You might think the closest planet to the sun would be easy to reach. After all, Mercury is our neighbor! But getting a spacecraft to Mercury is actually one of the hardest trips in the solar system. On March 29, 1974. A small NASA spacecraft called Mariner 10 became the first to fly close to Mercury and take detailed photos. What it found surprised everyone. Mercury's surface was covered in thousands of craters. Bowl-shaped holes from space rocks hitting the planet over billions of years. It looked remarkably like Earth's moon. But here is what makes Mercury strange: one side can reach 800 degrees Fahrenheit during the day. While the other side plunges to negative 290 degrees at night. That is because Mercury has almost no atmosphere -- no blanket of air to spread the heat around. Getting there is so difficult because Mercury orbits very close to the sun. Which means a spacecraft has to fight the sun's huge gravity. Mariner 10 solved this problem by using Venus as a slingshot. It flew past Venus first, using that planet's gravity to bend its path and fling it toward Mercury. This technique, called a gravity assist, was new. Mariner 10 made three passes by Mercury before running out of fuel. It would be over thirty years before another spacecraft visited.