booster for space vehicles, particularly the U.S. Mercury spacecraft series, designed originally as an intercontinental ballistic missile (ICBM). Atlas is equipped with three engines—two boosters that are jettisoned after about 2 ¹/₂ minutes of operation and a sustainer that operates until orbital velocity is attained. Atlas can lift about 5,800 pounds (2,600 kg) to about a 350-mile (560-kilometre) orbit. Coupled with Agena, the combined Atlas–Agena rocket is used for launching lunar and planetary probes, as well as Earth-orbiting satellites, such as Seasat, where the Agena is also the spacecraft. Atlas–Centaur, or Centaur, combines the first-stage Atlas, which burns kerosene, with a second stage fueled with liquid hydrogen; it was the first rocket to use liquid hydrogen as fuel.