Mars Direct is a sustained humans-to-Mars plan developed by Dr. Robert Zubrin that advocates a minimalist, live-off-the-land approach to exploring the planet Mars, allowing for maximum results with minimum investment. They produced a detailed study of a Design Reference Mission based on the Mars Direct plan but scaled up about a factor of two in expedition size compared to the original concept. They then produced a cost estimate for what a Mars exploration program based upon this expanded Mars Direct approach would cost.