搞科研很難,，搞尖端科研更難，這需要堅持和無限的耐心,。逾15年前,，我們開始籌劃飛往冥王星的項目。當時我們就知道,，和大家說的一樣,，這將是一場遠足。但我們也知道,，它有可能帶來巨大的收獲,。70年來我們都只能遙望冥王星，而“新視野號”將首次近距離觀察那個世界,。這樣的挑戰(zhàn)你很難抗拒,，而我選擇了迎難而上。

最初的喧囂

首次關于飛抵冥王星的認真探討出現(xiàn)在20世紀80年代末,。當時,，人們用望遠鏡觀察比冥王星更遠的星體（被稱為恒星掩星）時發(fā)現(xiàn)冥王星存在甲烷大氣層，而且可能正在快速向宇宙空間流失,。鑒于冥王星個頭很小,，看到其引力竟然足以“拴住”大氣層很讓人意外。

就在那段時間,，幾位行星科學家開始認真地研究冥王星,。在整個20世紀90年代，誰是冥王星科學知識的最熱心的貢獻者顯而易見，我們這些人也自然而然地在科學會議上聚在一起,，討論探訪這顆第九大行星的問題,。美國國家航空航天局（NASA）曾構想過一些探冥項目，但由于各種各樣的原因均未成行,，其中最主要的因素是經(jīng)費不足,。2000年NASA表示有可能啟動這樣的項目時，大家都已成竹在胸,，對于如何通過飛近探測冥王星來優(yōu)化我們的科學認知有了一些相當清晰的計劃,。

Science is hard and good science is harder — it takes persistence and tons of patience. When we began planning a mission to Pluto over 15 years ago, we knew it was going to be, as they say, a long haul. But we also knew it had a huge potential payoff. New Horizons would be the first closeup look at a world that we’d known about only distantly for 70 years. That kind of challenge is hard to pass by. And I didn’t.

Earliest rumblings

The first serious discussion of a mission to Pluto occurred in the late 1980s. That’s when telescope observations of distant stars going behind Pluto (known as stellar occultations) showed that it had a methane atmosphere, probably rapidly evaporating to space. Considering Pluto’s small size, it was a surprise to see that its gravity was strong enough to hold on to an atmosphere at all.

Several planetary scientists began working on Pluto in earnest around that time. Throughout the 1990s, it was pretty obvious who the enthusiastic contributors to Pluto science were, and we naturally congregated at scientific meetings to speculate about a mission to the ninth planet. Several attempts at a Pluto mission were formulated by NASA, but for various reasons none came to fruition, usually because of insufficient funding. By the time NASA announced the opportunity for a Pluto mission in 2000, everyone was ready with some pretty clear plans for how to optimize the science that could be done via a flyby.

冥王星的神秘大氣層

1984年,，我在加州理工學院讀研究生時對冥王星產(chǎn)生了興趣,。當時人們對冥王星大氣層所知甚少，無法用計算機建模來預測,。但后來當那些恒星掩星揭示了冥王星出人意料的大氣層后,，我就迷上了這方面的研究。我關注的重點是其大氣中的化學成分,?！靶乱曇疤枴表椖渴紫芯繂T艾倫·斯特恩開始召集研究者時，邀請我加入了大氣科學團隊,。其他小組的研究課題包括表面地質(zhì),、內(nèi)部結(jié)構以及冥王星五衛(wèi)星體系的形成等等。

我的主要興趣是了解冥王星大氣層的化學組成和穩(wěn)定性,。在太陽系45億年的歷史中,，冥王星怎么來約束自己的大氣層？它的引力很弱,，這些氣體早該在宇宙空間流失殆盡,。但情況顯然并非如此。

我感興趣的另一個問題是冥王星大氣中的甲烷是否形成了復雜的碳氫化合物,，以及后者是否凝聚成氣溶膠并沉降,，最終留在了冥王星的表面。經(jīng)歷了太陽系的漫長歷史,，這些碳氫化合物在冥王星表面的厚度應該有幾十米,。

而實際上，“新視野號”沒有在冥王星上發(fā)現(xiàn)可能由這些光化學物質(zhì)（冥王星大氣中的甲烷吸收陽光中的紫外線后產(chǎn)生）形成的大片非常暗的區(qū)域,。在太陽系外圍,，通過化學反應形成的類似碳氫化合物比比皆是。這些略帶紅色的物質(zhì)統(tǒng)稱為索林斯（tholins）,。我們計劃進一步觀測冥王星大氣層,，以便弄清楚可形成氣溶膠的乙烷、乙炔和乙烯等碳氫化合物氣體的分布情況。

Pluto’s mysterious atmosphere

I became interested in Pluto as a graduate student at Caltech in 1984. So little was known about the atmosphere at the time that you could get a computer model to predict just about anything. But after those stellar occultations revealed its unexpected atmosphere, I was hooked on Pluto atmosphere research. I focused on the planet’s chemistry. When Alan Stern, the New Horizons principal investigator, started pulling together researchers to work on the mission, he asked me to join the atmosphere science team. Other mission members are focused on surface geology, interior structure, and formation of the Pluto-Charon system of five moons, among other things.

My main interest is understanding the chemistry and stability of Pluto’s atmosphere. How could Pluto hold onto its extended layer of gases over the 4.5 billion year age of the solar system? It should have long ago escaped to space because of tiny Pluto’s low gravity. But apparently it hasn’t.

Also, I’m interested to see whether the methane in its atmosphere produced complex hydrocarbons that would condense onto aerosols, settle downward, and be deposited on Pluto’s surface. Over the age of the solar system, these molecules should accumulate so that now we’d expect to see tens of meters of the stuff on Pluto’s surface.

And in fact, New Horizons does see large very dark regions on Pluto that might be made up of these photochemical products, generated when the methane in Pluto’s atmosphere absorbs ultraviolet sunlight. Similar types of chemically processed hydrocarbons are seen elsewhere in the outer solar system. They are generally known as “tholins” — a generic term to describe this reddish material. We have further observations planned that will probe Pluto’s atmosphere and map the distributions of hydrocarbon gases such as ethane, acetylene and ethylene that condense to form the aerosols.