1962年7月9日“多米尼克行動”（Operation Dominic）中
“海星一號”高空核試驗爆炸的照片
圖片來源：U.S. Air Force 1352nd Photographic Group, Lookout Mountain Station – Image courtesy of US Govt. Defense Threat Reduction Agency, Public Domain
在太空中引爆核彈聽起來不是個好主意，但應該很容易避免？有些人可能不同意。美國國防部曾經警告稱俄羅斯可能會這麼做，不過並沒有任何確切證據。
如果太空中真的發生了核彈爆炸，會有哪些後果？這是一個值得討論的問題。
圍繞地球的軌道上，其實早已經用上了核能，核動力衛星使用的就是空間核反應堆，但歷史上也發生過不少“太空事故”，例如在 1978 年，一顆由核反應堆驅動的蘇聯衛星就發生了故障，碎片最終墜落在加拿大。
但對於處在繞地球軌道上並可能在軌道上爆炸的核武器，就完全是另一個問題了。
人類已經知道地球上的核爆炸是什麼樣的了。1945 年，美國在日本廣島和長崎各投下了一枚原子彈，造成 20 多萬人死亡，此後人類還進行了數百次核武器試驗。不止一部影視劇中出現過這種場景，比如《輻射》（Fallout）、《奧本海默》（Oppenheimer）和《奇愛博士》（Dr. Strangelove）。
首先，爆炸產生的火球會將一定半徑內的所有東西都蒸發殆盡。火球進而會引發衝擊波，造成毀滅性的影響，包括夷平建築物和引發火災。然後，火球和衝擊波都會讓炸彈投下的地方爆發出標誌性蘑菇雲。
從長遠來看，更令人擔憂的還是核輻射。
核輻射會持續數十年。它可以在幾分鐘內致人死地，也可以在幾十年後殺人於無形。而在太空中，這些情況幾乎不會發生，因為那裡的大氣非常稀薄，不會有火球，不會有衝擊波，也不會有蘑菇雲。所以，最終得到的就是爆炸中噴湧而出的大量輻射，這種輻射以伽馬射線和X射線的形式存在，沒有大氣的阻礙它們可以傳播得很遠。
軌道上的衛星會被最初的爆炸損壞或完全毀掉，相應的電磁脈衝則會摧毀地球和太空中一定半徑範圍內沒有保護措施的電子裝置。也許最令人擔憂的是，我們會得到一條類似於範艾倫帶（Van Allen belt）的輻射帶，並持續存在數年。範艾倫帶是一在地球周圍自然存在的高能帶電粒子區，衛星運營商會盡量避免讓衛星穿過它。
美國和蘇聯在 1958-1962 年間進行了十幾次高空核武器試驗，其中最著名或者說最臭名昭著的一次，被稱為“海星一號”（Starfish Prime）。在 1962 年 7 月 9 日當地時間晚上 11 點，一枚導彈從太平洋中部距離夏威夷不遠的約翰斯頓環礁發射，在地球表面 400 千米之上被引爆。這個高度屬於近地軌道，是今天大多數衛星所在的高度，也是國際空間站和天宮空間站所在的軌道，而這枚炸彈比美國投在廣島的炸彈要強 100 倍。
試驗前，報紙頭條曾報道過這次爆炸會“令人眼花繚亂”，人們可能會看到“極佳的景緻”，事實證明，確實如此。在太平洋看到這一現象的人將其描述為一種“極光”，但與我們常聽說的任何一種極光描述都不一樣。爆炸產生的輻射激發了上層大氣中的分子，不同的分子產生了不同顏色的光。
有報道稱，爆炸剛發生時出現了一道鮮亮的綠光，儘管當時是晚上 11 點，但天空卻像白晝一樣明亮。後來，亮綠色逐漸褪為黃色、橙色，最後穩定在怪異的紅色。它產生的電磁脈衝摧毀了瓦胡島大約 300 盞路燈，科學家們似乎並沒有真正預料到會發生的這一切，它的強度讓科學家們措手不及。它還摧毀了當時大約三分之一的在軌衛星，其中包括一顆名為“電星1號”（Telstar 1）的通訊衛星，它在爆炸試驗的第二天發射升空，受到的輻射比預期的要高 100 倍，直接被摧毀了。
美國和蘇聯在 1967 年（五年後）簽署了《外層空間條約》（Outer Space Treaty），該條約禁止將核武器——或者更廣泛地說，大規模殺傷性武器——送入軌道。
如今太空中的東西比 1962 年要多得多。在軌執行的衛星有 1 萬顆左右，其中大多數位於近地軌道，還有兩個空間站。根據爆炸的高度、強度以及位置，可以預料到很多衛星都會受損或被毀，然後變成太空垃圾，在地球周圍漫無目的地漂著，而我們目前沒有真正可以回收它們的方法。
除了無生命的衛星，更須要擔心的是太空中的人。國際空間站和中國的天宮空間站上都有人類宇航員，核武器爆炸產生的電磁脈衝可能會擾亂空間站上的重要系統，這也可能導致宇航員在一堆太空垃圾中航行，對空間站造成潛在損害。
而輻射本身，取決於爆炸發生的地點，可能會造成直接傷害。在 2010 年美國國防部的一份報告中，研究人員測試了 17 種可能的情況，最壞的情況顯示：國際空間站上的宇航員在 2~3 個小時內死亡的機率……[檢視全文]
What Happens if a Nuclear Weapon Goes Off in Space?
Rachel Feltman: This is probably going to blow your mind, but guess what: it’s a bad idea to set off a nuclear bomb in space. Shouldn’t be an issue, right? Seems like an easy thing to avoid doing.
Unfortunately it seems like some folks may disagree. The United States Department of Defense has sounded the alarm on a potential threat from Russia in the form of a hypothetical program aimed at putting a nuclear weapon into orbit. While there’s no evidence that such a device is on its way into space, let alone already up there, I think it’s safe to say we’d all rather be sure that Russia, like, definitely wasn’t going to do that.
But let’s not get ahead of ourselves—what actually happens when a nuke goes off in space? Thanks to the hubris of humankind, that’s a question we can answer from experience.
For Scientific American’s Science Quickly, I’m Rachel Feltman. Associate news editor Allison Parshall is joining me today to tell us more.
So, Allison, what are we talking about when we talk about nukes in space?
Allison Parshall: Yeah, it’s definitely worth being specific. But, like, what we’re talking [about] is specifically about nukes in orbit. I mean, there’s also the question of nuclear power in space. We power satellites with nuclear power. There’s some fun tales—and by fun, I mean distressing—from, you know, the 1960s and 1970s about attempts to power satellites with nuclear fuel.
In 1978 a Russian satellite that was powered by a nuclear reactor failed, and debris fell— scattered over Canada. So that was fun. But if we’re talking specifically about nuclear weapons positioned in orbit to potentially be exploded in orbit, that’s, like, a whole separate question.
That’s not something that we’ve necessarily had before, and it would be super bad. So let’s not do it.
Feltman: Yeah, no—100 percent agree. I think we can all safely end up on the same side of that debate. So what is the difference between a nuclear explosion on Earth and one in space?
Parshall: Yeah, I mean, we’re really familiar with what a nuclear explosion would look like on Earth, right? Like, we have the—the United States dropped bombs on Hiroshima and Nagasaki in 1945, killing over 200,000 people, and we’ve had hundreds of nuclear weapons tests since. And you’ve probably seen what this looks like in movies and TV shows—you know, Fallout, Oppenheimer, Dr. Strangelove.
You’ve got that first fireball that just kind of vaporizes everything around it within a certain radius. That fireball causes a shock wave that can be really devastating: It can level buildings. It can start firestorms. And then both of those things result in the really famous, distinctive mushroom cloud that erupts over where the bomb was dropped. And then, of course, you have the thing that is probably really concerning in the long term, which is the nuclear fallout.
You’ve got all of that radiation that lingers for decades. It can kill in minutes, or it can kill in decades. And in space almost none of this happens because we’re in somewhere with very little atmosphere, so there can'’ be a fireball, there can’t be a shock wave, and there can’t be a mushroom cloud.
Feltman: Mm-hmm.
Parshall: So what you get instead is just so much radiation spewing out from the explosion. You get that radiation in the form of gamma rays and x-rays, and those can travel really far because, like I said, there’s little to no atmosphere to interfere with them.
Feltman: Mm-hmm.
Parshall: We know that satellites in the line of sight would be damaged or taken offline entirely by that initial blast.
It would create an electromagnetic pulse that would fry unshielded electronics within a certain radius on Earth and in space. And perhaps most concerningly, we know that we would get this belt of radiation that is similar to something called a Van Allen belt.
A Van Allen belt exists naturally in space—satellite operators try to avoid having their satellites go through them because it can damage them. This artificial belt of radiation loops out from the Earth, and it can last for years.
Fetlman: Wow. Okay, so, uh, not great—something worth avoiding. So how do we know that this is what would happen if it’s so bad and worth avoiding?
Parshall: I’m glad you asked. This is a fascinating story. I did not know about this before I started reporting on it. I kinda can’t believe I didn’t know about it. But basically we tested this—by we, I mean humanity—the hubris of humanity…
Feltman: Of course we did.
Parshall: The U.S. and the Soviet Union conducted over a dozen high-altitude tests of nuclear weapons between 1958 and 1962. The most famous of them—I guess the most notorious, you could say—was called Starfish Prime. It happened on July 8, 1962, at 11 P.M. local time, relative to Hawaii. It launched on a missile from Johnston Atoll, which is in the middle of the Pacific, not too far from Hawaii.
It was detonated 250 miles above the Earth’s surface. So that’s low-Earth orbit. It’s where most satellites orbit today. It’s where the International Space Station orbits today. And this bomb was 100 times stronger than the one the U.S. dropped on Hiroshima.
Feltman: Wow. Why?
Parshall: Why? I mean, to test.
Feltman: To see what would happen—okay, sure.
Parshall: Yeah, to see what would happen. I mean, it’s relevant now. But it’s interesting—people in Hawaii knew that this was going to happen. There are newspaper headlines from before the test that advertised the, quote, “n-blast” would be “dazzling” and that people might have a “good view,” and people did have a good view.
Feltman: Wow.
Parshall: And it seems like it was dazzling. The people who saw this from the Pacific describe auroras that are just unlike anything that I’ve ever heard described. Basically the radiation from the blast excites molecules in our upper atmosphere, and depending on what those molecules are, you get different colors. And so accounts say that when the blast first happened, there was this startling flash of green that kind of lit the sky up like daylight, even though it was 11 P.M.
And that green kind of faded to yellow, to orange and then settled on this eerie red. There’s really startling pictures. I would recommend looking them up.
Also, the electromagnetic pulse it generated knocked out about 300 streetlights in Oahu. And it doesn’t seem like the scientists really expected any of this to happen.
I mean, they certainly knew some of it—this was not the first test. But it seems like the strength of it took scientists by surprise, as well as that belt of radiation that lingered for years. It also killed about a third of the satellites that were in orbit. It’s nothing compared to the number we have now.
It was maybe, like, two dozen satellites, about eight of which were killed or damaged. That included one called Telstar 1 that launched the next day after Starfish Prime. That thing got 100 times more radiation than they expected, and it was toast.
Feltman: Oh, wow.
Parshall: Yeah. And a source I spoke with—his name is Jonathan McDowell at the Center for Astrophysics | Harvard & Smithsonian—he called Starfish Prime the, quote, “poster child for why we don’t like nukes blowing up in space,” which, I think, is a fair summary.
Feltman: That seems fair.
Parshall: Yeah, and we know that the governments that were doing these kind of decided we shouldn’t be doing this anymore because both the U.S. and the U.S.S.R. signed on to the Outer Space Treaty in 1967, five years later, which forbade putting nuclear weapons—or really, more generally, weapons of mass destruction—into orbit.
Feltman: Yes. Speaking of that treaty, if I’m understanding correctly, it’s not totally clear how seriously Russia is considering putting a nuclear weapon into orbit—if they’re considering it at all.
But some government officials in the U.S. certainly were concerned enough to talk about it publicly. So what’s the actual worst-case scenario here?
Parshall: Yeah, I mean, I definitely is not clear the extent to which they’re seriously doing this. One of the sources I spoke with said that, you know, this could just be a PowerPoint from some general, or it could be a serious program. But it might not necessarily be a serious program designed for them to actually—endgame—detonate a nuke in space.
The State Department said that there was no, quote, “imminent threat.”
So I don’t want people to be afraid of a nuke off above their head. But, yeah, if this happened, it would be bad. Geopolitically it’s the kind of thing that we spent a lot of the cold war worrying about—just very escalatory. Of course, you worry about those tensions escalating even further into more nuclear exchanges and global thermonuclear war—don’t want that.
Feltman: Yeah, would love to avoid.
Parshall: But as far as the direct effects of what you’d see if this went off today, like I mentioned, it would be worse because there’s more things in space now than there were in 1962. We have 10,000-ish satellites in orbit. Most of those are in low-Earth orbit. We’ve got two space stations.
Of those 10,000 satellites, 6,000 of them are Starlink satellites. Depending on the height and the strength of the blast and where it went off, you could expect a lot of those to be damaged or killed. They would effectively be turned into space junk—just aimlessly floating without a real way to recover them.
And it’s not just inanimate satellites that we’re worrying about. There are people in space. There’s people on the International Space Station and China’s Tiangong space station. An electromagnetic pulse from a nuclear weapon going off could mess with vital systems aboard. It could also leave the astronauts to be navigating a bunch of space junk, which could potentially harm the space stations.
And the radiation itself, depending on where the blast went off, could be really directly harmful. I spoke with Victoria Samson. She works for the space sustainability [organization] Secure World Foundation. And she said that a nuclear blast in orbit could limit the safety of people on these space stations to mere hours or days.
And she was citing this 2010 Department of Defense report. They tested 17 possible scenarios—worst-case scenario showed…[full transcript]
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