VENUS Venus has no natural satellites, so before the days of space probes it was not easy to get an accurate determination of its mass. In recent years, however, this has changed, with a number of orbiters and landers visiting the planet, so we now know GM V fairly accurately, along with the J 2 , C etc terms. The mass of Venus is 4.871 10 24 kg (0.851 M E ) and its equatorial radius (R equ ) is 6051.3km. It rotates retrograde in 243.01 Earth days and has an orbital period of 224.7 earth days, sod of 224.7 earth days, so there are 2 solar rises and falls per year. (The Venus day is 116.75 earth days long.) Another way of looking at this is that it rotates almost 4 times as seen from Earth per inferior conjunction, and 5 times as seen from the Sun. The slow rotation rate has lead to speculation about the superrotation of the atmosphere counteracting the tidal effect to leave the residual rotation. The J 2 term is very small for Venus. It is difficult to see if this is because the
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