The Tesla and SpaceX CEO posted a challenge on Twitter after Boeing CEO Dennis Muilenburg said on Thursday that his aerospace giant would transport the first humans to Mars. “Eventually we’re going to go Mars and I firmly believe the first person that sets foot on Mars will get there on a Boeing rocket,” Muilenburg told CNBC’s Jim Cramer this morning.

In response to a tweet from the “Fortune Tech” Twitter account linking to Fortune’s news article about Muilenburg’s bold bet, Musk simply wrote, “Do it.”

Here’s Musk’s tweet:

Do it

— Elon Musk (@elonmusk) December 7, 2017

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Muilenburg has made the same proclamation in the past, including last year at a tech conference. However, Musk’s own very public ambitions to not only reach Mars, but to colonize the Red Planet, seem to have linked the celebrity billionaire’s name with the idea of Mars exploration in the public imagination. In October, Musk outlined the SpaceX strategy for reaching Mars and even his goal of settling large groups of people on the planet as part of his vision of making humans into a “multiplanetary species.”

Musk believes that SpaceX could begin landing spaceships on Mars as soon as 2022, while Muilenburg said on Thursday’s that Boeing’s own next generation rockets would be tested starting in 2019.

Musk is an active Twitter user who is known to jump into the Twitter conversations among Tesla customers and critics. Last year, the billionaire even engaged in a brief Twitter spat with Fortune (including blocking Fortune‘s primary Twitter account) over its reporting in the wake of a Tesla driver’s fatal crash and Musk’s post-crash sale of Tesla shares in a public offering.