The National Aeronautics and Space Administration (NASA) has announced that the asteroid Apophis will not crash into the Earth 23 years from now in 2036. The asteroid has been tracked by NASA since 2004 when it was discovered. Imagine a rock about the length of the air craft carrier USS Nimitz (CVN-68) (about 1100 ft) and 300 times its 97,000 ton displacement hurtling into the Earth’s atmosphere at several miles per second. The energy from that impact could destroy a city the size of New York or Chicago. Apophis made its closest approach to Earth on 9 January.
“Asteroid” means “little planet,” or “planet-like.” Most of them orbit between the orbits of Mars and Jupiter, however there is a class of asteroids called “Near Earth Asteroids” or NEAs that orbit the Sun close to Earth. NASA’s Jet Propulsion Laboratory (JPL) tracks these objects and determines their orbits once they are discovered. JPL lists NEAs and comets in its New Earth Object database, which is in turn used by professional and amateur astronomers to keep track of them. Apophis posed no threat with its near-earth approach on the 9th of January 2013, and will not pose one in 2036. It will approach within 19,400 miles of the Earth in 2029.
Outer Space is by no means empty, and though the distances in our solar system may be huge, there are a variety of objects in it. The largest object is the Sun, called “Sol” by science fiction writers and others. Its gravity keeps most of the other objects within the Solar System orbiting around it. There are eight objects that rank next in size: the planets, --sometimes called “classical planets”-- of which Jupiter is the largest and Mercury is the smallest. Dwarf planets are smaller than planets but like them orbit the Sun. There are four dwarf planets: Pluto (with four moons), Eris (with one moon), Haumea (with two moons) and Makemake (with none). The International Astronomical Union (IAU) lumps the eight “classical” planets and the “dwarf planets” together simply as planets. The IAU definition of "planet" gives the Solar System 12 planets altogether.
The next largest objects are moons, which orbit the planets. Six of the eight “classical” planets have moons; Earth has only one and Saturn has 63, including provisional moons. Provisional moons are objects around planets that have been identified and observed, but not named. Ganymede and Titan are the largest in the Solar System. Like the classical planets, Titan even has an atmosphere.
Asteroids are smaller than planets and orbit the Sun. Both they and the planets orbit the sun nearly paralell to the Sun's equator. Comets are smaller than asteroids, and orbit the Sun in eccentric (stretched out) circuits extending from the edge of the Solar System almost to the orbit of Mercury, the closest planet to the Sun, then back out into the edges of the Solar System. The most famous of these are Halley’s Comet and Shoemaker-Levi , which crashed into Jupiter in 1994. The objects that often enter the Earth’s atmosphere are meteors—often the detritus from comets that break up.
There are good reasons to track large objects approaching near Earth. An object, probably a small NEA, crashed into the Siberian tundra near Tunguska in 1908. Fifty thousand years ago, Meteor Crater in Arizona, the most famous meteor crater, was formed by a meteor impact. The object was just 150 feet across, but it produced a crater about a mile wide. The “granddaddy” of all impacts is the theorized impact of a 10km (6 mile)-wide asteroid near the Yucatan Peninsula, Mexico, which is said to have caused the extinction of the dinosaurs 65 million years ago.
Earth's major space agencies such as NASA, observatories around the world, deep space networks maintained by various countries, and thousands of astronomers world-wide constantly scan the skies for asteroids--and both NASA and Russia are developing intercept and destroy plans--just in case.