The long-simmering space rivalry between Elon Musk and Jeff Bezos is heating up.

Bezos’s aerospace firm, Blue Origin, turned up the temperature on Monday, by filing a protest against NASA’s choice of Musk’s SpaceX for a mission to the Moon.

Earlier this month, NASA awarded SpaceX a $2.9 billion contract to build a lunar lander. Blue Origin and defense contractor Duentics had also submitted proposals for the project.

NASA had been expected to choose two of the three firms, to ensure they had a backup and a healthy dose of competition. But the space agency said it plumped for a sole awardee due to budgetary constraints.

[Read: 3 new technologies ecommerce brands can use to connect better with customers]

The explanation hasn’t satisfied Blue Origin. The company has now filed a 50-page protest with the federal Government Accountability Office (GAO) over the decision.

The firm argues that it wasn’t given a chance to revise its bid after the funding requirements changed, the New York Times reports. The protest further claims that NASA had misjudged aspects of Blue Origin’s proposals, while downplaying the risks in SpaceX‘s design.

In a statement, Blue Origin said NASA had “moved the goalposts at the last minute.”

In NASA’s own words, it has made a ‘high risk’ selection. Their decision eliminates opportunities for competition, significantly narrows the supply base, and not only delays, but also endangers America’s return to the Moon.

Inevitably, Elon Musk responded to the complaint by tweeting a penis joke.

The two richest people in the world have been battling over the cosmos since a 2004 dinner meeting. At the time, the duo were only getting warmed up for their space race.

Bezos had founded Blue Origin in 2000, while Musk had launched his rocket company two years later. But the later starter was unimpressed by the Amazon founder’s approach.

Musk recounted their meeting to author Christian Davenport for his book, “The Space Barons.” He recalled warning Bezos that SpaceX had already tested some of the Blue Origin’s ideas:

Dude, we tried that and that turned out to be really dumb, so I’m telling you don’t do the dumb thing we did. I actually did my best to give good advice, which he largely ignored.

The two tycoons have been trading barbs and competing for contracts ever since.

With no rapprochement in sight, it might be time to settle their 15-year feud the old-fashioned way: with a fight. In zero-gravity. May the best astronaut win.

Partnerships advent solutions.