March 27, 1998
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.