Earth Drags Space and Time Around Itself as It Turns

GREENBELT, Maryland, October 25, 2004 (ENS) - The Earth is dragging space and time around itself as it rotates, an international team of NASA and university researchers has found. The phenomenon was first predicted in 1918 by using Albert Einstein's theory of relativity. Though small for Earth, the effect has far reaching implications for the nature of matter and the structure of the universe.

The researchers measured the effect 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 planet Earth pulls space and time around itself as it rotates. (Photo courtesy NASA Johnson Space Center)

LAGEOS II, launched in 1992, and its predecessor, LAGEOS I, launched in 1976, are passive satellites dedicated to laser ranging. They send laser pulses to the satellite from ranging stations on Earth and record the round-trip travel time.

Given the known speed of light, this measurement enables scientists to determine precisely the distances between laser ranging stations on Earth and the satellite.

The team was led by physicist Dr. Ignazio Ciufolini of the University of Lecce, Italy, and geophysicist Dr. Erricos Pavlis of the Joint Center for Earth System Technology, a research collaboration between NASA's Goddard Space Flight Center, Greenbelt, and the University of Maryland Baltimore County.

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.

"General relativity predicts massive rotating objects should drag space-time around themselves as they rotate," Pavlis says.

He describes the effect as similar to 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," he said, "as the Earth rotates, it pulls space-time in its vicinity around itself. This will shift the orbits of satellites near Earth."

The relief of the planet Earth indicates the deviation of the true gravitational field from that of a perfect spheroid with uniform mass distribution. Red is higher deviation, blue is lower deviation. Inset: The LAGEOS satellite provided the data to test the Einstein theory. The GRACE mission provided the underlying gravitational data. (Artwork by F. Ricci of the University of Rome and I. Ciufolini of the University of Lecce. Earth model courtesy of GFZ-Potsdam, Germany)

The 1998 measurement was much less accurate than the current work, due to inaccuracies in the gravitational model available at the time the authors say.

But now data from a collaboration between U.S. and German scientists known as the Gravity Recovery and Climate Experiment (GRACE) has improved the accuracy of the model, which made this new discovery possible.

GRACE is a two spacecraft, joint partnership of NASA and the German Aerospace Center.

Launched in March 2002, GRACE tracks changes in Earth's gravity field, sensing minute variations in gravitational pull from local changes in Earth's mass. To do this, GRACE measures, to 100th the width of a human hair, changes in the separation of two identical spacecraft in the same orbit 220 kilometers (137 miles) apart.

GRACE maps these variations from month to month, following changes imposed by the seasons, weather patterns and short term climate change. Understanding how Earth's mass varies over time is necessary to study changes in global sea level, polar ice mass, deep ocean currents, depletion and recharge of continental aquifers, and now the way the planet drags space and time as it rotates.

The measurements of frame dragging required the use of an extremely accurate model of the Earth's gravitational field, 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.

Physicist Dr. Ignazio Ciufolini of the University of Lecce, Italy (Photo courtesy Brazilian Ministry of Science and Technology)

Ciufolini's team, using the LAGEOS satellites, previously observed the effect. It has recently been observed around distant celestial objects with intense gravitational fields, such as black holes and neutron stars.

But the new research around Earth is the first direct, precise measurement of this phenomenon at the five to 10 percent level.

"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," Pavlis said. "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 earlier this year, should reduce this error margin to less than one percent, the researchers said.