NASA探測器週二飛掠冥王星事件
Nine and a half years and three billion miles later, NASA's New Horizons spacecraft will zoom past Pluto on Tuesday. And mission managers are confident that they have aimed precisely enough that the spacecraft, traveling 31,000 miles an hour, will pass through a rectangle just 60 miles by 90 miles at its closest approach to Pluto.
經過九年半、30億英里（約合48億公里）的飛行，NASA的“新視野號”(New Horizons)探測器將在週二掠過冥王星。此次探測任務的管理人員相信，他們已經瞄準得足夠精確，時速3.1萬英里的探測器將在飛經一個90英里長、60英里寬的矩形區域時，與冥王星的距離達到最近。In terms of accuracy, that's like driving from New York to San Francisco and ending up within five feet of the parking spot you had selected before setting out.
就準確度而言，這就像是開車從紐約去舊金山，最終抵達的地點離出發前選定的停車位誤差不到5英尺。A computer program calculated the necessary trajectory, including a swing by Jupiter to pick up velocity. That process is essentially unchanged from decades ago, when the Pioneer and Voyager probes similarly navigated to the outer solar system.
一個電腦程序計算了必要的飛行軌跡，包括藉助木星引力獲得加速度。這個過程和數十年前以類似方式探索外太陽系的“先驅者號”(Pioneer)和“旅行者號”(Voyager)探測器基本一樣。
The essential equation is simply F=ma, Newton's Second Law, where F is the sum of all the forces.
基本公式就是牛頓第二定律F=ma，F指各種力的總和。Nonetheless, the details matter. "We used to lob spacecraft at the moon and miss," said Mark Holdridge, the mission manager for the Pluto encounter. "It's very easy to make a mistake."
然而，細節很重要。“我們過去向月球發射過探測器，但發生了偏離，”冥王星探測任務負責人馬克·霍爾德里奇(Mark Holdridge)說。“非常容易出錯。”The calculations have to factor in the gravitational tug of the sun and the planets. The tiny thrusters that spin New Horizons around also knock it off course slightly. Even the heat from the chunk of plutonium that acts as the spacecraft's battery produces a smidgen of thrust, and so does the small pressure of sunlight hitting the spacecraft. All must be taken into account.
計算得把太陽和各行星的引力作用考慮進去。小型推進器在讓“新視野號”旋轉的同時，也導致其稍稍偏離航向。就連充當探測器電池的那一大塊鈈散發的熱量，也產生了少量的推力。照射到探測器上的太陽光產生的少量壓力也一樣。所有因素都必須考慮進去。Two teams of navigators do the calculations independently and then compare and reconcile their answers. That avoids mistakes like the one in 1999 when a mix-up between metric and imperial units pushed NASA's Mars Climate Orbiter slightly off course. It dipped too far into the atmosphere as it was entering orbit and was torn apart.
兩個導航團隊獨立計算，然後對結果進行對比和校準。這麼做會避免出現1999年那樣的錯誤。當時，公制單位和英制單位混淆導致NASA的火星氣候軌道器(Mars Climate Orbiter)稍稍偏離航向。進入軌道時，該軌道器進入大氣層太多，最終被撕碎。The calculations are not perfect, so New Horizons has also fired its thrusters nine times during the nine and a half years for small course corrections, most recently two weeks ago.
計算是不完美的，因此在九年半的時間裏，“新視野號”還曾九次點燃推動器，進行小的航向修正。距今最近的一次發生在兩週前。For the 11 million miles since then, New Horizons has been coasting, aimed at the middle of that 60-by-90-mile rectangle.
自那以後的1100萬英里飛行中，“新視野號”一直是在朝着前述矩形區域的中心滑行。"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。
