The Soviet Union launched a dog known as Laika into orbit on November 3, 1957.
Tardigrades—otherwise known as "water bears" or "moss piglets"—are considered extremophiles, organisms that thrive in extreme environments.
"Uncontrolled biological contamination of the Moon's surface is not scientifically ideal," said Lisa Pratt, director for NASA's Office of Planetary Protection.
The creatures "can reversibly enter a state where outwardly observable signs of metabolic activity are paused under conditions that are essentially incompatible with life."
Some tardigrades can survive extreme radiation, air deprivation and even the vacuum of space.
How do you design a spacesuit for a microscopic critter with eight legs?
The tiny creatures can also survive liquid nitrogen and boiling water.