Asteroid M is the name of an asteroid converted by the mutant Magneto into his home/orbiting base. It has been destroyed and reconstructed by the Master of Magnetism many times since it was first introduced. Each iteration has had several internal levels, including an observation deck, hangar bays and medical facilities. The various facilities have had technology that keep the asteroid concealed from standard detection technology. They have all been positioned in a low geosynchronous orbit. Each asteroid was relatively small yet large enough that it could not be shot down, or destroyed externally, without the risk of massive devastation to Earth.
Magneto completes construction on Asteroid M just before recruiting his children, Quicksilver and the Scarlet Witch into his group, the Brotherhood of Mutants, as seen in X-Men vol. 2, issue 1. This first version is destroyed in a battle with the original team of X-Men in X-Men #5.
Asteroid M is reconstructed just after Magneto imprisons the X-Men in his Antarctic citadel, reducing them to the level of six-month-old babies, around the time of X-Men #113. The space station is severely damaged when Warlock, an mutant-alien being from a techno-organic civilization with rigid rules and regulations, is fleeing to Earth. He collides with Asteroid M, breaking it to pieces, as seen in New Mutants, Vol. 1, #21.
During his brief association with the Hellfire Club, Magneto builds a fancier version of Asteroid M. Wanting a contingency plan if things don't work out with the Club, Magneto proceeds to rebuild his base of operations to survey the Earth from high above. From there, he hears of the troubles of his daughter, the Scarlet Witch. He descends to Earth to bring her back to his space station. This version was first seen in Avengers West Coast #57. When the West Coast Avengers attack Asteroid M to free their team-mate, we learn that Magneto has service robots on the station. The service robots are instrumental in alerting Magneto to the presence of the Wasp on his base. Furthermore, we learn that the entire asteroid isn't made out of metallic objects. Quicksilver demonstrates this when he rips plastic tubing out of one the consoles. After events involving Zaladane, in the Savage Land, Magneto returns to Asteroid M to live in peace, but his peace doesn't last very long. Acolytes, led originally by Fabian Cortez, request and are granted sanctuary by Magneto on Asteroid M, to escape American soldiers that are chasing them. In X-Men, vol. 2, #1, it is stated that the asteroid is in geosynchronous orbit and 250 kilometers above the Earth (geosynchronous orbit is actually much farther from Earth than this; in all likelihood it was being magnetically levitated at that altitude by Magneto through the use of his mutant power). The asteroid also has selective inhibitor fields that may keep mutants or superhumans from using their powers. This version of the asteroid has medical and science facilities, an observation deck, sleeping quarters, and a swimming pool. This third version of Asteroid M is destroyed in X-Men vol. 2, #3. From an escape pod, Cortez sets off nuclear missiles that were around the asteroid. Although Magneto survives the re-entry of Asteroid M to Earth, the rest of the original Acolytes do not. The remains of this Asteroid M crash in the Middle East. Forge, Henry Peter Gyrich, and other government officials, inspect the wreckage in Uncanny X-Men #299.
After the X-Men are chased out of their new base in San Francisco by Norman Osborn and the Dark X-Men, in the conclusion to the Dark Reign storyline Utopia, Cyclops sets up the new headquarters and mutant sanctuary on the ruins of the original Asteroid M which was raised from the bottom of the ocean. Christening it "Utopia," this version is more of a haven than a base of operations, brought on not by a desire to make a place among mankind, but to simply get away from it. However many see Utopia as a new Genosha and fears the same fate that had fallen on the once proud island nation.