Astronomers using NASA’s Hubble Space Telescope have found evidence that the spinning of a small comet slowed and then reversed its direction of rotation, offering a dramatic example of how volatile activity can affect the spin and physical evolution of small bodies in the solar system. This is the first time researchers have observed evidence of a comet reversing its spin.
天文学家使用美国宇航局的哈勃太空望远镜发现了证据，表明一颗小彗星的旋转速度减慢，然后反转了其旋转方向，这提供了一个戏剧性的例子，说明不稳定的活动如何影响太阳系中小天体的旋转和物理演化。这是研究人员第一次观察到彗星反转的证据。
The object, comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, likely originated in the Kuiper Belt, and was flung into its current trajectory by Jupiter’s gravity, now visiting the inner solar system every 5.4 years.
该物体名为 41P/Tuttle-Giacobini-Kresák 彗星，简称 41P，可能起源于柯伊伯带，并被木星引力抛入当前轨道，现在每 5.4 年访问一次内太阳系。
After its 2017 close passage around the Sun, scientists found that comet 41P experienced a dramatic slowdown in its rotation. Data from NASA’s Neil Gehrels Swift Observatory in May 2017 showed the object was spinning three times more slowly than it had in March 2017 when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.
2017 年近距离绕太阳运行后，科学家发现 41P 彗星的自转速度急剧减慢。美国宇航局尼尔·格尔尔斯·斯威夫特天文台 2017 年 5 月的数据显示，该物体的旋转速度比 2017 年 3 月亚利桑那州洛厄尔天文台探索频道望远镜观测到的速度慢了三倍。
A new analysis of follow-up Hubble observations has shown the spin of this comet took an even more unusual turn.
对哈勃后续观测的新分析表明，这颗彗星的自转发生了更加不寻常的转变。
Hubble images from December 2017 detected the comet spinning much faster again, with a period of approximately 14 hours, compared to the 46 to 60 hours measured by Swift. The simplest explanation, researchers say, is that the comet continued slowing until it almost stopped, and was then forced to spin in the near-opposite direction by outgassing jets on its surface.
2017 年 12 月的哈勃图像发现，这颗彗星的自转速度再次加快，周期约为 14 小时，而 Swift 测量的周期为 46 至 60 小时。研究人员说，最简单的解释是，彗星继续减速，直到几乎停止，然后由于其表面的排气喷流而被迫沿几乎相反的方向旋转。
The science paper detailing this finding published Thursday in The Astronomical Journal.
周四在《天文学杂志》上发表的科学论文详细介绍了这一发现。
Hubble also constrains the size of the comet’s nucleus, measuring it at around 0.6 miles across (about a kilometer), or about three times the height of the Eiffel Tower.
哈勃望远镜还限制了彗星核的大小，测量其直径约为 0.6 英里（约一公里），大约是埃菲尔铁塔高度的三倍。
This is especially small for a comet, making it easy to torque, or twist.
这对于彗星来说尤其小，因此很容易扭转。
As a comet approaches the Sun, heat causes frozen ices to sublimate, venting material into space.
当彗星接近太阳时，热量导致冰块升华，将物质排放到太空中。
“Jets of gas streaming off the surface can act like small thrusters,” said paper author David Jewitt of the University of California at Los Angeles. “If those jets are unevenly distributed, they can dramatically change how a comet, especially a small one, rotates.”
加州大学洛杉矶分校的论文作者 David Jewitt 表示：“从表面喷出的气体喷射流可以起到小型推进器的作用。” “如果这些喷流分布不均匀，它们可以极大地改变彗星，尤其是小彗星的旋转方式。”
The comet was originally spinning in one direction, but gas jets pushing against that motion gradually slowed it down. Because the jets kept pushing, they ultimately caused the comet to start rotating in the opposite direction.
这颗彗星最初朝一个方向旋转，但推动该运动的气体喷射逐渐减慢了它的速度。由于喷流不断推动，最终导致彗星开始沿相反方向旋转。
“It’s like pushing a merry-go-round,” said Jewitt. “If it’s turning in one direction, and then you push against that, you can slow it and reverse it.”
“这就像推旋转木马，”朱伊特说。 “如果它朝一个方向转动，然后你逆向转动，你就可以减慢它的速度并使其逆转。”
The study also shows that the comet’s overall activity has declined significantly since earlier returns. During its 2001 perihelion passage, 41P was unusually active for its size. By 2017, its gas production had decreased by roughly an order of magnitude.
研究还表明，自早期返回以来，彗星的整体活动已显着下降。在 2001 年经过近日点期间，41P 因其尺寸而异常活跃。到2017年，其天然气产量下降了大约一个数量级。
This change suggests that the comet’s surface may be evolving quickly, possibly as near-surface volatile materials become depleted or covered by insulating dust layers.
这一变化表明，彗星的表面可能正在快速演变，可能是因为近地表挥发性物质耗尽或被绝缘尘埃层覆盖。
Most changes in comet structure occur over centuries or longer. The rapid rotational shifts observed in comet 41P provide a rare opportunity to witness evolutionary processes unfolding on a human timescale.
彗星结构的大多数变化都会发生几个世纪或更长时间。在 41P 彗星中观察到的快速自转变化提供了一个难得的机会来见证人类时间尺度上展开的进化过程。
Modeling based on the measured torques and mass loss rates suggest that continued rotational changes could eventually lead to structural instability for comet 41P. If a comet spins too rapidly, centrifugal forces can overcome its weak gravity and strength, potentially causing fragmentation or even disintegration .
基于测得的扭矩和质量损失率的建模表明，持续的旋转变化最终可能导致 41P 彗星的结构不稳定。如果彗星旋转太快，离心力可以克服其微弱的重力和力量，可能导致碎片甚至解体。
“I expect this nucleus will very quickly self-destruct,” said Jewitt.
“我预计这个核将很快自毁，”朱伊特说。
Yet, comet 41P has likely occupied its present orbit for roughly 1,500 years.
然而，41P 彗星可能已经在其当前轨道上运行了大约 1,500 年。
Hubble has been collecting imaging and spectroscopic data from across the cosmos for over 35 years, and all of those observations are available in the Mikulski Archive for Space Telescopes , a central repository for data from more than a dozen astronomical missions, including Hubble.
35 年来，哈勃一直在收集整个宇宙的成像和光谱数据，所有这些观测结果都可以在米库斯基太空望远镜档案中找到，这是一个中央存储库，包含来自包括哈勃在内的十多项天文任务的数据。
Jewitt found these observations while browsing the archive, and realized they were yet-to-be analyzed.
朱伊特在浏览档案时发现了这些观察结果，并意识到它们还有待分析。
By making NASA’s science data open to all, observations made years, or even decades ago, can be revisited to answer new scientific questions. In many cases, scientists continue to make discoveries not just with new observations, but by mining the archive built over decades of space exploration.
通过向所有人开放美国宇航局的科学数据，可以重新审视几年甚至几十年前的观察结果，以回答新的科学问题。在许多情况下，科学家不仅通过新的观察，而且通过挖掘数十年太空探索建立的档案来不断取得发现。
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
哈勃太空望远镜已经运行了三十多年，并不断做出突破性的发现，塑造我们对宇宙的基本理解。哈勃望远镜是 NASA 和 ESA（欧洲航天局）之间的国际合作项目。美国宇航局位于马里兰州格林贝尔特的戈达德太空飞行中心负责管理望远镜和任务操作。总部位于丹佛的洛克希德·马丁航天公司也支持戈达德的任务运营。位于巴尔的摩的太空望远镜科学研究所由天文学研究大学协会运营，为美国宇航局进行哈勃科学操作。