Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earth's orbit. Some NEAs' orbits intersect Earth's so they pose a collision danger. On the other hand, NEAs are most easily accessible for spacecraft from Earth; in fact, some can be reached with much less delta-v than it takes to reach the Moon. The most famous near-Earth asteroid is 433 Eros that was visited by NASA's Near Earth Asteroid Rendezvous probe. An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Earth (often referred to as The Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. ... General In general physics delta-v is simply the change in velocity. ... Bulk composition of the moons mantle and crust estimated, weight percent Oxygen 42. ... The asteroid 433 Eros (eer-os) was named after the Greek god of love Eros. ... This article or section is missing references or citation of sources. ... -1...

A few hundred such near-Earth asteroids are known, ranging in size up to ~32 kilometres (1036 Ganymed). Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometre in diameter at up to 2,000. 1036 Ganymed is the largest Amor asteroid. ...

Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated by orbital decay and accretion by the Sun, collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, so they are resupplied on a regular basis. The Sun is the star at the center of Earths solar system. ...

NEA classification

Some NEAs with highly eccentric orbits are probably extinct comets that have lost all their volatile constituents, and a few NEAs still show faint comet-like tails. These NEAs were probably derived from the Kuiper belt, a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter. Comet Hale-Bopp For other uses, see Comet (disambiguation). ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ...

There are three families of NEAs:

• The Atens, which have average orbital radii closer than one astronomical unit (AU, the distance from the Earth to the Sun) and aphelia of greater than Earth's perihelion, placing them usually inside the orbit of Earth.
• The Apollos, which have average orbital radii greater than that of the Earth and perihelia less than Earth's aphelion.
• The Amors, which have average orbital radii in between the orbits of Earth and Mars and perihelia slightly outside Earth's orbit (1.017 - 1.3 AU). Amors often cross the orbit of Mars, but they do not cross the orbit of Earth. The two moons of Mars, Deimos and Phobos, appear to be Amor asteroids that were captured by the Red Planet.

Notice that all Atens and Apollos have eccentric orbits that cross the orbit of the Earth, making them potential threats to our planet, while Amors do not cross Earth's orbit but some may come very close. The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered (2062 Aten, discovered January 7, 1976 by Eleanor F. Helin). ... The astronomical unit (AU or au or a. ... A diagram of Keplerian orbital elements. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... The Apollo asteroid 25143 Itokawa. ... The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. ... For the Roman god, see Mars (mythology). ... Deimos (IPA or ; Greek Δείμος: Dread), is the smaller and outermost of Mars’ two moons, named after Deimos from Greek Mythology. ... Phobos (IPA , Greek Φόβος: Fright), is the larger and innermost of Mars two moons, and is named after Phobos, son of Ares (Mars) from Greek Mythology. ...

Also sometimes used is the Arjuna asteroid classification for asteroids with extremely Earth-like orbits. Near-Earth asteroid is a more restrictive term than near-Earth object. The Arjuna asteroids are a class of near-Earth asteroids whose orbits are very Earth-like in character, having low inclination, orbital periods close to one Earth year, and low eccentricity. ... Near-Earth objects (NEO) are asteroids, comets and large meteoroids whose orbit intersects Earths orbit and which may therefore pose a collision danger. ...

The NEA threat

The general acceptance of the Alvarez hypothesis, explaining the Cretaceous-Tertiary extinction event as the result of a large asteroid or comet impact event, has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit. The Alvarez hypothesis is the theory that the mass extinction of the dinosaurs and many other living things was caused by the impact of a large asteroid on the Earth sixty-five million years ago, called the Cretaceous-Tertiary extinction event. ... Badlands near Drumheller, Alberta where erosion has exposed the KT boundary. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on July 16, 1994. Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May 17, 1994. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... July 16 is the 197th day (198th in leap years) of the year in the Gregorian Calendar, with 168 days remaining. ... 1994 (MCMXCIV) was a common year starting on Saturday of the Gregorian calendar, and was designated the International year of the Family. ...

On March 23, 1989 the 300 metre (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometres (400,000 miles) passing through the exact position where the earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history. March 23 is the 82nd day of the year in the Gregorian Calendar (83rd in Leap years). ... 1989 (MCMLXXXIX) was a common year starting on Sunday of the Gregorian calendar. ... 4581 Asclepius is a small asteroid of the Apollo group that can approach Earth’s orbit to within 600,000 km; it was discovered in 1989 by the American astronomers Henry E. Holt (1929-) and Norman G. Thomas (1930-). Asclepius (1989 FC) passed by the Earth on March 22, 1989...

Asteroids with a 1 kilometre diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometre objects happen approximately once every ten million years. Small collisions occur a few times each month.

Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid (29075) 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on December 31, 2000. Proper calculation of its orbit then demonstrated that it has a potential Earth impact on March 16, 2880. (29075) 1950 DA has a diameter of roughly a kilometre. Asteroid 1950 DA, radar image (29075) 1950 DA is an asteroid considered to be the near Earth object with the highest known probability of crashing into Earth, according to the Palermo Technical Impact Hazard Scale. ... December 31 is the 365th day of the year (366th in leap years) in the Gregorian Calendar. ... This article is about the year 2000. ... March 16 is the 75th day of the year in the Gregorian Calendar (76th in Leap years). ... (Redirected from 2880) (28th century - 29th century - 30th century - more centuries) The 29th century (Gregorian Calendar) comprises the years 2801-2900. ...

On March 18, 2004, LINEAR announced a 30 metre asteroid 2004 FH which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years. March 18 is the 77th day of the year in the Gregorian calendar (78th in leap years). ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... The word linear comes from the Latin word linearis, which means created by lines. ... 2004 FH is a near-Earth asteroid that was discovered on March 15, 2004 by the NASA-funded LINEAR asteroid survey. ...

It is difficult to determine the chances of an impact more accurately. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect". The Sun is the star at the center of Earths solar system. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... In physics, the Yarkovsky effect is a force felt by a body caused by the momentum carried away by the thermal photons that it emits. ...

This effect was discovered by a Russian engineer named I. O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's pole direction and rotation rate.

Projects to ameliorate the threat

Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the LINEAR which began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 70% of all asteroid detections. LINEAR uses two one-metre telescopes and one half-metre one based in New Mexico. The word linear comes from the Latin word linearis, which means created by lines. ... 1996 (MCMXCVI) was a leap year starting on Monday of the Gregorian calendar, and was designated the International Year for the Eradication of Poverty. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ...

Spacewatch, which uses an old 90 centimetre telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis equipment to search the skies for intruders, was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan. The Spacewatch project has acquired a 1.8 metre telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimetre telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more. Spacewatch is a project at the University of Arizona that specializes in the study of minor planets, and including various types of asteroids and comets. ... The Kitt Peak National Observatory (KPNO) is located on a 6,875 ft peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oodham Nation, 55 miles southwest of Tucson. ... 1980 (MCMLXXX) was a leap year starting on Tuesday. ... Tom Gehrels is a Dutch-American astronomer. ... Robert S. McMillan is an astronomer at the University of Arizona, and heads the Spacewatch project, which studies minor planets. ... The University of Arizona (UA) is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona. ...

Other near-earth asteroid tracking programs include 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. Near Earth Asteroid Tracking (NEAT) is a program run by NASA and Jet Propulsion Laboratory to discover near-Earth objects. ... Lowell Observatory Near-Earth-Object Search (LONEOS) is a program run by NASA and Lowell Observatory in Flagstaff, Arizona, to discover near-Earth objects. ... Catalina Sky Survey is a project to discover comets and asteroids, and to search for near_earth objects. ... Campo Imperatore Near-Earth Objects Survey (CINEOS) is a project that looks for near-earth objects. ... Japans National Space Development Agency (NASDA), the National Aeronautic Laboratory, and the Space and Technology Agency have allocated the necessary funds to bring the Japanese Spaceguart Association (JSGA)on-line. ... The Asiago-DLR Asteroid Survey (ADAS) is a project to search for comets and asteroids, with special emphasis on near_Earth objects. ...

"Spaceguard" is the name for these loosely affiliated programs, some of which receive NASA funding to meet a U.S. Congressional requirement to detect 90% of near-earth asteroids over 1 km diameter by 2008. A 2003 NASA study of a follow-on program suggests spending US$250-450 million to detect 90% of all near-earth asteroids 140 metres and larger by 2028. The term Spaceguard loosely refers to a number of efforts to discover and study near-Earth objects (NEO). ... 2008 (MMVIII) will be a leap year starting on Tuesday of the Gregorian calendar. ... 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... Centuries: 20th century - 21st century - 22nd century Decades: 1970s 1980s 1990s 2000s 2010s - 2020s - 2030s 2040s 2050s 2060s 2070s Years: 2023 2024 2025 2026 2027 - 2028 - 2029 2030 2031 2032 2033 The year 2028 (MMXXVIII) in the Gregorian Calendar corresponds to 5788-5789 in the Hebrew Calendar. ...

Nonetheless, the fact that an impact of an NEA a kilometre or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating an explosive nuclear device above the surface of an NEA would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. This is a form of nuclear pulse propulsion. It has been suggested that Nuclear explosive be merged into this article or section. ... An artists conception of a spacecraft powered by nuclear pulse propulsion Nuclear pulse propulsion (or External Pulsed Plasma Propulsion, as it is termed in one recent NASA document) is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. ...

However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. In some ways, being struck with a loose cloud of smaller asteroids is worse than being struck with just one big one. This has led to a variety of other ideas for dealing with the threat: In astronomy, rubble pile is the informal name for an asteroid that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. ...

• Setting up "mass drivers" on the object to scoop up dusty material and shoot it away, giving the object a slow, steady nudge.
• Flying a big sheet of reflective aluminize PET film to wrap itself around the asteroid, acting as a "solar sail" to use the pressure of sunlight to shift the object's orbit.
• Dusting the object with powdered chalk or soot to perform a similar adjustment, utilising the Yarkovsky effect.

Thinking on the matter continues - see Asteroid deflection strategies - and if there is no prospect of immediate action, the issue isn't going away, either. A mass driver for lunar launch (artists conception) A mass driver or electromagnetic catapult is a method of spacecraft propulsion that would use a linear motor to accelerate payloads up to high speeds. ... Biaxially-oriented polyethylene terephthalate (boPET) polyester film is used for its high tensile strength, chemical and dimensional stability, transparency, gas and aroma barrier properties and electrical insulation. ... Concept image of a solar sail spacecraft in the process of unfurling sails. ... In physics, the Yarkovsky effect is a force felt by a body caused by the momentum carried away by the thermal photons that it emits. ... Artists impression of a major impact event. ...

As our space technology and space infrastructure advance, our choices improve. For example, it might become possible to bring asteroids (that are not flying rubble piles) to orbits near Earth, then maneuver them so that their small gravitational influence can move the NEAs to orbits that do not threaten us. Satellite navigation Space technology is a term that is often treated as a category. ... In astronomy, rubble pile is the informal name for an asteroid that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. ...

An example of a recent asteroid impact

On June 6, 2002 an object with an estimated diameter of 10 metres collided with Earth. The collision occurred over the Mediterranean Sea, at approximately 34°N 21°E and the object detonated in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a medium-size nuclear weapon [1]. At that time India and Pakistan were at a heightened state of alert, ready to initiate a nuclear war with each other. If this asteroid impact had hit in this area the results might have been catastrophic. June 6 is the 157th day of the year in the Gregorian calendar (158th in leap years), with 208 days remaining. ... For album titles with the same name, see 2002 (album). ... Satellite image The Mediterranean Sea is a part of the Atlantic Ocean almost completely enclosed by land, on the north by Europe, on the south by Africa, and on the east by Asia. ... A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the hypocenter. ...

See also

• Near-Earth object
• List of NEAs by distance from Sun
• Asteroid deflection strategies
• Sentry monitoring system

Near-Earth objects (NEO) are asteroids, comets and large meteoroids whose orbit intersects Earths orbit and which may therefore pose a collision danger. ... This list contains many notable Near-Earth asteroids organised by their average distance from the Sun Categories: Lists of asteroids | Lists of Solar system objects | Near-Earth asteroids ... Artists impression of a major impact event. ... Sentry is a highly automated collision monitoring system that continually scans the most current asteroid catalog for possibilities of future impact with Earth over the next 100 years. ...

External links

• JPL Near Earth Asteroid Tracking program (NEAT)

There is an excellent article in the November 2003 issue of Scientific American regarding NEA's and long term strategies for protecting Earth from them. 2003 : January - February - March - April - May - June - July - August - September - October - November - December A timeline of events in the news for November, 2003. ... Scientific American is a popular-science magazine, published monthly since August 28, 1845, making it the oldest continuously published magazine in the United States. ...