Venus Spinning Slower Than Thought—Scientists Stumped

Mysterious decrease could affect future exploration missions.

In the early 1990s scientists with NASA's Magellan mission calculated that a single rotation of Venus takes 243.015 Earth days, based on the speed of surface features passing beneath the orbiting spacecraft.

But scientists now mapping Venus's surface with the European Space Agency's Venus Express orbiter were surprised to find the same features up to 12.4 miles (20 kilometers) from where they were expected to be, based on the previous measurements.

According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.

(Related: "Japan Earthquake Shortened Days, Increased Earth's Wobble.")

"When the two maps did not align, I first thought there was

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