Elon Musk mocked President Joe Biden over the weekend after Saturday's successful splashdown of SpaceX's Inspiration4 flight carried four tourists on a three-day orbital mission.

When someone asked Musk on Twitter why Biden hadn't acknowledged the accomplishment, the SpaceX and Tesla CEO offered some choice words.

"He's still sleeping," Musk responded in the Sunday afternoon tweet, echoing the insult of former President Donald Trump, who referred to Biden by the nickname "Sleepy Joe."

Later, someone else tweeted with an image from the movie "Alien," labeling the alien grabbing onto the face of an astronaut as "UAW" and the victim as "Biden." Musk responded with a tweet that said, "Seems that way." Musk has been in battles with the United Auto Workers union, which thus far has failed to organize workers at Tesla's factories.

The SpaceX flight received multiple tweets of congratulations during and after the flight from Bill Nelson, the former senator who Biden appointed as NASA administrator.

"Congratulations #Inspiration4! With today's splashdown, you've helped demonstrate that low-Earth orbit is open for business," Nelson tweeted on Saturday.

"Low-Earth orbit is now more accessible for more people to experience the wonders of space," he tweeted last week after lift-off. "We look forward to the future — one where NASA is one of many customers in the commercial space market. Onward & upward."

Biden has been good for Musk

In a number of ways the Biden administration has been very good to Musk's companies. SpaceX has NASA contracts to deliver both supplies and US astronauts to the International Space Station. Many predate Biden taking office, but the company has gotten additional NASA contracts this year. In April it was awarded a US$2.9 billion NASA contract to build spacecraft that will land astronauts on the moon for the first time in five decades, despite protest from Blue Origin, the rocket company led by Amazon founder Jeff Bezos, which had also sought the contract.

Tesla has greatly benefited from regulatory tax credits it sold to other automakers who were not complying with tougher emission rules. Tesla, which sells only EVs, has an excess supply of those credits that can be sold. Those credits could get more valuable if the Biden administration toughens emissions rules as it has proposed to do.

Until the most recent quarter, Tesla would never have turned a profit without the benefits from the sale of those credits, and back when it was still losing money the credits also helped fund its early operations.

Tesla also benefited in the past from a US$7,500 tax credit to buyers of electric vehicles, a fact that allowed it to charge more for the cars.

Buyers of EVs made by Tesla and General Motors no longer qualify for that tax credit because of the number of plug-in cars they have delivered in the past.