What are Near-Earth Asteroids?
Near-Earth asteroids (NEAs) are asteroids that travel to within 1.3AU (195 million kilometers/121 million miles) of the Sun. An AU is an astronomical unit and it is equal to the mean distance from the Sun to the Earth (approximately 150 million kilometers or 93 million miles). As of June 2015, over 872 NEAs larger than 1 kilometer (.6 mile) have been discovered. NEAs are classified into three groups:
Classes of Near-Earth Asteroids
|Amor||Crosses the orbit of Mars, does not cross orbit of Earth. (representative example = 433 Eros — visited by the NEAR Shoemaker spacecraft)|
|Apollo||Crosses Earth's orbit, orbital period greater than one year. (example = 1620 Geographos)|
|Aten||Crosses Earth's orbit, orbital period less than one year. (example = 2340 Hathor)|
Most Near-Earth asteroids are believed to be main belt asteroids that were knocked out of the belt by collisions with other asteroids and/or by the gravitational forces of Jupiter. Some NEAs may actually be remnants of dead comets. The orbits of Near-Earth asteroids are thought to be influenced by gravitational interactions with the Sun or terrestrial planets or by collisions with other bodies.
The largest known Near-Earth asteroid is 1036 Ganymed with a diameter of nearly 41 kilometers (25.5 miles). Scientists are working to identify Near-Earth asteroids with diameters of 1 kilometer (0.6 mile) or more that may pose a danger of colliding with the Earth.
What's a PHA?
Asteroids that are larger than 150m (500 ft.) across and that travel closer than 7.5 million kilometers (4.6 million miles) to the Earth are classified as Potentially Hazardous Asteroids (PHAs). Classification as a PHA does not mean an asteroid will necessarily impact the Earth, but this is the class of asteroids whose paths should be tracked and monitored.
For more information on different kinds of NEAs and programs to monitor them, check out NASA's Near-Earth Object Program.
Asteroid and comet impacts are not uncommon in the solar system. The Earth is continually hit by small objects (meteorites) and occasional larger impacts occur. In 1908, an asteroid measuring 100 meters (330 feet) in diameter exploded over Siberia destroying over half a million acres of forest. Asteroid impact is still the main theory for the cause of mass biological extinctions of the Cretaceous period—including the extinction of the dinosaurs. That asteroid is thought to have been 10 kilometers (6 miles) in diameter.
In 1989, an asteroid 0.4 kilometers (0.25 miles) wide travelling 74,000 kilometers/hr. (46,000 miles/hr.) came within 640,000 kilometers (400,000 miles) of the Earth. The closest approach recorded was in October 2008 when a Near-Earth asteroid came within 6,500 kilometers (about 4,000 miles) of the Earth. (The mean distance between the Earth and the Moon is 384,400 kilometers (239,000 miles). The Minor Planet Center (under the auspices of the International Astronomical Union) keeps a record of the closest approaches to Earth by asteroids and comets.
These radar images of Toutatis were taken when the asteroid made a close approach to Earth in 1992, coming within 4 million kilometers (2.5 million miles) of the planet. The images are from radar observations made from the Goldstone Deep Space Communications Complex in California over a five day period. Toutatis measures approximately 4.6 x 2.4 x 1.9 kilometers (2.9 x 1.5 x 1.2 miles). Radar imagery revealed complex surface features including numerous craters and ridges. These surface features and the asteroid's strange tumbling rotation suggest a history of frequent collisions. Toutatis made another close approach to within 7,510,000 kilometers (4,670,000 miles) of the Earth in 2008.
In November 2011 the asteroid provisionally named 2005 YU55 safely passed about 324,900 kilometers (201,900 miles) from Earth. The path of this asteroid is well-known and it regularly travels by Earth. This latest pass was the closest it has come to our planet in 200 years. It does not, however, pose a threat to Earth for the foreseeable future. See a video of Asteroid 2005 YU55.