Space Agency (ASI) spacecraft. The research, which is reported in
the current edition of the journal Science, is the first direct
measurement of a bizarre effect called "frame dragging."
The team was led by Dr. Ignazio Ciufolini of the National
Research Council of Italy and the Aerospace Department of the
University of Rome, and Dr. Erricos Pavlis of the Joint Center for
Earth System Technology, a research collaboration between NASA's
Goddard Space Flight Center, Greenbelt, MD, and the University of
Maryland at Baltimore County.
"General relativity predicts that massive rotating objects
should drag space-time around themselves as they rotate," said
Pavlis. "Frame dragging is like what happens if a bowling ball
spins in a thick fluid such as molasses. As the ball spins, it
pulls the molasses around itself. Anything stuck in the molasses
will also move around the ball. Similarly, as the Earth rotates,
it pulls space-time in its vicinity around itself. This will
shift the orbits of satellites near the Earth.
"We found that the plane of the orbits of LAGEOS I and II
were shifted about six feet (two meters) per year in the direction
of the Earth's rotation," Pavlis said. "This is about 10 percent
greater than what is predicted by general relativity, which is
within our margin of error of plus or minus 20 percent. Later
measurements by Gravity Probe B, a NASA spacecraft scheduled to be
launched in 2000, should reduce this error margin to less than one
percent. This promises to tell us much more about the physics
involved."
Einstein's theory of general relativity has been highly
successful at explaining how matter and light behave in strong
gravitational fields, and has been successfully tested using a
wide variety of astrophysical observations. The frame-dragging
effect was first derived using general relativity by Austrian
physicists Joseph Lense and Hans Thirring in 1918. Known as the
Lense-Thirring effect, it was previously observed by the team of
Ciufolini using the LAGEOS satellites and has recently been
observed around distant celestial objects with intense
gravitational fields, such as black holes and neutron stars. The
new research around Earth is the first direct detection and
measurement of this phenomenon.
The team analyzed a four-year period of data from the LAGEOS
satellites from 1993 to 1996, using a method devised by Ciufolini
three years ago. The other team members are Dr. Federico Chieppa
of Scuola d'Ingegneria Aerospaziale of the University of Rome, and
Drs. Eduardo Fernandes and Juan Perez-Mercader of Laboratorio de
Astrofisica Espacial y Fisica Fundamental (LAEFF) in Madrid.
The measurements required the use of an extremely accurate
model of the Earth's gravitational field, called the Earth Gravity
Model 96, which became available only recently due to the
collaborative work of the Laboratory for Terrestrial Physics at
Goddard, the National Imagery and Mapping Agency (formerly the
Defense Mapping Agency), Fairfax, VA, and the Ohio State
University, Columbus, OH. It was developed over a four-year period
using tracking data from approximately 40 spacecraft.
Dr. John Ries, an expert in satellite geodesy at the
University of Texas at Austin, cautions that it is very
challenging to remove the much larger effects of tidal changes and
small zonal influences in the Earth's gravitational field, so that
estimating the possible errors in the measurement of the Lense-
Thirring effect is itself uncertain.
"The relativistic effect being sought is about ten million
times smaller than classical Newtonian disturbances on the plane
of the LAGEOS orbits, requiring an enormously accurate treatment
of background effects," said Dr. Alan Bunner, science program
director for the Structure and Evolution of the Universe in the
Office of Space Science at NASA headquarters, Washington, DC.
LAGEOS II, launched in 1992, and its predecessor, LAGEOS I,
launched in 1976, are passive satellites dedicated exclusively to
laser ranging, which involves sending laser pulses to the
satellite from ranging stations on Earth and then recording the
round-trip travel time. Given the well-known value for the speed
of light, this measurement enables scientists to determine
precisely the distances between laser ranging stations on Earth
and the satellite.
LAGEOS is designed primarily to provide a reference point for
experiments that monitor the motion of the Earth's crust, measure
and understand the "wobble" in the Earth's axis of rotation, and
collect information on the Earth's size, shape, and gravitational
field. Such research is part of NASA's Earth Science enterprise,
a coordinated research program that studies the Earth's land,
oceans, ice, atmosphere and life as a total system.
-end-
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Marcelo Puhl
mark@plug-in.com.br
marcelo@ee.pucrs.br