美國國家航空航天局（NASA）表示，旅行者2號探索器在發射42年后進入了星際空間。同年發射的旅行者1號和旅行者2號幾乎完全相同，并在2013年率先脫離太陽磁場，原因是其路徑和旅行者2號不同，而太陽磁場呈橢圓形。兩個探測器的運行時間最初定為五年。

本次“穿越”讓NASA進一步確認了太陽形成的磁性“氣泡”，也就是日光層的界線，以及其外部為何物——一個充滿了銀河宇宙射線的空間。旅行者2號11月5日的觀測結果所體現的“內外差異”和旅行者1號五年前觀測到的情況相同。

雖然兩位“旅行者”都已脫離太陽磁場，但它們絕對沒有離開太陽系，盡管NASA偶爾會因口誤而使用這樣的表述。日光層是太陽的磁場范圍，而天文學家基本上都認為太陽引力場才等同于太陽系。

旅行者1號走出日光層時，NASA和許多天文學家都覺得它會探測到磁場的90度轉向，就像把一塊條狀磁鐵轉90度那樣。但這種情況并未出現。相反，旅行者1號和如今的旅行者2號都探測到了周圍粒子的明顯變化。

在日光層內部，起主導作用的是太陽風，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的成分，如電子、質子和阿爾法粒子。而在日光層以外，處于統治地位的是銀河宇宙射線。這些射線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。

盡管“年事已高”而且經過長途跋涉，但兩個探測器上的科學儀器仍在工作，這些儀器為人們提供了必要的數據，以辨別它們探測到的不同種類的粒子和能量。

為兩個探測器供能的發電機使用放射性元素衰變產生的熱量。由于放射性元素的質量會隨著衰變而逐漸減少，它產生的熱量及轉換的電力將越來越少，而NASA也在逐步關閉那些不那么重要的儀器和功能。在兩個探測器順利飛越木星和土星以及旅行者2號飛越天王星和海王星后，它們就不再使用照相機了。

至少在2020年之前，兩位“旅行者”應該會繼續傳回科學數據，而且有可能多工作幾年，然后它們將永遠“旅行”下去。

太陽系的范圍一直延伸到奧爾特云，其推測半徑為地球到太陽距離（即1個天文單位）的10萬倍。旅行者1號和2號目前分別距地球145和120個天文單位，要達到距地球1000個天文單位的奧爾特云內側還需要300多年，要完全穿過奧爾特云并最終脫離太陽系可能還需要3萬年。

跟在兩位“旅行者”身后的探測器是新地平線號，它傳回的冥王星及其衛星卡戎的照片及數據前所未有。目前新地平線號正在奔向柯伊伯帶中的一個目標，后者距地球僅有40億英里，或40多個天文單位。（財富中文網）

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.