When there are two full moons in a single calendar month, the second one is referred to as a blue moon. This month, the first full moon was on May 1, and a second one will occur early Sunday morning. May’s blue moon is also a micromoon: It will look smaller and dimmer than usual because our lunar companion is farther away from Earth.

"Once in a blue moon" has become synonymous with something rare, yet the astronomical event itself is neither especially unusual nor related to the Moon's color.

At an event at NASA Headquarters, space agency officials unveiled the first rovers and landers headed to the future site of its planned lunar south pole outpost