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 comets 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 Plutoís 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 ëmoveí 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 ñ 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.

Spacewatch web page http://lpl.arizona.edu/spacewatch/