[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

New Masses and Densities Are Computed For The Three Largest Asteroids

Public Affairs Office
U.S. Naval Observatory
Washington, D.C.

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

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 =B1 0.1) x 10**20 kilograms
(9.5 x 10**17 tons), the mass of Pallas is (3.18 =B1 0.08) x 10**20 kilograms
(3.50 x 10**17 tons), and the mass of Vesta is
(3.0 =B1 0.2) x 10**20 kilograms (3.3 x 10**17 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. Confu=sion
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.


An html version of this press release can be found at

   http://aa.usno.navy.mil/ephemerides/asteroid/masses/PressRelease.htm 
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.


[Image: Orbits of the asteroids Ceres, Pallas, and Vesta,
http://aa.usno.navy.mil/ephemerides/asteroid/masses/Orbits.gif]

IMAGE CAPTION:

The orbits of the asteroids Ceres (cyan), Pallas (magenta), and Vesta
(grey) in the solar system. Improved masses for these three asteroids
were presented by Dr. James L. Hilton of the U.S. Naval Observatory at
the American Astronomical Society meeting in Washington, D.C. on
January 9, 1998. The orbits of the Earth (dark blue), Mars (red), and
Jupiter (green) are included for perspective.