Not only is Jean Grey an X-Man, but she is also an astronaut. When she was in space, she became possessed by the Phoenix. In classic mythology, the phoenix is a bird that dies and rises from its ashes. In the Marvel world, the Phoenix is an evil super thing that tries to destroy universe. When Jean Grey is the Phoenix, she beats up her friends and tries to destroy the world. She actually succeeds at destroying another planet, which causes many aliens to die. Even though Jean was evil, she at least got a costume upgrade.
(Drawing by John Byrne.)