Of course we have no prospect¹ of such a journey. 当然，我们没有打算去做这样一次旅行。

A trip of 240,000 miles to the Moon still represents a very big undertaking² for us. 做一次38.6万公里远的月球旅行，对我们来说依然是一件了不起的大事。

A manned mission to Mars, called for by the first President Bush in a moment of passing giddiness, 老布什总统曾一时头脑发昏，提出要执行一次去火星的载人任务，

was quietly dropped when someone worked out that it would cost $450 billion and probably result in the deaths of all the crew 但后来不了了之。有人估计，这要花费4500亿美元，最后很可能落个全体乘员命归黄泉的结局

their DNA³ torn to tatters by high-energy solar particles from which they could not be shielded. 他们无法遮挡高能的太阳粒子，DNA会被撕得粉碎。

Based on what we know now and can reasonably imagine, 根据我们目前掌握的知识和理智的想像，

there is absolutely no prospect that any human being will ever visit the edge of our own solar system—ever. 任何人都绝对不会前往我们自己太阳系的边缘--永远不会。

It is just too far. As it is, even with the Hubble telescope, 实在太遥远了。事实上，即便使用哈勃望远镜，

we can't see even into the Oort cloud, so we don't actually know that it is there. 我们也看不到奥尔特云，因此我们实际上不知道它在哪里。

Its existence is probable but entirely⁴ hypothetical. 它的存在是可能的，但完全是假设的。

Properly called the Opik-Oort cloud, it is named for the Estonian astronomer⁵ Ernst Opik, 也许该称呼它为奥匹克 - 奥尔特云，它是根据爱沙尼亚的天文学家恩斯特·奥匹克命名的，

who hypothesized its existence in 1932, 因为是他在1932年发现奥匹克 - 奥尔特云的，

and for the Dutch astronomer Jan Oort, who refined the calculations eighteen years later. 同时也是为纪念荷兰天文学家奥尔特扬，因为他18年后细化计算定位了奥匹克 - 奥尔特云。

About all that can be said with confidence about the Oort cloud is that 关于奥尔特云，有把握的只能说到这种程度：

it starts somewhere beyond Pluto⁶ and stretches some two light-years out into the cosmos⁷. 它始于冥王星以外，向宇宙里伸展大约两光年。

The basic unit of measure in the solar system is the Astronomical⁸ Unit, or AU, 太阳系里的基本计量单位是天文单位（AU），

representing the distance from the Sun to the Earth. 代表太阳和地球之间的平均距离。

Pluto is about forty AUs from us, the heart of the Oort cloud about fifty thousand. 冥王星距离我们大约40个天文单位，奥尔特云的中心离我们大约5万个天文单位。

In a word, it is remote. 一句话，它非常遥远。