The Kuiper Belt

Pluto Mission Comets Kuiper Belt Solar System Asteroids
Sun Saturn Jupiter Galaxies Furthest Planets


Since 1992 astronomers have known that an immense population of small objects that orbit the sun beyond Neptune exists. With over 70,000 of these objects having a diameter larger than 100km in the radial zone extending outwards from the orbit of Neptune. Studies have shown that they are most probably confined within a thick band around the ecliptic, which lead the experts to believe that they occupy a rind or a belt surrounding the sun. This ring is generally referred to as the Kuiper Belt and the objects within the belt are referred to as the Kuiper Belt Objects.

bk (28K)

It is believed that the Kuiper Belt Objects are primal leftovers from the early growth phase of the solar system. The dense, inner element of the pre-planetary disk compresses into the main planets while the outer elements were not as dense and growth development gradually. It is also believed that the Kuiper Belt is the source of the short-period comets by acting as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets.

Until recently the biggest Kuiper Belt Object (KBO) was named Varuna and is approximately 40 percent the size of Pluto. Quaoar is a newly discovered Kuiper Belt object, found in June 2002 by Chad Trujillo and Mike Brown at Caltech in Pasadena. It's the largest Kuiper Belt object currently known, half the diameter of Pluto (about 1/8 the volume), and 1.6 billion kilometers (1 billion miles) further away than Pluto.

Quaoar is about 1250 km in diameter, roughly the size of Pluto's moon Charon. Nothing larger has been found in our solar system since Pluto was discovered in 1930 and Pluto's moon Charon in 1978.

bk (28K)


Prior to the discovery of Charon in 1978 by Jim Christy it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon is the largest moon in relation to its main planet in the Solar System. It is so large, relatively speaking, that some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known. As mentioned earlier, Pluto and Charon are unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. This makes the phases of Charon as seen from Pluto very interesting. Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved. Charon may have been formed by a massive impact similar to the one that formed the Earth’s moon but it is doubtful that it has any significant atmosphere.

Observing The Galaxy

Anyone who takes the time to go out after sunset or before dawn and observe the sky from a dark sky location away from city lights will be rewarded with some moving stars! These are actually Earth orbiting artificial satellites. There are over 7,000 objects in space for which orbital information is kept by military and civilian space organizations. Most of these objects are small but quite a few are large enough to be seen with the unaided eye. And some, like the Space Shuttle and the Russian Mir space station, are very large and appear very bright.

Since satellites do not give off light of their own, they can only be seen when there is sunlight reflecting off of their surface. But the daytime sky is too bright to see their reflected light. So only during a comparatively brief period after sunset and before dawn, when the sun is below the horizon for Earth based observers but is still illuminating space overhead, are they reflecting light in a dark sky. For that hour to hour-and-a-half it is easy, if you are patient, to see artificial satellites. The darker the sky, of course, the more (and fainter) you can see.

bk (28K)

The best technique is to lie on the ground or in a reclining lounge chair and look at a group of stars overhead. Keep an eye out for a star-like object that is moving through the fixed stars. It may look like an airplane but will not change course, will not have flashing or colored lights and will, as you follow it, fade away before it reaches the horizon. You might be able to see more than one in the sky at the same time.

Satellites will orbit from west to east or from nearly pole to pole. If you learn the names of some of the constellations and stars in the sky you can direct others to your sighting. Saying that the satellite is, "over by that star," is not particularly helpful but saying, "it is moving through Cygnus the Swan," is much more instructive (if your fellow viewers know where to find Cygnus).

Space Shuttle, Mir and "Iridium" Observing
On some Space Shuttle missions, there are opportunities for many observers in the continental United States and Hawaii to see the orbiter fly overhead. Observers south of 28.5 degrees north latitude lie within the band overflown by all Shuttle flights. Shuttles are launched from the Kennedy Space Center in Florida which is at a latitude of 28.5 degrees north. That means that for most missions the orbiter's path is a circle around Earth that is inclined (tilted) 28.5 degrees relative to Earth's equator. Observers more than a little bit farther north than 28.5 degrees will not be able to see the orbiter. It will never rise above their local horizon.

But for some missions the "orbital inclination" will be as high as 57 degrees. That means that everyone in the band between 57 degrees north and south of the equator has a potential Shuttle over-flight. Because of the height of the Shuttle's orbit, just about every spot in this band, and for several degrees to the north and south, will be within the field of view of the orbiter and above the horizon for ground based observers. Hopefully, at some point during its mission, the Shuttle Orbiter will pass overhead during a dawn or dusk period when it can viewed from your location. The Russian Mir space station orbits at an inclination of 51.6 degrees. It is very large, quite bright, in space all the time and impressive to view. The International Space Station will be in 51.6 degree orbit as well. Another set of excellent viewing targets are the satellites of the "Iridium" commercial satellite network. They produce "flares" of several seconds in length that occur, often, several times a day and some are very bright. There are now several WWW pages that ask for your geographic location and automatically generate predictions for you.

Correct a Misconception!
Most illustrations of the Space Shuttle and low Earth orbiting spacecraft flight paths greatly exaggerate the altitude of the orbiter. To give students a true sense of the orbiter's relationship to Earth use a pair of calipers or a paper ruler to measure approximately 200 miles on the surface of a globe. Distances between local landmarks will make the best impression on children. Then turn the calipers perpendicular to the globe so that they show altitude. You will see that Shuttle orbits just graze the globe. Also note that our atmosphere is less than 1/4 as high!

Orbit Tracking Programs
Generally it is going to be difficult to determine what satellite you are viewing. But you can usually figure it out if you have an orbit tracking program and current spacecraft orbital elements. Even more exciting, when there is a current space shuttle mission underway or if you want to see the Mir space station you can determine when they will actually be visible and plan to go out and observe at the right time. With bright objects like the shuttle and Mir you can view from even light polluted city skies.

There are orbit tracking programs for Macintosh and IBM compatible computers. They are either free or shareware (where the user can get the program for free but is obligated to send the (generally low) shareware fee to the developer if they decide to keep the program) and can be obtained from public access sites via a modem or Internet connection. They require updated orbital elements which can also be easily obtained.

Getting the Programs
The National Aeronautics and Space Administration (NASA) has public access site called NASA Spacelink with a tremendous amount of material for students, teachers and the public. Follow these directions to get the right program(s) and data. Connect to NASA Spacelink via the World Wide Web at http://

courtesy of NASA


pluto home page