Defending Earth from aliens sounds like a big job, but the biggest one is actually defending alien life from us. The position NASA is hiring for-with the amazing title of the Planetary Protection Officer-is probably nothing like the image conjured by the trending headlines.
The Planetary Protection Officer does technically defend against aliens. It's just less Will-Smith-in-Independence-Day and more the-background-characters-in-The-Martian. Battling aliens sounds awesome. ... A little less awesome. But that's what the Office of Planetary Protection does.
See, the real challenge is not so much keeping foreign life away from Earth. In many cases, that's exactly what we are searching for. We do not want it accidentally stuck to a Mars rover's treads, but we still want it. What we definitely do not want is a bunch of Earth contaminating another planet or moon. This is for two reasons. One, we just should not be spreading our own life to alien ecosystems, since that could disrupt a huge portion of organisms. Two, we do not want to accidentally "find" life on another planet only to realize that it's our own.
Imagine if the Mars rovers had arrived on the red planet full of bacteria. There's a chance that some of the micro-organisms would be able to survive there. Years in the future, if we found life forms that seem awfully Earth-like, we'd have a much harder time figuring out if they were aliens, or just the descendants of some interplanetary bacterial hitchhikers. Keeping the rovers clean in the first place.
Even spacecraft that are going on flybys have to be sterilized, lest they crash land on the sphere.
This is a lot of harder than it seems. Bacteria are excellent survivors. That means that every single tiny bit of a spacecraft has to be sterilized, then has to stay that clean until lift off. When Curiosity went to Mars, there was something of a scandal after a team. That's how seriously they take contamination.
On the other hand, we did not bother to sterilize the Voyagers before they headed out to deep space, since they were not destined to land on any planets. Assuming they do not run headlong into anything, they should not be contamination risks. And, as one What If? XKCD post pointed out, that means that thousands of Earth.
So if you get the NASA job, you will not get to battle for control of Earth from aliens (that, by the way, are unlikely to come in any of our lifetimes). You'll get to do something that's way more important.
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