While I enjoy space opera (Star Trek, Star Wars, etc) my favorite fictional genre is the rarer hard science fiction, stories that involve science and engineering which doesn’t yet exist, but, unlike soft SF and fantasy, don’t can be rigorously explained as possible by present-day science, making clear assumptions that themselves are at least not very implausible according to present-day science. Though I’ve not seriously written fiction (which is to say, actually started, finished, and published a story in any form) since the late 20th century, I’ve kept a journal of “story ideas” since the 1990s, and kept to a personal quest to build a diamond-hard SF universe in which individual Earth people travel interstellar distances. Ignoring initially the sociological issues, such as why individuals would want to do this, and how to pay for it, we have an high-level, nuts-and-bolts engineering question: how, with unacceptably soft SF staples like warp drives, traversable wormholes, how can you deliver a single, few, or many effectively living (the definition of this is tricky - more later) Homo sapiens sapiens from Earth to somewhere on the order of 10 lightyears distant? That is, how to do interstellar travel? I start with the assumption that people have perfected the basic problem of living in outer space – that is, how to keep people alive and healthy for their entire lifetimes outside of Earth’s atmosphere given nothing but a reasonably small amount of per person physical power and a mass of expendable supplies not many much greater than the mass of the people, or a total mass greater than on the order of 10^-10 Earth masses. Next, I assume an upper limit on the “shelf life” for a live human of on the order of 1000 years. Near future medical technology can allow a person to live about 10 times longer than normal, or, more likely I think, some sort of suspended animation like the science fiction staple “cryosleep” is available. I’ll lift this assumption later in this post. Next, I conclude that an “automobile” - a vehicle that carries all it needs for its trip - isn’t feasible. Even using an ultimately high energy-density fuel such as stored antimatter and super-advanced reaction mass accelerators, no rocket can reach the speed needed to travel on the order of 10 light years. I’m left, then, with two high-level solutions: The spacecraft obtains rocket reaction mass, and or fuel from the interplanetary and interstellar medium. The best know example of this idea is the Bussard ramjet. The spacecraft is accelerated by a system it doesn’t carry. The most feasible example of a system of this of which I know is light-sail propulsion system described in physicist Robert Forward’s 1982 novel Rocheworld. As best I’ve been able to glean from the literature, there’s presently a small scientific consensus that Robert Bussard’s original 1960 proposal, in which the ramjet spacecraft was to not only used the ionized hydrogen it “scoops” as reaction mass, but fuse it to produce sufficient power to accelerate the reaction mass to produce sufficient rocket thrust that the spacecraft’s maximum speed relative to the interplanetary/stellar medium when the drag of due to its scoop equals that thrust is too low (less than .00017 c which is about 3 time the speed of Voyager 1) for the scheme to be useful. So for a ramjet spacecraft - perhaps better termed a scoop craft - to be acceptable for my hard SF world needs, it would require power in addition to that from the fusion of the scooped hydrogen. I’m left, then, with artificial light (or other EM radiation) pushed “sail” spacecraft like Forward’s design, or possibly craft propelled by other kinds of streams/beams, such as “fountains” of macroscopic particles. Comments?