Getting humans to Mars and actually getting them to live there will be no easy task. But this week, Elon Musk tried to put some numbers to what it would take to make it happen.

In a series of tweets on Thursday, Musk said that SpaceX will need to build a whopping 1,000 Starship rockets to create what he calls a "sustainable Mars city." At a cost of about $2 million per launch, that would suggest a total of $2 billion for those flights just to get people and supplies to Mars to create that city.

The problem, however, is that it won't be quick. Since Earth and Mars only align twice a year, there are only two windows to actually launch spaceships. Musk said in his tweets that it could take up to 20 years of successful launches before Mars has everything it needs to support a truly sustainable city.

Musk's comments come after he spoke earlier this week about the viability of reusable rocket technology. He estimates that the SpaceX Starship, which can transport 100 passengers into orbit, will cost $2 million per launch. That's a 99 percent discount off the $152 million the average NASA launch costs. Musk claims SpaceX could bring that price down by employing reusable rockets.

The idea of having a sustainable city on the surface of Mars seems straight out of a science-fiction novel, yet it's become a more plausible and perhaps possible endeavor in recent years, thanks to innovations in reusable technology.

It's also become big business. SpaceX certainly stands to gain from any move to Mars, but other companies may also benefit by providing the supplies, support, and other technologies to boost SpaceX's efforts. It also stands to reason that as innovations occur, companies can use technologies developed as part of the Mars plan to develop new products and services.

Alas, whether it's actually feasible for humans to go to Mars is something that is very much up for debate. Many scientists fear that Mars doesn't have the requisite carbon dioxide to support human life. Musk himself has said that detonating nuclear weapons that heat up frozen water on Mars to evaporate it and potentially release CO2 into the atmosphere might be a possibility. However, a Nature study called that theory into question.

Ultimately, we just don't know right now if Mars is in humanity's future. But for Musk, it is. And for now, that means big spending -- and plenty of Starship production -- to try and make it happen.