At NASA, the idea of sending humans to Mars and Venus was first proposed in the run-up to Neil Armstrong’s historic moonwalk, during the Apollo program, when the space agency was giddy with exploration and rich in funds. Public support for the space agency was also on its way to heights it would never otherwise reach. It had begun sending probes to Venus in 1961, as part of the Mariner program. For the humans-to-Venus proposals, which would repurpose Apollo hardware, engineers determined that the three astronauts on board would only have time for between 45 minutes and two days for close observations. Even with nuclear engines (also theoretical) this wouldn’t be enough time to make the 400-day trip worth it. Going the extra distance to Mars, meanwhile, was considered practically impossible. To explore these planets, robots would have to be enough.