Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids. The term "asteroid" has historically been applied primarily to minor planets of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and comets is made on visual appearance: Comets show a perceptible coma while asteroids do not.
A comet is a small solar system body bigger than a meteoroid that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.
Our planet has been hit by very big asteroids before. The events were:
(a) 1908 Tunguska Event
It is now commonly believed that on 30 June 1908 a stony asteroid exploded over Tunguska with the energy of the explosion of 10 megatons of TNT. The explosion occurred at a height of 8.5 kilometers. The object that caused the explosion has been estimated to have had a diameter of 45-70 meters.
(b) 2002 Eastern Mediterranean event
On June 6, 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the Mediterranean Sea, between Greece and Libya, at approximately 34°N 21°E and the object exploded in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a small nuclear weapon.
(c) 2008 Sudan event
On 5 October 2008, scientists calculated that a small Near-Earth asteroid 2008 TC3 just sighted that night should impact the Earth on 6 October over Sudan, at 0246 UTC, 5:46 local time. The asteroid arrived as predicted. This is the first time that an asteroid impact on Earth has been accurately predicted. However, no reports of the actual impact have so far been published since it occurred in a very sparsely populated area. A systematic search for fragments found a total of 280 fragments, with a mass of 3.9 kilograms. The object is confirmed to have entered Earth's atmosphere as a meteor above northern Sudan at a velocity of 12.8 kilometres per second (29,000 mph).
Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with approximately one in a million on a potential impact date of February 1, 2019.
Asteroid (29075) 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting of its orbit, and then rediscovered on December 31, 2000. The chance it will impact Earth on March 16, 2880 during its close approach has been estimated as 1 in 300. This chance of impact for such a large object is roughly 50% greater than that for all other such objects combined between now and 2880. It has a diameter of about a kilometer (0.6 miles).
Films that have been made about an asteroid impact with Earth are:
(a) The Green Slime (1968), film. A rogue asteroid hurtles toward Earth. The astronauts leave Space station Gamma 3 and place bombs on the asteroid, finding it inhabited by strange blobs of glowing slime that are drawn to the equipment. Unfortunately for everyone some of the slime is carried back on a space suit and soon evolves into tentacled creatures. The movie inspired the classic board game “The Awful Green Things From Outer Space”.
(b) Meteor (1979), film. The asteroid Orpheus hurtles toward Earth after its orbit is deflected by a comet. The movie was inspired in part by a MIT. student report Project Icarus (1968).
(c) Starship Troopers (1997), film, based on the 1959 novel by Robert A. Heinlein. Aliens launch an asteroid at Earth, completely wiping out Buenos Aires. This is the opening move in the war.
(d) Deep Impact (1998). film. Based on Arthur C. Clarke's novel The Hammer of God, although the asteroid becomes a comet. An unsuccessful attempt to alter the course of the asteroid by detonating nuclear devices on its surface, after which the astronauts involved pilot their ship into the asteroid's path to prevent it hitting Earth.
(e) Armageddon (1998), film. An asteroid is prevented from impacting the Earth by drilling into its core and planting nuclear bombs which split the asteroid in half. The two halves move in different directions and miss the Earth.