What are some of the known risks towards long term sustainability of manned space missions? Or put another way: What are the limited resources that could keep a future mankind grounded?
What are some of the known risks towards long term sustainability of manned space missions? What are the limited resources that could keep us sadly grounded?
These days there aren't any special and essential resources for manufacturing spaceflight hardware that aren't also used more widely. Liquid fuels are produced or refined materials that are also produced and refined for other applications. If we can't make and refine kerosene and produce liquid oxygen any more, for example, or aircraft-grade alloys, 3D metal printers, cable harnesses, computer chips, etc. then worldwide manufacturing itself has broken down.
The ugly prototype rockets that SpaceX is building for it's next big rocket sort-of show that with a big knowledge and experience base, one can make spacecraft increasingly less fancy and still get them to go to space.
So perhaps a big knowledge and experience base is the most critical resource. If we lost the institutional knowledge contained in the heads of people in the aerospace industry in several spacefaring countries simultaneously, we'd be SOL.
Those remaining on Earth would be left scraping Space Exploration SE for the clues as to how to build and launch rockets!
A good example of this are the answers to Were the Saturn V construction plans destroyed? which point out that it's the specialized knowledge that's been lost, not the blueprints, that makes building a reliable Saturn V today unlikely.
Your question is very broad, which is why it might be better suited to Worldbuilding.SE .
There's a big difference between no longer being able to reach and operate an orbiting lab, and not being able to send humans to, e.g., Mars or the Kuiper Belt.
For that matter, a nice global thermonuclear war would preclude sufficient resources to build a rocket.
There are plenty of reasons that travel to Mars is almost impossible now, including radiation protection and body degradation due to long-term zero-g living.
A few ASAT bombs and whatnot could lead to the so-called "Kessler Syndrome," in which the density of orbiting debris precludes any safe launch.