Kathy

>From ScienceDaily --Source: National Aeronautics And Space Administration
Posted 3/30/98 on ScienceDaily

Earth Dragging Space And Time As It Rotates

An international team of NASA and university researchers has found the
first direct evidence of a phenomenon predicted 80 years ago using
Einstein's theory of general relativity -- that the Earth is dragging space
and time around itself as it rotates.

Researchers believe they have detected the effect by precisely measuring
shifts in the orbits of two Earth-orbiting laser-ranging satellites, the
Laser Geodynamics Satellite I (LAGEOS I), a NASA spacecraft, and LAGEOS II,
a joint NASA/Italian 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.