Mercury taken by Mariner 10 in 1974

Mercury, taken by Mariner 10 in 1974    [Source]

Mercury is the innermost planet of the Solar System. It orbits the Sun in only 88 Earth days, about a quarter of Earth‘s orbital period. But due to a resonance between Mercury’s orbit and rotation, in reference to the Sun, a Mercurian day lasts twice as long as a Mercurian year!

Mercury is a heavily cratered planet, and like the Moon it contains no notable atmosphere. For this reason, it has the highest surface temperature variation of any of the planets, ranging from 427°C (801°F) in during the day to -173°C (-279°F) at night.

Mercury-orbit precession

An exaggerated display of Mercury’s orbital precession     [Source]

It also has the highest orbital eccentricity among the 8 planets, taking it from 46,000,000 to 70,000,000 km (0.31 to 0.47 AU) away from the Sun. Its elongated orbit appears to slowly precess over time (as shown to the right). Though a small deviation (less than 10 arcminutes per century), by 1859 it became clear to French mathematician Urbain Le Verrier (1811 – 1877) that Newton’s laws of motion failed to account for 43 arcseconds of this precession! Given Verrier’s successful use of celestial mechanics to independently discover Neptune in 1846 (by studying its gravitational effects on the orbit of Uranus), it was proposed that an additional planet might exist within Mercury’s orbit to account for the precession’s discrepancy. This hypothetical planet even received the name “Vulcan”, but it was never found. Instead, it wasn’t until Einstein’s Theory of General Relativity (published in 1916) that this puzzle was finally resolved, by way of the curvature of spacetime by the Sun.

Mercury-false color

A false color image of Mercury by MESSENGER in 2013, enhanced to show physical features     [Source]

Due to the planet’s proximity to the Sun, Mercury can attain a maximum separation of only 28 degrees as seen from the vantage point of Earth. This means it can typically be observed only during morning or evening twilight. The planet exhibits phases like Venus, though Mercury’s disk appears smaller. Mercury appears brightest when its phase is near its fullest, reaching a theoretical peak apparent magnitude of -2.6 when it passes behind the Sun.

The difficulties of observing Mercury have understandably made it far less examined than most of the other planets. To date, only two spacecraft missions have involved the study of Mercury. Mariner 10 performed three flybys of the planet in 1974 and 1975 (as well as one of Venus) before running out of fuel, fixing the probe in a solar orbit. MESSENGER became the first spacecraft to orbit Mercury in 2011, and ultimately met its fateful end in April 2015 when it deorbited as planned, impacting Mercury’s surface. Tentatively, it will not be until at least 2024 until another orbiter visits Mercury: BepiColumbo of the European Space Agency (ESA).


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