical orbit, through the inner solar system, around the Sun, and back out into space, only to return, in many cases, after a set period of time.
There are, however, a number of “meteor” showers which originate from asteroids; the recent Geminid meteor shower, debris from asteroid 3200 Phaethon (see last week’s article), being an example.
The vast majority of the comet-sourced particles burn up in the Earth’s upper atmosphere, providing a brief but bright flash of light (the “shooting stars”), while some of the asteroid-sourced rocky debris may survive and reach Earth’s surface.
There are also numerous instances where a fiery object is seen to streak through the Earth’s atmosphere, either at night or during the daytime, with the passage lasting several seconds (much longer than the typical brief, tenths-of-a-second passage of a comet-sourced “meteor”), sometimes accompanied by a sonic boom followed a huge explosion in our planet’s lower atmosphere. Sometimes, with luck, some part of the object survives the explosion and buries itself “into” the ground, or scatters pieces of the parent body over a vast area; these are true “meteorites”.
Our planet bears the scars of numerous historical impacts from meteorites, e.g., Gosses Bluff (4.5 km dia.), Northern Territory, Australia; Pingualuit Crater (3.4 km dia.), Quebec, Canada; Lonar Crater (1,830 metres dia.), Maharashtra, India; and Barringer Crater (1,300 metres dia.), Arizona, USA.
Some meteorite impacts, however, can have catastrophic results. The object that crashed into the Yucatan Peninsula in present-day Mexico approximately 66 million years ago created a 200 km-wide crater (the Chicxulub Crater) and resulted in what is referred to as the “Cretaceous – Paleogene extinction event”, the mass extinction of 75% of all plant and animal species on Earth at that time, including all non-avian dinosaurs, is believed to have been a 10 km-wide asteroid, technically, at the moment of impact, a “meteorite”.
Famous Meteorite Explosions
There have also been a number of famous, recorded meteorite explosions that have occurred just above the Earth’s surface which, although not leaving an impact crater in the ground, did, nonetheless, leave their mark. P
erhaps the most famous of such events is the 1908 Tunguska Event in eastern Siberia, Russia, where, on the morning of June 30, an estimated 50-60 metre-wide stony asteroid detonated at a height of between 5 -10 km above the Earth’s surface (referred to as a “meteor airburst”) with the power of between 3-50 megatons of TNT, flattening an estimated 80 million trees over a 2,150 sq. km radius.
Another such event was the 18 meter-wide Chelyabinski meteor (a near-Earth asteroid) that entered Earth atmosphere in the southern Ural region of Russia on Feb.15, 2013, which detonated (meteor airburst) with a force of 400-500 kilotons of TNT at a height of 30 km above the ground, produced a huge, hot, shockwave of dust and gas that reached to within 26 km of the ground, injured nearly 1,500 people, and damaged over 7,000 buildings.
As with the 1908 Tunguska asteroid event, the Chelyabinsk asteroid event did not produce an impact crater on the ground.
The Main Categories of Meteorites
Meteorites are divided into three broad categories – stony meteorites, iron meteorites, and stony-iron meteorites. Stony meteorites are further subdivided into two types – chondrites and achondrites.
Chondrites, comprising approximately 85% of all meteorites discovered on Earth, contain chondrules – molten or partially-molten droplets formed in space before they were accreted to their parent asteroid.
Most are what are referred to as “ordinary chondrites”, which are subdivided into three broad classes – H, L, and LL – based on their iron content, plus the distribution of iron and magnesium in the major silicate minerals olivine and pyroxene, the primary blocks of our solar system. H-chondrites, the most common group of meteorites, have a High iron content of 20-25% by weight; L-chondrites, the second most common group of meteorites, have a Low iron content of less than 10%, while LL-chondrites, the least common, have a Low (total) iron content and are Low in metals.
Carbonaceous or C-chondrites contain, in addition to high amounts of water and minerals, a high proportion of carbon (up to 3%) in the form of graphite, carbonates and organic compounds, including amino acids. Scientists believe C-chondrites perhaps formed during the explosion of a nearby supernova, or in the vicinity of a pulsating red giant star, before being pulled into the cosmic matter cloud from which our solar system eventually formed. Achondrites, another type of stony meteorite, consisting of material similar to terrestrial basalts and plutonic rocks, lack the distinctive molten chondrules found in the chondrites, having experienced differentiation and reprocessing due to melting and recrystallization on a parent meteorite body.
Where Meteorites Originate From
Studies have demonstrated that, while some meteorites originate from the Moon and from Mars, the vast majority of them originate from just a handful of asteroid breakups, and possibly from individual asteroids, within the asteroid belt, a huge swath of millions of rocky chunks, left over from when our solar system first formed, occupying the area between the orbits of Mars and Jupiter.
Within the asteroid belt there are three large groups or “families” of asteroids – the Koronis family, along with its Karin sub-family group, located in the outer regions of the main-belt, and the Massalia and Flora families in the inner regions of the belt.
The Largest Meteorites
Astronomers believe the asteroids 158 Koronis (the largest asteroid in the Koronis family) and 832 Karin (a minor planet, and the largest member of the Karin sub-family group) are the primary source of H-chondrites, with the Massalia family the predominant source of L-chondrites, and the Flora family the source of LL-chondrites.
A recent report indicates that astronomers have now managed to pinpoint decameter-sized asteroids (10s of metres across) within the main asteroid belt between Mars and Jupiter.
While they have a fairly accurate count of the large, potential “planet-killer” asteroids present within the solar system, astronomers have always found it difficult to determine how many small to medium-sized asteroids capable of striking Earth there are in our solar system. Being able to spot such asteroids, which are the ones most likely to escape the main-belt and become near-Earth objects, is a huge step forward, as these asteroids, being frequent interlopers to the inner solar system, pose a significant risk to our planet should they, like the 1908 Tunguska and 2013 Chelyabinskasteroids, come too close. The ability to locate such objects will enable future planetary defense measures, such as space missions to potentially intercept and deflect the asteroids well before they reach Earth.
This Week’s Visibility
Mercury (mag. +2.2, in Scorpius – the Scorpion) is not observable at the beginning of this coming week, as it is only 8 degrees above the eastern horizon at dawn. It will, however, continue to rise higher in the pre-dawn sky throughout the week, reaching 14 degrees above the southeast horizon at sunrise on Dec. 21, its highest elevation for its December 2024 – January 2025 morning apparition.
Venus (mag. -4.2, in capricorn – the Sea Goat) becomes visible around 4:50 p.m., 20 degrees above the southern horizon as dusk fades to dark, before sinking towards the horizon and setting around 7:55 p.m.
Mars (mag. -0.7, in Cancer – the Crab) becomes visible 8 degrees above the northeast horizon around 8:20 p.m., reaching 65 degrees above the southern horizon shortly after 3 a.m., before getting lost in the dawn twilight around 7:25 a.m., 32 degrees above the western horizon. Look for Mars sitting less than 3 degrees below the waning, gibbous Moon on the evening of Dec.17.
Jupiter (mag. -2.8, in Taurus – the Bull) becomes accessible in the eastern evening sky 9 degrees above the horizon around 4:50 p.m., reaching 65 degrees above the southern horizon around 11:50 p.m., before becoming inaccessible when it sinks below 7 degrees above the northwest horizon shortly after 6 a.m..
Saturn (mag. +1.0, in Aquarius – the Water-bearer) becomes accessible about 5:10 p.m., 35 degrees above the southern horizon as dusk yields to darkness, its highest point of the evening, remaining observable until around 9:40 p.m., when it sinks below 11 degrees above the southwest horizon.
Uranus (mag. +5.6, in Taurus) becomes accessible 32 degrees above the eastern horizon by about 5:45 p.m., reaching a height of 62 degrees above the southern horizon by about 9:55 p.m., before becoming inaccessible shortly after 3 a.m., when it drops below 21 degrees above the western horizon.
Neptune (mag. +7.9, in Pisces – the Fish) becomes accessible by about 5:45 p.m., 40 degrees above the southern horizon, reaching its highest point in the sky 41 degrees above the southern horizon around 6:20 p.m., and remaining observable until about 9:55 p.m., when it drops below 21 degrees above the southwest horizon.
Special Events
The Winter Solstice, the official start of the 2024-2025 winter season here in the Northern Hemisphere, and the start of the summer season in the Southern Hemisphere, occurs at 5:21 a.m. AST (5:51 a.m. NST) on Saturday, Dec. 21. The Winter Solstice occurs when either of Earth’s poles reaches its maximum tilt away from the Sun. It also marks the shortest day of the year in the Northern Hemisphere, with the days beginning to get longer thereafter, slowly at first, but at ever-larger daily intervals as we head towards the Spring Equinox in March.
This year’s Ursid meteor shower peaks during the pre-dawn hours of Dec. 22. The shower’s radiant (its apparent point of origin in the sky as viewed from Earth) lies near the bright star Kochab, located at the end of the bowl of the Little Dipper asterism in the constellation of Ursa Minor – the Little Bear, visible in the northern sky all night long. The Ursid meteors are debris from Comet 8P/Tuttle, named after the American astronomer Horace P. Tuttle (1837-1923). Although relatively few in numbers per hour (expect about 5-10 per hour from a dark site under a clear sky), the Ursids have been known to periodically display bursts of 100+ meteors per hour, as they did in 1945 and 1986. Unfortunately, this year the 65% illuminated, waning, gibbous Moon (nearing Last Quarter on the evening of Dec. 22) rises around 11:25 p.m. on the 21st, and remains up the entire night, washing out the fainter Ursid meteors. As when viewing last week’s Geminid meteor shower, remember to dress appropriately, and limit your meteor observing to short periods to avoid getting chilled.
Until next week, clear skies,
Events:
Dec. 17 – Mars sits less than 3 degrees below the waning, gibbous Moon; evening
Dec. 21 – Winter Solstice (the official start to winter); 5:21 a.m. AST / 5:51 a.m. NST – Mercury at highest elevation for Dec. 2024 – Jan. 2025 morning apparition; sunrise
Dec. 22 – Last Quarter Moon; 6:18 p.m. AST / 6:48 p.m. NST – Ursid meteor shower peaks; pre-dawn
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