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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.
水星是太阳系里我们探索最少的行星。细想来，还挺奇怪的。
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.
与其他行星相比，水星离我们不算远，大气层又不是很稀薄，天气也不是很恶劣，不至于损坏航天器。
But it turns out that the conditions on Mercury aren't actually the problem.
但其实水星的自身条件并非问题所在。
It's the whole getting there part.
问题在于去水星太难了。
Getting into orbit around the planet is such an astronomical and technical challenge that we only figured out how to do it in 1985.
进入水星附近的轨道无论是从天文学还是技术的角度来讲都是一项挑战，所以，人类1985年才突破如何进入水星轨道这个难题。
In other words, we knew how to land people on the Moon before we knew how to get a machine around Mercury.
换言之，我们攻克载人飞月的时间都要早于让机器进入水星轨道的时间。
But we did figure it out. And now, we're going back.
不过，好在这个问题已经解决了，言归正传：
The biggest challenges in all this involve Mercury's small size and its closeness to the Sun.
之所以进入水星轨道存在巨大挑战是因为水星的体型太小，离太阳又最近。
At about 4900 kilometers in diameter, Mercury is the solar system's smallest planet.
水星直径近4900公里，是太阳系里最小的行星。
It's also the closest to the Sun, about 60 million kilometers away on average, compared to Earth's 150 million.
也是离太阳最近的行星——大约6000万公里；而地球离太阳却有1.5亿公里。
That means Mercury also has a very short orbit; it's only 88 Earth days.
这就说明水星的轨道也很短，大概只需要88个地球日就能环绕完毕。
These two things alone make it a pretty hard target to hit. But there's also the issue of the Sun itself.
光是这两点就让水星成为很难抵达的目的地。不过还有一个原因在于太阳本身。
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.
航天器飞离太阳的时候，比如从太阳飞向火星或者木星的时候，其实是在摆脱太阳的引力并获得速度。
But when they're moving towards the Sun, like on the way to Mercury, the Sun provides extra speed.
但当航天器飞向太阳的时候，比如终点站是水星的时候，太阳会为航天器提供动能。
So if you aren't careful, your spacecraft could overshoot its target and fall into the fiery abyss that is our star.
所以，稍有不慎，航天器就会飞过头，并进入炽热的深渊，即太阳。
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.
在科幻片中可能会看到航天器解决的这样的问题，方式是进行强有力的反转和燃烧动作，以实现减速，从而随时随地想停就停。
Basically, the idea is that if you fire your thrusters hard enough in the opposite direction, you can stop.
它的基本理念是：如果向反方向启动推进器，并且足够用力的话，就能停下来。
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.
但如果用现在的技术，飞向水星的路上进行减速就成了件难事，因为路上会获得太多的动能。
To generate enough thrust to slow down, your mission would need to carry more fuel than it's capable of launching with.
要产生足够的推力以实现减速的话，航天器上就要配有超负荷的燃料。
And, like, I'm not an engineer or anything, but that sounds like a design problem.
虽然我不是工程师之类的岗位，但我认为这个问题听起来像是设计的问题。
The very first mission to Mercury, Mariner 10, got around this by being a flyby mission.
第一次飞往月球的是水手10号，它当时进行的是飞近探测任务，解决了这个问题。
In 1973, it took a fast, relatively direct route to the planet, getting there in only five months.
1973年，水手10号沿着相对直接的路径快速飞往水星，用了仅5个月的时间就抵达了水星。
As expected, it was going really fast by the time it arrived, but that was okay, since it was never intended to orbit Mercury.
不出所料，其抵达水星的时候速度过快，但这不是事儿，因为水手10号本来也就没打算环绕水星飞行。
Instead, it flew by it three times, mapping about half of its surface.
相反，它飞越了水星3次，勾画了其表面的图谱。
Over the years, though, scientists have found a way into orbit.
不过，过去这些年来，科学家找到了进入水星轨道的办法。
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.
为了近距离观测水星更长的时间，科学家们设计了一个航天器。该航天器依靠太阳能、燃料和引力助推的合力来移动。
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.
引力助推是轨道力学上的一种常见工具，即航天器通过其所经过的行星的引力来改变自己的方向或者速度。
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.
科学家一直通过引力助推作用来让航天器抵达外太阳系，但抵达水星所需的引力助推尤为复杂，因为水星体型太小，离太阳又太近。
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.
虽然上世纪80年代我们终于搞明白如何实现，但直到2004年，人类才首次成功试行这个方案。当时的项目是美国宇航局推出的信使号。
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.
信使号在引力助推的作用下飞行了6年半的时间，但2011年的时候，信使号终于成为第一个进入水星轨道的航天器。
MESSENGER circled the planet for four years before it ran out of fuel and was intentionally crashed into the surface.
信使号环绕水星的时间长达4年之久，最终，由于燃料用尽而坠落到水星表面。
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.
但故事还没结束。因为目前科学家正在思考如何通过引力助推的作用再次实现飞到水星的任务——贝皮可伦坡号。
It's named for the Italian scientist who helped calculate Mariner 10's path to Mercury, and it launched in October 2018.
贝皮可伦坡号是以意大利科学家的名字命名的，因为这位科学家曾助力计算了水手10号去往水星的路径。贝皮可伦坡号已于2018年10月发射。
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.
贝皮可伦坡号由2个探测器组成，分别来自欧洲太空总署和日本的宇宙航空研究开发机构。这次任务中，贝皮可伦坡号可以完成分析水星磁场、地形、表面组成等任务。
The information it gathers will help astronomers better understand exoplanets that reside close to their stars, as well how our solar system formed.
贝皮可伦坡号收集的信息将助力天文学家更好地理解离自己所环绕恒星很近的系外行星，同时也能更好地理解太阳系的形成方式。
But getting there at the right speed will be an intense, seven-year adventure.
但以合适的速度抵达火星将会耗费7年的时间，强度很高。
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.
贝皮可伦坡号采取的策略与信使号相似——它将进行9次飞近探测，然后才会于2025年进入水星轨道。
Its path looks like you took one of those old Spirograph stencils and went to town, making a series of large ellipses.
其路径看起来很像是一个人带着呼吸描记器模板去城里，画了一系列椭圆。
The first of its major milestones will happen in April 2020, when it will visit home and make its sole flyby of Earth.
贝皮可伦坡号的第一次重大里程碑将在2020年4月出现，那时候，它会经过地球，并对地球做一次单独的飞近探测。
Throughout the rest of 2020 and 2021, the BepiColombo will make a couple flybys of Venus.
其后一直到2021年，贝皮可伦坡号将对进行做数次飞近探测。
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.
再往后直到2025年，航天器将在水星执行任务——进行6次飞近探测，并环绕太阳做几次飞行。在将速度和轨道调整合适后，将于12月进入水星轨道环行。
The numerous flybys won't be wasted though.
不过，在那之前的飞近探测都不是无用功。
They'll serve as opportunities for researchers to test the equipment onboard, and prepare for the moment of truth: the final arrival.
而都将是研究人员的机会，可以通过它们来检测设备，并为关键时刻——最后抵达水星做准备。
So even along the way, we'll likely be learning a whole lot about the innermost planet.
所以，即便是在研究的过程中，我们也很有可能对水星这颗最里面的行星了解许多。
It might look like we should just cruise right on over to Mercury in no time.
看起来可能像是马上就可以抵达水星了。
But in space, just because it looks easy, definitely doesn't mean it is.
但在太空里做什么事都不是那么容易。
Orbital mechanics and gravity make things a bit more complicated, but it just makes you appreciate all those scientists and engineers even more.
轨道力学和引力让情况变得更为复杂，但也会让我们更加欣赏科学家和工程师，
And I'd say it makes things a little more exciting, too.
在我本人看来，也让这一切更激动人心了。
Thanks for watching this episode of SciShow Space!
感谢收看本期的《太空科学秀》！
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.
如果大家想对探索太阳系工程学知识了解更多的话，可以观看我们的一期视频，它与登陆火星的巨大难度有关。
