Among the thousands and thousands of asteroids are some that are particularly notable. Some, such as Ceres, are significant in the history of astronomy. Others, such as Vesta, are geologically interesting.
The dwarf planet Ceres is the largest known asteroid in the asteroid belt and the first asteroid to be discovered. Ceres revolves around the Sun once in 4.61 Earth
Series of six images showing the rotation of Ceres, taken by the Hubble Space Telescope. ESA/STScI/NASA
years in a nearly circular, moderately inclined (10.6°) orbit at a mean distance of 2.77 AU (about 414 million km [257 million miles]). Ceres has the shape of a flattened sphere with an equatorial radius of 490 km (304 miles) and a polar radius of 455 km (282 miles), equivalent in volume to a sphere with a diameter of 940 km (94 miles)—i.e., about 27 percent that of Earth's Moon. Although Ceres is the largest asteroid, it is not the brightest. That honour belongs to the second largest asteroid, Vesta, which orbits closer to the Sun than Ceres (Vesta's mean distance is 2.36 AU) and has a surface reflectivity more than three times as high (its albedo is 0.34, compared with 0.10 for Ceres). The mass of Ceres, which accounts for more than one-third the total mass of the main asteroid belt, is about 9.1 * 1020 kg (2 * 1021 pounds), and its density is 2.2 g/cm3 (1.3 oz/in3; about two-thirds that of the Moon). Ceres's shape and density are consistent with a two-layer model of a rocky core surrounded by a thick ice mantle. Ceres rotates once in 9.1 hours, showing no large-scale colour or brightness variations over its surface. Compositionally, the asteroid's surface resembles the carbonaceous chondrite meteorites.
Ceres has been designated a dwarf planet, a new category of solar system objects defined in August 2006 by the IAU.
Eros was the first asteroid found to travel mainly inside the orbit of Mars and the first to be orbited and landed on by a spacecraft. Eros was discovered in 1898 by the German astronomer Gustav Witt at the Urania Observatory in Berlin. It is named for the god of love in Greek mythology.
A near-Earth asteroid, Eros can pass within 22 million km (14 million miles) of Earth. During a close approach in the 1930s (before the development of direct radar ranging), astronomers were able to observe the asteroid's parallax displacement against the background stars to refine their measurement of Earth's mean distance from the Sun, the basis for the astronomical unit. Eros was the first asteroid found (1901) to display variations in brightness due to its rotation. These periodic light fluctuations were later used to determine its rotation period (5.27 hours), its elongated shape, and, with other observations, its size, which is about twice that of New York City's Manhattan Island. Spectral observations established that Eros belongs to the S compositional class, the most common class among asteroids located in the inner part of the main asteroid belt.
In 2000 the Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft (launched 1996) entered orbit around Eros and collected a full year of data on its surface composition, topography, mass, gravity field, internal structure, and other properties before touching down gently on its surface. NEAR Shoemaker obtained precise dimensions (33 * 13 * 13 km [20.5 * 8 * 8
distinct composition—and so may be a pristine sample of primordial solar system material.
Geographos is an asteroid that passes inside Earth's orbit. Geographos was discovered in 1951 by American astronomers Albert Wilson and Rudolf Minkowski at the Palomar Observatory. In 1994 radar observations found that Geographos has dimensions of 5.11 x 1.85 km (3.18 x 1.15 miles) and is thus the most elongated object in the solar system. That same year the U.S. spacecraft Clementine was scheduled to fly by Geographos after leaving lunar orbit, but a computer malfunction canceled that portion of the mission.
Hermes miles]), found evidence of geologic phenomena that could have originated on a much larger parent body from which Eros was derived, and obtained thousands of images revealing numerous ridges, grooves, crater chains, and boulders. A significant discovery was that Eros is an undifferentiated asteroid—i.e., it was never subjected to extensive melting and segregation into layers of
The binary asteroid Hermes has an eccentric orbit that brings it near Earth. It was discovered in October 1937 by German astronomer Karl Wilhelm Reinmuth when it approached within about 742,000 km (461,000 miles) of Earth; announcement of this near passage occasioned some fear that it might collide with Earth. Hermes was subsequently lost and was not observed again until 2003. Radar observations of Hermes showed that it was actually two asteroids that orbit each other every 14 hours. The asteroids are 630 and 560 metres (2,070 and 1,840 feet) in diameter.
The asteroid Icarus has a very eccentric orbit with an eccentricity of 0.82 and also approaches quite near to the Sun (within 30 million km [19 million miles]). It was discovered in 1949 by Walter Baade of the Hale Observatories (now Palomar Observatory), California. Its orbit extends from beyond Mars to within that of Mercury; it can approach within 6.4 million km (4 million miles) of Earth. In June 1968 Icarus, the first asteroid to be examined by radar, proved to have a diameter of about 0.8 km (0.5 miles), considerably smaller than previous estimates, and a rotation period of about 2.5 hours.
Pallas is the second largest asteroid in the asteroid belt and the second such object to be discovered, by the German astronomer and physician Wilhelm Olbers on March 28, 1802, following the discovery of Ceres the year before. It is named after Pallas Athena, the Greek goddess of wisdom.
Pallas's orbital inclination of 34.8° is rather large, but its moderate orbital eccentricity (0.23), mean distance from the Sun of 2.77 AU (about 414 million km [257 million miles]), and orbital period of 4.61 years are typical for asteroids located between the orbits of Mars and Jupiter.
Pallas has an ellipsoidal shape with radial dimensions of 285 * 262 * 250 km (177 * 163 * 155 miles), equivalent to a sphere with a diameter of 530 km (329 miles)—i.e., about 15 percent of the diameter of the Moon. Pallas's albedo (reflectivity) is 0.15. Its mass is about 2.2 * 1020 kg (4.8 * 1020 pounds), and its density is about 2.9 g/cm3 (1.7 oz/in3; nearly 90 percent that of the Moon). Pallas turns once on its axis every 7.8 hours. Compositionally, Pallas resembles the carbonaceous chondrite meteorites. Its surface is known to contain hydrated minerals.
Vesta is the third largest and the brightest asteroid of the asteroid belt and the fourth such object to be discovered, by the German astronomer and physician Wilhelm Olbers on March 29, 1807. It is named for the ancient Roman goddess of the hearth.
Vesta revolves around the Sun once in 3.63 years in a nearly circular, moderately inclined (7.1°) orbit at a mean distance of 2.36 AU (about 353 million km [219 million miles]). It has an ellipsoidal shape with radial dimensions of 280 * 272 * 227 km (174 * 169 * 141 miles), equivalent to a sphere with a diameter of 520 km (323 miles)—i.e., about 15 percent of the diameter of Earth's Moon. Although Vesta is only about half the size of the largest asteroid, Ceres, it is about four times as reflective (Vesta's albedo, averaged over its rotation, is 0.40, compared
The asteroid Vesta, in three renditions based on observations made with the Hubble Space Telescope (HST) in May 1996 during a relatively close approach of the asteroid to Earth. The mottling on the model is artificially added and does not represent true brightness variations on Vesta. Source: Ben Zellner, Georgia Southern University; Peter Thomas, Cornell University; NASA © Encyclopedia Britannica, Inc.
with 0.10 for Ceres), and it orbits closer (Ceres's mean distance is 2.77 AU). Vesta is the only main-belt asteroid visible to the unaided eye. Its mass is about 2.7 * 1020 kg (5.6 * 1020 pounds), and its density is 3.5 g/cm3 (2 oz/in3; about the same as that of the Moon). It rotates once in 5.3 hours, showing large-scale colour and brightness variations over its surface.
Compositionally, Vesta resembles the basaltic achondrite meteorites and is widely believed to be the parent body of the meteorites known as basaltic achondrite HEDs (a grouping of the howardite, eucrite, and diogenite types). In other words, most and perhaps all of these meteorites were once part of Vesta.
Meteors and Meteorites
If you look long enough at the night sky, eventually you will see a shooting star. This glowing streak in the sky is a meteor, and its cause is a relatively small stony or metallic natural object from space called a meteoroid that enters Earth's atmosphere and heats to incandescence. In modern usage the term meteoroid, rather than being restricted to objects entering Earth's atmosphere, is applied to any small object in orbit around the Sun having the same nature as those that result in meteors.
When a meteoroid enters Earth's atmosphere, it is traveling at very high velocity—more than 11 km per second (25,000 miles per hour) at minimum, which is many times faster than a bullet leaving a gun barrel. Frictional heating, produced by the meteoroid's energetic collision with atmospheric atoms and molecules, causes its surface to melt and vaporize and also heats the air around it. The result is the luminous phenomenon recognized as a meteor. The vast majority of meteoroids that collide with Earth burn up in the upper atmosphere. If a meteoroid survives its fiery plunge through the atmosphere and lands on Earth's surface, the object is known as a meteorite.
The term meteoroid is usually reserved for chunks of matter that are approximately house-sized—i.e., some tens of metres across—and smaller, to distinguish them from the larger asteroidal bodies. Meteoroids are believed to be mostly fragments of asteroids and comets and are placed, with them, in the category of solar system objects known as small bodies. A few meteoroids also appear to have come from the Moon and Mars. The smallest meteoroids, those less than a few hundred micrometres across (about the size of a period on a printed page), are called interplanetary dust particles or micrometeoroids.
The terms meteoroid and meteor (and meteorite as well) are sometimes confus-ingly interchanged in common usage. Meteor in particular is often applied to a meteoroid hurtling through space, to an incandescent meteoroid (rather than just its luminous streak) in the atmosphere, or to an object that has hit the ground or a man-made object. An example of the last case is found in the name Meteor Crater, a well-known impact structure in Arizona, U.S.
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