Comet Elenin

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.