solar system


You will learn about Asteroids and over objects left over from the formation of the Solar, Kuiper bodies and more about the planets

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Asteroids are primoridal objects left over from the formation of the Solar System (see schematic diagram of nomenclature and relationships among asteroids, Edgeworth-Kuiper bodies, and comets and their different subgroups by William K. Hartmann). While some have suggested that they are the remains of a protoplanet that was destroyed in a massive collision long ago, the prevailing view is that asteroids are leftover rocky matter that never successfully coalesced into a planet. Most planetary astronomers still believe that the planets of the Solar System formed from a nebula of gas and dust that coalesced into a disk of dust grains around the developing Sun. Within the disk, tiny dust grains coagulated into larger and larger bodies called planetesimals, many of which eventually accreted into planets over a period as long as a 100 million years. However, beyond the orbit of Mars, gravitational interference from Jupiter's huge mass prevented protoplanetary bodies from growing larger than about 1,000 km (620 miles).



Most asteroids are rocky bodies that orbit the Sun between Mars and Jupiter in a "Main Asteroid Belt" that is centered around 2.7 times the Earth-Sun distance (astronomical unit or AU) from Sol. Two "clouds" of asteroids 60� ahead and behind Jupiter (and at or near Jupiter's orbital distance from the sun) are called "Jupiter Trojans" (diagram), while two similar objects in Mars orbit are called "Martian Trojans." Some asteroids have been found inside Earth's orbit (including many Near Earth Objects), while others -- including burnt out or dormant comets, such as perturbed Edgeworth-Kuiper Belt objects called "Centaurs" -- are located beyond Saturn's orbit.

Indeed, many have orbits that cross Earth's path (see orbit diagram of near-Earth Asteroid 4179 Toutatis), and while small asteroidal fragments hit the Earth every day as meteorites, bigger asteroids are surmised to have landed with impacts that killed off a significant share of life on the planet in times past (see schematic diagram of the life history of cometary and asteroidal material by William K. Hartmann). While most asteroids may be only the size of pebbles, 16 asteroids have a diameter of 240 km (150 miles) and Ceres, the largest, has a diameter of about about 914 km (568 miles). (See an animation of Ceres's orbit around the Sun, with a table of basic orbital and physical characteristics.)



It has been estimated that the total mass of the Main Asteroid Belt may total less than 1/1000th of the mass of the Earth. Indeed, if all asteroids down to the size of meter- or yard-sized boulders or less were combined together, the resulting object would measure less than 1,300 to 1,500 km (810 to 930 miles) across, which is less than one third to one half the diameter of the Earth's Moon. The Main Asteroid Belt is only a small remnant of the material that once resided in the region between Mars and Jupiter, but once may have contained between two to 10 Earth masses of material (Dan Durda, "Ask Astro," Astronomy, December 2000). However, T-Tauri-type Solar winds from a very young Sun, gravitational perturbations from Jupiter developing nearby, and dynamic interactions with other large planetesimals and protoplanets during the first 100 million years, and continuing collisional grinding over the following 4.5 billion years after the formation of the planets, interfered with the formation of a substantial, single planet and caused most of the mass to be lost to the rest of the Solar System and interstellar space.

planetary disk

Based on the composition of meteorites found on the Earth, most asteroids may be composed of three materials: mostly (92.8 percent) silicates (stone); metals (5.7 percent) iron and nickel; and the rest as a mix of the those materials and carbon-rich substances. Asteroids located closer to Mars and Earth that exhibit the same spectra are composed of rocky minerals ("stone") mixed with iron. In contrast, asteroids located farther away from the Sun on the Jupiter side of the Main Asteroid Belt are generally darker and redder, presumably because they were not well heated by the Sun and so have a composition more like the primordial, circum-Solar dust disk out of which the planets accreted about 4.5 billion years ago. Thus, the outer asteroids may more closely resemble the icy planetary bodies of the Edgeworth-Kuiper Belt and Oort Cloud.

951 Gaspra


The larger asteroids or protoplanets may have accumulated enough internal heat to "differentiate," whereby denser metals settled downwards (and perhaps even form metallic cores) and left lighter rocky ("stoney") residues in their outer layers. On some asteroids, internal heat may also have formed metamorphosized rocks, and volcanoes may even have erupted. Although no asteroid in the Main Belt grew big enough to hold on to an atmosphere, minerals found in some meteorites suggest that liquid water was often present.

433 Eros


The near-Earth asteroid Eros is made of material that has not been much altered since its accretion from the circum-Solar dust disk 4.5 billion years ago, never subjected to the melting and separation of its materials into distinct layers -- e.g., differentiated into metallic core, mantle, and crust -- as happened with the four inner planets (Earth, Mars, Venus, and Mercury). With an uniform density like the Earth's crust, it is probably a fractured chip off a larger body, perhaps another asteroid. Eros is roughly the size of Manhattan, NYC, USA (see Astronomy Picture of the Day).

As Jupiter accreted into a giant planet, its gravitational pull began to disturb the orbits of the nearest planetesimals so that collisions became more violent. As a result, the larger, differentiated protoplanets tended to be shattered into smaller asteroids, and so many asteroids are believed to have formed from their rocky or metallic debris based on spectral analysis of meteorites that landed on Earth. In contrast, the asteroids found farther out are thought to be made from planetary bodies that are less well differentiated, if at all.

433 Eros Rotating


Near-Earth Objects (NEOs) are asteroids and comets that have been nudged by the gravitational pull of nearby planets into orbits that cause them to enter the Earth's vicinity. In terms of orbital elements, NEOs are asteroids and comets with a perihelion (q) distance less than 1.3 times the Earth-Sun distance (AU). Near-Earth Comets (NECs) are further restricted to include only short-period comets -- that is, with an orbital period (P)less than 200 years. However, the vast majority of NEOs are asteroids, called Near-Earth Asteroids (NEAs). NEAs are subdivided into the Aten, Apollo, and Amor groups (that are categorized by their perihelion distance (q), aphelion distance (Q), and their semi-major axes (a) of orbit) and the Potentially Hazardous Asteriods (PHAs). PHAs are objects that can some day harm life on Earth because of their proximity and size -- Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less with an absolute magnitude (H) of 22.0 or brighter.



One of the largest known PHAs is Toutatis, an asteroid that is nearly a mile (1.6 km) long. No other PHA larger than 0.6 miles (one km) has been found that moves around the Sun in an orbit so nearly coplanar -- inclined less than half a degree -- with Earth's, extending from just inside Earth's orbit to a point deep within the Main Asteroid Belt between Mars and Jupiter. On September 29, 2004, Toutatis will pass just four Lunar distances from Earth, which is closer than any other known PHA over the next 30 years.

Analysis of data collected by spacecraft sent to a sample of asteroids since the 1990s and estimation of rotation speeds for a large number of asteroids have led to a startling conclusion. As of late 2001, no known asteroid larger than 200 meters (656 feet) across has been found to rotate faster than once every 2.2 hours. As a result, planetary astronomers infer that, over time, repeated collisions appear to have reduced most asteroids larger than a couple hundred meters (or yards) into "rubble piles" that are only loosely held by their weak gravity, which fly apart if spun too fast. For example, it has been estimated that asteroid Mathilde has a density close to that of water. Smaller asteroids, however, should be solid bodies because the shattered fragments of an asteroidal collision should easily escape their feeble gravity and fly off into space instead of settling back on their surface as regolith.

Toutatis and Earth


Asteroids are small bodies that are believed to be left over from the beginning of the solar system 4.6 billion years ago. They are rocky objects with round or irregular shapes up to several hundred km across, but most are much smaller.

More than 100,000 asteroids lie in a belt between Mars and Jupiter. These asteroids lie in a location in the solar system where there seems to be a jump in the spacing between the planets. Scientists think that this debris may be the remains of an early planet, which broke up early in the solar system. Several thousand of the largest asteroids in this belt have been given names.

The chances of an asteroid colliding with Earth are very small! But some do come close to Earth, like Hermes (closest approach of 777,000 km).



This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6,500 miles). The images used are from the sequence in which Ida's moon was originally discovered; the moon is visible to the right of the asteroid. This picture is made from images through the 4100-angstrom (violet), 7560 A (infrared) and 9680 A (infrared) filters. The color is 'enhanced' in the sense that the CCD camera is sensitive to near infrared wavelengths of light beyond human vision; a 'natural' color picture of this asteroid would appear mostly gray.

Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right- hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron- bearing minerals in these areas. Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.

Asteroid Names

Amphitrite Camilla Ceres Chiron Cybele
Daphne Davida Doris Egeria Elpis
Eros Eugenia Eunomia Euphrosyne Europa
Freia Hebe Hygiea Interamnia Iris
Juno Kalliope Mathilde Pallas Vesta

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