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Asteroids have historically been observed from Earth. The first close-up observation of an asteroid was made by the Galileo spacecraft. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres.
C-type (carbonaceous / ˌ k ɑːr b ə ˈ n eɪ ʃ ə s /) asteroids are the most common variety, forming around 75% of known asteroids. [1] They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks and minerals. They have an average density of about 1.7 g/cm 3.
Stony asteroids with a diameter of 4 meters (13 ft) enter Earth's atmosphere about once a year. [19] Asteroids with a diameter of 7 meters enter the atmosphere about every 5 years with as much kinetic energy as the atomic bomb dropped on Hiroshima (approximately 16 kilotons of TNT), but the air burst is reduced to just 5 kilotons. [19]
Barring detailed mass determinations, [4] the mass can be estimated from the diameter and assumed density values worked out as below. = Besides these estimations, masses can be obtained for the larger asteroids by solving for the perturbations they cause in each other's orbits, [5] or when the asteroid has an orbiting companion of known orbital radius.
They have a relatively high albedo and form about 17% of the total asteroid population. [74] M-type (metal-rich) asteroids are typically found in the middle of the main belt, and they make up much of the remainder of the total population. [74] Their spectra resemble that of iron-nickel.
An asteroid burned up in the Earth’s atmosphere just hours after it was detected last month, the European Space Agency revealed in its latest newsletter.. The asteroid, called 2024 UQ, was first ...
A menacing asteroid named Apophis is projected to have a close encounter with Earth in 2029, but scientists have long ruled it out as an impact risk. Asteroids safely fly by Earth all the time ...
Asteroids as small as 20 metres (66 ft) in diameter can cause significant damage to the local environment and human populations. [5] Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact the sea. Interest in NEOs has increased since the 1980s because of ...