NASA SPACECRAFT TO HUNT FOR ELUSIVE GRAVITY RIPPLE [Archive]

KTMCDO

11-21-2001, 07:43 PM

<title>NASA SPACECRAFT TO HUNT FOR ELUSIVE GRAVITY RIPPLE KENT MCDONALD KTMCDO ktmcdo@netzero.net</title>
NASA SPACECRAFT TO HUNT FOR ELUSIVE GRAVITY RIPPLES 
 
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 
accuracy." 
 
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 
distance. 
 
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 <a target=_new href="http://www.jpl.nasa.gov/cassini">http://www.jpl.nasa.gov/cassini</a> . 
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. 
 
 
 
By nasa 
- - <a target=_new href="http://www.nasa.gov">http://www.nasa.gov</a>