Photocentre and barycentre positions of asteroid (21) Lutetia (brown and blue bullets) as projected in the direction of Gaia (normal to the image plane) for three epochs. The z-axis of the equatorial coordinate system is denoted by the green bars. North points to the top. The Sun illuminates the asteroid from the directions denoted by the red bars. From left to right, the solar phase angle takes the values of 16.7, 24.2, and 20.9 degrees, and the photocentre-barycentre offset takes the values of 7.42, 5.96, and 10.91 km.