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Nuclear waste.
核废料。
We’ve got a lot of it, it'll stay dangerous for tens of thousands of years, and we don't really know what to do with it.
我们有很多这样的东西，它会在几万年内保持危险状态，而我们真的不知道该怎么处理它。
So why don't we just send it into space and crash it into the sun?
那我们为什么不直接把它送到太空，然后撞向太阳呢?
Well, first, it's really dangerous to put nuclear waste on a rocket, since rockets have a tendency to occasionally explode while launching, making any nuclear-waste-filled exploding rocket into a really big dirty bomb.
首先，把核废料放在火箭上是非常危险的，因为火箭在发射时偶尔会发生爆炸，使得任何充满核废料的爆炸火箭都会变成一枚巨大的脏弹。
But the bigger reason is that it's actually really really hard to get to the sun.
但更重要的原因是，实际上要到达太阳是非常非常困难的。
It might seem like it should be easy, since the sun's gravity is always pulling us towards it.
这看起来似乎很容易，因为太阳的引力总是把我们拉向它。
But we're also orbiting really fast sideways around the sun, so that as we fall towards it, we miss it.
但我们也在绕着太阳快速旋转，所以当我们靠近太阳时，我们会错过它。
In order to crash into the sun, you have to slow down so that you're not going sideways really fast.
为了撞向太阳，你必须放慢速度，这样你就不会很快地转到旁边去了。
The earth - and everything on it - is moving around the sun at around 30 kilometers per second, so you'd have to accelerate to a speed of 30kilometers per second backwards away from the earth in order to stop moving around the sun and do a sun dive.
地球，以及地球上的一切，都在以每秒30公里的速度绕着太阳转，所以你必须加速到每秒30公里的速度，向远离地球的方向运行，才能停止绕着太阳转，从而向太阳俯冲。
And you have to slow down all the way – with even a little bit of sideways speed, you'll miss the sun and whip around, not crashing.
而且你必须一路减速——即使是一点点的侧向速度，你都会错过太阳，飞驰而过，而不是撞上它。
Ok, so a speed of 30 kilometers per second is really fast, but just how fast?
那么，每秒30千米的速度确实很快，但到底有多快呢？
Well, from earths’s orbit, you only need to be going 11 kilometers per second faster than the earth in order to escape from the entire solar system.
从地球的轨道出发，你只需要比地球快11千米/秒就能逃离整个太阳系。
Which means that it's much, much harder to crash into the sun than to escape it altogether.
这意味着撞上太阳要比干脆逃离太阳困难得多。
Let me say that again: it takes less acceleration to get to other stars than it does to get to our own sun. Crazy.
我再说一遍：到达其他恒星所需的加速度比到达我们自己星系里的太阳所需的加速度要小。太疯狂了。
But it gets weirder: because the gravity from an object is stronger the closer you are to it, the smaller your orbit is, the faster your orbital speed.
但更奇怪的是：因为一个物体的引力越强，你离它越近，你的轨道越小，你的轨道速度就越快。
For example, Mercury goes around the sun at a speed one and a half times faster than earth, while Pluto goes only a sixth as fast.
例如，水星绕太阳公转的速度是地球的1.5倍，而冥王星绕太阳公转的速度只有地球的六分之一。
And that means it's actually way harder to crash into the sun from Mercury than from the earth, even though you're closer, because you'd have to accelerate to a speed of 48 kilometers per second backwards instead of 30.
这意味着，即使你离太阳更近，从水星撞向太阳实际上要比从地球撞向太阳难得多，因为你必须向后加速到48公里每秒，而不是30公里每秒。
And it's way easier to crash into the sun from Pluto, since you only have to accelerate to a speed of five kilometers per second backwards.
而且从冥王星撞向太阳要容易得多，因为你只需要向后加速到每秒5公里的速度。
In fact, if you're trying to crash into the sun just using rockets, it's far more efficient to first go to the outer solar system where your speed is much lower, then do a second burn to counteract that slow orbital speed and allow you to fall directly into the sun.
事实上，如果你正试图只是用火箭撞向太阳，先去到你的速度要慢得多的太阳系，然后进行第二次燃烧，以抵消缓慢的轨道速度，然后让你直接坠入太阳，这样效率会高得多。
And that's precisely why early mission trajectories for NASA's spacecraft to study the sun proposed going out to Jupiter first – to make it easier to slow down and get to the sun.
这也正是为什么美国国家航空航天局(NASA)早期研究太阳的航天器的轨道时建议先去木星——这样就更容易减速并到达太阳。
Ultimately they decided instead to use repeated flyby's of Venus to slow down the probe and save on rocket fuel getting to the sun.
最终，他们决定改用反复飞越金星的方式来减慢探测器的速度，并节省火箭到达太阳所需的燃料。
But how gravity assists work is a topic for another day.
但重力是如何辅助工作的就是另一个话题了。
Speaking of which – how long would a day be on the sun?
说到这——太阳上的一天有多长？
