  Interstellar radar is a potential intermediate step between passive
observation of exoplanets and interstellar exploratory missions. Compared to
passive observation, it has the traditional advantages of radar astronomy. It
can measure surface characteristics, determine spin rates and axes, provide
extremely accurate ranges, construct maps of planets, distinguish liquid from
solid surfaces, find rings and moons, and penetrate clouds. It can do this even
for planets close to the parent star. Compared to interstellar travel or
probes, it also offers significant advantages. The technology required to build
such a radar already exists, radar can return results within a human lifetime,
and a single facility can investigate thousands of planetary systems. The cost,
although high, is within the reach of Earth's economy, so it is cheaper as
well.
