An experiment by Italian scientists using data from NASA’s Cassini
spacecraft, currently en route to Saturn, confirms Einstein’s theory
of general relativity with a precision that is 50 times greater than
previous measurements.
The findings appear in the Sept. 25 issue of the journal Nature. They
are part of a scientific collaboration between NASA and the Italian
Space Agency. The experiment took place in the summer of 2002, when
the spacecraft and Earth were on opposite sides of the Sun separated
by a distance of more than 1 billion kilometers (approximately 621
million miles).
Researchers observed the frequency shift of radio waves to and from
the spacecraft as the waves passed near the Sun. They precisely
measured the change in the round-trip light time of the radio signal
as it traveled close to the Sun. The round-trip light time is the
time it takes the signal transmitted from the Deep Space Network
station in Goldstone Calif., to the spacecraft on the other side of
the Sun and back traveling at the speed of light.
"The scientific significance of these results is the important
confirmation of the theory of general relativity and the agreement
with Einstein’s formulations to an unprecedented experimental
accuracy," said Sami Asmar, manager of the Radio Science Group, which
acquired the data for this experiment at NASA’s Jet Propulsion
Laboratory in Pasadena, Calif. "The technological significance of the
experiment is the ability to overcome the harsh solar environment on
radio links."
The researchers measured how much the Sun’s gravity bent an
electromagnetic beam, in this case the radio signal transmitted by the
spacecraft and received by the ground stations.
According to the theory of general relativity, a massive object like
the Sun causes space-time to curve, and a beam of radio waves (or
light) that passes by the Sun has to travel further because of the
curvature. The extra distance that the radio waves travel from
Cassini past the Sun to the Earth delays their arrival; the amount of
the delay provides a sensitive test of the predictions of Einstein’s
theory. Although deviations from general relativity are expected in
some cosmological models, none were found in this experiment.
Tests of general relativity have important cosmological implications.
The question is not whether general relativity is true or false, but
at which level of accuracy it ceases to describe gravity in a
realistic way.
Past tests of general relativity confirmed Einstein’s prediction to an
accuracy of one part per thousand. This accuracy was achieved back in
1979 using the Viking landers on Mars. The Cassini experiment
confirmed it to an accuracy of 20 parts per million. The key to this
improvement has been the adoption of novel technologies in space
telecommunications.
The experiment could not have been conducted to this level of accuracy
in the past because of noise on the radio link introduced by the solar
corona. With the Cassini experiment, this hindrance was overcome by
fitting the spacecraft communication system with multiple links at
different frequencies. This new capability on the Cassini spacecraft
and on the 34-meter (112 foot) diameter antenna at Goldstone, allowed
scientists to remove the effects of the interplanetary and solar
plasma from the radio data. In addition, the noise from Earth’s
atmosphere was strongly reduced by special equipment installed at the
Goldstone complex. These technological breakthroughs developed for
the Cassini mission have led to unprecedented accuracies in the
velocity measurements with benefits for future scientific experiments
as well as deep space navigation.
The experiments are part of a series of radio science experiments
planned for the cruise phase of the mission, including the search for
low frequency gravitational waves.
Cassini will begin orbiting Saturn on July 1, 2004, and release its
piggybacked Huygens probe about six months later for descent through
the thick atmosphere of the moon Titan.
Cassini-Huygens is a cooperative mission of NASA, the European Space
Agency and the Italian Space Agency. JPL, a division of Caltech,
manages the mission for NASA’s Office of Space Science, Washington,
D.C. Authors of the Nature paper, "A New Test of General Relativity
With the Cassini Space Mission," are Dr. Bruno Bertotti of the
University of Pavia, Italy; Dr Luciano Iess of the University of Rome
"La Sapienza", Italy; and Dr. Paolo Tortora of the University of
Bologna, Italy.
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