At this stage of the space industry evolution, with the sufficient but still limited technology readiness of propulsion (for deep space exploration) and the much lower technology readiness of resistance in extreme environments (for onsite exploration and colonization), it seems to my modest rationale that such five startups are just aiming at minor or lateral goals? Especially considering that YC can really put anyone / anything interesting enough in direct contact with the most important and active people in the field? And the bottleneck, for both branches (propulsion, colonization) is current materials science & engineering, so that 20 years ago the world needed developers for the dotcom revolution, while today it needs materials? Just food for thought, obviously, so take my ramble for what it is.