Venus, our closest planetary neighbor, has the slowest rotational period of any world in our solar system—and according to data recently gathered by the European Space Agency's Venus Express orbiter, it's getting slower. In the 1990s, NASA's Magellan probe measured the Venusian day, the length of time needed for the planet to complete one rotation, to be 243.0185 Earth days. But new measurements by Venus Express (artist's concept above), which has been orbiting the cloud-shrouded planet since 2006, reveal the current rotational period to be about 6.5 minutes slower, researchers report this month in Icarus. Although that difference seems minor, it places some features on Venus about 20 kilometers away from where scientists were expecting—a big deal for future missions looking to set a lander or rover down at a particular site. Reasons for the rotational slowdown aren't clear. Friction caused by fierce weather systems may be slowing the planet's rotation, just as weather and tides cause Earth's day to vary. Or, gravitational interactions between Earth and Venus when the planets pass near each other in orbit may be sapping our neighbor of its angular momentum. Finally, the researchers suggest, Magellan's 4-year mission may simply have occurred at a time when the Venusian rate of rotation was temporarily faster than normal, because the new data actually match long-term measurements made by radar from Earth.
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