So what is this anyway?
"Project Heimdall" is a collaborative writing project.

Okay, got that. …What about?
Ah, yes, right. The Foundation would have contingency plans for just about everything - after all, it's all about maintaining normalcy for humanity in general. In contingency planning, there are events which are considered low-probability, high-impact (sometimes colloquially called "wild card events"). In other words, events which are not very likely, but would be a really big deal if they happened. Historically speaking, if you'd considered the reunification of Germany or the breakup of the Soviet Union prior to their actually happening, they'd have been considered low-probability, high-impact.

Project Heimdall is the Foundation's planning for one of the major wild cards of speculative fiction - an invasion of Earth by hostile extraterrestrials.

Uh, hold on - an alien invasion is science fiction, not horror.
That comes down to a difference of execution. There's a big difference between the titular Alien and the Vulcans in Star Trek. Some scifi is light and optimistic - scifi/horror tends to be a lot heavier on humans losing (often bloodily) than on aliens coming in peace.

What are the rules?
Of course all the usual site rules apply. That said, Project Heimdall has a number of key assumptions, listed above.

Okay, those are in bureaucratese. Care to put them in English?
Sure.

Extraterrestrial intelligence will display vigorous, active hostility to human life and/or civilization.

The aliens do not come in peace. At least, that's what we're assuming. Why? Well, first, the Foundation would probably assume hostility first and shake hands in friendship later. Second, this wiki is for scary things. Friendly aliens who drop by for tea (probably) aren't scary. (Of course, if you want to write a dissenting opinion, arguing to the Foundation highers-up that Heimdall is misguided fear-mongering which fails to consider a reasonable possible occurrence, I'd personally upvote that if you did it well.)

Hostile Extraterrestrials (HEs) will possess no technology forbidden by the laws of physics. (While numerous SCPs exist which operate in manners that are forbidden by the laws of physics, these are less-than-completely-understood exceptions, and have accordingly not supplanted the generally accepted laws of physics, even among Foundation researchers. Speculations of impossible technologies make for entertaining "soft" science fiction, not good analysis grounded in scientific fact.)

Basically, this means we're aiming for "hard" science fiction. Why? (Especially when so much of this wiki is "soft" science fiction?)

• There are a surprising dearth of alien invasion stories which have any sort of grounding in real-world science.
• The goal here is not to worldbuild anomalous science fiction aliens and technology or claim alien origin of existing SCPs. Do NOT use SCPs as the hypothetical aliens or alien tech. No, not even especially not 682.
• Who knows what sort of aliens might exist, and how they might better understand the universe and its natural laws than humanity? But don't sell humans too short. The Foundation is one of the world's foremost scientific entities and has access to entities which defy conventional understanding of scientific laws. And yet the Foundation still operates under the basic assumption that the scientific laws known by the non-affiliated scientific community are correct (or at minimum less wrong than any other known alternative) except in a tiny handful of cases. The point is, SCPs are anomalies which may be impossible to replicate, no matter what galaxy you're from.
• There's plenty of reason to be downright terrified of scientifically plausible extraterrestrial threats; from a pragmatic standpoint, it makes more sense to develop plans for fighting the devil that you know first.
• Perhaps most significantly, assuming hard science for aliens and their technology keeps all collaborators on the same page.

Unless their goal is simply to destroy civilization and/or life (human or otherwise) on Earth, any attempt by HEs to attack the planet will necessarily be limited in the level of damage that it will cause to the biosphere.

Earth is, to date, the only planet known to mankind that is capable of sustaining life in any recognizable form. That carrying capacity is delicate, and takes years (or decades, or centuries, or millennia) to recover from major shocks to the system. If aliens care about living here and reaping the benefits of their successful conquest, they can't render the planet uninhabitable. If they're interested in Earth, their physiology may well be dependent upon an ecosystem not dissimilar to our own (else they'd go invade some planet where the natives would cause fewer problems).

Attempts by HEs to conquer, enslave, or exterminate humanity can be either overt (exempli gratia armed invasion) or covert (exempli gratia infiltration).

So, we could be looking at either War of the Worlds or Invasion of the Body Snatchers. Different threats, different Foundation responses.

Knowledge of any overt attempt by HEs to conquer, enslave, or exterminate humanity will be impossible to contain from the world at large.

Even the Foundation does not have the resources to hide a fleet of ships appearing in orbit. It is possible, given a clear night and a lack of light pollution, to see orbiting satellites with the naked eye. If you can see something the size of a bus as bright moving light with no magnification, anyone with simple binoculars or telescopes would be able to see some reasonable detail on spacecraft the size of buildings. And any serious attempt to conquer, enslave, or exterminate humanity would require transporting sufficient military forces to defeat the combined military-industrial power of the planet.

"Simple" solutions to attempts by HEs to conquer, enslave, or exterminate humanity will prove insufficient.

Could we simply point SCP-682 or SCP-076-2 at the invaders, or asking SCP-343 to "make the aliens go away"? Yes, we could. This would probably also be either ineffectual or boring, so exercise some creativity and try something else. Also, it seems unlikely the Foundation would risk exposing dangerous and sapient SCPs to the aliens lest we find ourselves fighting two enemies rather than one. In the course of writing this, you're sure to come across "spoilers" that would instantly solve everything. Why it doesn't work, I'll leave up to you to explain.

Who can participate in Project Heimdall?
Anyone! If something strikes your fancy, write it up. This is supposed to be cross-disciplinary, so documents could range from theories on world political and military reaction (political science, military science, economics, etc.) to potential technological countermeasures (physics, engineering, etc.) to what our attackers are like (biology, psychology, etc.) There's a list of suggestions below that people are free to add to or use.

Is this official canon?
Everything involved in Project Heimdall is officially canon within Project Heimdall. Beyond that, it's up to you.

Who is running this project, out of universe?
Well, Hornby came up with the idea and got the ball rolling, but he isn't Senior Staff. While he's up for helping to herd cats coordinate things related to Project Heimdall, he's not "running" things in any sort of formal, moderating, or administrative way. In many ways, there isn't really one particular person "running" this - it is, after all, a collaborative exercise.

How do I get involved?
Go. Write something. Link it here when done under the "Project Documents" heading. Tag it with "heimdall". Hopefully, it'll stick, else it will be downvoted into deletion like any other work on the site. But hopefully it will stick.

My reaction to this project is "Meh."
That's fine; nothing on the wiki is for everyone.