KEY POINTS:
Despite being small and inhospitable worlds, the inferior planets, Mercury and Venus, have contributed a lot to science.
Their rare transits across the face of the Sun provided, until 1900, one of the few practical methods of measuring the distance from the Earth to the Sun - one of the most fundamental numbers in astronomy. Called the Astronomical Unit, it is the yardstick to determine the distances to the nearest stars.
Detailed study of Mercury's motion revealed another secret. The orientation of the orbit of Mercury slowly changes relative to the distant stars. Newton's theory of gravity predicted only half the measured motion, leading astronomers to speculate that there may be an undiscovered planet (dubbed Vulcan) causing the perturbation.
Decades were spent searching for Vulcan, but the anomaly of Mercury's motion was not explained until Einstein published his theory of gravity in 1916. His General Theory of Relativity, treating gravity as curvature of space-time, correctly accounted for the observed rotation of Mercury's orbit; marking the first success for the theory that describes the Universe's structure.
Now New Zealander Professor David Stevenson, working with Alex Alemi at the California Institute of Technology, has offered an interesting explanation for the slow, backward axial rotation rate of Venus - and its lack of a moon. Using computer simulations, they have discovered a plausible scenario for Venus' early history that echoes the formation of our own Moon.
They argue that the young Venus underwent a titanic collision with a substantial object forming a Venusian moon that then moved slowly away from the planet - similar to the Earth's moon.
About 10 million years later, following a second oblique collision with another massive body, the rotation of Venus was slowed and reversed. The change in the planet's rotation caused its moon to move back towards the planet until it got so close that it was ripped apart and destroyed.
Just after sunset, Jupiter is seen low in the west with Mercury, which will transit the Sun on November 9. Saturn rises in Leo and is low in the northeastern sky before dawn.
* Grant Christie is an astronomy researcher who writes a monthly column for the Herald.
