From Halopedia, the Halo wiki
Atlas is a planet known to have multiple natural satellites, collectively known as the Atlas Moons.
The prolonged Siege of the Atlas Moons during the Human-Covenant War was fought around the planet's moons. Vice Admiral Danforth Whitcomb commanded the UNSC Naval forces in the battle, gaining reputation for his accomplishments.
The planet (like the UNSC Atlas) is named after Atlas, a titan from the Greek mythology who bore the world on his back.