Two orbiters begin their long journey to Mercury Friday night

ESA

One might think it's a relatively easy thing to reach Mercury, the innermost planet in the Solar System. At its closest approach, Mercury is just 77 million kilometers from Earth, or not all that much further than the closest that Earth comes to Mars. The Earth-Mars transit typically only takes about six months.

However, the Sun's enormous gravity makes putting a spacecraft into orbit around Mercury quite difficult. How much gravity are we talking about? The g-force at the surface of the Earth is 9.8 meters/second^2. By comparison, the Sun's gravity is nearly 30 times greater, at 274 m/s^2.