Elon Musk has thrown down the gauntlet. The SpaceX CEO responded to Boeing CEO Dennis Muilenburg’s remarks that Boeing could get to Mars first with two words, posted to Musk’s Twitter page in the early hours of Friday morning: “Do it.”

Muilenburg was asked in an interview with The Street on Friday about who would make it first out of the two. After making his declaration, he explained: “We are working jointly with NASA and building that first rocket space launch system. It’s about 38 stories tall, the first story is being built right now. It has 9.2 million pounds of thrust on that rocket, it’s the biggest rocket ever. We will begin test flights starting next year. I firmly believe that the first person to step foot on Mars will get there on a Boeing rocket.” The two CEOs have been locked in battle over the goal, and the pair had a near-identical exchange of words in December 2017.