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" ... There are two major problems with ramjets, however. The first, as Schwartz noted, is that we don't know how to build one yet. The second is that interstellar hydrogen is not easy to fuse. It is made up mostly of the lightest hydrogen isotope, protium. Also, earth is located in an interstellar medium of particularly low density. While ramjets can't be ruled out, even enthusiasts say they are unlikely. ... "