After four attempts, SpaceX has successfully landed its Falcon 9 rocket on a floating barge. It's a huge milestone for the private company, and one step closer to reusable rocket technology, something scientists believe will be crucial to one day taking humans to Mars, not to mention making space travel much more affordable.

SpaceX has successfully landed its Falcon 9 rocket back on Earth before, but that occurred at a site in Cape Canaveral, Florida. Landing on a drone ship at sea is much more difficult because both the rocket and the barge are moving; it's essentially like finding a needle in a haystack in terms of difficulty.

SpaceX has gotten close to sticking the landing before but today's feat marks a significant milestone, not just for the company, but for humanity. Now that SpaceX has demonstrated its rockets are capable of landing on land and at sea, it could lead to many more missions down the road, and make going to space a hell of a lot less expensive.

Elon Musk, SpaceX's CEO, said drone ship landings are "needed for high-velocity missions." Musk added that altitude and distance don't mean much for orbit—speed does.

"Ship landings are not needed for flexibility or to save fuel costs," Musk explained. "Just not physically possible to return to return to launch site."

You can see the moment SpaceX made history in the video below.