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  • [♪ INTRO]

  • 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 wholegetting therepart.

  • 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 powerfulflip

  • and burnactions 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]

[♪ INTRO]

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なぜ水星に着くまでにこんなに時間がかかるのか? (Why Does It Take So Long to Get to Mercury?)

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    林宜悉 に公開 2021 年 01 月 14 日
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