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 revealed 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.
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 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 disagreement 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.
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