March 29 – Anniversary of the First Mercury Fly-by

Posted on March 29, 2016

Mercury is one of the four rocky inner planets (and the smallest one - that's it on the far right), so it seems it would be fairly easy to send robotic space probes to this planet. But Mercury is so close to the Sun, it was tricky for the NASA scientists and engineers to design Mariner 10 to be tough enough to endure that environment.

The fact that Mercury is so close to the Sun
makes it difficult for us to see it in our skies.

It tends to be lost -- or almost lost -- in the
Sun's glow during dawns and sunsets.
Even though the Sun is the ultimate resource necessary for all of Earth's life – giving the light and heat that we need – being close to the Sun means getting a whole lot of radiation, including way too much heat as well as punishing doses of high-energy radiation.

So the Mariner needed shielding. It needed thermal blankets and sunshades. The sunshade scientists devised ended up being aluminized Kapton and glass-fiber sheets covered with Teflon! The solar panels, of course, could not be covered – their whole purpose was to soak up the sun's light so that they could make energy that the space probe needed – but solar panels do not work if they get too hot. So scientists created a way to tilt the solar panels so that the right temperature was maintained. Finally, the hydrazine rocket nozzle had to face the Sun in order to work; the exposed parts of the rocket nozzle were painted with special paint to reduce the heat flow to the rest of the probe.

Mariner 10 was launched on November 3, 1973, and it did its first fly-by on this date in 1974.

Mariner 10 ended up flying past Mercury three times. It mapped between 40% and 45% of the planet's surface by taking 2,800 photos. It discovered that Mercury has an extremely thin and unstable atmosphere made up mostly of helium, and it discovered that the planet has a magnetic field, which shows that it has a large iron-rich core.

This is one of the Mariner 10 photos
of Mercury.
The radiometer readings reveal that Mercury has nighttime temperatures of – 183 degrees Celsius ( -297 degrees Fahrenheit) and maximum daytime temperatures of 187 degrees Celsius (369 degrees Fahrenheit). So that's way more than twice as cold AND twice as hot as Earth.

Mercury endures the greatest temperature
extremes of any planet in the Solar System.
You may think to yourself, okay, I get the twice as hot part, since Mercury is so much closer to the Sun. But why are the nights more than twice as cold as Earth?

The answer is that it is Earth's atmosphere that keeps us relatively warm at night, compared to places with little or no atmosphere – like the Moon and Mercury. In those places, the heat from the rocks radiates away quite quickly, and then the cold of space is all that is left...

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