为什么在不直接以轴向向上或向下发射探测器,而要用螺旋状的呢?那不会更快到达星际空间吗?Why don’t we launch a probe axially out of the Solar System instead of always radially? Wouldn’t we reach interstellar space relatively quickly?

【翻译自quora, Jeffrey Naujok的回答 】

The biggest reason we don’t send things axially is that there’s not a lot of science benefit. There’s just “nothing up there” to look at.

我们不轴向发射探测器最大原因是太空很多地方并没有多大科研价值——那儿“什么也没有”。

(The five craft currently leaving the solar system.)

(目前离开太阳系的五个航天器。)

But more importantly, it’s really not even possible.

但更重要的是,这还是有可能的。

Let’s say you take a Falcon Heavy, in fully expendable mode, to put a 60 metric ton payload into orbit. That means it’s got roughly 8 km/s of delta-v.

让我们假设你坐上了猎鹰重型火箭,充分利用了它,将60公吨有效载荷送入轨道。 这意味着大约有8 km / s的delta-v。

Now, assuming that 60 tons represents a kick stage with a satellite, in order to reach Solar escape velocity, you need to get to 42.1 km/s of velocity. Normally, we get 29.8km/s of that for free, because Earth imparts that velocity — in the plane of the ecliptic — but you don’t get to use it, because you’re going straight up out of the ecliptic, so your starting velocity “up” (or “down”) out of the Solar System is zero.

现在,假设60吨是装着卫星的 kick stage ,为了达到太阳逃逸速度,您需要达到42.1 km / s的速度。 正常情况下,我们已经有了29.8km / s的速度,因为地球将速度传递到黄道平面上,但是您却无法使用它,因为您将直接从黄道上走,因此 离开太阳系的起始速度“上升”(或“下降”)为零。

That means you need to come up with at least 42.1 km/s of delta-v all from that 60 ton payload.

这意味着你需要在带着60吨载荷的情况下,拥有42.1km/s的delta-v。

Luckily, we can determine exactly what payload you could accelerate to 42.1 km/s using the “Rocket Equation”.

幸运的是,我们可以用“火箭方程”计算出你能把多少载荷加速到42.1 km/s。

Well, you have 21.9 kg of useful payload left over if you use the most efficient chemical engine ever produced. Mind you, that 21.9 kg has to include the body of the rocket as well.

好,如果你使用目前能生产的最有效的化学引擎,你最终可以剩余29.1kg的有效载荷。但是要提醒你,21.9kg也包括火箭本身。

So, we could use ion propulsion, that’s better, right?

所以,我们用离子推进剂,效果是不是更好呢?

In fact, it is. We could use 26 tons of xenon to push a 34 ton satellite (and rocket body) up to solar escape velocity.

对,我们可以使用26吨氙气来推起34吨的卫星(包括火箭)到太阳系逃离速度。

Of course, 26 tons of xenon costs about $1000 per kilogram, so you need $26 million worth of xenon just to feed the beast. You’d need massive solar panels to provide the energy, or a RTG (nuclear radio-thermal generator) power supply as you move away from the Sun. All those reduce your science payload mass. Speaking of which, you’d need lots of ion drives, because each one can only “burn” a few grams of propellant an hour, and those are really heavy as well.

当然,26吨氙气每公斤的成本约为1000美元,因此仅需要2600万美元的氙气就可以喂饱火箭。 当您远离太阳时,您将需要庞大的太阳能电池板来提供能量,或者需要RTG( nuclear radio-thermal generator ,核辐射热发生器)电源。 所有这些都会减少您可用的科学实验装备质量。 而且,您将需要大量的离子驱动器,因为每个离子驱动器一个小时只能“燃烧”几克推进剂,而且它们也很重。

Basically no one has ever built a spaceship anywhere near that scale, and I’m not sure the 34 tons of leftover mass would be enough.

基本上没有人建造过如此规模的太空船,而且我不确定这34吨的剩余重量是否足够。

And, given the acceleration of a typical 5 KW ion thruster, it would take about 4 years to get up to that speed, during which time you’d be going through lots of different orbits around the sun.

而且,考虑到一般的5KW离子推进器的加速度,要达到该速度大约需要4年的时间,在此期间,您将绕着太阳行驶过许多不同的轨道。

Of course, you can’t really get any useful slingshots, because you’re passing planets at 90 degrees to their rotational movement, meaning you can’t get a useful slingshot, though you might be able to convert some of that momentum that Earth gave you to a different plane.

当然,您实际上无法使用任何引力弹弓,因为您正在以与行星公转方向的的90度角通过他们,这意味着您无法使用有效的弹弓,尽管您可用将一些地球给你动量 给了你的另一个航天器。

Still, after 4 years (and I’m pretty sure no Xenon thruster has ever been fired for four continuous years, or used literal tons of fuel) you could reach escape velocity, having spent hundreds of millions of dollars.

不过,经过4年的时间(而且我很确定,目前人类还没有连续使用了四年的氙气推进器,也没有使用过那么多的燃料),花费了数亿美元,您达到了逃逸速度。

And you would still be the slowest craft to ever leave the solar system, because Voyager and Pioneer and New Horizons all got slingshot boosts from Jupiter (along with their head start from Earth) and left Jovian orbit at a speed of about 50 km/s, faster than your half-billion dollar “probe to nowhere” could ever reach.

而且您仍然是有史以来最慢的飞船,因为 Voyager , Pioneer 和 New Horizons 都从木星那里得到了引力弹弓的推动(自从它们从地球发射开始),并以约50 km / s的速度离开 Jovian 轨道 ,这比您五亿美元的“不知道在探测什么”任务快得多。

And the heliosphere (the hint is in the name) around the sun is roughly spherical, so going “up” or “down” doesn’t really help you reach interstellar space any faster. In fact, Voyager 2 actually hit the heliopause (where the solar wind goes sub-sonic) closer to the sun than Voyager 1 did, which is heading out of the solar system at a 35 degree angle to the ecliptic. So, staying in the plane of the solar system meant a closer exit for Voyager 2.

太阳周围的太阳风层大致是球形的,因此“向上”或“向下”并不能真正帮助您更快地到达星际空间。 实际上,旅行者2号实际上比旅行者1到达了离太阳更近的太阳风层顶 (太阳风是亚音速的),这个太阳风层顶与黄道成35度角。 因此,留在这个黄道面内意味着旅行者2离出口更近。

Exactly hitting solar escape velocity means that by the time you’re out to the heliopause, your ship has lost so much energy to gravity that you’d only be moving about 5 km/s. That’s about a third of Voyager 1’s speed.

在一开始拥有太阳系逃逸速度,意味着当你到达太阳风层顶时,你由于太阳的引力损失率很多能量,仅剩5km/s左右,只有旅行者1号速度的三分之一。

Finally, Interstellar space isn’t some grand exciting place to go and study. It’s mostly just… empty. Like really empty. Like 1 atom per cubic meter empty. There’s just not that much to see there.

最后,星际空间并不是值得研究的地方。 大多只是……空着。 真的空的。 每立方米空1个原子。 那里没什么可看的。

And looking down at the solar system isn’t like the pictures you see in the books either, it’s really pretty dull. (Jeffrey Naujok’s answer to Should Voyager take a panoramic set of images at least once a year?)

向下看着太阳系,也并不像在其他书上看到的那样。真的非常单调。 (Jeffrey Naujok’s 对于旅行者号每年是否应该至少拍摄一次全景图像的回答)

(A simulated view of the Solar System at 200 AU of distance. All the planets are lost in the glare of the sun.)

(距离的太阳系200AU( astronomical unit ,天文单位,150,000,000km )的模拟视图。所有行星都在太阳的眩光消失了。)

So, yeah, there’s really no point. At least Voyager, Pioneer, and New Horizons got to do some science along the way.

所以,是的,真的没有意义。 至少Voyager,Pioneer和New Horizons沿途还做一些科学研究。

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