How to Drive a Spacecraft 3 Billion Miles and Get Where You Want to Go
By Kenneth Chang

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.

In terms of accuracy, that’s like driving from New York to San Francisco and ending up within about five inches of the parking spot you had selected before setting out.

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.

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

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.”

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.

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.

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.

For the 11 million miles since then, New Horizons has been coasting, aimed at the middle of that 60-by-90-mile rectangle.

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

9年飛越50億公里！ 美飛船14日擦身冥王星

美國太空總署（NASA）的 「新地平線號」無人太空船已進入與冥王星的「相遇模式」，預計本月十四日從冥王星上方約一萬公里處飛掠，成為造訪這個遙遠矮行星的第一艘探測船，兩者交會的一刻，將是科學家探索太陽系逾五十年來的高潮。

時速約五萬公里的「新地平線號」將在飛行逾九年、五十億公里後與冥王星相遇，這趟行程約相當於繞行地球十二萬次。隨著探測船一天天接近，拍攝的冥王星照片也越來越清晰。出乎意料，冥王星是一個桃紅色調，表面地形明暗對比鮮明的星球。

一月時， 「新地平線號」從沉睡中醒轉，展開觀測冥王星和它主要衛星冥衛一（Charon）的任務。科學家於1930年發現體積小於月球的冥王星，將它當作太陽系第九大行星，不過，國際天文學會已於2006年 「新地平線號」升空六個月後將冥王星降級為矮行星。神秘的冥王星位於太陽系最外圍的古柏帶，古柏帶發現於1992年，古柏帶的發現燃起了科學家觀測冥王星的興趣，他們相信古柏帶內有四十六億年前太陽系形成時的化石。

國際天文學會作成將冥王星降級的決定後，科學家在古柏帶已知的四十多個星體外，又發現了約兩千個星體。

一個電腦程式將主導太空船從十四日開始飛掠冥王星，其後電腦會當機，暫停作業三天。

時機的掌握將無比重要。「新地平線號」僅有卅分鐘可執行此行最重要的任務，包括拍攝冥王星與冥衛一的影像，判別兩個冰冷世界的組成，以及掃描冥王星的大氣，這些任務必須在探測船與冥王星交會之際完成。

預估「新地平線號」與冥王星最靠近的時間是在台灣時間十四日晚間七時四十九分，兩者相距約一萬兩千五百公里。

原文參照：
http://www.nytimes.com/interactive/projects/cp/summer-of-science-2015/latest/how-to-drive-a-spacecraft-3-billion-miles-and-end-up-where-you-want-to-be

紐約時報中文版翻譯：
http://cn.nytimes.com/science/20150713/c13pluto/zh-hant/

2015-07-12．聯合報．A13．國際．編譯王麗娟