A green comet will appear again in Earth’s sky for the first time in 50,000 years. The American space agency NASA says the comet's last visit came during Neanderthal times. It will come within 42 million kilometers of Earth on Wednesday before continuing on its path, unlikely to return for millions of years. Discovered less than a year ago, this green comet can be observed in the northern night sky with devices such as binoculars and telescopes. It is also possible to see with one’s own eyes in the darkest corners of the Northern Hemisphere.

一颗绿色彗星将在 5 万年来首次出现在地球天空中。美国宇航局表示，这颗彗星最后一次造访是在尼安德特人时代。它将在周三到达距地球 4200 万公里以内，然后继续前进，数百万年内不太可能返回。这颗绿色彗星被发现不到一年前，可以用双筒望远镜和天文望远镜等设备在北方夜空中观测到。在北半球最黑暗的角落也可以亲眼看到。

The comet will be best seen in the early morning hours. It is expected to brighten as it comes closer and rises higher over the horizon through the end of January. By February 10, the comet will be near Mars. Viewers in the Southern Hemisphere will have to wait until next month to see it. Paul Chodas is director of the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory in California.

这颗彗星最好在清晨看到。到 1 月底，随着它越来越近并在地平线上上升得更高，预计它会变亮。到 2 月 10 日，这颗彗星将靠近火星。南半球的观众要等到下个月才能看到。保罗·乔达斯 (Paul Chodas) 是加利福尼亚州美国宇航局喷气推进实验室近地天体研究中心的主任。

Chodas said that while several comets have appeared in the sky over the past year, “this one seems probably a little bit bigger and therefore a little bit brighter and it’s coming a little bit closer to the Earth’s orbit." The comet was discovered last March by researchers using the Zwicky Transient Facility, a special camera at Caltech’s Palomar Observatory. That explains its official, difficult name: comet C/2022 E3 (ZTF). On Wednesday, the comet will pass between the orbits of Earth and Mars at a relative speed of 207,000 kilometers. Its nucleus, or center, is thought to be about 1.6 kilometers across, with its tails extending millions of kilometers.

Chodas 说，虽然过去一年天空中出现了几颗彗星，但“这颗彗星看起来可能更大一点，因此也更亮一点，而且它离地球轨道也更近了一点。”这颗彗星于去年三月被发现由研究人员使用加州理工学院帕洛玛天文台的特殊相机 Zwicky Transient Facility 拍摄。这解释了它的官方名称：彗星 C/2022 E3 (ZTF)。周三，这颗彗星将在地球和火星轨道之间以相对速度为 207,000 公里。它的核或中心被认为大约有 1.6 公里宽，其尾部延伸数百万公里。

The comet is not expected to be nearly as bright as Neowise in 2020, or Hale-Bopp in the mid to late 1990s. Karen Meech of the University of Hawaii said in an email to the Associated Press that the comet “will be bright by virtue of its close Earth passage ... which allows scientists to do more experiments and the public to be able to see a beautiful comet.” Scientists are confident in their calculations, putting the comet’s last pass through the solar system at 50,000 years ago. However, they do not know how close it came to Earth or whether it was even visible to the Neanderthals, said Chodas.

预计这颗彗星不会像 2020 年的 Neowise 或 1990 年代中后期的海尔波普那样明亮。夏威夷大学的卡伦·米奇在给美联社的一封电子邮件中说，这颗彗星“将因其接近地球的通道而变得明亮……这让科学家们可以做更多的实验，让公众能够看到一颗美丽的彗星”科学家们对他们的计算充满信心，认为这颗彗星最后一次穿过太阳系是在 5 万年前。然而，他们不知道它离地球有多近，也不知道尼安德特人是否能看到它，Chodas 说。

When the comet returns, though, is more difficult to estimate. Every time the comet passes the sun and planets, their gravitational pulls change the comet’s path by small amounts. However, these small effects lead to major path changes over time. There is another source of measurement difficulty. Large amounts of dust and gas come off the comet as it heats up near the sun. “We don’t really know exactly how much they are pushing this comet around,” Chodas explained.

然而，彗星返回的时间更难估计。每次彗星经过太阳和行星时，它们的引力都会稍微改变彗星的路径。然而，随着时间的推移，这些小的影响会导致主要的路径变化。还有另一个测量困难的来源。当彗星在太阳附近升温时，大量的尘埃和气体从彗星上脱落。 “我们真的不知道他们到底在多大程度上推动了这颗彗星，”乔达斯解释道。

The comet — an ancient object produced from the emerging solar system 4.5 billion years ago — came from what is known as the Oort Cloud well beyond Pluto. This area is believed to extend more than one-quarter of the way to the next star. While comet ZTF began in our solar system, we cannot be sure it will stay there, Chodas said. If it gets sent out of the solar system, it will never return, he added. However, you should not worry if you miss it. “In the comet business, you just wait for the next one because there are dozens of these,” Chodas said. “And the next one might be bigger, might be brighter, might be closer.”

这颗彗星是 45 亿年前新兴太阳系产生的古老物体，它来自远在冥王星之外的所谓奥尔特云。据信，该区域延伸到下一颗恒星的路径超过四分之一。 Chodas 说，虽然 ZTF 彗星开始出现在我们的太阳系中，但我们不能确定它会留在那里。他补充说，如果它被送出太阳系，它将永远不会返回。但是，如果您错过了，也不用担心。 “在彗星业务中，你只需等待下一个，因为有几十个这样的彗星，”Chodas 说。 “下一个可能更大，可能更亮，可能更近。”