Barely perceptible fluctuations in the speed of a distant
NASA spacecraft coasting away from Earth could provide
science’s first direct detection of gravitational waves, a
basic feature of how the universe behaves.

A 40-day search beginning Nov. 26 will use the Cassini
spacecraft and specially upgraded ground facilities of NASA’s
Deep Space Network. “We’ve tried this before with other
spacecraft, but this time we have new instrumentation on the
spacecraft and on the ground that gives us 10 times the
sensitivity,” said astronomer Dr. John Armstrong of NASA’s Jet
Propulsion Laboratory, Pasadena, Calif. “We’re able to measure
the relative velocity between Earth and Cassini with exquisite

Cassini’s speed relative to Earth will vary during the 40
days, but will typically be about what it would take to zip
from New York to Chicago in five minutes. In contrast, this
experiment could detect any change in speed so small it would
lengthen or shorten that trip by a mere fraction of a second.

Gravitational waves are ripples in the fabric of space
and time that are set off by accelerations of massive bodies,
such as black holes or supernovas. Albert Einstein theorized
they exist, and indirect evidence confirmed his prediction in
the 1970s.

“Gravitational waves are at the frontier of astrophysics.
There’s no question they exist, but they have not yet been
detected directly,” said Armstrong, leader of an international
team that has been preparing for years to conduct this search.

“Gravity waves can give us another window into the
universe, the way Galileo’s telescope did in the 17th century
and radio telescopes did in the 1940s,” said JPL’s Randy
Herrera, lead operations engineer. The ability to detect
gravitational waves could lead to their use as a way to study
black holes and other massive phenomena, he added.

Cassini is in a quiet cruise phase of its mission, 11
months past Jupiter but still more than 30 months from its
destination at Saturn. The researchers will use radio
transmissions between Cassini and Earth to search for
gravitational waves measurably warping space between the two.
The transmissions reveal velocity changes by the Doppler
effect, the same phenomenon that raises the pitch of an
approaching train’s whistle or lengthens the light waves from
a receding galaxy. If gravitational waves within a particular
range of long wavelengths are passing through our solar
system, they will alternately stretch and compact space in a
way that would rhythmically affect the Earth-to-Cassini

Italian scientists Dr. Bruno Bertotti of the University
of Pavia and Dr. Luciano Iess of the University of Rome are
co-leaders of the experiment. Italy’s national space agency,
Agenzia Spaziale Italiana, provided crucial equipment aboard
Cassini enabling the gravitational-wave experiment to use
higher-frequency radio transmissions than have been used in
earlier gravitational-wave searches with Galileo, Mars
Observer, Ulysses and Mars Global Surveyor spacecraft. The
higher frequency suppresses noise from the solar wind,
allowing more precise measurements of velocity changes.

JPL engineers have carefully instrumented a large dish
antenna at the Deep Space Network’s Goldstone complex near
Barstow, Calif., to send and receive the higher frequencies
with unprecedented Doppler sensitivity. The upgrade includes
refined pointing capability needed to exploit the higher
frequencies, said Sami Asmar, supervisor of JPL’s Radio
Science Group. Other new equipment at Goldstone will allow
researchers to correct for the atmosphere’s distortion of
radio transmissions and improve performance of the search.

The experiment will use links at lower radio frequencies
between Cassini and Deep Space Network antennas near Madrid,
Spain, and Canberra, Australia. This will enable around-the-
clock observations. Taking data with independent equipment at
three sites will help discriminate subtle instrumental effects
from signals that might be gravitational waves.

The scientific importance of detecting gravitational
waves has also prompted ground-based projects, most notably
the highly sensitive Laser Interferometer Gravitational Wave
Observatory, coordinated by the California Institute of
Technology, Pasadena, and Massachusetts Institute of
Technology, Cambridge. The two approaches complement each
other because the Cassini experiment is sensitive to million-
fold longer wavelengths of gravitational waves than the
ground-based laser interferometers are, Armstrong said.

The Cassini experiment is timed so that Earth is on a
line between the Sun and the spacecraft, minimizing noises on
the radio link. Measurements taken during the 40 days will
take several months to analyze. The experiment will be
repeated twice more in the next two years when the
spacecraft’s position will make the measurements sensitive to
gravitational waves from different directions in the sky.

Information about the Cassini-Huygens mission is
available online at .
Cassini, launched in 1997, will begin orbiting Saturn on July
1, 2004, and drop its piggybacked Huygens probe onto the haze-
wrapped moon Titan about six months later. The mission is a
collaboration of NASA, the European Space Agency and the
Agenzia Spaziale Italiana. JPL, a division of Caltech,
manages the Cassini program for NASA’s Office of Space
Science, Washington, D.C.

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NOTE TO BROADCASTERS: A video file with a new animation
portraying gravitational waves will air on Nov. 20 and 21 at
noon, 3 p.m., 6 p.m., 9 p.m. EST. NASA Television is
available at GE-2, Transponder 9C at 85 degrees West
longitude, with vertical polarization. Frequency is on 3880.0
megahertz with audio on 6.8 megahertz.