发射42年后，旅行者2号进入星际空间

美国国家航空航天局（NASA）表示，旅行者2号探索器在发射42年后进入了星际空间。同年发射的旅行者1号和旅行者2号几乎完全相同，并在2013年率先脱离太阳磁场，原因是其路径和旅行者2号不同，而太阳磁场呈椭圆形。两个探测器的运行时间最初定为五年。

本次“穿越”让NASA进一步确认了太阳形成的磁性“气泡”，也就是日光层的界线，以及其外部为何物——一个充满了银河宇宙射线的空间。旅行者2号11月5日的观测结果所体现的“内外差异”和旅行者1号五年前观测到的情况相同。

虽然两位“旅行者”都已脱离太阳磁场，但它们绝对没有离开太阳系，尽管NASA偶尔会因口误而使用这样的表述。日光层是太阳的磁场范围，而天文学家基本上都认为太阳引力场才等同于太阳系。

旅行者1号走出日光层时，NASA和许多天文学家都觉得它会探测到磁场的90度转向，就像把一块条状磁铁转90度那样。但这种情况并未出现。相反，旅行者1号和如今的旅行者2号都探测到了周围粒子的明显变化。

在日光层内部，起主导作用的是太阳风，也就是从日冕射向宇宙的高能等离子体流，其中包含从原子中分解出来的成分，如电子、质子和阿尔法粒子。而在日光层以外，处于统治地位的是银河宇宙射线。这些射线由完整的原子核组成，所有的电子已被甩脱而且加速到了接近光速的状态。

尽管“年事已高”而且经过长途跋涉，但两个探测器上的科学仪器仍在工作，这些仪器为人们提供了必要的数据，以辨别它们探测到的不同种类的粒子和能量。

为两个探测器供能的发电机使用放射性元素衰变产生的热量。由于放射性元素的质量会随着衰变而逐渐减少，它产生的热量及转换的电力将越来越少，而NASA也在逐步关闭那些不那么重要的仪器和功能。在两个探测器顺利飞越木星和土星以及旅行者2号飞越天王星和海王星后，它们就不再使用照相机了。

至少在2020年之前，两位“旅行者”应该会继续传回科学数据，而且有可能多工作几年，然后它们将永远“旅行”下去。

太阳系的范围一直延伸到奥尔特云，其推测半径为地球到太阳距离（即1个天文单位）的10万倍。旅行者1号和2号目前分别距地球145和120个天文单位，要达到距地球1000个天文单位的奥尔特云内侧还需要300多年，要完全穿过奥尔特云并最终脱离太阳系可能还需要3万年。

跟在两位“旅行者”身后的探测器是新地平线号，它传回的冥王星及其卫星卡戎的照片及数据前所未有。目前新地平线号正在奔向柯伊伯带中的一个目标，后者距地球仅有40亿英里，或40多个天文单位。（财富中文网）

译者：Charlie

审校：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.