美國(guó)國(guó)家航空航天局（NASA）表示，旅行者2號(hào)探索器在發(fā)射42年后進(jìn)入了星際空間。同年發(fā)射的旅行者1號(hào)和旅行者2號(hào)幾乎完全相同，并在2013年率先脫離太陽(yáng)磁場(chǎng)，原因是其路徑和旅行者2號(hào)不同，而太陽(yáng)磁場(chǎng)呈橢圓形。兩個(gè)探測(cè)器的運(yùn)行時(shí)間最初定為五年。

本次“穿越”讓NASA進(jìn)一步確認(rèn)了太陽(yáng)形成的磁性“氣泡”，也就是日光層的界線(xiàn)，以及其外部為何物——一個(gè)充滿(mǎn)了銀河宇宙射線(xiàn)的空間。旅行者2號(hào)11月5日的觀測(cè)結(jié)果所體現(xiàn)的“內(nèi)外差異”和旅行者1號(hào)五年前觀測(cè)到的情況相同。

雖然兩位“旅行者”都已脫離太陽(yáng)磁場(chǎng)，但它們絕對(duì)沒(méi)有離開(kāi)太陽(yáng)系，盡管NASA偶爾會(huì)因口誤而使用這樣的表述。日光層是太陽(yáng)的磁場(chǎng)范圍，而天文學(xué)家基本上都認(rèn)為太陽(yáng)引力場(chǎng)才等同于太陽(yáng)系。

旅行者1號(hào)走出日光層時(shí)，NASA和許多天文學(xué)家都覺(jué)得它會(huì)探測(cè)到磁場(chǎng)的90度轉(zhuǎn)向，就像把一塊條狀磁鐵轉(zhuǎn)90度那樣。但這種情況并未出現(xiàn)。相反，旅行者1號(hào)和如今的旅行者2號(hào)都探測(cè)到了周?chē)Ｗ拥拿黠@變化。

在日光層內(nèi)部，起主導(dǎo)作用的是太陽(yáng)風(fēng)，也就是從日冕射向宇宙的高能等離子體流，其中包含從原子中分解出來(lái)的成分，如電子、質(zhì)子和阿爾法粒子。而在日光層以外，處于統(tǒng)治地位的是銀河宇宙射線(xiàn)。這些射線(xiàn)由完整的原子核組成，所有的電子已被甩脫而且加速到了接近光速的狀態(tài)。

盡管“年事已高”而且經(jīng)過(guò)長(zhǎng)途跋涉，但兩個(gè)探測(cè)器上的科學(xué)儀器仍在工作，這些儀器為人們提供了必要的數(shù)據(jù)，以辨別它們探測(cè)到的不同種類(lèi)的粒子和能量。

為兩個(gè)探測(cè)器供能的發(fā)電機(jī)使用放射性元素衰變產(chǎn)生的熱量。由于放射性元素的質(zhì)量會(huì)隨著衰變而逐漸減少，它產(chǎn)生的熱量及轉(zhuǎn)換的電力將越來(lái)越少，而NASA也在逐步關(guān)閉那些不那么重要的儀器和功能。在兩個(gè)探測(cè)器順利飛越木星和土星以及旅行者2號(hào)飛越天王星和海王星后，它們就不再使用照相機(jī)了。

至少在2020年之前，兩位“旅行者”應(yīng)該會(huì)繼續(xù)傳回科學(xué)數(shù)據(jù)，而且有可能多工作幾年，然后它們將永遠(yuǎn)“旅行”下去。

太陽(yáng)系的范圍一直延伸到奧爾特云，其推測(cè)半徑為地球到太陽(yáng)距離（即1個(gè)天文單位）的10萬(wàn)倍。旅行者1號(hào)和2號(hào)目前分別距地球145和120個(gè)天文單位，要達(dá)到距地球1000個(gè)天文單位的奧爾特云內(nèi)側(cè)還需要300多年，要完全穿過(guò)奧爾特云并最終脫離太陽(yáng)系可能還需要3萬(wàn)年。

跟在兩位“旅行者”身后的探測(cè)器是新地平線(xiàn)號(hào)，它傳回的冥王星及其衛(wèi)星卡戎的照片及數(shù)據(jù)前所未有。目前新地平線(xiàn)號(hào)正在奔向柯伊伯帶中的一個(gè)目標(biāo)，后者距地球僅有40億英里，或40多個(gè)天文單位。（財(cái)富中文網(wǎng)）

譯者：Charlie

審校：夏林

The Voyager 2 space probe has entered interstellar space, according to NASA, 42 years after it and the Voyager 1, its nearly identical sibling, were launched. Voyager 1 first passed across the edge of the magnetic bubble generated by the sun in 2013, as it took a different path, and the bubble is elliptical. The probes’ original missions were slated for five years.

This latest transition provided NASA with more confirmation about the separation between the interior of the magnetic bubble, called the heliosphere, and what lies outside it: a medium full of galactic cosmic rays. Measurements taken by Voyager 2 on Nov. 5 showed the same pattern of differences measured five years ago by Voyager 1.

While the Voyager probes have both left the sun’s magnetic pull, they decidedly have not left the solar system, even though NASA occasionally slips up and uses that term. The heliosphere defines the sun’s magnetic reach, but astronomers largely agree that the gravitational pull of the sun encompasses the solar system.

At the time Voyager 1 left the heliosphere, NASA and many astronomers believed that it would measure a 90-degree rotation in the orientation of detected magnetic fields, like rotating a bar magnet a quarter turn. However, that magnetic effect didn’t appear. Instead, Voyager 1 and now 2 found a distinct difference in the kinds of particles detected.

Inside the heliosphere, the solar wind prevails, a high-energy plasma that streams from the sun’s corona into space, and which is made up of stripped bits of atoms, like electrons, protons, and alpha particles. But outside the bubble, galactic cosmic rays predominate. These rays comprise full atomic nuclei, the core of atoms, with all the electrons shorn away and accelerated to nearly the speed of light.

Despite their advanced age and long space journeys, the probes have active scientific instruments, which provided the data necessary to detect differences in the kinds of particles and energy detected.

The ships’ power comes from generators that produce heat from the decay of radioactive elements. Because the decay reduces the amount of material over time, ever less heat and power is generated, and NASA has gradually shut down less critical instruments and functions. Cameras stop being used after successful flybys of Jupiter and Saturn by both probes, and of Uranus and Neptune by Voyager 2.

Voyager 1 and 2 should continue to provide scientific data through at least 2020, and perhaps for a few years thereafter, at which point it will continue to travel indefinitely.

The solar system extends to the Oort Cloud, a ring of objects that may stretch as far as 100,000 times the distance of the Earth to the sun (a measure known as an astronomical unit or AU). Voyager 1 and 2, currently at 145 and 120 AU from Earth, respectively, will take about 300 more years before the probes reach the anticipated closest edge of the cloud at 1,000 AU, and potentially 30,000 years before they pass beyond it entirely and finally exit the solar system.

The next nearest active craft is New Horizons, which delivered unprecedented photos and data about Pluto and its moon, Charon, and is en route to an object in the Kuiper Belt, a mere four billion miles or 40-odd AU from Earth.