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Kuiper Belt
I INTRODUCTION
Kuiper Belt (pronounced KY-per), a collection of frozen objects extending from just beyond the orbit of the planet Neptune to well beyond the orbit of the planet Pluto. The larger members of the belt are called Kuiper Belt Objects, or KBOs. Smaller members of the Kuiper Belt are usually referred to as comets. The orbits of these objects show that the belt is actually disk-shaped.
The existence of the Kuiper Belt was first predicted during the mid-20th century, most notably by Dutch American astronomer Gerard Kuiper. Kuiper and other astronomers expected that a debris belt, similar to the asteroid belt of rocky material that orbits the Sun between Mars and Jupiter but composed of icy material, might lie beyond Neptune. The first searches for the Kuiper Belt, however, were unsuccessful. Astronomers now know the early searches failed because the photographic technology in use at the time was not sensitive enough to find KBOs. By the late 1980s, astronomers had access to a kind of electronic camera called a charge-coupled device (CCD). CCDs are much more sensitive than traditional photography, allowing them to detect fainter objects. In 1992 astronomers Jane Luu and David Jewitt found the first Kuiper Belt Object. This KBO, designated 1992QB1, is more than 1,000 times fainter than the planet Pluto.
Over 500 Kuiper Belt Objects had been found by 2002. Astronomers estimate that over 100,000 KBOs larger than 50 km (30 mi) in diameter may exist. The Kuiper Belt therefore likely contains a greater collection of objects than the asteroid belt. Including the billions of comets that are believed to orbit in the Kuiper Belt, it is estimated that the belt's total mass is about one-tenth the mass of Earth.
II CHARACTERISTICS
Astronomers have been studying the Kuiper Belt since the first KBO was detected. The orbits of objects within the belt fall into two major categories. The majority of KBOs, called classical KBOs, orbit between 30 and 55 astronomical units (AU) from the Sun. (An astronomical unit is the average distance from Earth to the Sun, just under 150,000,000 km.) A small percentage of KBOs, called Scattered Disk Objects (SDOs), have orbits stretching far beyond 55 AU. Some SDOs have orbits that stretch out beyond 1,000 AU.
Astronomers have discovered KBOs as large as 1,100 km (700 mi) and as small as 20 km (12 mi) in diameter. The largest known KBO, tentatively named Sedna, is more than half the diameter of Pluto. Small KBOs greatly outnumber large ones. Computer simulations indicate that the Kuiper Belt, like the asteroid belt, formed early in the history of the solar system. Based on the sizes and orbits of KBOs, it appears that the Kuiper Belt was coalescing into one or more large planets when the growth process was interrupted. Astronomers believe that the formation of Neptune disturbed the region gravitationally and interrupted this growth.
The Kuiper Belt differs from the asteroid belt in two significant ways. First, KBOs formed more than ten times as far from the Sun as most asteroids. Second, they contain far more ice than asteroids, which are primarily rocky objects.
Based on their knowledge of the composition of Pluto and its moon, Charon, astronomers expect that KBOs consist of water-ice and rock, with some organic and other complex compounds as well. KBOs have a wide range of surface colors, varying from almost gray to very red. Their surfaces are usually quite dark, only reflecting from 3 percent to perhaps 15 percent of the light that falls on them. As a result, if one were to stand on a KBO, the surface would appear blacker than dirt. Based on the distance of KBOs from the Sun and their surface reflectivity, astronomers estimate that KBO surface temperatures are typically about -220C (-360F), not much warmer than absolute zero.
Collisions in the Kuiper Belt create craters on the surfaces of KBOs and reduce their mass. KBOs shrink over time because their gravity is so weak that material thrown up when impacts occur never falls back to their surfaces. Astronomers studying results from collision models believe that most KBOs smaller than about 50 km (30 mi) in diameter cannot have survived the collisional bombardment over the lifetime of the solar system. KBOs smaller than about 50 km in diameter, therefore, must be either remnants of larger KBOs or fragments created by collisions. It is now widely accepted by researchers that almost all Jupiter family comets, which originate in the Kuiper Belt, are bits of KBOs chipped off by collisions within the belt.
In 2001 astronomers learned that some KBOs have moons. Seven KBOs with moons were discovered in 2001 and 2002, suggesting that 1 to 2 percent of all KBOs have moons. Surprisingly, the KBO satellites are much larger, relative to their parent KBOs, than planetary moons are relative to their planets. Whereas most moons have diameters just a few percent as large as that of the planet they circle, the KBO moons so far discovered are typically half as large as their parent KBO. One KBO has a companion that is apparently as large as the KBO itself. Such a pair is known as a binary KBO.
III RESEARCH
The Kuiper Belt is an exciting area of research in astronomy. The discovery and astronomical exploration of the Kuiper Belt over the past decade has fueled a revolution in scientists' views of the solar system. Today astronomers recognize the Kuiper Belt as the third major region of the solar system (the other two regions are the inner solar system, with its small rocky planets, and the outer solar system, with its gas-giant planets). They feel that the belt helps explain Pluto's small size, eccentric orbit, and icy composition—characteristics so different from those of other planets but so similar to KBOs. They also see the belt as the site of the initial stages of planet-building in this part of the solar system long ago. This recognition, combined with the intense scientific interest in Pluto and its moon, Charon, prompted astronomers to request the first mission to explore Pluto, Charon, and the Kuiper Belt Objects.
In 2001 the National Aeronautics and Space Administration (NASA) selected the New Horizons Pluto-Kuiper Belt mission for funding. The mission is scheduled to launch in January 2006. After swinging past Jupiter for a gravity-assist boost in March 2007, New Horizons would reach Pluto in 2015 or 2016 (depending on which rocket is selected to launch it). The New Horizons spacecraft will explore the Pluto-Charon system with cameras, spectrometers, and other instruments, and then fly on to visit up to three KBOs in the following few years.
In the meantime, astronomers will continue to use telescopes on the ground and the Hubble Space Telescope in Earth orbit to discover new KBOs and to learn more about this population of ancient relics left over from the birth of the solar system. NASA's Spitzer Space Telescope, launched in 2003, is an extremely useful tool for observing KBOs. Spitzer can determine the temperatures, reflectivities, and sizes of KBOs directly by detecting their infrared emissions.
Contributed By:
Alan S. Stern
Kuiper Belt
The Kuiper Belt is a disk-shaped collection of small, icy objects in the outer solar system. The belt extends from just inside Neptune's orbit far beyond the orbit of Pluto.
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