What just happened? Bill Gates and Elon Musk are two of the wealthiest people in the world, but they certainly don't see eye to eye. The pair have had public spats in the past, and now the former Microsoft boss has taken a shot at Musk's ambitions for sending missions to and colonizing Mars, saying he would rather pay for vaccines than travel to the red planet.

In an interview with the BBC, Gates was asked by journalist Amol Rajan if going to Mars is not a good use of money. "It's actually quite expensive to go to Mars. You can buy measles vaccines and save lives for $1,000 per life saved," he replied. "And so [that] just kind of grounds you, as in - don't go to Mars."

Gates' reply seems aimed chiefly at Elon Musk, whom he's clashed with in the past, though fellow billionaires Jeff Bezos and Richard Branson both have space exploration companies in Blue Origin and Virgin Galactic, respectively.

Gates has poured billions into vaccine research and delivery around the world through the Bill & Melinda Gates Foundation, founded alongside his ex-wife Melinda. He criticized the US government back in 2020 over its Coronavirus strategy, saying it did not act fast enough to prevent a shutdown, and has called for the creation of a $1 billion-per-year Global Epidemic Response and Mobilization (GERM) team in preparation for the next pandemic.

Gates' focus on vaccines and Covid-19 led to some conspiracy theorists claiming he started the pandemic, used the vaccinations to track people, or profited from the outbreak. "I did not expect that," he said, "These messages sort of inverted that. I guess people are looking for the 'boogeyman' behind the curtain, the over-simplistic explanation. Malevolence is a lot easier to understand than biology."

Gates did have some positive things to say about Musk. When asked if he would encourage the Twitter owner to join the "club of megaphilantropists," Gates said, "Yeah, I think someday he'll be a great philanthropist. Obviously things like Tesla are having a positive impact even without being a form of philanthropy."

Gates once said that Musk, who previously spoke out against the lockdowns and Covid-19 infection numbers, should keep his comments to himself. In addition to arguing over colonizing Mars and cryptocurrencies, Gates said he feared Musk's free-speech plans would make Twitter worse.

Musk, meanwhile, once said he confronted Gates about shorting Tesla stock, which he followed with a now-deleted Tweet (above) mocking Gates' appearance.