I've read quite a bit about von Braun and by him, and this seems to come out of nowhere. As a bit of coffee-hour BS-ing, okay. And some of it is based on reality.

The Advanced Research Projects Agency (ARPA, later to become DARPA) embarked on Project Defender in 1960. One element of Defender was BAMBI (ballistic missile boost intercept) / SPAD (space patrol active defense). During this time, the Soviet Union was developing concepts and systems for a fractional-orbital bombardment system (FOBS). Everything cooled off upon resolution of the Cuban Missile crisis and BAMBI/SPAD was discontinued in 1963. It was later revived in 1983 under the Strategic Defense Initiative, graduated to the "Brilliant Pebbles" concept and then was again put in the cooler after the fall of the Soviet Union. As a result, we still do not have a space-based defense system. It's not hard to develop the technology; it was all in place by the late 1980s. Just hard to make the commitment (and, until now and SpaceX, fairly expensive to emplace the required constellation---but cheap at twice the price when you consider the cost of replacing a destroyed metropolis).

As for terrorism, I think the space contribution has been from surveillance and reconnaissance satellites, both optical and radio-communications.

Deflecting asteroids heading for Earth is a big problem. NASA (in my opinion) is mincing around the problem with entirely dilatory and effete methods such as kinetic impactors (i.e., deflect a charging bull elephant by shooting a spitwad at it). They get to claim they are "working the problem," when they are just playing with their favorite rice bowls. The necessity of 100-Mt devices to provide a sufficient kick is evident to anyone who is trying to solve the problem---under the realization that most of the serious close shaves have been detected on short notice, not a leisurely years-long campaign of spitwads. That would require Big Time international concurrence and coordination.

Aliens? Well, that's the last category, but only as an intellectual exercise. It is on the level of thinking you can out-wrestle a great white shark in its element.