Near Earth Asteroids
What are Near Earth Asteroids?
Near Earth asteroids are asteroids that travel to within 1.3AU (195 million kilometers/121 million miles) of the Sun. There are over 250 near Earth asteroids known and they 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.
How Big?
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.
Asteroid Impacts
Asteroid and comet impacts are not uncommon in the solar system. The Earth is continually hit by small objects (meteorites) and occassional 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.
How Near?
In 1989, an asteroid 0.4 kilometers (0.25 miles) wide travelling 74,000 km/hr (46,000 m/hr) came within 640,000 kilometers (400,000 miles) of the Earth. The closest approach recorded was on Dec. 9, 1994 when a near Earth asteroid came within 0.0007AU (103,500 km/64,200mi) of the Earth. [The mean distance between the Earth and the Moon is 0.0026AU (384,400km/239,000mi)]. In June 2002, an asteroid passed about 120,000 km (74,000 mi) from the Earth. 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.
235K Quicktime
MovieAsteroid 2002 NY 40 was photographed passing near the Earth on August 15 and 16, 2002. At its closest approach it passed about 524,000 km (326,000 mi.) from the Earth.
Courtesy National Optical Astronomy Observatory, Tucson, AZ.
Toutatis - A Potentially Dangerous Near Earth Asteroid
 25k JPEG Radar image of Toutatis Goldstone/NASA image P-41525 |
This radar image of Toutatis was
taken when the asteroid made a close approach to Earth in 1992, coming
within 4 million kilometers (2,500,000 miles) of the planet. The 4 images
are from radar observations made from the Goldstone Deep Space Communications
Complex in California over a 5 day peroid. Toutatis measures approximately
4.6 x 2.4 x 1.9 kilometers (2.9 x 1.5 x 1.2 miles) and 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.
Because of its size and irregular orbit, the Minor Planet Center lists Toutatis as a "potentially dangerous" near Earth asteroid. It is scheduled to make another close approach to within 0.0104AU (1,537,600 km/953,300 mi) of the Earth in September of 2004. |
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