How Long is a Day on Venus? Astronomers Make Their Best Measurement Yet

Universe Today | 10/18/2019 | Staff
reantes (Posted by) Level 3
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There’s a problem with Venus. We don’t know how fast it rotates. For a space-faring civilization like ours, that’s a problem.

Measuring the length of day, or rotation rate, of most bodies is pretty straightforward. Mark a prominent surface feature and time how long it takes to rotate 360 degrees. But Venus is blanketed in thick clouds. Those clouds give it its reflectivity, and make it bright and noticeable in the sky, but they make it hard to measure Venus’ day length.

End - Venus - LOD - Subsequent - Radar

That wasn’t the end of measuring Venus’ LOD. Subsequent radar observations came up with different values, sometimes by up to six minutes. Maybe a spacecraft would do a better job.

In 1989, NASA launched the Magellan spacecraft. Magellan arrived at Venus in August 1990, and entered a three hour, near-polar elliptical orbit. After 487 days, and almost 1800 orbits, Magellan completed its mapping mission, and also measured Venus’ LOD at 243.0185 days, with an uncertainty of nine seconds.

Scientists - Venus - Rotation - Rate - Answer

Scientists have been measuring Venus’ rotation rate ever since, and can’t get a consistent answer. There’ve been different proposed explanations for this, like atmospheric drag from Venus’ thick atmosphere, or solar tidal torque. But an exact number has been elusive.

Besides being in the uncomfortable position of not knowing how fast our close neighbour rotates, there’s a practical reason for wanting to know: landing spacecraft there.

Venus - Place - Blistering - Temperatures - Pressure

Venus is an inhospitable place. Blistering temperatures and crushing atmospheric pressure have limited the surface exploration of the planet to a handful of Soviet probes. They were the Venera family of probes, which were sent to Venus starting in 1961.

But there are plans to send more spacecraft to explore Venus. Without knowing the rotation rate, it’s very difficult for a spacecraft to stick a landing. The current uncertainty in the rotation rate means that a spacecraft could miss its target...
(Excerpt) Read more at: Universe Today
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