When Space X’s Falcon Heavy lifts off, hopefully by this Sunday, it will be the most powerful operational rocket in the world by a factor of two. The Space X’s most mysterious launch to date has been delayed for nearly two months and was just delayed once again. If Falcon Heavy does launch on Sunday it will include the launch of “Zuma” in its mission, but what exactly Zuma is, though, is still a big secret.

Northrop Grumman contracted Elon Musk’s commercial space company to do the launch on behalf of an unnamed US government agency, but no other details about the payload have been provided to the media.

With the ability to lift into orbit over 54 metric tons (119,000 lb)–a mass equivalent to a 737 jetliner loaded with passengers, crew, luggage and fuel, the Falcon Heavy rocket can lift more than twice the payload of the next closest operational vehicle, the Delta IV Heavy, at one-third the cost. Falcon Heavy draws upon the proven heritage and reliability of Falcon 9.

Its first stage is composed of three Falcon 9 nine-engine cores whose 27 Merlin engines together generate more than 5 million pounds of thrust at liftoff, equal to approximately eighteen 747 aircraft. Only the Saturn V moon rocket, last flown in 1973, delivered more payload to orbit. Falcon Heavy was designed from the outset to carry humans into space and restores the possibility of flying missions with crew to the Moon or Mars.