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When Mariner 2 flew by Venus in 1962 at a distance of 6.6 planetary radii (Rv), it did not detect any evidence of an Earth-size magnetosphere. Mariner 5, passing within 1.4 Rv in 1967, detected the signatures in the solar wind of deflection around an 'obstacle' at Venus. The small inferred size of that obstacle placed an upper limit on the magnetic dipole moment of Venus of ~ 10-3 that of Earth. Later Venera 4 made magnetic measurements down to 200 km altitude, still detecting no planetary field but providing data that reduced this estimate by about an order of magnitude. In a 1974 flyby, Mariner 10 merely confirmed the existence of a small, nearly planet- size obstacle. Venera 9 and 10 were put into orbit around Venus in 1975, but did not approach Venus closer than ~ 1500 km. Nevertheless, the data that these spacecraft obtained in the wake of the planet provided the first evidence that an Earth-like magnetotail was absent, and that instead a structure related to the interplanetary magnetic field occupied that region of space. The most definitive measurements of the magnetic moment of Venus were obtained during the Pioneer Venus Orbiter mission in its first years of operation (1979-1981). Repeated low-altitude (~ 150 km) passes by that spacecraft over the antisolar region, coupled with dayside observations to the same altitude, proved the insignificance of a field of internal origin in near-Venus space. The observed fields for the most part could be explained as solar wind interaction-induced features, to be described below. The new upper limit on the dipole moment obtained from the Pioneer Venus Orbiter wake measurements placed the Venus intrinsic magnetic field at ~ 10-5 times that of Earth.

Of course, the weakness of the present measurement does not imply that Venus has always been bereft of an intrinsic field. Theories of the dynamos operating in the liquid cores of the newly accreted terrestrial planets suggest that there was a magnetic moment of Venus of the same order as Earth's for about the first billion years of Venus' life. During that time, thermal convection from the heat left over from accretion drove the dynamo. However, after that energy source diminished, there was apparently no source to replace it. While solid core formation in Earth's interior maintains its dynamo to this day by virtue of the related 'stirring' of the molten core around it, Venus appears to either lack the necessary internal ingredients (chemical or physical) for solid core formation, or to have ceased such processes at an earlier time if they resulted in complete core solidification or arrested core solidification. It is important to note that, contrary to popular belief, dynamo theory does not credit the smallness of the magnetic moment to the slow rotation of Venus (a Venus day of ~ 243 Earth days is almost equal to the length of its year of ~ 224 days, and its sense of rotation is retrograde). It is also notable that Venus would not have maintained any remanent crustal magnetic fields from its proposed early period of dynamo activity because the temperatures in the crust are expected to be above the Curie point (below which such fields could persist in rocky materials).

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