More important is the need to collect a precise understanding of local gravity conditions. Controllers at the Jet Propulsion Laboratory switch off the ion engine periodically for a figurative finger in the wind, to see which way gravity is pulling the spacecraft. Optical navigation data from the spacecraft is combined with its thruster history and ground tracking, and the necessary adjustments are made.

“Once we actually got into orbit, as Dawn revolved around Vesta and Vesta rotated beneath it, we could measure the higher-order gravity terms, looking for in some sense the same kind of thing that was found on the Moon,” Rayman says, referring to irregularities in the two bodies' gravity fields that impact spacecraft orbiting at low altitudes.

Engineers of robotic spacecraft have been perfecting navigation techniques using gravity assists since the beginning of the space age. “We're trying to learn how to use these gravity lanes to maneuver around space with humans,” says Gerstenmaier. “It takes a little bit longer, but it's a much more effective way to move things along. So we're learning from the robotic community.”