On a night with no apparent moon in the sky, you look up in a place far from the urban hustle and bustle and, suddenly, a light scratches the starry ceiling and disappears. You may even think: “what a beautiful shooting star!”, but, astronomically speaking, what did you really see? A meteor, a meteorite… or a meteoroid? But what if it was an asteroid or even a comet?
All of these answers are possible, since our Solar system it is filled with countless asteroids, comets and small objects left over from the original disk of dust and gas that formed the planets 4.5 billion years ago.
Most of these celestial bodies are far from Earth, in the distant reaches of the Solar System, but some of them come closer to our planet, penetrate our atmosphere and, when not incinerated by friction and air compression, fall to the surface.
Therefore, it is possible that that shooting star is a meteoroid rock, resulting from the fragmentation of an asteroid or detachment from the tail of a comet. Upon entering the atmosphere, it can produce a luminous phenomenon and become a meteor, or fall to the ground and become a meteorite.
See below the main characteristics of each of these astronomical objects.
Asteroids
Asteroids are mostly small, rocky or metallic objects, located in the so-called asteroid belt, between the orbits of Mars and Jupiter, although some come close to and even cross the Earth’s orbit. By definition, they are larger than a meter in diameter.
Floating rocks smaller than asteroids are called meteoroidsand transform into meteors upon entering our atmosphere. Asteroids massive enough to become spherical due to the force of their own gravity are called dwarf planets, as is the case with Pluto.
Asteroids are rocks that contain the primitive materials from which the planets of our Solar System were built. But unlike Earth’s rocks, shaped by eons of erosion and geological processes, asteroids have remained untouched.
Considered orbital time capsules, these rocks have recently been visited by terrestrial probes, such as OSIRIS-REx and Hayabusa2, to collect samples that provide clues about the origin and evolution of our planetary system.
Potentially dangerous asteroids
But there is another reason to know asteroids (and also comets) well: its proximity to Earth. The so-called near-Earth objects (NEOs) are asteroids between 3 meters and 40 kilometers in diameter that are constantly monitored by NASA’s Center for Near-Earth Object Studies (United States space agency).
As the orbits of these objects are elongated or elliptical, they can move up to 195 million kilometers away from the Sun, getting dangerously close to us. Although the probability of a major collision with our planet is considered relatively low, the devastating potential of these objects requires continuous monitoring.
Meteors
An easy way to understand what a meteor is is to remember that it is a luminous phenomenon which occurs when a meteoroid (which may be a small fragment of a comet or asteroid) enters the Earth’s atmosphere and burns up due to friction with the air. It is the “shooting star”.
I.e, when the small rock approaches the Earth, it is called a meteoroid; when it enters the atmosphere, it is a meteor; and, if it survives the rapid entry into the atmosphere and falls to the ground, it is a meteorite.
When the Earth passes through a cloud of debris left by a comet, or an asteroid in certain cases, we say that a “Meteor rain”. This astronomical phenomenon occurs at regular intervals throughout the year. These are the Quadrantids (January), Perseids (August), Leonids (November), and the Geminids (December).
Comets
The dramatic appearances of comets have made these celestial objects the most mythical and fascinating throughout history. Today we know that the impressive shiny head and long tail (or tails) that so impressed our ancestors are nothing more than a small, icy, dusty object orbiting the Sun.
Made from a mixture of ice, rock and gas, these objects are actually “leftovers” from the formation of the Solar System. However, as they form in their outer regions, far beyond Neptune’s orbit, their ice remains stable for a long time.
Although most of them (according to NASA, there are 3,956 known comets) inhabit a distant region of space called Kuiper belt, some have orbits that pass through the inner Solar System. This is when, closer to the Sun, its ice sublimates (goes directly from ice to gas), releasing dust and forming the coma (nebulous cloud of gas) and the tail, which are its characteristic elements.
One of the most famous comets in history, Shoemaker-Levy 9 (SL9) collided, exactly 30 years ago, with the planet Jupiter. The explosive astronomical event, observed by astronomers around the world, showed how SL9 was captured by the gas giant’s gravitational tidal forces, and destroyed into fragments that fell into the planet’s atmosphere.
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