San Francisco, Sep 21 (IANS): Tech billionaire Elon Musk took to Twitter to mock US President Joe Biden for failing to congratulate his company SpaceX's historic space flight.

One of Musk's followers on Twitter asked him, "The President of the United States has refused to even acknowledge the 4 newest American astronauts who helped raise hundreds of millions of dollars for St. Jude. What's your theory on why that is?"

Musk replied, "He's still sleeping."

The tweet drew sharp reactions from social media users.

"Dude is busy running a nation during a pandemic. Post-war intelligence briefings. Cybersecurity threats. SCOTUS shenanigans and human rights. Budget and tax legislation," wrote one user.

"You're awake, you may be better than the sleeper. Why don't you nominate yourself to be the next president of America instead of the sleeper" wrote another user.

After spending three days on-orbit, the world's first civilian mission of SpaceX's Inspiration4 returned to Earth on Sunday.

The crew, onboard the Dragon spacecraft re-entered Earth's atmosphere for a soft water landing off the coast of Florida at 7.06 p.m. EDT (4.36 a.m. Sunday India time).

The mission lifted off to orbit aboard SpaceX's Falcon 9 rocket and Dragon spacecraft on last Wednesday (India time Thursday).

It was commanded by tech entrepreneur Jared Isaacman and joined by Medical Officer Haley Arceneaux, a physician assistant at St Jude Children's Research Hospital and pediatric cancer survivor; Mission Specialist Chris Sembroski, an Air Force veteran and aerospace data engineer; and Mission Pilot Dr Sian Proctor, a geoscientist, entrepreneur, and trained pilot.

The fully automated Dragon capsule reached an unusually high altitude of 585 kilometres, surpassing the International Space Station by 160 kilometres. The Dragon's dome window, inspired by the Cupola on the ISSl provided the crew with incredible views of Earth.