Discovering water ice on the Moon, illustration. Water ice was potentially discovered at the Moon's poles in 1994 by the spacecraft Clementine that reached orbit around the Moon. Known as the Bistatic Radar Experiment, radio signals (purple) from the spacecraft were reflected from craters in the polar regions, with the reflections detected by Deep Space Network radio telescopes on Earth (top). The middle image shows rock scattering the radio waves and ice reflecting them. The bottom image shows unpolarized (purple and blue) radio waves becoming polarized (purple only) by the reflection. Initial results suggested that water ice had been detected, though later analysis cast doubt on the findings. Since then, evidence has accumulated that water ice is present at the Moon's poles, with direct observations made in 2018.