2024 YR4真的會撞地球嗎 @Unsplash
有顆小行星可能要撞地球了？這並非謠言，但事實可能沒有你想象的那麼可怕。
這顆名為2024 YR4的小行星直徑約40~90米，在2024年12月27日被“小行星撞擊地球警報系統站（ATLAS）發現。2024 YR4屬於阿波羅型近地天體（Apollo-type Near Earth Object），也就是說它的軌道與地球軌道相交，對生活在地球上的我們而言，它是一個來自太空的潛在威脅。
ATLAS由美國夏威夷大學（University of Hawaii）與美國航空航天局（NASA）聯合建設，目前共有四個天文臺（夏威夷兩個，智利、南非各一個）參與資料收集。大多數情況下，觀測到的小行星對於人類而言是已知且被編目的，僅需快速與資料庫中的軌道資料進行擬合，就能知道它是哪顆星星。望遠鏡系統也一直在發現新的天體，不過大多數都被證實對地球毫無威脅，只是正好“路過”太陽系。然而，偶爾還是會出現一些令人擔憂的天體，比如2024 YR4。
被發現時，2024 YR4與地球相距828,800千米，約為地月距離的兩倍，且仍在靠近地球，人們由此得以觀察到它的軌道。經過兩天的初步追蹤，研究人員認為，2024 YR4將在2025年4月初離開望遠鏡的觀測範圍，等到2028年6月才能被再次觀測到。2028年12月17日，它會再次接近地球；到2032年12月22日，當這顆小行星第三次接近地球之時，我們和它之間的距離會變得格外近，這也是最有可能發生撞擊的時間節點。
這顆具有潛在危險的小行星撞擊地球的可能性實際上非常低，為何還會引起如此大的轟動？原因是在現行的國際標準中，小行星撞擊地球的機率達到或超過1%就已經是件大事了。官方閾值規定，如果某個天體的直徑在50米以上，撞擊地球的機率大於1%，則可能會對地球造成重大損害。
2月11日，歐洲空間局（ESA）曾估計，這塊正在太空中漂移的石頭在2032年12月22日與地球相撞的機率約為2%。2月中旬，NASA的最新資料顯示這一機率已達2%~2.3%；2月18日，機率又上升到了3.1%（有史以來最大的此類威脅）；2月19日晚間，經過持續監測和計算，天文學家將該天體的撞擊風險下調至1.5%，但仍然值得擔憂。直到2月20日，NASA在社交媒體上更新了小行星2024 YR4的撞擊風險評估，將撞擊機率大幅降低至約0.28%，這意味著小行星撞擊威脅已從“需要高度關注”降低至“撞擊幾乎不可能發生，公眾無需擔憂”。
科學家指出，由於觀測時間較短，對小天體的軌道計算精度較低，只有當觀測結果累積到一定程度，對小天體的軌道以及撞擊機率的推算才可能更準確。比如等到2028年，當它再次接近地球，那時我們會得到一個比現在準得多的結果，對於撞擊機率的判斷也要準很多。目前，世界各地的天文學家和天文望遠鏡仍在密切關注2024 YR4的動向。
小行星撞擊地球預警系統觀測到的天體，多數無需為之感到擔憂。人們上次發現撞擊機率超過1%的小行星還是在20年前，2004年6月19日，一顆名為Apophis（埃及神話中的黑暗、混亂和毀滅之神）的小行星佔據各大報紙的頭版。這顆小行星被位於美國亞利桑那州的基特峰國家天文臺（Kitt Peak National Observatory）發現，一度被認為是對地球構成重大威脅的小行星。
最初，科學家計算出Apophis在2036年撞擊地球的可能性比其他已知小行星要高得多，約為2.7%，這引起了廣泛關注。當時預計，它將在2029年4月13日飛掠地球，與地球的距離可能比一些地球同步衛星還要近。後續經過更精確的觀測和軌道計算，科學家們現在已經排除了Apophis在可預見的未來（至少一百年）撞擊地球的可能性。通常情況下，小行星撞擊地球的機率波動大致遵循的也是這一模式：剛發現時會有一些波動和震盪，接下來，隨著科學家進一步最佳化小行星的執行軌道，撞擊機率將呈斷崖式下跌，最後得出結論：地球還是安全的。
正如前文所述，到2028年，2024 YR4再度接近地球時，天文學家基本上就可以確定地球會不會受到它的影響。在此之前，科學家將嘗試進一步計算它的軌道和撞擊地球的可能性，不過如果到2028年，科學家發現它仍然有很大的機會在2032年撞擊地球，災難即將來臨，彼時才開始做出響應可能已經太晚了。
因此，有些討論認為，如果在未來幾個月內還無法準確掌握2024 YR4的軌道，我們可能必須先發制人地採取行動和計劃，做好萬全的準備，以便一旦需要就能迅速行動起來。2月初，聯合國下屬官方機構空間任務規劃諮詢組（Space Mission Planning Advisory Group）召開了一次會議，首要議題正是2024 YR4。該機構專門負責協調國際社會對所有直徑大於50米、在未來50年內撞擊地球的機率大於1%的小行星的應對措施。但由於情況比較微妙，專家組決定先觀察等待，並在今年四月底或五月初再次開展評估，根據屆時情況協助協調應對措施。
合理的應對措施不應只是非常“溫柔”地推動它偏離軌道。把小行星的一側塗成白色，透過改變它的反射率來微調它的執行軌跡；或是傳送一艘飛船繞著它執行，用引力拖拽效應輕輕地把它推開，這些方式都不可取，因為這麼做至少需要10年時間。我們必須做一些更“暴力”的事：利用動能撞擊器把它撞偏，改變它的軌道，就像NASA 2022年在雙小行星重定向測試（DART，NASA執行的首次行星防禦任務，也是人類歷史上首次嘗試改變小行星軌道的實驗，該任務成功改變了小行星Dimorphos的軌道週期）中對另一顆小行星所做的那樣；或者對它進行核打擊……[檢視全文]
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: This one’s a little special, though. T he reason why it’s special has to do with what we already know about its orbit and how long it takes to prepare any adequate response to try to prevent disaster.
Essentially, this thing is going to come back to Earth in 2028. We know we’re safe from it then. It’s already headed away from Earth. It’s whizzing away from us in the solar system. It’s going to be too faint to see with telescopes by late April or early May. So we have this window of time in which we can try to pin down the chances of it striking Earth. If we don’t pin it down, there’s really not much we can do until 2028 rolls around. And if 2028 rolls around and this thing comes barreling by the Earth and we look at it and, and we can get a better gauge of its orbit then and we see that it still has the significant chance of striking Earth, that doesn’t give us much time at all …
Feltman: Mm.
Billings: Twenty thirty-two is right around the corner from there.
Feltman: Right.
Billings: So we’re left with very few options if we find that it’s actually on a collision course. So there is some discussion now that we might have to almost preemptively act and plan and just have our ducks in a row so that we can get something going very quickly if we need to, if we can’t pin this thing down in the next couple months.
Feltman: What would that kind of prep look like?
Billings: Well [laughs], that’s a really good question. Officially, there is a body called the Space Mission Planning Advisory Group; they’re associated with the UN. And as luck would have it, they met in early February. So this was already on the books, and they happened to meet, and of course, top of the agenda was this asteroid, 2024 YR4.
They are supposed to coordinate international responses to any asteroid that is greater than 50 meters that bears a greater than 1 percent chance of striking Earth within the next 50 years. And of course, this asteroid still exceeds that threshold right now. But because it’s so marginal, they decided they’re gonna just watch and wait and reevaluate in late April or early May and see where things stand then. But the idea is that they would help coordinate the response. But what, what would that response be?
There’s really only two options. You can’t do anything that just really, really, really gently nudges her out of the way—you have to be a little more violent. You can’t do something like paint one side of the asteroid white and cause its reflectivity to change, which nudges the orbit. You can’t park a spacecraft around it to just orbit around it and use a gravitational-tractor effect, is what it’s called, to gently nudge it out of the way over time. That takes at least 10 years. Instead, you have to hit it. You can either hit it with a kinetic impactor, like NASA did with the DART mission back in 2022 with a different asteroid, and change its orbit. Or you have to nuke it…[full transcript]
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