Why the US can’t send humans to Mars©JPL/NASA

• Humans have long imagined life on Mars, though our understanding of the planet has changed a lot.
• Some of the US’s earliest plans assumed humans could reach the Red Planet by the 1980s.
• Over the decades, technology and funding challenges have hampered the nation’s hopes of crewed flights.

Earlier this month, NASA announced it was funding a revolutionary high-thrust rocket — called a Pulsed Plasma Rocket — that could make crewed missions to Mars in just two months.

That’s seven months faster than it’d take with current technology, and it would drastically reduce the risk and cost of a crewed Mars mission, according to Howe Industries, which is developing the concept. It “holds the potential to revolutionize space exploration,” NASA said in a statement.

The PPR is just one of the latest developments in the US’s decadeslong discussion to send humans to Mars. In the early ’60s, for example, nuclear-bomb-powered spaceships were proposed for the trip.

Since well before NASA landed the first humans on the moon, the US has poured money and time into proposals for a crewed Mars mission, only to see its attempts never leave the ground. But technology isn’t the only thing standing in the way. Politics also plays a big role.

“That’s kind of like a joke within the space community or the Mars community,” Matthew Shindell, a curator with the National Air and Space Museum, told Business Insider. “Putting humans on Mars is always 20 years away.”

He said it was short enough to seem tangible but long enough that the political situation would change before it could be realized.

To fully understand why the US hasn’t sent humans to Mars despite sending more robots there than any other country, it just takes a trip down memory lane. Here’s a history of the US’s most promising crewed Martian missions that never were.