When Clyde Tombaugh discovered the planet Pluto 70 years ago, the picture of the solar system seemed clear and orderly. It showed our central star, its nine planets, the asteroid belt, and comet visiting occasionally from the dark and chilly fringes of the solar system. Today, thanks to the pioneering asteroid survey Spacewatch and similar projects, our planetary system appears as a humming hive populated with countless asteroids circling the sun like a swarm of bees.

Jeffrey Larsen, a principal research specialist with the Spacewatch group at the University of Arizona (UA) Lunar and Planetary Laboratory in Tucson, and his undergraduate students recently used the 80-year-old, 36-inch Spacewatch telescope on Kitt Peak in discovering dim Centaur asteroids and Trans-Neptunian Objects (TNOs).

Larsen is talking about it today at the 24th General Assembly of the International Astronomical Union in Manchester, England.

Centaurs orbit between Jupiter and Neptune, typically 5 to 30 astronomical units (AU) away. (An AU is Earth-to-sun distance.) "Their orbits are very elliptical and cross the orbits of these planets, so if anybody lived on Jupiter these objects would be as close as Near Earth Asteroids (NEAs) are to Earth," Larsen explains. "Centaurs also seem to be related to comets. From the point of view of their orbits, you cannot tell the difference between the two. Comets get comas and tails as they get closer to the sun, Centaurs never do."

Trans-Neptunian Objects are even more remote. They circle the sun at a distance of at least 38 AU in circular orbits comparable with Pluto's orbit.

"There are at least as many of them in Pluto¹s vicinity as have been found in the main asteroid belt, a region between the orbits of Mars and Jupiter. TNOs are very difficult to spot because they are very distant," says Larsen.

Larsen and his students looked 20 degrees above and below the plane of the ecliptic, or the plane through which the planets circle the sun, using the 36-inch Spacewatch telescope, the oldest telescope on Kitt Peak. They also analyzed data collected previously by Spacewatch to locate the extremely dim objects from between 24 and 27 magnitude. (By comparison, Jupiter seen from Earth is bright, or at minus 2 magnitude.)

During Spacewatch observations May through August 1999, Nichole Danzl, Arianna Gleason, and graduate student Anne Descour discovered five TNOs, three Centaurs and two 'scattered disc objects'. These asteroids wander between 40 and 150 AU, or farther away than Pluto.

To complicate the matter even more, the team also discovered two asteroids that met both criteria - they crossed planetary orbits like Centaurs and wandered far away from the solar system like scattered disc objects. Based on these two discoveries, the Centaurs and scattered disc objects are now considered a single class of asteroids.

Spacewatch astronomers search for asteroids by taking repeated scans of certain patches of the sky. Such scans are six-tenths of a degree wide (approximately as wide as the full moon) and about 12 full moons long. It takes a CCD detector about 30 minutes to acquire a single image. The astronomers then compare images taken at different times. If there is an asteroid in any of the images, it will appear to 'move' across the picture. Asteroids travel at different rates, depending on where they are in the solar system.

"Near Earth Asteroids move between three-tenths of a degree to 30 degrees a day. Main belt asteroids at opposition (when the asteroids are on the opposite side of the Earth as seen from the sun) move on average about two-tenths of a degree a day. Trans-Neptunians are so distant that, when observed from the Earth, they seem to barely move at all, " Larsen says.

UA astronomers Tom Gehrels and Robert McMillan, who currently leads Spacewatch, founded the Spacewatch Project in 1980 to survey for asteroids and comets that might pose a threat to Earth.

The 1989 discovery of Chiron - known now to be a Centaur asteroid - came as a surprise. Originally, astronomers assumed that it was just a single object drifting on the outskirts of the solar system. But in 1991 Spacewatch observers found the second Centaur, Pholus. Today a whole population of 16 Centaur asteroids is known, and Spacewatch discovered about half of them.

In 1992 astronomers David Jewitt of the University of Hawaii and Jane Luu of the University of California ­ Berkeley discovered the first Trans Neptunian Object, an object named 1992 QB1 which orbits the sun at about 40 AU. "That was further away than Pluto and proved that asteroids inhabit even the farthermost parts of the solar system," Larsen says.

These distant bodies may hold the key to the origin of our planetary system. What intrigues astronomers is their peculiar location in the solar system. They are at the same distance from the sun as protoplanetary discs surrounding very young stars, such as Beta Pictoris. By mapping Centaurs and TNOs, learning how abundant they are and how they are distributed, theorists hope to form a model of how the solar system came into existence.

"The other reason why the brighter of these objects are important is that they are the easiest to study with spectroscopy, " Larsen says. Spectroscopy reveals the chemical composition of objects, and by asteroid spectra astronomers may be able infer the composition of the primordial solar nebula.

Also, the physical rules that triggered the solar system's formation and how long that process took remain a mystery. Hidden near at the periphery of the solar system, many TNOs have not been influenced by the gravitational forces of planets, especially Jupiter. Their orbits may represent the original configuration of matter in the very early solar system.

"The size distribution of these bodies is important for making models of the solar system's formation. Whatever the original protosolar nebula was doing, the TNOs probably have some signature of it left in their orbits," Larsen says.

After the discovery of Pluto, astronomers looked without success for similar objects. Many then thought that Pluto was unique and, apart from the Oort Cloud of comets beyond Pluto, believed that the outer solar system was devoid of any sizable objects.

"Today the picture looks more messy. Our solar system seems to be populated with many objects that even a few years ago were not seen because they are extremely faint. We still do not know yet how far out they go, " Larsen says.

Spacewatch cannot presently look far enough to see the most distant objects that may exist.

"We are only able to see the inner rim of the TNO belt. Nobody has found objects in circular orbits past 50 AU. Does this mean the belt ends at that distance? The scattered disc objects exist past 50 AU, though we have not discovered many of them. I'll be willing to bet money that we just haven't looked right," Larsen speculates.

The fuss over whether to continue calling the little world of Pluto a "planet", or switch to calling it an big ice asteroid (or even a giant comet nucleus), has drawn a great deal of public attention in recent years.

But in the April 20 "Nature", Dr. Don Yeomans points out that astronomers will soon be faced with a new naming problem that dwarfs the Pluto controversy -- for the distinction between "asteroids" and "comets" has now started to blur seriously.

Up to now, "comets" have been defined as small icy objects ("comet nuclei") surrounded by a visible coma (a cloud of dust and gas) boiled off them by the heat of the Sun as they approach it from the outer Solar System -- while all small Solar System objects without a coma have been called "asteroids", on the grounds that they must be made of rock rather than ice.

But now this distinction is breaking down -- because our telescopes are powerful enough that they can routinely see small objects so far out in the Solar System that they have no comas even if they are made of ice.

Chiron -- the big "asteroid" discovered between the orbits of Saturn and Uranus in 1977 -- was the first of these; just a few years after its discovery, as it moved closer to the Sun, it suddenly developed an unmistakable coma, and thus received double designation as a comet.

Since then, the Kuiper Belt -- the huge region beyond Neptune, containing millions of icy objects ranging from a few hundred km in diameter down to only a few km -- has been discovered.

They're all too cold to possess comas, but the vast majority of them are definitely made of ice rather than rock -- in fact, they are comet nuclei.

More "Centaurs" like Chiron -- similar icy objects that have wandered inside Neptune's orbit -- have also been discovered.

And to complicate matters further, it turns out not to be safe to assume that every object in the far outer Solar System IS ice rather than rock.

Yeomans in Nature points out that recent computer simulations show that as much as three percent of Kuiper Belt objects are likely to be rocky asteroids that formed in the outer fringes of the Asteroid Belt -- but then, at some point over the eons, flew close enough to Jupiter to be catapulted by its gravity into the outer Solar System.

And the same is possible for the far more distant Oort Cloud, made of objects that formed in the realm of the giant planets and then flew close enough to one of them during the following eons to be flung into incredibly distant orbits taking them up to thousands of times farther from the Sun than Pluto - an appreciable part of the distance to the nearest star.

Indeed, the first such provable "outer asteroid" (my own term) may have been discovered in 1996; like many of the comets, its orbit is incredibly elongated, suggesting that it has probably wandered back into the region of the outer planets from the far reaches of the Oort Cloud -- but although it's now close enough to the Sun that it should definitely be showing a coma, it has none.

Since then, at least three more "outer asteroids", with the elongated orbits of regular comets but with no trace of a coma, have been discovered -- and two of them are orbiting the Sun backwards, a trait previously limited entirely to occasional comets.

So if you can't be sure whether a small object you find in the outer Solar System would develop a coma or not if it was close to the Sun, what do you call it -- an asteroid or a comet?

The naming conventions, after all, are completely different: asteroids are given whatever name their discoverer wishes (preceded by a number), while comets are named AFTER their discoverers (preceded by a "P").

This problem is holding up all attempts to name any of the Kuiper Belt objects; instead, they're all retaining their initial post-discovery labels such as "1996 PW".

To make matter worse, we're now confirming what astronomers have suspected for decades -- some of the "asteroids" in the inner Solar System are really dried-out comets, jockeyed (like some still-active comets) into orbits completely in the inner Solar System by a chance encounter with Jupiter, and exposed to the Sun's heat so many times since that all their ices have evaporated, leaving only a lump made out of the dust and rock mixed with the comet's ice.

Two recently discovered "asteroids" have turned out to have been seen years earlier while they still had comas and were labeled as comets -- that must have been their very last gasp of outgassing before they "died".

And IR spectral measurements have raised the suspicion that some of the other "near-Earth" asteroids closer to the Sun than the Asteroid Belt are also dried-out comet nuclei -- although the indications are still that most near-Earth asteroids really are authentic rocks from the Asteroid Belt, nudged by chance into the realm of the inner planets by Jupiter's long-distance gravitational tuggings.

Finally, over the past couple of years, meteorites have been found still containing significant traces of water trapped inside them -- which means that "Far from being the dry rocky bodies they were once thought to be, it would seem that some asteroids, along with with comets, might be significant sourcees of water" -- especially those in the outer fringes of the Asteroid Belt, or the "Trojan" asteroids that hover in the Lagrange points of Jupiter's orbit.

So all our old categories are evaporating.

Not only can we no longer draw a clear boundary between "planets" and "asteroids" [especially since the odds are excellent that the Kuiper Belt and Oort Cloud hold a fair number of undiscovered objects ranging in size from the biggest asteroid Ceres (914 km diameter)to as large as Pluto (2274 km) and possibly even as big as Mercury (4,878 km,] but we can no longer distinguish at all convincingly between "asteroids" and "comets".

You could even make a case for calling Pluto any of the three!

At some point, the International Astronomical Union will finally have to grasp this nettle and change the way we name newly discovered Solar System objects -- which may to some extent require changing the long-established official titles we've given to the asteroids and comets (and even planets!) we've already discovered.

But while this is a mess for the nomenclaturists, it's yet another facet of our growing knowledge of the solar system and the great variety of objects orbiting the Sun.