On Tuesday, SpaceX CEO Elon Musk announced his ambitious — if crazy — plan to colonize Mars within the next 40 to 100 years. But Musk isn’t the only one with Mars dreams and ambitions. In fact, people have been drawing out missions to the Red Planet for at least the last 70 years — and we’re still waiting for one to take flight.

HOW DO THE MOST PROMINENT PLANS STACK UP?

One of the early proposed missions that set the stage for today’s Mars plans was the Mars Direct proposal, which was published by aerospace engineers Robert Zubrin and David Baker in 1991. They thought it might be possible to land the first crewed mission to Mars as early as 1999. That didn’t happen of course, but some of their ideas — like sending a return vehicle to Mars before sending a human crew and making propellant from the planet’s resources — are still considered viable options.

Among Musk’s competition there’s NASA, of course, which intends to send astronauts to Mars sometime in the 2030s. And then there’s Mars One, which is planning a one-way trip to the Red Planet by the year 2026. All missions to Mars will be massive — and expensive — undertakings. There’s still a lot we need to figure out to land heavy cargo on Mars and to protect people from radiation while traveling in space. Then there’s the life-support system on the planet itself. We need to figure out how to create habitats that have electricity, sanitation, clean air, and potable water. And what about food supplies and spare parts when things break down?