An international team of NASA and university researchers has found
the first direct evidence the Earth is dragging space and time around
itself as it rotates.

The researchers believe they have measured the effect, first predicted in
1918 by using Einstein’s theory of general relativity, by precisely
observing shifts in the orbits of two Earth-orbiting laser-ranging
satellites. The researchers observed the orbits of the Laser Geodynamics
Satellite I (LAGEOS I), a NASA spacecraft, and LAGEOS II, a joint
NASA/Italian Space Agency (ASI) spacecraft.

The research, reported in the journal Nature, is the first accurate
measurement of a bizarre effect that predicts a rotating mass will drag
space around it. The Lense-Thirring Effect is also known as frame
dragging.

The team was led by Dr. Ignazio Ciufolini of the University of Lecce,
Italy, and Dr. Erricos C. 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 Baltimore County.

“General relativity predicts massive rotating objects should drag
space-time around themselves as they rotate,” Pavlis said. “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 Earth.”

The study is a follow-up to earlier work in 1998 where the authors’ team
reported the first direct detection of the effect.

The previous measurement was much less accurate than the current work, due
to inaccuracies in the gravitational model available at the time. Data
from NASA’s GRACE mission allowed for a vast improvement in the accuracy
of new models, which made this new result possible.

“We found 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. “Our measurement agrees 99 percent with what is predicted by
general relativity, which is within our margin of error of plus or minus
five percent. Even if the gravitational model errors are off by two or
three times the officially quoted values, our measurement is still
accurate to 10 percent or better.” Future measurements by Gravity Probe B,
a NASA spacecraft launched in 2004, should reduce this error margin to
less than one percent. This promises to tell researchers much more about
the physics involved.

Ciufolini’s team, using the LAGEOS satellites, previously observed the
Lense-Thirring effect. It 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, precise measurement of
this phenomenon at the five to 10 percent level. The team analyzed an
11-year period of laser ranging data from the LAGEOS satellites from 1993
to 2003, using a method devised by Ciufolini a decade ago.

The measurements required the use of an extremely accurate model of the
Earth’s gravitational field, called EIGEN-GRACE02S, which became available
only recently, based on an analysis of GRACE data. The model was developed
at the GeoForschungs Zentrum Potsdam, Germany, by a group who are
co-principal investigators of the GRACE mission along with the Center for
Space Research of the University of Texas at Austin.

LAGEOS II, launched in 1992, and its predecessor, LAGEOS I, launched in
1976, are passive satellites dedicated exclusively to laser ranging. The
process entails sending laser pulses to the satellite from ranging
stations on Earth and then recording the round-trip travel time. Given the
known value for the speed of light, this measurement enables scientists to
precisely determine the distances between laser ranging stations on Earth
and the satellite. NASA and Stanford University, Palo Alto, Calif.
developed Gravity Probe B. It will precisely check tiny changes in the
direction of spin of four gyroscopes contained in an Earth satellite
orbiting 400-miles directly over the poles. The experiment will test two
theories relating to Einstein’s Theory of General Relativity, including
the Lense-Thirring Effect. These effects, though small for Earth, have
far-
reaching implications for the nature of matter and the structure of the
universe. For graphics and other material about this research on the Web,
visit:

http://www.nasa.gov/vision/earth/lookingatearth/earth_drag.html