Landmark-free optical navigation around small bodies: Application to the Hayabusa2 touchdown on Ryugu

As for missions that explore small bodies such as asteroids or comets, landmark-based optical navigation is widely used in such operations as descent or landing. The Japanese asteroid explorer Hayabusa2 successfully performed two touchdowns on the asteroid Ryugu in 2019, using one of the landmark-based optical navigation. Hayabusa2 realized the guidance, navigation, and control with accuracy of less than 5 m at the touchdowns. On the other hand, this navigation method strongly depends on the terrain surface of the target celestial bodies, and also requires laborious work to detect sufficient number of landmarks in images. This paper presents an optical navigation method that is independent of landmarks as an advanced study for future missions. The movement of a global surface, rather than a local point, is focused to enable visual tracking without relying on landmarks. The result of the visual tracking yields the pose of the probe via perspective projection equation. The function of the developed method is simulated using the flight data of Hayabusa2.