The linear resolution of details on the surface of Mars through a telescope is about 500 km. How, with the help of ground-based

The linear resolution of details on the surface of Mars through a telescope is about 500 km. How, with the help of ground-based telescopic observations, were Mars satellites with a characteristic size of only 20 km discovered?

The contrast of satellites illuminated by the sun against a dark sky background is significantly higher than the contrast of fine details on the surface of Mars.

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