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).