If intelligent aliens exist, they might be smart enough to realize how dangerous we are, warns a new report by scientists at NASA and Pennsylvania State University. In fact, if such brainy aliens ever see how much we've damaged our own planet, they may even wipe us out to protect other ecosystems throughout the cosmos.

This is all sci-fi speculation, of course, and it's just one theory in a broader effort to imagine what contact with extraterrestrial intelligence ("ETI") would be like. The paper's authors divide our imaginary visitors/invaders into three basic categories — beneficial, neutral and harmful — and propose various ideas for how each might react to what they find on Earth.

These ideas range from "cooperative extraterrestrials," who would share technology and help us solve problems, to "selfish ETI," who would "attack us," "enslave us" or "eat us." There are also plenty of alternatives offered in between, such as aliens with "no desire to communicate," others who are "uninteresting and non-useful," or still others who pose a "mild nuisance." The report's authors compare the latter group to the prawn-like aliens in the movie "District 9."

The report is timely, considering NASA and the U.S. Defense Department are working on a project called the 100 Year Starship Study, designed to lay the groundwork for sending probes — and eventually people — to other star systems. The Defense Advanced Research Projects Agency, or DARPA, plans to award a $500,000 grant later this year to jump-start the process, with hopes of eventually exploring deep space in search of habitable planets and extraterrestrial life.

But as its name makes clear, the 100 Year Starship Study isn't in any rush. It will take centuries for humans to reach the stars, so we're more likely to be on the receiving end of any interstellar journeys for quite a while. And that's where the NASA/Penn State report comes in. The authors admit it's conjecture, but they also say it's a worthwhile effort to "prepare for actual contact" with aliens, "even if the details of contact do not fully resemble any specific scenario." And, according to one of their theories, it should also offer yet another reason to take better care of the Earth: Aliens could judge us harshly for our lackluster environmental stewardship.

An ETI may be wary of civilizations that grow too quickly, the report's authors suggest, perhaps fearing they'll endanger life on other planets as they expand. (We've all seen "Avatar.") And such an assumption wouldn't necessarily be wrong, considering humanity's growth has already taken a heavy toll on Earth's ecosystems, from tropical rain forests to the polar ice caps. That might lead some aliens to avoid contacting us, but it might prompt others to kill us off for the good of the universe.

"A preemptive strike would be particularly likely in the early phases of our expansion because a civilization may become increasingly difficult to destroy as it continues to expand," the report states. "Humanity may just now be entering the period in which its rapid civilizational expansion could be detected by an ETI because our expansion is changing the composition of Earth's atmosphere (e.g. via greenhouse gas emissions), which therefore changes the spectral signature of Earth."

But before you start preparing for an invasion of cosmic climatologists, the report does offer a ray of hope. It's not too late to change our ways, it suggests, both to save ourselves from aliens and to save the planet from ourselves. "While it is difficult to estimate the likelihood of this scenario, it should at a minimum give us pause as we evaluate our expansive tendencies," the authors write. "As we continue the search for extraterrestrials into the future, perhaps our thinking about the different modes of contact will help human civilization to avoid collapse and achieve long-term survival."

If that's what it takes. Thanks, imaginary aliens.

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