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Space is large. If you don't look in some direction, you don't see what's there. Big, or not. Let's take the Orion engine. It keeps blasting away thermonuclear

Space is large. If you don't look in some direction, you don't see what's there. Big, or not. Let's take the Orion engine. It keeps blasting away thermonuclear bombs. Are we looking straight at it? Why? Ah, maybe because they are in the neighborhood of a star. But then we are also looking at the star, which is trillions or quadrillions times brighter. Same for fusion plasma drives or anything else. If the Orion engine is traveling between stars, we'd have a chance, but then how would we know where to look ?

dredmorbius 4y ago
If they're approaching us, they might be using the Orion engine to decelerate on long final.
- credit_guy 4y ago
  Here's an amusing thing. Even if they were within Jupiter's orbit and decelerating and pointing straight at us, we wouldn't be able to detect them. Nuclear bombs produce a fireball here on Earth. In the first microsecond when the explosion actually happens, they produce gamma rays and X rays. The atmosphere is opaque to those rays, which is to say, it absorbs them. Their energy is enormous, so the part of the atmosphere that absorbs them becomes superheated. That's what the fireball is. That's what produces the bright flash. In space it goes without saying that there's no air. The fireball doesn't form. The explosion itself will produce gamma rays and x-rays (the blackbody radiation at 100 million degrees). They will go in all directions, including towards Earth. But once they get to Earth, they'll be absorbed by the upper layers of the atmosphere. As seen from Earth a nuclear explosion happening somewhere in the vacuum of the Solar system will be invisible. But we do have gamma ray and X-ray telescopes. On satellites in space. Only not that many of them. If they don't look in the right direction exactly when the explosion happens, they see nothing. Another difference between a nuke detonated in space and one on Earth is that on Earth you can see things (like the mushroom cloud) for hours after the explosion. In space, the explosion happens, and one second later nothing. There is absolutely nothing happening one second after the nuke goes boom. So our telescope needs to point exactly in the right direction, exactly at the right time to have a chance to see something.
  - dredmorbius 4y ago
    Rocket propulsion benefits from directional discharge. Project Orion lead to the investigation prospects for nuclear shaped charges, in which the blast is highly directional. The concept was developed further in the colourfully-named "Casaba-Howitzer": <https://en.wikipedia.org/wiki/Casaba-Howitzer https://en.wikipedia.org/wiki/Casaba-Howitzer> Odds are reasonably good that any space-faring civilisation would create similarly directional reaction devices for space travel if they were utilising explosive nuclear fission. ... Though, thinking this through, the shaped charge would probably be directed at rather than away from the pusher-plate in an Orion design. There might of course be other directed reaction-engine designs. They'd be generating similar-magnitude energy signatures, however, and again, in a deceleration phase, would be directed toward us. And whilst naked eye observations would of course be difficult, that's really not what we're talking about, but instead sensors which could pick up specific radiation. And I strongly suspect that sudden-onset x-ray and gamma-ray sources within the Solar System would in fact be detectable. (It was devices that were looking for such bursts on and near Earth from Earth orbit via the Vela satellites as part of nuclear war early-warning systems which first detected astronomical gamma-ray bursts.) Given that small meteor impacts on the Moon are quite detectable from Earth, nukes near Jupiter should be readily detectable. A 2013 lunar meteor impact from what's thought to have been a 40kg mass traveling at 25 km/s was visible to the naked eye as a 4th magnitude event. That's about 5 tons TNT equivalent. <https://science.nasa.gov/science-news/science-at-nasa/2013/16may_lunarimpact/ https://science.nasa.gov/science-news/science-at-nasa/2013/1...> The Orion concept was based on 0.15 kt (150 ton TNT) charges, or 30x larger than the meteor impact discussed here, exploding repeated at a rate of 1 Hz. That would be a pretty significant signature.
    - credit_guy 4y ago
      Cool link about the Casaba howitzer. Thank you.
      - dredmorbius 4y ago
        A relatively recent discovery of mine, and ... interesting to contemplate.

dredmorbius 4y ago

If they're approaching us, they might be using the Orion engine to decelerate on long final.

credit_guy 4y ago
Here's an amusing thing. Even if they were within Jupiter's orbit and decelerating and pointing straight at us, we wouldn't be able to detect them. Nuclear bombs produce a fireball here on Earth. In the first microsecond when the explosion actually happens, they produce gamma rays and X rays. The atmosphere is opaque to those rays, which is to say, it absorbs them. Their energy is enormous, so the part of the atmosphere that absorbs them becomes superheated. That's what the fireball is. That's what produces the bright flash. In space it goes without saying that there's no air. The fireball doesn't form. The explosion itself will produce gamma rays and x-rays (the blackbody radiation at 100 million degrees). They will go in all directions, including towards Earth. But once they get to Earth, they'll be absorbed by the upper layers of the atmosphere. As seen from Earth a nuclear explosion happening somewhere in the vacuum of the Solar system will be invisible. But we do have gamma ray and X-ray telescopes. On satellites in space. Only not that many of them. If they don't look in the right direction exactly when the explosion happens, they see nothing. Another difference between a nuke detonated in space and one on Earth is that on Earth you can see things (like the mushroom cloud) for hours after the explosion. In space, the explosion happens, and one second later nothing. There is absolutely nothing happening one second after the nuke goes boom. So our telescope needs to point exactly in the right direction, exactly at the right time to have a chance to see something.
- dredmorbius 4y ago
  Rocket propulsion benefits from directional discharge. Project Orion lead to the investigation prospects for nuclear shaped charges, in which the blast is highly directional. The concept was developed further in the colourfully-named "Casaba-Howitzer": <https://en.wikipedia.org/wiki/Casaba-Howitzer https://en.wikipedia.org/wiki/Casaba-Howitzer> Odds are reasonably good that any space-faring civilisation would create similarly directional reaction devices for space travel if they were utilising explosive nuclear fission. ... Though, thinking this through, the shaped charge would probably be directed at rather than away from the pusher-plate in an Orion design. There might of course be other directed reaction-engine designs. They'd be generating similar-magnitude energy signatures, however, and again, in a deceleration phase, would be directed toward us. And whilst naked eye observations would of course be difficult, that's really not what we're talking about, but instead sensors which could pick up specific radiation. And I strongly suspect that sudden-onset x-ray and gamma-ray sources within the Solar System would in fact be detectable. (It was devices that were looking for such bursts on and near Earth from Earth orbit via the Vela satellites as part of nuclear war early-warning systems which first detected astronomical gamma-ray bursts.) Given that small meteor impacts on the Moon are quite detectable from Earth, nukes near Jupiter should be readily detectable. A 2013 lunar meteor impact from what's thought to have been a 40kg mass traveling at 25 km/s was visible to the naked eye as a 4th magnitude event. That's about 5 tons TNT equivalent. <https://science.nasa.gov/science-news/science-at-nasa/2013/16may_lunarimpact/ https://science.nasa.gov/science-news/science-at-nasa/2013/1...> The Orion concept was based on 0.15 kt (150 ton TNT) charges, or 30x larger than the meteor impact discussed here, exploding repeated at a rate of 1 Hz. That would be a pretty significant signature.
  - credit_guy 4y ago
    Cool link about the Casaba howitzer. Thank you.
    - dredmorbius 4y ago
      A relatively recent discovery of mine, and ... interesting to contemplate.