It came up as a question during the KS project while we were playing with the PnP editions. You would have answered it there too - but I don't remember seeing it in a rulebook version.
Resources are unlimited. In several hundred games I've witnessed, it has happened once that players needed to use something else to represent resources. Occasionally I've seen players run out exactly (both on a resource or astronauts), but then inevitably someone sells one or launches one before another is needed.
Some scoring schemes push this a bit more than others (sufficiency for resources, scarcity for astronauts, in particular).
Hmm, I just searched the rulebook and you are correct it doesn't mention it. That's odd, because I know the line "Resources are not meant to be limited" has appeared in previous rulebook versions. Doh!