Synthetic Aperture Ladar (SAL) could provide high resolution optical/infrared imaging of planetary surfaces from airborne or spaceborne platforms, using only modest-sized optics feasible for high-altitude flight or orbital missions. We discuss the characteristics of a planetary observing SAL (range and azimuth resolutions, field of regard, imaging swath size, altitude, aperture size, laser power, wavelength, etc.) and model the imaging performance of a SAL. Required laser power grows exponentially with range from the sensor platform ground track, due to atmospheric extinction, and also depends on wavelength as λ-1 for a shot-noise-limited receiver. Planetary observing by SAL from space may be feasible in the terahertz band (~ 100 μm) with ~10 W of laser power. A planetary-observing SAL at shorter wavelengths (e.g. 1-2 μm) would require correspondingly higher laser power and would be much more challenging. SAL imaging may be attractive from low orbit around other planets, in particular those with little or no atmosphere (e.g Mars, Mercury, the Moon and many other planetary satellites). Beam stabilization, motion compensation and autofocus are among the most challenging aspects of a SAL mission.