NASA, STEM and PBL meet the needs of 21st century education because the NASA piece adds interest for students.  When teaching Earth Science, my colleagues and I dedided to teach the Astronomy unit first, in order to grab the students' attention.  It works--students have a natural curiosity about space.
Then, when you have the students' attention, Web 2.0 tools are a natual fit to collaborate on a product to express their findings and communicate to their peers and the outside world, both parts of STEM and PBL.