What objects have been explored outside Pluto's orbit?
We do not know of any planet-sized, solar system bodies beyond the orbit of Pluto. There are, however, lots of objects in the rest of the universe that have been examined, but of course none from close up. It all depends on how you define 'explored'.
Astronomers have explored, via remote observation with telescopes and observatories, all sorts of objects in the universe and from this we feel we know most of what defines these objects in terms of their physical parameters, mass, temperature, luminosity, size, composition, and other such properties. In terms of sending something out there to examine them first hand, the farthest body we have explored in this way is the planet Neptune via the Viking 2 spacecraft. Eventually, NASA may send a similar probe to the Pluto/Charon system, but that will be in the next century. Got to leave something for our children to do!!
There are a number of solar system bodies which have been discovered out beyond the orbit of Neptune. These trans-Neptunian bodies are considered to be the inner edge of the so-called Kuiper Belt which consisting of millions of objects 1-100 kilometers in diameter orbiting from 30 to several hundred AU from the Sun.
Here is what you can find about them at theLunar and Planetary Laboratory pages.
Recently, trans-Neptunian bodies have been discovered that are widely believed to constitute the long-sought Kuiper Belt. This is a primordial disk of planetesimals beyond Neptune which have survived since the formation of the planetary system. The Belt is of scientific interest on many levels. It is the suspected source of the short-period comets. Trans-Neptunian objects may contain some the least processed Solar System material, and thus ultimately provide a window on processes operative in the epoch of planet formation. Mutual collisions in the Kuiper Belts of other stars are suspected sources of circumstellar dust, perhaps providing a link with such systems as the unexpectedly dusty main-sequence star Pictoris. Collisions in our own Kuiper Belt may also be a source of observable dust: COBE data are being independently analyzed in search of the anticipated low temperature diffuse thermal emission.
The discovery of the trans-Neptunian objects of the Kuiper Belt, coming on the heels of the Voyager explorations of the giant planets and their satellites, has sparked continued scientific vigor in the study of the deep outer Solar System, and provided grist for the mills of those interested in the origin of such disparate entities as Pluto, Triton, the Centaurs (the type member of which is the asteroid 2060 Chiron), comets and the planetary system itself. In addition to extensive observational efforts on moderate to large aperture telescopes, several theoretical studies have been sparked by the discovery of the Belt. The remarkable orbital similarity between Pluto and some of the newly detected objects has provoked new work on the origin of that planet. Dynamicists are beginning to address the mechanism of capture into the 3:2 mean-motion resonance with Neptune. One result of the new work is the necessity to understand the relationship of Pluto to the smaller trans-Neptunian objects. Further the Kuiper Belt is a dynamically plausible source for short period comets, opening the possibility of a link between large outer Solar System solid bodies (such as Pluto and Triton) and short-period comets as the planetesimals from which they formed. The key features of the Kuiper Belt are:
1.At the time of writing(1996), 28 trans-Neptunian bodies exceeding 100 kilometers diameter (in addition to Pluto and Charon) have been directly observed from ground-based telescopes, all with low inclination orbits and small to moderate orbital eccentricities (Jewitt and Luu, 1993, 1995). Additionally, a population of smaller, perhaps comet-sized bodies moving consistent with Kuiper Belt orbits have been reported using Hubble Space Telescope data.
2.The orbits of many trans-Neptunians cluster near the 3:2 mean- motion resonance with Neptune at a = 39 AU. It is likely that these objects are stabilized against Neptune perturbations by the resonance, much like Pluto. Other objects (e.g. 1995 DA2 and 1995 DB2) may be in the 3:4 resonance, although further astrometry will be needed to prove this.
3.The total number of Kuiper Belt objects larger than 100 km diameter in the 30 AU to 50 AU heliocentric distance range is about 35,000 (Jewitt and Luu, 1995). If recent Hubble Space Telescope observations are correct, the number of km sized and larger bodies may approach 1 billion.
4.The Kuiper Belt is the suspected source of the Jupiter-family short-period comets (SPCs). These small, ice-rich bodies have dynamical and physical lifetimes that are short compared to the age of the Solar System. If a steady state population is to be maintained, the comets in the inner Solar System must be resupplied from a longer-lived source elsewhere. While it has long been thought that SPCs are captured from long-period orbits by the action of the gas giant planets (especially Jupiter), this explanation has recently been shown to be invalid. In particular, the highly anisotropic distribution of orbital inclinations of the Jupiter-family SPCs argues for a flattened (disk-shaped) source, exactly as is observed among the Kuiper Belt objects (Duncan et al., 1988). Therefore, in the presently accepted view, the long-period comets are eroded from the Oort Cloud by external gravitational perturbations, while the SPCs have a separate and distinct source in the trans-Neptunian region. A mission to the Kuiper Belt therefore is a mission to the birth site of the comets.
5.The Kuiper Belt is likely a remnant of the much more extensive (and long gone) protoplanetary disk of gas and dust from which the solid objects of the Solar System formed, a conclusion which has been strengthened by very recent dynamical simulations (Duncan et al., 1995).
6.With the density of 100 km diameter Kuiper Belt objects being of order 1 per , the characteristic separation of these bodies is of order 1 AU. This means that, without any extra efforts on the part of Pluto Express, the post-Pluto encounter trajectory would pass (on average) about 1/2 AU from one or more large Kuiper Belt Objects. This is a pessimistic estimate of the distance of closest approach for two reasons. First, there will be many opportunities (roughly one for each year of flight) for close encounters along the spacecraft trajectory in the years following the Pluto flyby. It is likely that several of them will occur at distances considerably smaller than 1 AU from the spacecraft. Second, and more importantly, Pluto Express will contain propellant sufficient to permit the spacecraft to be steered towards known Kuiper Belt objects. Accordingly, one or more post-Pluto encounters with objects in the Kuiper Belt almost certainly will be possible (encounters prior to arrival at Pluto are ruled out by the tight requirements imposed on the Pluto encounter geometry).
For more details about these bodies, visitDavid Jewitt's Kuiper Belt page.
Here is theofficial list which is current as of 2003.
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