Slow interstellar travel designs such as Project Longshot generally use near-future spacecraft propulsion technologies. As a result, voyages are extremely long, starting from about one hundred years and reaching to thousands of years. Crewed voyages might be one-way trips to set up colonies. The duration of such a journey would present a huge obstacle in itself. A generation ship is a type of interstellar ark in which the travellers live normally (not in suspended animation) and the crew who arrive at the destination are descendants of those who started the journey. A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of a method to replicate conditions in a uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots which would replace human parents.