At www.space.com/19537-venus-comet-atmosphere.html … it seems Venus sometimes has the appearance of a comet rather than a planet according to a European Space Agency source. Part of the upper atmosphere of Venus, namely the ionosphere, behaves oddly on different occasions, as it is buffeted by the solar wind (issuing from the Sun). One side of Venus ionosphere, facing away from the Sun, can billow outward like the tail of a comet while the other side, facing the Sun, remains tight. The discovery was made by ESAs Venus Express spacecraft in 2010.
The same story is at www.esa.int/Our_Activities/Space_Science/When_a_planet_behaves_like_a_co… … which goes on to explain how Earth's magnetic field stops this happening with our atmosphere. Venus is different as far as a magnetic field is concerned and the solar wind can shape the ionosphere. The story is old – going back to January 29th and already mentioned on In the News (at the time) but having come up on Eric Aitchison's email thread (discussion of ancient history and chronology) here it is again.