Venus Express, its fuel reserves almost exhausted, is started a new and risky mission phase.
Its altitude will be allowed to decay naturally so that it is closer to the planet ever before, passing only 135 km above the planet’s surface.
At this low altitude, the spacecraft is passing through the upper reaches of the atmosphere, such that the spacecraft experiences significant deceleration (“drag”), changing its orbit.
This “aerobraking” experiment is described in greater detail on ESA’s website
www.esa.int