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.

Musk’s comments may sound like disbelief, but it’s part of the reason why he founded SpaceX in the first place. Fresh from his success with PayPal, Musk formed the company in 2002 after growing despondent that NASA wasn’t planning a mission to the red planet. His hope was to send a greenhouse to Mars, reignite the industry and kickstart a second Space Race. He’s continued to express this goal: after launching the Falcon Heavy rocket in February, Musk said that he hoped it encouraged nations and firms to “raise their sights.”

SpaceX and Boeing are in an intense competition to reach Mars first. Musk unveiled the BFR rocket in September of that year with a plan for SpaceX test flights in 2019, an unmanned mission in 2022 and a manned mission as soon as 2024. Boeing, on the other hand, is developing the Space Launch System due for testing next year in collaboration with NASA — which means the first team to get to Mars could arrive on a Boeing rocket, but as part of a NASA mission.

The test flights may prove the first hint at who is likely to reach Mars first. Musk stated earlier this week that tests of the Raptor engine are making good progress, 31 of which will power the BFR.

The new era of space exploration is reaching fever pitch.

Photos via SpaceX