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Nuclear waste. 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. So why don't we just send it into space and
crash it into the sun?
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.
But the bigger reason is that it's actually really really hard to *get* to the sun. It
might seem like it should be easy, since the sun's gravity is always pulling us towards
it. But we're also orbiting really fast sideways around the sun, so that as we fall towards
it, we miss it.
In order to crash _into_ the sun, you have to slow down so that you're _not_ going sideways
really fast. 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. 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.
Ok, so a speed of 30 kilometers per second is really fast, but just how fast? 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. Which means that it's much, much harder
to crash into the sun than to escape it altogether. Let me say that again: it takes less acceleration
to get to _other_ stars than it does to get to our own sun. Crazy.
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. 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. 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. 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.
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.
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. 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.
But how gravity assists work is a topic for another day. Speaking of which – how long
would a day be on the sun?