【聯合報╱By DENNIS OVERBYE／陳世欽譯）】
Space Telescope’s Planet Hunt May Go On
MOUNTAIN VIEW, California — The little spacecraft that could may still have some life left in it.
In the spring a critical wheel on NASA’s Kepler spacecraft got stuck, leaving its telescope unable to point precisely enough to continue prospecting for Earthlike planets in a starry patch of the Milky Way.
But Kepler’s managers say they have a plan that could keep it hunting for these exoplanets for three or four more years.
In the past four years , Kepler identified 3,500 possible exoplanets by seeing stars dim when planets crossed in front of them. It also enabled the first rough estimate of the abundance of habitable planets in the Milky Way: about one in five sunlike stars have potentially habitable Earth-size planets, meaning billions of chances for the existence of extraterrestrial life.
But the spacecraft was only beginning to zero in on planets with orbits like ours, Earth-size worlds that take a year to orbit suns similar to our own, candidates for Earth 2.0, in the vernacular.
The new approach would no longer train Kepler on only one set of stars. Instead, it would skip around the sky, monitoring the stars in one spot continuously for up to 80 days. It would not detect any planets with orbital periods more than a few weeks. But it would detect planets with habitable orbits around the smaller, dimmer stars known as red dwarfs.
The problem arose when the loss of a second reaction wheel — Kepler was launched in 2009 with four — left the spacecraft prone to rolling about its line of sight.
The solution is a neat bit of rocket science. Tests have shown sunlight can be used to stabilize Kepler. The key is to keep it pointed in directions that leave its solar panels evenly illuminated. “When it’s stable, it’s really stable,” said Charles Sobeck, deputy project manager of Kepler. As a result, Kepler’s planet hunting will now be limited to fields of stars that lie along a circle known as the ecliptic — the apparent path traversed by the sun through the zodiac.
Part of the sun’s path goes right across the center of the galaxy, in the constellation Sagittarius, and other parts, away from the Milky Way, cross outside galaxies, where Kepler could record the rise and fall of supernova explosions that have proved critical to understanding cosmic history. Other fields of opportunity include stormy regions strewn with clouds of gas and dust where new stars and presumably new planets are being born, so Kepler, if the plan goes through, will be able to study stars and planets in a variety of environments.
The idea is still being tested; a decision will come next spring. Regardless , the Kepler team has three more years to analyze the data already obtained.
Mr. Sobeck is hopeful for more. “Nobody wants to turn off a spacecraft in orbit,” he said.
中譯
這艘小型太空船可能氣數還沒有盡。
今年春天，美國太空總署凱卜勒太空船一個很重要的輪子卡住了，以致它的望遠鏡失了準頭，無法繼續搜尋銀河一處多恆星帶內與地球類似的行星。
然而凱卜勒任務的經理們表示，他們的新計畫或許可以讓它繼續搜尋這些太陽系以外的行星三、四年。
過去四年，凱卜勒太空船藉由觀察行星通過恆星前方時恆星變暗的過程，辨識出3500顆可能的太陽系外行星。它還讓我們得以首次粗估銀河當中適合生命存活的行星多到什麼程度：像太陽這樣的恆星大約五分之一可能有大小與地球相當，而且可能適合生命存活的行星。這意味，外星生命很可能存在。
然而凱卜勒太空船才剛開始鎖定軌道類似地球的行星。這些大小與地球相當的世界環繞類似太陽的恆星需時一年，是行話所說地球2.0的候選者。
新方法使凱卜勒不只對準一組恆星。它將在太空中來回跳躍，在一個定點持續觀察天體80天。它不會發現公轉周期超過數周的行星，卻會偵知在適合生命存活軌道上環繞較小、較暗的紅矮星運行的行星。
凱卜勒2009年發射時有四個反應輪，失去第二個輪子時問題來了，視線很容易搖晃。靈巧的火箭科學解決了這個問題。測試顯示可用陽光穩定凱卜勒。關鍵是使它維持定向，讓太陽能板平均受光。凱卜勒計畫副經理索貝克說：「當它穩定時還真穩。」在這種情況下，凱卜勒的行星搜索將限於黃道沿線的恆星。這也是太陽通過黃道帶的明顯路徑。
太陽的部分途徑通過銀河正中央的射手座，遠離銀河的其他部分則越過外圍的銀河。凱卜勒可以在這裡觀察並記錄超級新星爆炸的起落。這攸關我們對宇宙歷史的理解。其他可能潛藏機會的地帶包括散落氣體與灰塵雲的風暴區，這兒新恆星正在誕生，理論上也可能產生新行星。如果一切按計畫進行，凱卜勒將可在多種環境下觀測恆星與行星。
這個構想還在測試，將於明年春季決定。儘管如此，凱卜勒小組還有三年時間可以分析已經蒐集的資料。
索貝克對未來抱著希望。他說：「沒有人會想關掉軌道上的太空船。」
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