Mercury in 1974; it is still the only source of flyby data for the planet.
By EarthΓÇÖs standards, conditions on Mercury are strange and harsh. The planet rotates on its axis once every 59 Earth days, but because it is also racing around the Sun at a rate of one revolution, or ΓÇ£year,ΓÇ¥ every 88 days, the Sun stays in the sky for 88 days. This means that a noon to noon ΓÇ£dayΓÇ¥ on Mercury is 176 Earth days, or two Mercury years. The planetΓÇÖs proximity to the Sun (46 million kilometers at the closest point in its orbit) creates daytime temperatures over 700 degrees Kelvin. That is hot enough to melt lead. During the planetΓÇÖs night, temperatures drop to around 93 degrees Kelvin, 180 degrees below the freezing point of water.
