Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0566.PR

Text File | 1993-04-21 | 6KB | 110 lines

OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE [phone]

FOR RELEASE:  FRIDAY, AMs, November 13, 1970


          PASADENA, Calif.--Albert Einstein's theory of general
relativity has been upheld by results thus far of Jet Propulsion
Laboratory radio experiments with two Mariner spacecraft as far
as 250 million miles out in space.
          Dr. John D. Anderson, chief investigator of the JPL
experimental team, said today that time delays in the round-trip
radio signals caused by the Sun's gravity verified Einstein's
54-year-old theory.
          These delays thus far cast doubt on more recent
theories, such as that of Drs. Charles Brans and Robert H. Dicke,
holding that Einstein's formulations about the effects of gravity
on radio and light signals could be in error by 7 per cent.
          "Our experiments to date show that Einstein's
predictions are right, to within 2 to 4 per cent," said Dr.
Anderson.  "The best measurements we have thus far indicate a
maximum delay of about 204 microseconds for Mariner VI, as
compared with an expected 200 microseconds using Einstein's
theory.
          "If the Brans-Dicke theory were correct, the time delay
would be much less than that, perhaps about 186 microseconds.  On
the other hand, if the much older Newtonian theory were correct,
there would have been no time delay at all."

                               -2-

          A microsecond is one millionth of a second.  The
effective slow-up in the signal at maximum delay corresponded to
about two-tenths of a mile per second.  The maximum elapsed
roundtrip time of the radio signal was 43 minutes--transmitted
from JPL's Goldstone Tracking Station on the Mojave Desert to
either Mariner VI or VII.
          The JPL experimenters have taken several hundred radio
measurements involving the two Mariners, now in wide-ranging
orbits around the Sun after completing their successful Mars
flyby missions in the summer of 1969.
          Dr. Anderson reported the findings to a Conference on
Experimental Tests of Gravitational Theories, held at the
California Institute of Technology.  Caltech operates JPL for the
National Aeronautics and Space Administration.
          Dr. Anderson headed a radio science team which included
Dr. Duane O. Muhleman, Caltech, Dr. Pasquale B. Esposito, JPL,
and Warren L. Martin, JPL.
          Martin and Dr. Richard Goldstein, JPL, devised the
precise distance-measuring system which, together with the
210-foot Goldstone antenna, enabled these first spacecraft tests
of Einstein's theory.
          Einstein's general theory of relativity is actually a
geometrical theory of gravitation.  It predicts that the velocity
of light (186,000 miles per second) should apparently be slower
in the gravitational field of the Sun.  Hence the same should be
true of radio signals.

                               -3-

          Analysis of the Goldstone-to-Mariner signals began
shortly after the spacecraft finished their Mars tasks in August,
1969, and will continue through 1971, Anderson said.  The JPL
investigator added there is no reason to believe further analysis
will reverse the apparent vindication of Einstein.
          The Mariner signal readings were processed by JPL
computers, with particular stress placed on the observations
obtained during the spring of 1970 when the radio signals passed
within one million miles of the Sun's surface.
          The experiments represent a fourth test of general
relativity proposed as early as 1964 by Muhleman and Dr. Irwin
Shapiro of the Massachusetts Institute of Technology.  In 1916
Einstein advanced his theory correcting the earlier theory of Sir
Isaac Newton.
          Einstein himself proposed three methods of testing his
theory.  One was by measuring spectral line frequency (red)
shifts in the light from massive stars.  Another was by measuring
the bending of star light rays by the Sun's gravitational field.
          The third test, measuring the slight change in
Mercury's orbit around the Sun, was considered valid until 1966.
Then Dicke and co-investigators at Princeton University
determined that the Sun is slightly oblate, not round.
          Dicke argued that this could be sufficient to throw
Einstein's prediction of Mercury's orbital shift off by 10 per
cent.  Brans and Dicke also theorized that, if such error existed

                               -4-

in the orbital prediction of Einstein's theory, his light or
radio time delay prediction would also be in error from 7 to 10
per cent.
          The Mariner data indicate that this is ___ the case,
Anderson said flatly.
          The year-and-a-half series of experiments has been the
farthest-out scientific test yet conducted by JPL.  From the
210-foot antenna at Goldstone, the radio signals are shot in a
narrow beam with up to 200,000 watts of power.  As they strike
the spacecraft antenna far out in space, the signals trigger a
transponder which amplifies and returns the signal to the desert
tracking station.
          The ranging performance of NASA's Deep Space Network
has been improved so elapsed time can be measured to within a
millionth of a second, or less.  Radio astronomers and trackers
thus can establish the actual distance to each spacecraft to
within 100 feet over a distance of a quarter of a billion miles.

BB-11/11/70 #566