ESA's Venus Express has made unique observations of Venus during a period of reduced solar wind pressure, discovering that the planet's ionosphere balloons out like a comet's tail on its night-side. For Earth, which has a strong magnetic field, the ionosphere is relatively stable under a range of solar wind conditions. By comparison, Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere. A similar effect is also expected to occur around Mars, the other non-magnetized planet in our inner Solar System.