This paper presents the Janus Habitat, a novel spacecraft architecture for Mars transit that achieves artificial gravitythrough a 200-meter tether system connecting a 50-tonne crew module to a 50-tonne counterweight composed primarilyof terrestrial recycled polyethylene terephthalate (R-PET). The system generates 0.5 g at 2.1 RPM with only ~200 kg oftether mass, representing a 98.5% mass reduction compared to traditional rotating cylinder designs. The counterweightutilizes standardized interlocking R-PET blocks manufactured on Earth at approximately 0.1% the cost of launchingdedicated ballast. An optional orbital debris processing system adds value by transforming aluminum space debris intosupplemental shielding. The complete 120-tonne system enables deployment with two heavy-lift launches, providing apractical path to artificial gravity for long-duration transits while creating terrestrial environmental benefits through space-based demand for recycled plastics.