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	Oort Cloud
		The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun.
		The structure of the cloud is believed to consist of a relatively dense core that lies near the ecliptic plane and gradually replenishes the outer boundaries, creating a steady state.
		The Oort cloud is the source of long-period comets and possibly higher-inclination intermediate comets that were pulled into shorter period orbits by the planets, such as Halley and Swift-Tuttle.
	www.solarviews.com /eng/oort.htm (719 words)

	Oort cloud
		The Oort cloud was proposed in 1950 by Dutch astronomer Jan Oort[?] to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had existed since the origin of the solar system, all would have been destroyed by now.
		The Oort cloud is a remnant of the original nebula that collapsed to form the sun and planets five billion years ago, and is loosely bound to the solar system.
		It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing the occasional intrusion of a comet into the inner solar system.
	www.ebroadcast.com.au /lookup/encyclopedia/oo/Oort_cloud.html (240 words)

	Oort cloud - Facts, Information, and Encyclopedia Reference article
		The Oort cloud (sometimes called the Öpik-Oort Cloud) is a postulated spherical cloud of comets situated about 50,000 to 100,000 AU from the Sun.
		The most widely-accepted hypothesis of its formation is that the Oort cloud's objects initially formed much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptical or parabolic orbits.
		The star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710.
	www.startsurfing.com /encyclopedia/o/o/r/Oort_cloud.html (715 words)

	Oort Cloud & Sol b?
		Oddly enough, Oort Cloud objects were probably formed in a region of the protoplanetary disk that was located closer to the Sun than the Edgeworth-Kuiper Belt objects that persist in the orbital plane of the planets (ecliptic) to this day.
		The structure of the Cloud is believed to consist of a relatively dense core that lies near the planetary plane (ecliptic) and gradually replenishes the outer boundaries, creating a steady state.
		In addition, tidal forces affecting the Oort Cloud come from the differential gravitational forces exerted by stars in the Milky Way's galactic disk and by the galactic core on the Sun and comets as a result of their relative location in the Solar System.
	www.solstation.com /stars/oort.htm (1852 words)

	Oort Cloud
		Studies of the orbits of long-period comets, which are believed to originate in the Oort Cloud, suggest that the Cloud extends out from a heliocentric distance of about 20,000 to 100,000 astronomical units, with a peak density of objects at 44,000 AU from the Sun.
		A star that approaches the Sun to within roughly the distance of the Oort Cloud might increase the rate of comet passages near the Earth by a factor of 300 for 2 to 3 million years, so greatly increasing the risk of a catastrophic impact.
		If it were the case that the Oort Cloud comets originated at a wide range of distances, from the orbit of Jupiter to that of Neptune, and therefore at a wide range of ambient temperatures, this would explain the differences in composition observed in long-period comets.
	www.daviddarling.info /encyclopedia/O/OortCloud.html (573 words)

	ORRERY: The Oort Cloud
		Oort discovered that not only were the orbits concentrated towards the very large, they actually peaked out, most falling in from around 50,000 AU.
		Piecing all the mathematical evidence together, Oort concluded that the solar system is surrounded by a vast sphere of icy rocks (comet nuclei) at a distance of 50,000 AU - over 1200× the distance of Pluto from the Sun - that occasionally fall in to visit us.
		The furthest edge of the Oort cloud is believed to be 0·8 ly away.
	www.harmsy.freeuk.com /oort.html (305 words)

	Oort Cloud -- Birthplace of Comets - Astronomy
		Oort’s prediction was later confirmed by the gathering of additional data.
		In fact, the cloud is considered the outer limit of the material part of the Solar System, and is thought to mark the limit of the Sun’s gravitational pull.
		The orbits of Oort Cloud comets are easily disturbed by passing stars, and other outside forces such as giant molecular clouds (masses of cold hydrogen in which stars and solar systems form).
	www.bellaonline.com /articles/art41582.asp (380 words)

	Oort cloud Summary
		The Oort cloud is a vast swarm of some 2 trillion comets orbiting our star in the most distant reaches of our solar system, extending from beyond the orbits of Neptune and Pluto out to 100,000 times the Earth-Sun distance—nearly one-third the distance to the nearest star.
		While the planets are confined to a flattened disk in the solar system, the Oort cloud forms a spherical shell centered on the Sun, which gradually flattens down to an extended disk in the inner region, called the Kuiper belt.
		The Oort cloud was "discovered" by Dutch astronomer Jan Hendrik Oort in 1950, not through telescopic observations, but through a theoretical study of the orbits of long-period comets (comets with periods greater than 200 years).
	www.bookrags.com /Oort_cloud (1768 words)

	Oort Cloud Sphere \| Guide \| Digital Universe \| Hayden Planetarium
		The Oort cloud is a region of space surrounding the Sun where comets are believed to originate.
		Proposed by Jan Oort in the 1950s, the Oort cloud is believed to extend from 20,000 to 100,000 Astronomical Units (AU), with its greatest concentration around 50,000 AU (1 AU = 149 million kilometers = 93 million miles, the average Earth-Sun distance).
		Occasionally, one comet in the cloud interacts with another or is perturbed by a passing star or a passing star's comet cloud.
	www.haydenplanetarium.org /universe/duguide/mwg_oort.php (296 words)

	::Interactive Astronomy::
		Comet Hale-Bopp is an object in the Oort Cloud.
		As the comets pass by Earth in all varying directions, Oort concluded that the “Oort Cloud” as he had named, is a sphere.
		By measuring the orbits of certain long trailing comets, scientists believe that the Oort Cloud is huge, between 5000 AU and 100,000AU.
	library.thinkquest.org /04apr/00533/Text-Only/oort_cloud.htm (770 words)

	What is the Oort Cloud? (Site not responding. Last check: 2007-10-17)
		The Oort cloud is a huge spherical cloud of comets and dust that extends three light years from the Sun in all directions.
		Oort clouds certainly overlap, and the Oort cloud of our sun likely overlaps with the cloud of Alpha Centauri.
		It is theorized that Oort cloud objects actually formed relatively close to the sun, closer than Neptune's orbit, but were ejected into huge parabolic orbits when they were slungshot by the gravity wells of huge planets like Jupiter.
	www.wisegeek.com /what-is-the-oort-cloud.htm (298 words)

	Oort Cloud Oort Cloud Oort Cloud Oort Cloud Oort Cloud (Site not responding. Last check: 2007-10-17)
		As a result, comets in the Oort Cloud are frequently perturbed by the gravity of passing stars.
		The combined effect is a "random walk" in the magnitude of the velocity perturbation such that, after 10,000 stars have passed by, the original orbit of the comet has been drastically altered.
		Closer to the sun, the Oort Cloud comets are tightly held and may never be dislodged by the gravity of passing stars.
	www.ifa.hawaii.edu /faculty/jewitt/oort2.html (417 words)

	Comets and the age of the solar system
		Oort proposed a large spherical cloud of comet nuclei that formed early in the history of the Solar System.
		The Oort cloud is supposed to be at a large distance from the Sun, placing the nuclei too far away to be observed.
		Oort himself apparently believed that the short period comets were best explained by the disruption of a planet that once orbited between Mars and Jupiter, an old idea that has been largely discarded, but still has its supporters (for instance, van Flandern).
	www.answersingenesis.org /tj/v11/i3/comets.asp (8195 words)

	Oort Cloud at Universe Guide
		The Oort Cloud in brief is an hypothesized area in space that is roughly one light year away from the Sun.
		The Oort Cloud is sometimes known as the Opik-Ort Cloud in recognition of the earlier hypothesis.
		Apart from being associated with the Oort Cloud, Jan also calculated that the centre of the Milky Way is 30,000 light years away in the direction of the Sagittarius constellation.
	www.universeguide.com /OortCloud.php (457 words)

	Oort cloud - Wikipedia, the free encyclopedia
		The Oort cloud is thought to be a remnant of the original solar nebula that collapsed to form the Sun and planets approximately 4.6 billion years ago, and is loosely bound to the solar system.
		A recent alternative hypothesis for the origin of the Oort cloud is that the comets were already present in the original solar nebula, even before the protosun and the protoplanetary disk was formed.
		It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing perturbations in the comets' orbits and thereby increasing the number of comets that enter the inner solar system.
	en.wikipedia.org /wiki/Oort_cloud (1174 words)

	Oort Cloud (Site not responding. Last check: 2007-10-17)
		Thus, the nuclei of comets are icy balls of frozen water, methane, and ammonia, mixed with small pieces of rock and dust, rather than the largely volatile-free stones and irons that typify asteroids.
		In the original version of the theory, the Oort cloud extended tens of thousands of times farther from the Sun than the Earth, a significant fraction of the way to the nearest stars.
		Part of the uncertainty concerns the reality of a hypothesized massive "inner Oort cloud" -- or "Kuiper belt" (if the distribution is flattened)--of comets that would exist at distances from the Sun 40 to 10,000 times that of the orbit of the Earth.
	abyss.uoregon.edu /~js/glossary/oort_cloud.html (420 words)

	Oort Cloud Oort Cloud Oort Cloud Oort Cloud Oort Cloud (Site not responding. Last check: 2007-10-17)
		Jan Oort noticed that the concentration towards large orbital size was very tightly peaked.
		He calculated that comets falling into the solar system from very large distances should suffer perturbations from the planets (especially Jupiter) and found that these perturbations were actually larger than the width of the LPC peak.
		Therefore, he suggested that the sun is surrounded by a spherical cloud of comets from which the observed LPCs are somehow perturbed.
	www.ifa.hawaii.edu /faculty/jewitt/oort.html (205 words)

	Comets' Oort Cloud (Site not responding. Last check: 2007-10-17)
		In this hypothesis, the Oort Cloud may account for a significant fraction of the mass of the solar system (perhaps even more than Jupiter itself).
		In fact, most of the comets are thought to come from the outer edge of the cloud, where the attraction to the sun is weak and passing stars have bigger effects.
		Another possible mechanism to make comets leave the Oort cloud can be the shock wave from an explosive event such as a supernova.
	spaceguard.esa.int /NScience/neo/neo-what/com-oort.htm (454 words)

	American Scientist Online - Perturbing the Oort Cloud (Site not responding. Last check: 2007-10-17)
		In the classical view, first proposed by the Dutch astronomer Jan Oort in 1950, these comets remain in the distant reservoir until a passing star perturbs the cloud, diverting some of the comets toward the inner solar system.
		According to the proponents of the theory, the Oort cloud was being greatly perturbed by something in the galactic midplane, and this caused a catastrophic rain of comets on the inner solar system (including the earth) every 30 million years.
		For the time being it appears that the Oort comet cloud and the galaxy will continue to be entangled with the dinosaur extinctions and extraterrestrial impactors.
	www.americanscientist.org /template/AssetDetail/assetid/24618 (1302 words)

	Astronomy Online - Solar System Basics
		Gerard Kuiper speculated that between Pluto and the Oort cloud is a population of icy bodies, similar to Pluto.
		As some nearby object disturbs the Oort cloud, an icy rock is "thrown" our way in the form of a comet.
		It is the random nature of the comets orbit that is the evidence of the Oort cloud.
	astronomyonline.org /SolarSystem/Introduction.asp?Cate=SolarSystem&SubCate=Introduction&SubCate2=SS00 (902 words)

	Oort Cloud solar system astronomy
		Within the cloud comets are thought to be typically tens of millions of miles apart.
		Only weakly bound to the Sun, they are easily disturbed when the Oort Clouds of passing stars pass through or close to the Sun’s Oort Cloud, hurling them into the inner solar system or out into interstellar space.
		The Oort Cloud’s inhabitants are most strongly perturbed, however, by the tidal forces of giant molecular clouds.
	www.myastrologybook.com /Oort-Cloud-solar-system-astronomy.htm (528 words)

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