Asteroid belt

• 1. Discovery And Naming History

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  In 1596, Johannes Kepler wrote in his book Mysterium Cosmographicum that he placed a planet between the orbits of Mars and Jupiter. He believed an invisible world must exist there to fill the gap in planetary spacing. This theoretical prediction remained unproven for two centuries until the 1st of January 1801. On that date, Giuseppe Piazzi discovered a tiny moving object while observing from Palermo, Sicily. The object had an orbit matching the radius predicted by the Titius-Bode Law. Piazzi named it Ceres after the Roman goddess of harvest who was also the patron of Sicily. Initially, astronomers thought this new body might be a comet because it moved quickly across the sky. However, its lack of a visible coma suggested it was actually a planet. Heinrich Olbers found a second object called Pallas about fifteen months later. Both Ceres and Pallas appeared as points of light even under high magnification telescopes. They looked indistinguishable from stars except for their rapid movement. William Herschel proposed they should be called asteroids instead of planets or comets. He stated that neither appellation could properly describe these star-like objects. By 1807, four such bodies were known: Ceres, Pallas, Juno, and Vesta. Counting them among the planets became increasingly cumbersome as more discoveries emerged. In 1845, Karl Ludwig Hencke detected a fifth object named Astraea. New objects began to be found at an accelerating rate thereafter. Eventually, astronomers dropped them from the official planet list. Alexander von Humboldt first suggested this reclassification in the early 1850s. Herschel's coinage gradually came into common use over the following decades.

• 2. Formation And Evolutionary Dynamics

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  The asteroid belt formed from the primordial solar nebula as a group of planetesimals. These smaller precursors eventually clumped together through sticky collisions during the first few million years of Solar System history. Once clumps reached sufficient mass, they drew in other bodies via gravitational attraction. This process led to the formation of protoplanets within the region between Mars and Jupiter. However, strong gravitational perturbations from Jupiter disrupted their accretion into a single planet. Orbital resonances occurred where an object's revolution period formed an integer fraction of Jupiter's orbital period. As Jupiter migrated inward following its formation, these resonances swept across the asteroid belt. They dynamically excited the region's population and increased velocities relative to each other. In regions where average collision velocity was too high, shattering dominated over accretion. Most of the material was ejected from the belt within about one million years of formation. Computer simulations suggest the original belt may have contained mass equivalent to Earth's. Only less than 0.1% of the original mass remains today. The current total mass is estimated at three percent that of the Moon. Some fragments found their way into the inner Solar System leading to meteorite impacts on Earth. During early history, asteroids melted to some degree allowing elements to differentiate by mass. A study of zircon crystals in an Antarctic meteorite suggested Vesta formed quickly within ten million years of origin. Surface melting from impacts and space weathering from radiation continued to modify them over billions of years.

• 3. Composition And Spectral Classification

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  The present day belt consists primarily of three categories of asteroids based on spectral analysis. Carbonaceous C-type asteroids dominate the outer regions and are rare in the inner belt. Together they comprise over seventy-five percent of visible asteroids. Their surface compositions match carbonaceous chondrite meteorites like the Allende stone that fell to Mexico in 1969. S-type silicate-rich asteroids are more common toward the inner region within two point five astronomical units. These spectra reveal presence of silicates and some metal but no significant carbon compounds. They form about seventeen percent of the total population. M-type metal-rich asteroids typically occupy the middle of the main belt. Their spectra resemble iron-nickel alloys though some may be silicate compounds instead. Within the belt, number distribution peaks at a semimajor axis of about 2.7 AU. One mystery remains regarding the relative rarity of basaltic V-type asteroids. Theories predict objects the size of Vesta should have crusts composed mainly of basaltic rock. Yet observations suggest ninety-nine percent of predicted basaltic material is missing. Until 2001 most basaltic bodies were believed to originate from Vesta itself. Discovery of asteroid 1459 Magnya revealed slightly different chemical composition suggesting another origin. Two additional asteroids found in 2007 further reinforced this hypothesis with differing basaltic compositions.

• 4. Orbital Mechanics And Resonance Gaps

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  Most asteroids within the belt have orbital eccentricities less than 0.4 and inclinations below 30 degrees. The orbital distribution reaches a maximum around an eccentricity of 0.07 and inclination under four degrees. In 1866 Daniel Kirkwood announced discovery of gaps in distances of these orbits from the Sun. These gaps occurred where revolution period formed an integer fraction of Jupiter's orbital period. Gravitational perturbations by the gas giant led to removal of asteroids from these specific orbits. When mean orbital period matches an integer fraction of Jupiter, a mean-motion resonance creates sufficient perturbation. Primordial asteroids entered these gaps due to migration of Jupiter's orbit over time. Subsequently asteroids primarily migrate into gap orbits due to the Yarkovsky effect or collisions. After entering, an asteroid is gradually nudged into a different random orbit. The 4:1 orbital resonance at radius 2.06 AU serves as inner boundary of the main belt. Perturbations send bodies straying there into unstable orbits. Most bodies formed within radius of this gap were swept up by Mars or ejected early on. The core region lies between strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 AU. This compact area contained ninety-three percent of all discovered minor planets as of the 8th of February 2006.

• 5. Collisional Families And Dust Production

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  In 1918 Japanese astronomer Kiyotsugu Hirayama noticed that some asteroid orbits had similar parameters forming families. Approximately one-third of asteroids in the belt are members of such groups sharing common origin. These share similar semi-major axis eccentricity and inclination along with spectral features indicating breakup of larger body. Graphical displays show concentrations representing presence of specific asteroid families like Flora Eunomia Koronis Eos and Themis. The Flora family contains more than eight hundred known members possibly formed from collision less than one billion years ago. Largest true member of a family is Vesta itself unlike Ceres which acts as interloper. Three prominent bands of dust have been found within the belt associated with Eos Koronis and Themis groupings. Fine material produced from collisions spirals inward toward Sun due to Poynting-Robertson effect. Average lifetimes of main-belt zodiacal cloud particles are about seven hundred thousand years. To maintain these bands new particles must be steadily produced inside the belt. Computer simulations attributed eighty-five percent of zodiacal-light dust to fragmentations of Jupiter-family comets rather than asteroid collisions. At most ten percent of dust comes directly from the asteroid belt itself. Some debris forms meteoroids entering Earth's atmosphere where ninety-nine point eight percent originate from this region.

• 6. Spacecraft Exploration And Mission History

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  The first spacecraft to traverse the asteroid belt was Pioneer 10 on the 16th of July 1972. Concern existed that debris would pose hazard but it safely passed through without incident. Multiple probes including Voyager 1 and 2 Ulysses Cassini Juno and Dawn have since crossed the region. Odds of probe running into an asteroid estimated at less than one in one billion. Galileo spacecraft imaged 951 Gaspra in 1991 and 243 Ida in 1993 during brief flybys. NEAR mission landed on near-Earth asteroid 433 Eros in February 2001 after imaging Mathilde in 1997. Cassini measured plasma and fine dust grains while traversing in 2000. Stardust imaged 5535 Annefrank in 2002 and New Horizons captured APL in 2006. Rosetta photographed Šteins in September 2008 and Lutetia in July 2010. Dawn orbited Vesta between July 2011 and September 2012 before moving to Ceres. It has orbited Ceres since March 2015 studying its surface features extensively. Lucy space probe made flyby of Dinkinesh in 2023 en route to Jupiter Trojans. ESA's JUICE mission will pass through belt twice with proposed flyby of Rosa in 2029. Psyche spacecraft targets large M-type asteroid named after the same figure.