SpaceX is about to conduct perhaps one of the biggest rocket tests in decade with a key test fire of the Falcon Heavy rocket that could one day carry astronauts to Mars. However, the much-awaited test has once again been delayed, raising some concern about the program and its near future.

SpaceX planned to fire the 27 Merlin engines on the Falcon Heavy rocket at Kennedy Space Center on Tuesday, but that date shifted to Friday at the earliest. Meanwhile, the United Launch Alliance will be launching a satellite from the complex on Thursday, which is part of the reason why SpaceX’s window is being pushed back.

Authorities hope to conduct the SpaceX Falcon Heavy test fire on Friday after 3 p.m., but there’s a chance it could continue to be delayed. The rocket tests and eventual launch are certain to be one of the most closely watched in decades.

“When Falcon Heavy lifts off in 2018, it will be the most powerful operational rocket in the world by a factor of two,” SpaceX says on its website. “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–Falcon Heavy 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.”