有颗小行星可能要撞地球了？这并非谣言，但事实可能没有你想象的那么可怕。
这颗名为2024 YR4的小行星直径约40~90米，在2024年12月27日被“小行星撞击地球警报系统站
（ATLAS）
发现。2024 YR4属于阿波罗型近地天体
（Apollo-type Near Earth Object）
，也就是说它的轨道与地球轨道相交，对生活在地球上的我们而言，它是一个来自太空的潜在威胁。
ATLAS由美国夏威夷大学
（University of Hawaii）
与美国航空航天局
（NASA）
联合建设，目前共有四个天文台（夏威夷两个，智利、南非各一个）参与数据收集。大多数情况下，观测到的小行星对于人类而言是已知且被编目的，仅需快速与数据库中的轨道数据进行拟合，就能知道它是哪颗星星。望远镜系统也一直在发现新的天体，不过大多数都被证实对地球毫无威胁，只是正好“路过”太阳系。然而，偶尔还是会出现一些令人担忧的天体，比如2024 YR4。
被发现时，2024 YR4与地球相距828,800千米，约为地月距离的两倍，且仍在靠近地球，人们由此得以观察到它的轨道。经过两天的初步追踪，研究人员认为，2024 YR4将在2025年4月初离开望远镜的观测范围，等到2028年6月才能被再次观测到。2028年12月17日，它会再次接近地球；到2032年12月22日，当这颗小行星第三次接近地球之时，我们和它之间的距离会变得格外近，这也是最有可能发生撞击的时间节点。
这颗具有潜在危险的小行星撞击地球的可能性实际上非常低，为何还会引起如此大的轰动？原因是在现行的国际标准中，小行星撞击地球的几率达到或超过1%就已经是件大事了。官方阈值规定，如果某个天体的直径在50米以上，撞击地球的概率大于1%，则可能会对地球造成重大损害。
2月11日，欧洲空间局
（ESA）
曾估计，这块正在太空中漂移的石头在2032年12月22日与地球相撞的几率约为2%。2月中旬，NASA的最新数据显示这一概率已达2%~2.3%；2月18日，几率又上升到了3.1%（有史以来最大的此类威胁）；2月19日晚间，经过持续监测和计算，天文学家将该天体的撞击风险下调至1.5%，但仍然值得担忧。直到2月20日，NASA在社交媒体上更新了小行星2024 YR4的撞击风险评估，将撞击概率大幅降低至约0.28%，这意味着小行星撞击威胁已从“需要高度关注”降低至“撞击几乎不可能发生，公众无需担忧”。
科学家指出，由于观测时间较短，对小天体的轨道计算精度较低，只有当观测结果累积到一定程度，对小天体的轨道以及撞击概率的推算才可能更准确。比如等到2028年，当它再次接近地球，那时我们会得到一个比现在准得多的结果，对于撞击概率的判断也要准很多。目前，世界各地的天文学家和天文望远镜仍在密切关注2024 YR4的动向。
小行星撞击地球预警系统观测到的天体，多数无需为之感到担忧。人们上次发现撞击概率超过1%的小行星还是在20年前，2004年6月19日，一颗名为Apophis（埃及神话中的黑暗、混乱和毁灭之神）的小行星占据各大报纸的头版。这颗小行星被位于美国亚利桑那州的基特峰国家天文台
（Kitt Peak National Observatory）
发现，一度被认为是对地球构成重大威胁的小行星。
最初，科学家计算出Apophis在2036年撞击地球的可能性比其他已知小行星要高得多，约为2.7%，这引起了广泛关注。当时预计，它将在2029年4月13日飞掠地球，与地球的距离可能比一些地球同步卫星还要近。后续经过更精确的观测和轨道计算，科学家们现在已经排除了Apophis在可预见的未来（至少一百年）撞击地球的可能性。通常情况下，小行星撞击地球的概率波动大致遵循的也是这一模式：刚发现时会有一些波动和震荡，接下来，随着科学家进一步优化小行星的运行轨道，撞击概率将呈断崖式下跌，最后得出结论：地球还是安全的。
正如前文所述，到2028年，2024 YR4再度接近地球时，天文学家基本上就可以确定地球会不会受到它的影响。在此之前，科学家将尝试进一步计算它的轨道和撞击地球的可能性，不过如果到2028年，科学家发现它仍然有很大的机会在2032年撞击地球，灾难即将来临，彼时才开始做出响应可能已经太晚了。
因此，有些讨论认为，如果在未来几个月内还无法准确掌握2024 YR4的轨道，我们可能必须先发制人地采取行动和计划，做好万全的准备，以便一旦需要就能迅速行动起来。2月初，联合国下属官方机构空间任务规划咨询组
（Space Mission Planning Advisory Group）
召开了一次会议，首要议题正是2024 YR4。该机构专门负责协调国际社会对所有直径大于50米、在未来50年内撞击地球的概率大于1%的小行星的应对措施。但由于情况比较微妙，专家组决定先观察等待，并在今年四月底或五月初再次开展评估，根据届时情况协助协调应对措施。
合理的应对措施不应只是非常“温柔”地推动它偏离轨道。把小行星的一侧涂成白色，通过改变它的反射率来微调它的运行轨迹；或是发送一艘飞船绕着它运行，用引力拖拽效应轻轻地把它推开，这些方式都不可取，因为这么做至少需要10年时间。我们必须做一些更“暴力”的事：利用动能撞击器把它撞偏，改变它的轨道，就像NASA 2022年在双小行星重定向测试（DART，NASA执行的首次行星防御任务，也是人类历史上首次尝试改变小行星轨道的实验，该任务成功改变了小行星Dimorphos的轨道周期）中对另一颗小行星所做的那样；或者对它进行核打击
……
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An Asteroid May Hit Earth in 2032. What Can We Do about It?
Rachel Feltman:
For
Scientific American
’s
Science Quickly
,
I’m
Rachel Feltman.
Have you heard that an asteroid might—just maybe—smack into Earth sometime in the next few years? The rumors are true, though perhaps not as frightening as you might think. The fact is that this asteroid, called 2024 YR4, is both literally and figuratively a moving target. As of February 11, the European Space Agency estimated that the space rock has a 2 percent chance of colliding with Earth on December 22, 2032. That’s slightly higher than the risk you’ll hear quoted in the episode that follows because we recorded it last week. Why are those numbers changing so quickly? We promise this isn’t a matter of the risk going higher and higher as time goes on. It’s a lot more complicated than that.
Here to explain what’s going on with this potentially hazardous asteroid is Lee Billings, a senior editor covering space and physics for
Scientific American
.
Lee, thanks so much for coming on to chat.
Lee Billings
: It’s my pleasure, as always, Rachel.
Feltman
: So there’s an asteroid with a very low chance of hitting us. Why did this make such a big splash in the news?
Billings
: So it made such a big splash in the news because, apparently, a 1 percent or greater chance of being struck by an asteroid is actually a big deal. And that’s what this thing is. It’s, it’s called 2024 YR4—really rolls off the tongue. It was discovered on December 27 by astronomers using the Asteroid Terrestrial-Impact Last Alert Sy stem, or ATLAS, which is a telescope in Chile. W hen it was roughly about two times farther out than our moon, it was zooming right by our planet, and folks were able to look at its orbit, which they’re still piecing together, and determined that it was gonna make another especially close pass on December 22, 2032.
Feltman
: Mm.
Billings
: And at that time it had about a 1.3 percent chance of striking our planet.
Feltman
: Got it. So how did we detect this? You mentioned ATLAS, but tell me more about what that telescope does and whose job it is to keep an eye out for these asteroids.
Billings
: Last I counted there’s somewhere between a half dozen and a dozen wide-field survey telescopes that look at the sky every night, and what they’re looking for is moving objects. What do we mean by moving? Everything’s moving. We’re looking for objects that seem to move against the background of the more distant stars, which don’t seem to move ’cause they’re so far away. You can do that by comparing images to each other over time—image A, image B; image A, image B—and you can see a difference, an offset in, in some little point of light. And that’ll tell you: that thing’s moving.
Most of the time, when they see these things, they know what they are; they’re already cataloged. They can quickly refine their orbit, figure out what it is—“Oh, it’s this thing in the catalog, sure.” But they discover new objects all the time, and most of them prove to be totally harmless, just whizzing by through the solar system.
Every now and then, however, one looks a little worrisome. The official threshold for being worrisome is if it’s somewhere between 50 meters [about 164 feet] or greater in size, ’cause then it could cause significant damage to Earth if it struck us, and if it’s greater than about a 1 percent chance.
And I keep using some caveats here about how certain we are about this thing striking us because this is not a settled story. This is still so fresh that we don’t actually have this object’s orbit totally refined. We’re still getting more information about it. We’re still observing this thing and trying to figure out what exactly it’s going to be doing, where exactly it’s going in space.
Late last month, when our story about this published, the odds of it striking were about 1.6 percent. And then, in the following couple of days, they rose to 1.7 percent. About three or four days after the story dropped the odds have gone back down to 1.4 percent [laughs]. So, you can see, it’s very active and fluid, but the upshot essentially is that if it stays above 1 percent, then folks should be worried about it.
Feltman
: As you said, we see lots of objects that we never end up having to worry about. How unusual is it to get something above that 1 percent threshold?
Billings
: The last time this happened was about 20 years ago ...
Feltman
: Okay!
Billings
: With an asteroid that you might have heard about called Apophis, right? And Apophis, for a while, also had a greater than 1 percent chance of striking Earth, and it made big headlines, and people got really worried about it—I think it helped spark Hollywood movies and various space missions ...
Feltman
: Mm.
Billings
: To try to deflect asteroids—so it doesn’t happen that often; that’s one reason why it’s newsworthy. But I just wanna emphasize, of course, that typically what does happen with these things is: you’ll see this initial oscillation in the chances of it striking Earth, and then they just fall off a cliff because we refine the orbit, we realize that we’re safe, and that’s that.
Feltman
: Sure.
Billings
