The European Space Agency, ESA, says Venus appears to be rotating on its axis slightly slower than it did in the early 1990s, adding 6.5 minutes to the length of the planet’s day.
ESA says scientists made the discovery when trying to reconcile a new map made with recent measurements taken by its orbiting Venus Express spacecraft with features observed some 16 years ago by NASA’s Magellan orbiter.
ESA researchers noticed the Venus Express data indicated that some objects on Venus were up to 20 kilometers from where they expected to find them, if the planet's rate of rotation were the same as when Magellan made its measurements. The scientists say the Venus Express map and the Magellan map aligned when they added 6.5 minutes to the length of Venus’ day.
ESA says the phenomenon requires further study.
The second closest planet to the Sun, Venus is situated between the orbits of tiny Mercury and the Earth.
Venus Express scientists say the speed of Venus’ rotation could be influenced by several factors, including weather cycles that span decades or even potential rotational effects that occur when Venus and Earth are relatively close to each other.
NASA’s Magellan probe orbited Venus from 1990 to 1994. ESA’s Venus Express spacecraft has been orbiting the planet since 2006.