A near-Earth object (NEO) is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids (NEAs), near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to the Earth, and active mitigations are being researched. A study showed that the United States and China are the nations most vulnerable to a meteor strike. The United States, European Union and other nations are currently scanning for NEOs in an effort called Spaceguard.

In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be expected to produce severe to catastrophic effects. As of October 2008, 982 of these mandated NEOs have been detected. It was estimated in 2006 that 20% of the mandated objects have not yet been found. Efforts are under way to use an existing telescope in Australia to cover the ~30% of the sky that has not yet been surveyed.

Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth. Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi), or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs. The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in Lunar Distances, and this usage has become the more usual unit of measure used by the press and mainstream media in discussing these objects.

As of 2008, two near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe, and 25143 Itokawa, by the JAXA Hayabusa mission.

There are two schemes for classification of impact hazards:

• the simple Torino Scale, and
• the more complex Palermo Technical Impact Hazard Scale

On 25 December 2004, minor planet 2004 MN4 (later named 99942 Apophis) was assigned a 4 on the Torino scale, the highest rating so far. On 27 December 2004, there was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but, due to a resonant return possibility the Torino rating for an April 2036 impact rose to 4 in early 2005, but by Oct 2009 the Torino rating was 0 (zero). The Palermo rating (October 2009) is -3.08.

While orbiting the sun, most potential impactors can be classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets can leave debris in their orbits. As of April 2011, 7,954 NEOs have been discovered: 87 near-Earth comets and 7,867 near-Earth Asteroids. Of those there are 647 Aten asteroids, 2,920 Amor asteroids, and 4,289 Apollo asteroids. There are 1,215 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 148 PHAs and 824 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.

Near-Earth meteoroids are objects with orbits in the vicinity of Earth's orbit having a diameter less than 50 meters.

A small number of NEOs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.

As of May 2010, 84 near-Earth comets have been discovered. Although no impact of a comet in earth history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke. Cometary fragmenting may also be responsible for some impacts from near-Earth objects.

These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.

Objects with diameters of 5-10 m impact the Earth's atmosphere approximately once per year, with as much energy as the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Every 2000–3000 years NEAs produce explosions comparable to the one observed at Tunguska in 1908. Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval. Large collisions with five kilometer objects happen approximately once every ten million years.

The rate of impacts of objects of at least 1 km in diameter is estimated as 2 per million years. Assuming that this rate will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth. However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between Mars and Jupiter. Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small proportion) hitting a planet.

The general acceptance of the Alvarez hypothesis, explaining the Cretaceous–Tertiary extinction event as the result of a large object impact event, raised the awareness of the possibility of future Earth impacts with other objects that cross the Earth's orbit. 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.

On 6 June 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.

On 6 October 2008, scientists calculated that a small near-Earth asteroid, 2008 TC3, just sighted that night, should impact the Earth on 7 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 600 fragments, with a mass of 10.5 kilograms. The object is confirmed to have entered Earth's atmosphere as a meteor above northern Sudan at a velocity of 12.8 kilometers per second (29,000 mph).

A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the Nagasaki atomic bomb. No injuries were reported.

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.

C/2010 X1 (Elenin) will come to perihelion (closest approach to the Sun) on 11 September 2011 at a distance of 0.4824 AU. On 16 October 2011, the comet will pass within about 0.2338 AU(34,980,000 km; 21,730,000 mi) of the Earth at a relative velocity of 86,000 km/hr. The Minor Planet Center ephemeris shows this relatively bright comet will reach about 6th magnitude near mid-October 2011, but until the activity level of the coma is better known it is still uncertain just how bright this comet will become.

Several surveys have undertaken "Spaceguard" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS),Catalina Sky Survey, Campo Imperatore Near-Earth Objects Survey (CINEOS),Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. The George E. Brown, Jr. Near-Earth Object Survey Act, calls for NASA to detect 90 percent of NEOs with diameters of 140 meters or greater by 2020. But this act has not yet become a law in the U.S.

A comet is an icy, small Solar System body which, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. Comets have been observed since ancient times and have traditionally been considered bad omens.

Comets are distinguished from asteroids by the presence of a coma or a tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.

As of January 2011 there are a reported 4,185 known comets of which about 1,500 are Kreutz Sungrazers and about 484 are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular. Particularly bright or notable examples are called "Great Comets".

Comet nuclei are known to range from about 100 meters to more than 40 kilometres across. They are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia. Because of their low mass, comet nuclei do not become spherical under their own gravity, and thus have irregular shapes. Officially, according to NASA guidelines, a comet has to be at least 85% ice in order to be considered an actual comet.

Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.

As a result of outgassing, comets leave a trail of solid debris. If the comet's path crosses Earth's path, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October.

Most comets have elongated elliptical orbits that take them close to the Sun for a part of their orbit, and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: the longer the period the more elongated the ellipse. Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years.

Short-period comets are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. At the shorter extreme, Encke's Comet has an orbit which never puts it farther away from the Sun than Jupiter. Short-period comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years). Short-period comets originate in the Kuiper belt, or its associated scattered disc, which lie beyond the orbit of Neptune.

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the solar system. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods". Long-period comets plunge towards the Sun from the Oort Cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars.

Single-apparition comets are similar to long-period comets since they also have parabolic or lightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change, and when they are beyond the planets, their osculating eccentricity is still hyperbolic with aphelia lying beyond the outer Oort Cloud. The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)). All comets with parabolic and slightly hyperbolic orbits belong to the Solar System and had certain orbital periods, generally hundreds of thousand, or millions of years before being perturbed onto an ejection trajectory. Only a few hundred comets have been seen to achieve a hyperbolic orbit when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System. No comets with an eccentricity significantly greater than one have been observed, so there are no confirmed observations of comets that are likely to have originated outside the Solar System.

Some authorities use the term periodic comet to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), while others use it to mean exclusively short-period comets. Similarly, although the literal meaning of non-periodic comet is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Some comets meet a more spectacular end—either falling into the Sun, or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Earth's Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth (about 4 billion years ago) brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Other researchers have cast doubt on this theory. The detection of organic molecules in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. There are still many near-Earth comets, although a collision with an asteroid is more likely than with a comet.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.

Forthcoming space missions will add greater detail to our understanding of what comets are made of. The European Rosetta probe is presently en route to Comet Churyumov–Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.

Of the thousands of known comets, some are very unusual. Encke's Comet orbits from outside the main asteroid belt to just inside the orbit of the planet Mercury while the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL3. Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.

A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. Comet C/2010 X1 (Elenin) is a long-period comet discovered by amateur Russian astronomer Leonid Elenin on December 10, 2010, remotely, using the International Scientific Optical Network's robotic observatory near Mayhill, New Mexico, U.S.A. At the time of discovery Elenin had an apparent magnitude of 19.5, making it about 150,000 times fainter than the naked eye magnitude of 6.5. The discoverer, Leonid Elenin, estimates that the comet nucleus is 3–4 km in diameter.

However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO's 2000th comet was discovered by Polish amateur astronomer Michal Kusiak on 26 December 2010, and the numbers are expected to continue rising steadily for the foreseeable future.

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