Just how advanced would a civilization have to be to attempt an invasion of Earth? In most sci fi the aliens are light-years ahead of us in technology, but would they really need to be?
I believe the answer to be, no, not really, they actually would only have to be about as advanced as us, at least when they left their home. They wouldn't need a fancy hyperspace engine, they could use something like Project Orion to get here. 10% light speed might take awhile but you'll get here just the same, and if they had anti-matter powered drives they could possibly travel at 80% light speed. There are Sol-like stars within 20 light-years of Earth, if they came from one of these they could make it within 200 years.
Once they were here they could easily assault Earth with impunity. All they'd have to do is stay out of range of our weapons, and let gravity assist theirs (nuclear weapons or meteors). If they had point defense lasers they'd be even more safe from us. A good example of this type of invasion can be seen in the novel Footfall.
I believe the answer to be, no, not really, they actually would only have to be about as advanced as us, at least when they left their home. They wouldn't need a fancy hyperspace engine, they could use something like Project Orion to get here. 10% light speed might take awhile but you'll get here just the same, and if they had anti-matter powered drives they could possibly travel at 80% light speed. There are Sol-like stars within 20 light-years of Earth, if they came from one of these they could make it within 200 years.
Once they were here they could easily assault Earth with impunity. All they'd have to do is stay out of range of our weapons, and let gravity assist theirs (nuclear weapons or meteors). If they had point defense lasers they'd be even more safe from us. A good example of this type of invasion can be seen in the novel Footfall.
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