home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Space & Astronomy
/
Space and Astronomy (October 1993).iso
/
pc
/
text
/
jplnews1
/
0224.pr
< prev
next >
Wrap
Text File
|
1993-04-21
|
4KB
|
91 lines
OFFICE OF PUBLIC EDUCATION AND INFORMATION
CALIFORNIA INSTITUTE OF TECHNOLOGY JET PROPULSION LABORATORY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA. TELEPHONE MURRAY 1-3661, EXTENSION 3111
FOR RELEASE: P.M.'s of Friday, December 28, 1962
RADIO TRACKING OF MARINER II AND ITS SCIENTIFIC IMPLICATIONS
Mariner II's fly-by of Venus on December 14 has produced
the most accurate estimate yet of the mass of our sister planet,
two scientists from the California Institute of Technology Jet
Propulsion Laboratory reported today. This information was re-
vealed at a meeting of the American Geophysical Union at Stanford
University, in a paper by John D. Anderson and George Null,
describing their preliminary analysis of the trajectory data
obtained during the 109-day flight of Mariner II from earth to
Venus. According to Anderson, who presented the paper, they find
the mass of Venus a value of 0.81485 times the mass of the earth,
with a probable error of 0.015 percent. They said that their
analysis is continuing, using additional data obtained before and
after the encounter with Venus, and that their final result will
probably alter the quoted value slightly and still further reduce
the probable error. For comparison, the mass of the earth is
known to be approximately 13,173,000,000,000,000,000,000,000
pounds (about 13 septillion pounds).
The only method known to astronomers for determining
the mass of other planets is through the observation of their
gravitational effects on other bodies in the solar system. Thus,
for planets having satellites (moons), the determination can be
made with considerable accuracy.
RADIO TRACKING -2-
In the case of Venus, which has no known satellites, no
natural object has ever been observed to pass close to it, and
hence all estimates of its mass made before 1940 were both
inaccurate and erroneous.
Two more recent determinations are in agreement with
the new Mariner value, but have much less precision. In 1943, G.
M. Clemence published a value equivalent to 0.813 times the
earth's mass, with a probable error of 0.34 percent, based upon
his study of the astronomical records of the observations of the
motions of the planet Mercury through the year 1767 to 1937.
In 1954, E. W. Rabe obtained a value equivalent to
0.8148, with a probable error of 0.05 percent, from records of
the motion of the minor planet, Eros, over two decades.
In contrast, the data required to deduce the new more
accurate mass of Venus were obtained by the Jet Propulsion
Laboratory's Goldstone Tracking Station during two 10-hour
observations of Mariner, on the day of its passage of Venus and
the previous day.
The data obtained was a so-called "two-way Doppler"
measurement, involving a round trip by a radio signal. A signal
at a frequency of approximately 960 megacycles per second was
sent from Goldstone and was received by Mariner, 3 minutes 12.5
seconds later. The spacecraft then shifted the frequency of the
signal slightly and sent it back to Goldstone, where it was
compared to the original signal.
From this comparison the spacecraft velocity relative
to the earth, approximately 40,000 miles per hour, can be
RADIO TRACKING -3-
calculated within about 0.01 miles per hour, and it was the
change in this velocity amounting to approximately 3,000 miles
per hour, produced by the gravitational field of Venus which gave
the scientists the necessary data to determine the mass of the
planet.
Anderson also said that further analysis of the data
will probably refine our knowledge of another particularly
important astronomical constant, the Astronomical Unit--the mean
distance between the sun and the earth.
At present, the measurement of this unit by a variety
of conventional astronomical techniques are slightly in disagree-
ment with those obtained by bouncing radar beams off of Venus, as
has recently been done again by the Goldstone station. The two-
way Doppler measurement is an independent measurement, and may
help to resolve the inconsistency.
224-12/62