Nine and a half years and three billion miles later,NASA's New Horizons spacecraft will zoom past Pluto on Tuesday. And mission managers areconfident that they have aimed precisely enough that the spacecraft, traveling 31,000 milesan hour, will pass through a rectangle just 60 miles by 90 miles at its closest approach toPluto.

經(jīng)過(guò)九年半、30億英里（約合48億公里）的飛行，NASA的“新視野號(hào)”（New Horizons）探測(cè)器在周二掠過(guò)冥王星。此次探測(cè)任務(wù)的管理人員相信，他們已經(jīng)瞄準(zhǔn)得足夠精確，時(shí)速3.1萬(wàn)英里的探測(cè)器將在飛經(jīng)一個(gè)90英里長(zhǎng)、60英里寬的矩形區(qū)域時(shí)，與冥王星的距離達(dá)到最近。

In terms of accuracy, that's like driving from New York to San Francisco and ending up withinfive feet of the parking spot you had selected before setting out.

就準(zhǔn)確度而言，這就像是開(kāi)車(chē)從紐約去舊金山，最終抵達(dá)的地點(diǎn)離出發(fā)前選定的停車(chē)位誤差不到5英尺。

A computer program calculated the necessary trajectory, including a swing by Jupiter to pickup velocity. That process is essentially unchanged from decades ago, when the Pioneer andVoyager probes similarly navigated to the outer solar system.

一個(gè)電腦程序計(jì)算了必要的飛行軌跡，包括借助木星引力獲得加速度。這個(gè)過(guò)程和數(shù)十年前以類(lèi)似方式探索外太陽(yáng)系的“先驅(qū)者號(hào)”（Pioneer）和“旅行者號(hào)”（Voyager）探測(cè)器基本一樣。

The essential equation is simply F=ma, Newton's Second Law, where F is the sum of all theforces.

基本公式就是牛頓第二定律F=ma，F(xiàn)指各種力的總和。

Nonetheless, the details matter. 'We used to lob spacecraft at the moon and miss,' said MarkHoldridge, the mission manager for the Pluto encounter. 'It's very easy to make a mistake.'

然而，細(xì)節(jié)很重要?！拔覀冞^(guò)去向月球發(fā)射過(guò)探測(cè)器，但發(fā)生了偏離，”冥王星探測(cè)任務(wù)負(fù)責(zé)人馬克·霍爾德里奇（Mark Holdridge）說(shuō)?！胺浅Ｈ菀壮鲥e(cuò)?！?/span>

The calculations have to factor in the gravitational tug of the sun and the planets. The tinythrusters that spin New Horizons around also knock it off course slightly. Even the heat fromthe chunk of plutonium that acts as the spacecraft's battery produces a smidgen of thrust, andso does the small pressure of sunlight hitting the spacecraft. All must be taken into account.

計(jì)算得把太陽(yáng)和各行星的引力作用考慮進(jìn)去。小型推進(jìn)器在讓“新視野號(hào)”旋轉(zhuǎn)的同時(shí)，也導(dǎo)致其稍稍偏離航向。就連充當(dāng)探測(cè)器電池的那一大塊钚散發(fā)的熱量，也產(chǎn)生了少量的推力。照射到探測(cè)器上的太陽(yáng)光產(chǎn)生的少量壓力也一樣。所有因素都必須考慮進(jìn)去。

Two teams of navigators do the calculations independently and then compare and reconciletheir answers. That avoids mistakes like the one in 1999 when a mix-up between metric andimperial units pushed NASA's Mars Climate Orbiter slightly off course. It dipped too far intothe atmosphere as it was entering orbit and was torn apart.

兩個(gè)導(dǎo)航團(tuán)隊(duì)獨(dú)立計(jì)算，然后對(duì)結(jié)果進(jìn)行對(duì)比和校準(zhǔn)。這么做會(huì)避免出現(xiàn)1999年那樣的錯(cuò)誤。當(dāng)時(shí)，公制單位和英制單位混淆導(dǎo)致NASA的火星氣候軌道器（Mars Climate Orbiter）稍稍偏離航向。進(jìn)入軌道時(shí)，該軌道器進(jìn)入大氣層太多，最終被撕碎。

The calculations are not perfect, so New Horizons has also fired its thrusters nine times duringthe nine and a half years for small course corrections, most recently two weeks ago.

計(jì)算是不完美的，因此在九年半的時(shí)間里，“新視野號(hào)”還曾九次點(diǎn)燃推動(dòng)器，進(jìn)行小的航向修正。距今最近的一次發(fā)生在兩周前。

For the 11 million miles since then, New Horizons has been coasting, aimed at the middle of that60-by-90-mile rectangle.

自那以后的1100萬(wàn)英里飛行中，“新視野號(hào)”一直是在朝著前述矩形區(qū)域的中心滑行。

'That’s pretty incredible, if you think about it,' Mr. Holdridge said.

“想想真是很不可思議，”霍爾德里奇說(shuō)。