How many asteroids are locked up in the Kuiper Belt?

How many asteroids are
locked up in the Kuiper Belt?
In 1930, American astronomer Frederick C. Leon ard proposed the existence of a band of small bodies in the solar system that extends from
Neptune's orbit to a diameter of about 50 astronomical units (AU). Leonard based this hypothesis on models of solar-system formation and on orbital dynamics of the outer planets. Nearly a decade later, Dutch-American astronomer Gerard Kuiper (1905-1973) published research on the proposed belt and suggested it was the source of many short-period comets. During the
DISTANT SUN. An artist's impression of noontime on Sedna, the most distant minor planet from the Sun, shows distant, feeble rays from our star. Sedna's body temperature is -400° F on this airless, frozen world.
NASA/ESA/ADOLF SCHALLER
/
1960s, 1970s, and 1980s, researchers searched for Leonard's outer solar system belt but found nothing.
All that changed in 1992. With the discovery of asteroid 15760, designated 1992 QBj, astronomers finally found a trans-neptunian object (TNO), the first real member of the hypothetical belt. Astronomers believed many
WANDERING ROCK. Sedna tracks its way across the sky in an image captured by the Hubble Space Telescope March 16,
2004. NASA/ESA/MIKE BROWN
other such objects lay in the region, and they named it the Edgeworth-Kuiper Belt, or alternatively, the Kuiper Belt.
The belt exists between about 30 AU, the outer edge of Neptune's orbit, and 50 AU, where Neptune's orbital resonance causes the number of objects to drop off rapidly. Farther out from the Kuiper
Kuiper Belt
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Oort Cloud
ROCKS AND ICEBALLS. The Kuiper Belt is a diskshaped collection of asteroids (inset diagram! that floats within the far larger, spherical Oort Cloud of
comets. NASA/ANN FEILD
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Belt are the so-called Scattered Disk and the Oort Cloud.
Over the past 14 years, the number of Kuiper Belt objects (KBOs) found has grown to more than 800. Among the noteworthy KBOs are Pluto and its largest moon,
Charon, which led to the debate over whether Pluto should be classified as a planet or an asteroid (see "Should Pluto be considered a planet?" p. 64.
Objects in the Kuiper Belt that have a 3:2 orbital resonance with Neptune (that is, whose orbital period measures 50 percent longer than Neptune's), like Pluto, are called plutinos. The first plutino discovered (after Pluto itself) was 1993 RO.The largest known plutino, 90482 Orcus, was found in 2004. Based on Orcus'absolute magnitude and assuming a standard reflectivity, this body likely measures 1,000 miles (1,600 km) across, as opposed to Pluto's diameter of 1,400 miles (2,250 km).
Astronomers call KBOs without the 3:2 neptunian orbital resonance cubewanos
after 1992 QB, (Q-B-1 - ohs).
Among the most notable cubewanos is 20000 Varuna, discovered in 2000.
Another, larger discovery occurred in 2002 — 50000 Quaoar. At the time of its discovery, Quaoar, at 780 miles (1,260 km) across, was the largest object found in the solar system since Pluto, in 1930. Other significant KBOs include 2003 EL., which has two moons and high reflectivity due to surface ice.
Major objects found in the Scattered Disk include 1996 TL66 (the first object cataloged as a Scattered Disk object) and Eris, formerly nicknamed Xena.This object is the largest known TNO, measuring some 1,500 miles (2,400 km) across. Another important asteroid discovery on the fringe of the solar system, that of 90377 Sedna in 2003, uncovered what is perhaps an inner Oort Cloud object that spans anywhere from 800 to 1,100 miles (1,300 to 1,800 km) across.
Although KBO numbers continue to grow, astronomers don't know the grand
total of icy bodies that inhabit the Kuiper Belt. American astronomer David Jewett, codiscoverer of the first TNO, believes at least 70,000 TNOs exist with diameters largerthan 60 miles (100 km) and with orbits as close as 50 AU from the Sun.
This vast reservoir of iceballs will keep astronomers busy with discoveries and cata loging for years to come. It also will allow planetary scientists to glean a clearer picture of how the solar system formed and how its outer reaches work today. EE
r
TWO FOR ONE. The distant Kuiper Belt asteroid 1998 WW31 appears as a pair of icy bodies on the fringe of the solar system in this artwork. This binary asteroid orbits the Sun every 301 years.
At
least 70,000 TNOs exist with diameters larger than 60 miles (100 km).
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