The Cherenkov Effect is caused by high-energy beta particles moving at velocities faster than the speed of light in water.
The effect causes the blue glow in nuclear reactors (right).
This process is similar to the sonic boom heard when an airplane exceeds the speed of sound.
Pioneer 2 re-entered Earth's atmosphere over Northwest Africa.
This was the last of the U.S. Air Force Project Able Probes.
This was America's third attempt to reach the Moon.