Elon Musk attacked astrophysicist Neil deGrasse Tyson after his comments about SpaceX.

The online spat between the two science giants began after Tyson said in a video shared on social media this September that SpaceX had done nothing “that NASA hasn’t already done.”

“What Elon pioneered was not what was impossible in space exploration, but what was unrealistic in terms of cost,” Tyson said in the video, which was taken about 17 minutes into an episode of the Moonshots podcast with Peter Diamandis.

“The engineering is being worked on, but consider that it hasn’t done anything that NASA hasn’t already done. The real space frontier is still held by NASA.”

“It remains to be seen if his new ventures have commercial backing, commercial value. Because if not, then it’s a one-off and it’s a spectacle … we’ll all applaud it, but then he’ll move on to his other projects.”

Responding to the clip shared on social media, Musk wrote on X: “Well then I think Neil will be pleasantly surprised by what SpaceX does in the future.”

It is hoped that a SpaceX spacecraft will one day carry people and cargo to Mars. Musk envisions an initial settlement where the first settlers would live and begin building infrastructure. The first missions would focus on carrying equipment to produce water, oxygen and fuel, and the first manned mission could take place in the early 2030s, depending on the success of unmanned missions and technological readiness.

Musk previously wrote for X: “Starship will make life multiplanetary and preserve life as you know it [sic] from extinction on Earth unless it is suffocated by bureaucracy.”

Also in the Moonshot interview, Tyson says he agrees that technical boundaries are being violated, but not the boundaries of space.

“When people say ‘Oh, now Elon’s leading us through space’ — no, he’s not. He’s cheapening everything we’ve been doing,” Tyson said.

In a video on his StarTalk channel in October, Tyson later clarified that he thinks SpaceX has taken some major steps forward, and that he simply means that SpaceX has yet to surpass NASA’s achievements.

“What private companies were doing was going back into orbit, where NASA has been since 1962,” he said in the video.

“No one has ever sent a rocket to Mars that could carry humans, which would push the space frontier,” he said. “Good luck SpaceX. God speed!”

However, months later on HBO’s Friday night episode Real Time with Bill MaherTyson went on to comment on Musk and SpaceX, saying that SpaceX’s plans to get to Mars were far-fetched and may never come off.

“My interpretation of the history of space exploration is that we only do big, expensive things when it’s geopolitically expedient, like when we feel threatened by an enemy,” Tyson said in an interview with guests Donna Brazile and Andrew Young.

“So to just say, let’s go to Mars, because that’s the next thing we have to do — what does the venture capital meeting look like, Elon, what do you want to do? I want to go to Mars. How much is it going to cost?” A trillion dollars, is it safe? What is the return on investment. Nothing?

Tyson noted that he “has nothing against Mars,” but he doesn’t think we’ll be going there anytime soon without a geopolitical stimulus.

“This is how we get to Mars – China leaked a report, it may not even be true, that they want to put military bases on Mars,” Tyson said.

“NASA doesn’t have a spaceship, but Elon does. Here’s my spaceship to Mars. After all, we’re paying him to use his spaceship to get to Mars. So I don’t see that happening until governments decide it’s geopolitically in our interest.” Otherwise, I don’t see it as just an exploration.’

Brazile pointed out that President-elect Trump has expressed interest in a mission to Mars.

“At some point somebody has to pay for it,” Tyson replied. “Just being interested in something is not the same as paying for it.”

Musk has long discussed his goal of establishing a self-sustaining human civilization on Mars, arguing that creating a backup for Earth’s population is necessary to ensure humanity’s survival in the face of existential threats such as natural disasters, climate change or potential global conflicts.

“If this is Earth’s plan B, what have you done to Earth that you now have to go to Mars?” Tyson said. “If you messed up Earth but you’re good enough to turn Mars into Earth, then you can turn Earth back into Earth and never have to go.”

Terraforming Mars—the process of changing its environment to make it habitable for humans—faces enormous scientific and technological challenges. Terraforming would aim to create a denser atmosphere rich in oxygen, higher temperatures and liquid water, as the Red Planet currently has an average temperature of about -80 degrees F and has an atmosphere made up of 95 percent carbon dioxide, only about 1 percent of Earth’s atmospheric pressure.

Musk responded to these comments, writing on X: “Wow, they really don’t get it. Mars is critical to the long-term survival of consciousness. Also, I won’t be asking any venture capitalists for money. I realize it doesn’t make sense as an investment, that’s why I’m raising resources.”

In another post, he made a personal comment against Tyson.

“The real problem is that Neil decided to crawl to the woke far left when he got hit with #MeToo,” he wrote on X. “You can avoid cancellation if you beg for forgiveness and push their nonsensical ideology. The truth hurts.”