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Mercury is one of the least explored planets in our solar system, which is kind of strange, if you think about it. Compared to other planets, Mercury isn’t too far away, and it doesn’t have any sort of terrifying atmosphere or weather that would damage a spacecraft.

But it turns out that the conditions on Mercury aren’t actually the problem. It’s the whole “getting there” part. Getting into orbit around the planet is such an astronomical and technical challenge that we only figured out how to do it in 1985.

In other words, we knew how to land people on the Moon before we knew how to get a machine around Mercury. But we did figure it out. And now, we’re going back.

The biggest challenges in all this involve Mercury’s small size and its closeness to the Sun. At about 4900 kilometers in diameter, Mercury is the solar system’s smallest planet. It’s also the closest to the Sun, about 60 million kilometers away on average, compared to Earth’s 150 million.

That means Mercury also has a very short orbit; it's only 88 Earth days. These two things alone make it a pretty hard target to hit. But there’s also the issue of the Sun itself.

See, when spacecraft are traveling away from our star, like to Mars or Jupiter, they’re fighting the pull of the Sun’s gravity and trying to gain speed. But when they’re moving towards the Sun, like on the way to Mercury, the Sun provides extra speed. So if you aren’t careful, your spacecraft could overshoot its target and fall into the fiery abyss that is our star.

In sci-fi shows you might see spacecraft solve this problem by performing powerful “flip and burn” actions to slow down and cruise to an easy stop anywhere they want. Basically, the idea is that if you fire your thrusters hard enough in the opposite direction, you can stop. But with current technologies, slowing down after a direct route to Mercury would be extremely difficult, because you’d pick up way too much speed.

To generate enough thrust to slow down, your mission would need to carry more fuel than it’s capable of launching with. And, like, I’m not an engineer or anything, but that sounds like a design problem. The very first mission to Mercury, Mariner 10, got around this by being a flyby mission.

In 1973, it took a fast, relatively direct route to the planet, getting there in only five months. As expected, it was going really fast by the time it arrived, but that was okay, since it was never intended to orbit Mercury. Instead, it flew by it three times, mapping about half of its surface.

Over the years, though, scientists have found a way into orbit. To get a longer and closer look at Mercury, they’ve designed spacecraft that rely on a combination of solar power, fuel, and most importantly, gravity assists. Gravity assists are a common tool in orbital mechanics, where spacecraft use the gravity of the planets they’re flying by to change direction or speed.

We use them all the time to get to the outer solar system, but the combination of assists needed to reach Mercury is especially complex because it’s so small and close to the Sun. Even though we figured out how to do this in the 80s, the first time we actually pulled it off was in 2004 when NASA launched the MESSENGER spacecraft. It had a six-and-a-half-year gravity assist journey, but in 2011, it became the first craft to insert itself into Mercury’s orbit.

MESSENGER circled the planet for four years before it ran out of fuel and was intentionally crashed into the surface. But that wasn’t the end. Because now, scientists are thinking a lot about gravity assists as part of a new mission to Mercury.

Meet BepiColombo. It’s named for the Italian scientist who helped calculate Mariner 10’s path to Mercury, and it launched in October 2018. The Bepi is made of two probes from the European Space Agency and the.

Japan Aerospace Exploration Agency, and it’s on a mission to do things like analyze. Mercury’s magnetic field, terrain, and surface composition. The information it gathers will help astronomers better understand exoplanets that reside close to their stars, as well how our solar system formed.

But getting there at the right speed will be an intense, seven-year adventure. BepiColombo is using a similar strategy to MESSENGER, and it will take nine planetary flybys before it’s finally able to enter Mercury’s orbit in 2025. Its path looks like you took one of those old Spirograph stencils and went to town, making a series of large ellipses.

The first of its major milestones will happen in April 2020, when it will visit home and make its sole flyby of Earth. Throughout the rest of 2020 and 2021, the BepiColombo will make a couple flybys of Venus. From then, until 2025, the spacecraft will hone in on Mercury, making six flybys, as well as additional trips around the Sun, getting its speed and trajectory just right until it enters the planet’s orbit in December.

The numerous flybys won’t be wasted though. They’ll serve as opportunities for researchers to test the equipment onboard, and prepare for the moment of truth: the final arrival. So even along the way, we’ll likely be learning a whole lot about the innermost planet.

It might look like we should just cruise right on over to Mercury in no time. But in space, just because it looks easy, definitely doesn’t mean it is. Orbital mechanics and gravity make things a bit more complicated, but it just makes you appreciate all those scientists and engineers even more.

And I’d say it makes things a little more exciting, too. Thanks for watching this episode of SciShow Space! If you’d like to learn more about the surprising engineering it takes to explore the solar system, you can watch our episode about why it’s so hard to land on Mars. [♪ OUTRO].