By why has it been nine years? And why is it SpaceX, not NASA sending them?

The space shuttle is arguably the most recognisable spacecraft ever built. Aside from the Saturn V, if I ask you to imagine people launching into space, the space shuttle will probably be it.

All told, there were six space shuttles built. The Enterprise was used for design and testing, and eventually moved to the Intrepid museum in New York. There were the original four operational shuttles - Challenger, Columbia, Discovery, and Atlantis. After the 1986 Challenger disaster, a sixth was built, Endeavour.

The first space test flight was in 1981, and over the next 30 years, the space shuttles would fly 135 missions. But it came with a big price tag. The whole space shuttle program cost $209 billion (yes, billion with a b), meaning it was about $1.5 billion per mission. A cheaper way was needed to send humans into space.

This doesn't mean the space shuttle wasn't successful, I think it was very successful. Numerous spacecraft and probes went up on the shuttle. The Hubble Space Telescope was not only launched from it, but the space shuttle had four service missions to fix it and keep it going, and it is why we just celebrated Hubble's 30th anniversary in April.

The original vision of the space shuttle, which started in 1972, was to have a space vehicle that could go up about once per week, and cost tens of millions, not a billion. Ultimately, the space shuttle was not the vision.

Queue the private sector.

In 2004, a year after the second space shuttle disaster, Columbia, it was announced the space shuttles would be retired. It wasn't just the disasters or the costs, it was being open to the idea that NASA couldn't do everything. If NASA really wanted to go back to the Moon, and to Mars, it needed help.

When George W. Bush announced the shuttle retirement, he also announced that private companies would more readily be able to get NASA contracts for launch services. Under Barack Obama, this program was expanded. This allowed NASA to invest in private companies, aid development at a fraction of the cost, and focus on bigger goals, like the Moon and Mars.

NASA has invested more than $3 billion in SpaceX's Crew Dragon development. That is two space shuttle launches. They have also invested in Boeing's CST Starliner program. Again, at the rate of about 2.5 space shuttle launches.

This allowed SpaceX and Boeing to develop cheap, cost effective ways which meets that original vision of the shuttle program nearly 50 years ago.

SpaceX and Boeing are not the only ones. Blue Origin and Virgin Galactic are getting close to human launches. And not just astronauts, but private citizens as well.

This launch is really the start of a new era of human space flight.