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Kuiper Belt Objects

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Kuiper belt objects (KBOs) are chunks of rock, dust and ice found in the area of the solar system just beyond the orbit of Neptune, starting at about 30 astronomical units (AU) to about 50 AU. In 1992, astronomers proposed that there must be at least 70,000 of these objects with diameters larger than 60 mi (100 km). It is estimated that there are many more such bodies beyond 50 AU, but these are so small and faint that they are outside the limits of detection by present-day instruments. Observations do show that the majority of the known KBOs are found within a few degrees of the ecliptic, or the plane of the solar system, just like all the planets except Pluto. This band of objects has been named the Kuiper belt after Gerard Kuiper, the astronomer who, in the 1950s, first hypothesized the existence of such a "ring" around the solar system. Pluto and its moon Charon are composed of much more ice than the other planets and orbit the Sun in a much less circular orbit at a high inclination, or tilt (about 17°). Because of this and fact that their composition seems to resemble that of the KBOs, they have been called, by some, the largest known members of this class of objects. In the late 1990s there was even a heated debate among astronomers as to whether Pluto should be reclassified and called a KBO rather than a planet. One argument against this idea, that a minor solar system body (a KBO, an asteroid, or a comet) cannot have a moon had been disproved years before when small asteroids orbiting larger ones had been found. However, the official decision, as of the year 2003 at least—though it continued to meet with disagreement—was that the solar system's complement of planets would remain at nine with Pluto as the mysterious oddball most distant from the sun.

Years before Kuiper proposed the existence of these objects, Dutch astronomer Jan Oort had noticed that orbital calculations revealed no comets arrived within detection range of Earth from outside the solar system. He also determined that many of them originally came from distances as close as just beyond the orbit of Pluto and as far away as a light year from the sun.

Oort proposed a huge sphere of icy, rocky objects surrounded the solar system that became known as the Oort cloud. Occasionally, these chunks are pulled by the gravity of one of the planets into a new orbit, which brings it close enough to be observed as a comet from Earth as its ice is warmed and evaporates, releasing gases and dust to form the well-known cometary tails. Most of the objects from the Oort cloud never come into the solar system at all, and still others probably leave the solar system due to the gravitational pull of nearby stars. The material in the Oort cloud appears to account for most long-period comets—those with that take more than 200 years to orbit the sun. The Kuiper belt seems to be responsible for the shorter-period comets. Both groups of objects taken together make up the main body of leftover debris from the formation of the solar system. As such, especially when observed at the distances of Neptune and Pluto where they remain in their original frozen state, the study of these objects is very important to understanding the early phases of solar system formation.

Most of the observed KBOs remain far from Neptune, even at perihelion, their closest approach to the sun. These are called "classical" (CKBOs) because they follow nearly circular orbits, as do most of the planets. This is what would be expected if they formed with the rest of the solar system. Some KBOs have much larger and more elliptical, tilted orbits that have perihelion distances near 35 AU. These are called "scattered" Kuiper Belt Objects (SKBOs), the first of which was discovered in 1996. Three more were discovered in wide field scans of the solar system in 1999 and it is expected that many more will be discovered as improved technology allows astronomers to probe larger areas of the sky in ways that allow fainter objects to be seen.

Since the SKBOs reach perihelion distances that are smaller than those of CKBOs, the gravitational pull of Neptune can deflect them into new orbits. This can have the same effect as the outer planets do on Oort cloud objects. It can send them into the inner solar system where they are eventually classified as comets. Other possibilities are that their orbits can change in a way that they remain in the distant reaches near Neptune but in the elliptical, tilted orbits that define them as "scattered," or they are ejected into the Oort cloud or out of the solar system into interstellar space. The SKBOs seem to form a fat doughnut that surrounds the classical KBOs in their flatter ring-like region, extending a little closer and also to much larger distances from the sun.

Both the Hubble Space Telescope (HST) and ground-based observatories have detected these populations of comet-like material at the cold fringe of the solar system. We now know, conclusively, that our solar system does not end at Neptune and Pluto. Obviously, the larger objects are easier to find and just as there are more pebbles than boulders on a beach, it is expected that many millions, billions, or even trillions of much smaller objects exist than may ever be found in the Kuiper belt and the Oort cloud. Detecting even the larger bodies in their distant icy state, at the dim edge of the solar system, pushed Hubble Space Telescope to its limits. One astronomer compared it to trying to see something the size of a mountain, draped in black velvet, located four billion miles (6.4 billion km) away.

The recent discovery of the Kuiper belt and the even newer information about the number and distribution of the objects in it fueled an interest in the possibility of using the Pluto Express spacecraft, already scheduled to fly past Pluto in 2012, to also explore this region of the solar system. The main scientific reason for attempting KBO flybys is the opportunity to explore a whole new region of the planetary system. The mounting evidence that the Kuiper belt is a region where planet-building processes ended is also an intriguing aspect of solar system evolution to study. The opportunity to study comet nuclei that have been undisturbed by the warming influence of the sun is an additional important goal for the mission. Such study may reveal many secrets about the formation of the sun and planets. With these compelling motivations in mind, the Pluto Express project has now been renamed the Pluto-Kuiper Express and plans are in place to continue observations beyond the most distant known planet.

The Pluto-Kuiper Express twin spacecraft are well suited for possible flybys of KBOs because, since the composition of Pluto is similar and the planet is also out on the dim, cold edge of the solar system, the scientific instrument packages already installed can adequately observe them. The high-resolution imager, infra-red spectral mapper, and the ultraviolet spectrometer, should be able to provide detailed information about many aspects of these as yet mysterious objects. Maps obtained even from many tens of thousands of miles away would have a feature resolution of a few miles across. This would provide geological and color information about of surface features. A Kuiper belt extension to the already ground breaking Pluto Express mission would be scientifically valuable and unquestionably historic. Not only will they be the first spacecraft from earth to observe Pluto at close range, but the only ones to travel to this distant region of the solar system since Voyagers 1 and 2 crossed the distance of Pluto's orbit in the early 1990s. It is also the only mission planned to reach this area in the first two decades of the twenty-first century.

While there currently are no KBOs identified for the flyby phase of the extended mission, there is at least a decade during which to find more and identify suitable targets for the Pluto-Kuiper Express. It is very likely that the course of one or both spacecraft can be changed after the Pluto encounters to allow close encounters of Kuiper belt objects. Even one of the 60 mi (96 km) diameter objects already detected by ground-based telescopes may be within reach if enough fuel remains in the spacecraft for a sufficient change in course. In theory, reaching one of the much smaller comet-sized objects in the Kuiper belt will be even easier because they are so much numerous than the intermediate- and large-sized objects. However, it will be more difficult to identify and to determine the orbit of smaller bodies in advance. But since the actual selection of specific objects to visit may not need to be made until well into the mission, the probability of reaching this goal is quite high. Success in this mission will mark the greatest milestone for solar system science that has been reached since the late 1970s when the Voyagers first visited all the gas giant planets.



Sagan, Carl, and Ann Druyan. Comet. Random House, Inc., 1985.

Zeilik, Michael. Astronomy. 7th ed. Wiley and Sons, Inc., 1994.


Jewitt, David. Kuiper Belt Page. University of Hawaii. <http://www.ifa.hawaii.edu/faculty/jewitt/kb.html>.

Clint Hatchett


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Astronomical unit

—The average distance between the Sun and Earth. One astronomical unit, symbol AU, is equivalent to 92.9 million mi (149.6 million km).


—An object usually seen in the inner solar system that results when a dusty, rocky chunk of ice left over from the formation of the solar system moves close enough to the sun that its ices evaporate. The resulting release of gases and dust surrounds the original object with a cloud called the coma and a tail that can extend for 100 million miles (161 million km) across space. This creates the sometimes spectacular objects observed from Earth known as comets.


—In the sky, the ecliptic is the apparent path of the sun against the star background, due to the earth orbiting the sun. The term ecliptic plane is used to describe the average location in space of the orbits of the planets of our solar system, except Pluto which orbits the sun at a 17° angle to the others.


—The orbital "tilt" of a planet or other object in the solar system. The first eight planets with very low inclinations to the eclipitic plane. Pluto's orbit has a 17° tilt or inclination.

Infrared Spectral Mapper

—A device used to detect heat (frequencies lower than visible light) and "map" the intensity of the radiation received in order to determine chemical processes at work in the object being observed.

Light year

—A unit of measure used between stars and galaxies. A light year is the distance that light travels (at about 186,272 mi or 300,000 km per second) in one year. One light year is equal to about six trillion miles (9.6 trillion km).


—The closest approach of an object to the sun in its orbit.

Ultraviolet spectrometer

—A device that receives, and breaks into its component frequencies, electromagnetic radiation in the region "above" (of higher frequency) than visible light. The spectrometer splits up the received energy allowing analysis of chemical elements and processes that caused the radiation.

Voyager spacecraft

—A pair of unmanned robot spacecraft that left earth in 1977 to fly by all the gas giant planets (Jupiter, Saturn, Uranus, and Neptune). The original mission called for them to also fly past Pluto in what was to be called the "Grand Tour" of the solar system, but mission delays made that impossible, necessitating the Pluto Express mission. These craft were the second set to reach Jupiter and Saturn and the only, so far, to reach Uranus and Neptune.

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