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A good point, but that still supposes the aliens are pretty stupid. Even ten thousand years ago we had structures that were a lot easier to detect than humans. So they would have had to have scanned us over 50,000 years ago, or more. If you're going to take on such a crucial mission, would you rely on intel that is 50,000 years old?

Let's say for argument's sake that they are 1000 light years away, so their scan data on us is 1000 years old. They detected no signs of advanced civilization (no atmospheric chemicals indicating large-scale combustion, no EM signals, and they were unable to detect us) - none of this would have existed in the year 1008. So, they decide to undertake the mission. Now, they can only travel sublight, but let's say that can travel at almost c. So it takes them 1000 years to get here. That means when they get here, it is 3008. We're not only advanced then, we're probably downright deadly. You see... relying on old intel for something like this is silly. In the space of a thousand years we went from Roman times to this... and we were slow! The dark ages held us back big time.

If you're gonna undertake a mission like this, you better make sure your information is right up to date... or you may be in for a nasty surprise.

The other thing to take into account is the farther away you allow this colonization attempt to be (so the older their info on us), then the more choice they have for planets. A safe number for stellar density is .003 stars per cubic light year. So if you want their info to be 100 years old, the number of stars they can choose from is:

0.003 * 4/3 * pi * (100)^3 = 12,566

A thousand years:

0.003 * 4/3 * pi * (1000)^3 = 12,566,371

It can't be that hard to find another Earth like planet in that massive number of stars that they would be willing to exterminate an intelligent species. Good grief, we don't have anywhere near the technology to undertake a colonizing mission yet, and we're already seriously considering terraforming Mars!!! That would mean there are two planets in this one solar system suitable for a colonization mission for us. How hard can it be for these aliens to find something suitable that they would have to travel a thousand light years and start a war?

(Actually, i might be makin a roleplay involving a colonizing trip to 37 Geminorum, which is only 56 light years away.)

Well that would make them pretty stupid again. ^_^;

How are they to know that they're not sending their ships right into the orbit of a VERY dangerous species, who might then turn around and follow their amazing space program's ship back to their homeworld, and turn it to a cinder?

No, we definitely would not send a colony ship to an un-surveyed world. ^_^; That's just idiotic. That could very well trigger a response that means the end of our civilization. Chances are small, but the risks are so huge that it's not worth it. (Unless they're just ******** crazy. And it's seriously unlikely that any species that can manage an interstellar trip is that nuts, or stupid. They'd destroy themselves first.)

You realize we didn't even go to the Moon without being very careful about it. They flew several survey missions before landing. Same for Mars. And these are planets that we are sure are dead. You don't get a ship in the air by being careless (and if you do, you get Columbias and Challengers), and getting a ship to another planet... that takes serious caution and planning. It's totally unrealistic to believe that someone could get a ship here without taking the planets current inhabitants into account.