An international team led by scientists at the Southwest Research Institute (SwRI) has discovered some curious properties of the largest asteroid, Ceres. The astronomers observed Ceres with the Hubble Space Telescope (HST) at ultraviolet wavelengths using a resolution higher than previously attained. The resulting images are the first to resolve detail on the surface of Ceres and show features as small as 50 kilometers across.

Images of Ceres that shows the Piazzi feature as a darkened spot near the center. Photo: Southwest Research Institute

Led by Principal Investigator Dr. Alan Stern of SwRI, the team detected a dark spot on the surface of Ceres, which it nicknamed "Piazzi" in honor of the discoverer of Ceres. "Although we can't determine the nature of the spot with these data, whether it is an area of different coloration or possibly a crater from an impact by another asteroid, it is pretty big," says Dr. Joel Parker, also of SwRI, who led the team in the analysis of the images. "The Piazzi feature has a diameter of about 250 kilometers, which is more than a quarter the size of Ceres. If it resulted from an impact, the object that hit Ceres would have been about 25 kilometers across. It must have really shaken things up."

The high-resolution images allowed the team to refine measurements of Ceres. Although Ceres is the largest known asteroid -- estimated to contain more than one-third of the total mass of all other asteroids combined -- researchers still dispute its size, even after 200 years of observations. The new HST measurements indicate that the asteroid is slightly flattened, with a diameter ranging from 930 to 970 kilometers. Spinning objects can have a flattened or "squashed" shape depending on how big they are, how fast they spin, and what kind of material they are made of. However, the amount of flattening seen on Ceres is more than expected and may indicate that the inner structure is not as homogeneous as previously assumed.

"These results are very tantalizing," says Stern. "What we need to be definitive are observations with better resolution and frequent enough to follow Ceres through a nine-hour rotation period to track surface features. This 'movie' would allow us to finally map the surface of Ceres and figure out what the Piazzi feature is." The team has already proposed such an experiment with a new instrument to be installed on HST next year.

The analysis of the Ceres images will be published in the January 2002 issue of The Astronomical Journal. Authors include researchers from SwRI, the Massachusetts Institute of Technology, Cornell University, the University of Arizona, and the Observatoire Midi-Pyrenees in France.

In addition to being the largest asteroid, Ceres was also the first asteroid to be discovered. In the latter part of the 18th century, astronomers noted a regular spacing in the planets of the solar system, but with a gap between Mars and Jupiter where they expected to find a planet. On January 1, 1801, the Sicilian astronomer Giuseppe Piazzi at the Palermo Observatory discovered a moving object in the region. Researchers at the time assumed that this object, Ceres, was the missing planet. However, early observations indicated that Ceres was too small to be a planet, and as more such objects were discovered in the region, they became known as "asteroids" or "minor planets." Ceres orbits the sun once every 4.6 years at a distance of 41 million kilometers, and it spins on its axis once every nine hours.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 2,700 employees and an annual research volume of more than $315 million.