In 1974, the Mariner 10 probe took 147 days to reach Mercury. Messenger, in 2011, took almost 7 years to make the journey… just like the BepiColombo probe, launched in 2018, which is set to arrive by the end of 2025.
Now, you might wonder, why does it take so long to reach a planet that is, on average, only 92 million kilometers away from Earth? This question becomes even more pressing when you consider that the Juno probe reached Jupiter, which is 600 million kilometers away, in just 5 years!
So, what’s the explanation for this? Well, the first key factor in interplanetary flight is the distinction between inner and outer planets. Surprisingly, heading towards the Sun, being pulled by its strong gravity, isn’t an advantage at all. It’s fine if you’re content with quick flybys…

but if you want to enter orbit around a planet like Mercury or Venus, you have only two options: either brake using retro rockets to counteract the Sun’s gravity, or lose speed by performing flybys with nearby planets. In the first case, you’ll need a massive amount of fuel; in the second, travel times and distances will increase dramatically.To understand this better, imagine having to pull an iron ball towards a pair of spherical magnets: one very large (the Sun) and one much smaller (Mercury).

You need to get the ball to be captured by the smaller magnet without first being caught by the overpowering magnetic field of the larger one… Quite difficult, isn’t it?
That’s why, among all the planets in our Solar System, Mercury is perhaps the most challenging to reach, despite being relatively close to Earth.