FOR RELEASE: 9:20 AM EST JANUARY 9, 1998 NEW MASSES AND DENSITIES ARE COMPUTED FOR THE THREE LARGEST ASTEROIDS Dr. James L. Hilton, an astronomer at the U.S. Naval Observatory, announced today at the American Astronomical Society meeting that he has computed more accurate masses and densities for the three largest asteroids. More accurate masses of the asteroids are important for improving our knowledge of the motions of the planets, the compositions of the asteroids, and the histories of the asteroids, including those that have the potential of colliding with the Earth. The asteroids are small rocky bodies ranging from a few hundred feet to a few hundred miles in diameter. Most asteroids are located in the main asteroid belt between the orbits of Mars and Jupiter. Ceres, Pallas, and Vesta are the three largest asteroids. Even though they are much smaller than the planets (Ceres, the largest asteroid, has only 1% the mass of the Moon), they are still capable of causing changes in the orbits of Mars and the Earth. These changes are seen in data returned by interplanetary satellites such as Viking, Mars Pathfinder, and Mars Global Surveyor. The best available planetary positions are calculated using perturbations from 300 asteroids; however, the masses of only a hand full of asteroids are known with any precision. The masses of the other asteroids are estimated by comparing them with asteroids which do have known masses. Computing the masses of the three largest asteroids, Ceres, Pallas, and Vesta, was part of a larger project computing the motions of fifteen of the largest asteroids. These computations will be used in producing future editions of The Astronomical Almanac, a yearly publication of the U.S. Naval Observatory and Her Majesty's Nautical Almanac Office, used by astronomers worldwide. The masses of Ceres, Pallas, and Vesta were determined by calculating their positions to high accuracy and then comparing those positions with tens of thousands of positions actually observed by astronomical observatories worldwide. The gravitational forces of the asteroids on each other affects the observed positions of the asteroids in their orbits. The observed positions of the asteroids are compared with positions calculated using a mathematical model of the solar system. The masses are determined by finding the values that make the differences between the observed positions and calculated positions as small as possible. Observations covered the period from 1801, shortly after Ceres was discovered, through 1996. Using observations over such a long period of time allows the most accurate calculation of both the positions of the asteroids and their masses. The masses of all three asteroids were calculated simultaneously using a special computer program known as the Planetary Ephemeris Program, or PEP for short. The mass for Ceres was found to be (8.7 +/- 0.1) x 1020 kilograms ( 9.5 x 1017 tons), the mass of Pallas is (3.18 +/- 0.08) x 1020 kilograms (3.50 x 1017 tons), and the mass of Vesta is (3.0 +/- 0.2) x 1020 kilograms (3.3 x 1017 tons). For comparison, this means that Ceres has 1.18% of the mass of the Moon while Pallas has 0.43% the Moon's mass and Vesta is 0.41% of the mass of the Moon. The mass of Pallas is greater than previously thought, while the mass of Ceres is somewhat smaller. The change in the mass calculated for Ceres is a direct result of the change in the mass calculated for Pallas. Confusion is caused by the fact that the two asteroids take nearly the same time to go around the Sun and have been in the same part of the sky ever since they were discovered. Since these two asteroids always appear close to one another, separating the amount of gravitational pull caused by each asteroid is difficult unless a simultaneous solution for their masses is used as was done at the Naval Observatory. Dr. Hilton said, "These results reduce the uncertainty in the mass of Pallas by a factor of five, and show that to get a good mass for Ceres you need to know the mass of Pallas. However, calculating the mass of Pallas is tricky and it would be nice to have some one else check this value." The mass calculated for Vesta is in very good agreement with previous calculations. In addition to the masses, densities can be calculated for these three asteroids because several groups of astronomers have determined their radii within a few kilometers. The density of Ceres is 1.98 +/- 0.03 grams per cubic centimeter, the density of Pallas is 4.2 +/- 0.2 grams per cubic centimeter, and the density of Vesta is 3.9 +/- 0.3 grams per cubic centimeter. For comparison, water has a density of 1 gram per cubic centimeter and the average rock has a density of 3 grams per cubic centimeter. Although Pallas has long been considered to be similar to Ceres, the new densities indicate that Pallas may, in fact, be more like Vesta. For more information contact Dr. James L. Hilton, U.S. Naval Observatory, 3450 Massachusetts Ave. NW, Washington, DC 20392. Phone: 202-762-1432. FAX 202-762-1612. E-mail: jhilton@usno.navy.mil. An html version of this press release can be found at http://aa.usno.navy.mil/ephemerides/asteroid/masses/PressRelease.html. There is also a scientific paper describing in detail the calculation of orbits, masses, and densities of the asteroids at http://aa.usno.navy.mil/ephemerides/asteroid/astr_alm/asteroid_ephemerides.html.