Billionaires Elon Musk and Jeff Bezos locked horns again today as the Amazon (NASDAQ:AMZN) boss lodged a formal protest over the award of NASA’s contract to return astronauts to the Moon.

Space-X, Musk’s rocket business, won the contract earlier this week sparking the official complaint and another war of Twitter words.

Bezos’ Blue Origin has accused US Space agency NASA of a last-minute change of terms for the contracts, sparking a mocking retort from Musk, who tweeted.

Tesla (NASDAQ:TLSA) CEO Musk also posted an article from 2019 detailing Blue Origin’s plans for landing and operating on the Moon, while his latest tweet contained pictures of Earth and Mars. 

“NASA has executed a flawed acquisition for the Human Landing System program and moved the goalposts at the last minute,” Blue Origin said in an emailed statement, reported by Reuters.

“Their decision eliminates opportunities for competition, significantly narrows the supply base, and not only delays, but also endangers America’s return to the moon. Because of that, we’ve filed a protest with the GAO.”

Blue Origin partnered with Lockheed Martin, Northrop Grumman Corp and Draper for its bid, while Space-X bid alone.

NASA has said it is aiming to get to the Moon as early as possible and potentially by 2024.