When NASA’s Mars Science Laboratory — scheduled to lift-off from Cape Canaveral later this week — touches down on the Red Planet in August of 2012, the one-ton probe will be the largest and most complex piece of unmanned machinery ever to land on another world.

But here’s a little secret: With current technology, nothing larger or heavier than MSL can be put on the surface of Mars. Anything more massive, including a human mission, which NASA estimates would require landing at least 40 to 80 tons of machinery, is completely out of the question.

“We’ve maxed out our ability to take mass to the surface of Mars,” said engineer Bobby Braun, former NASA chief technologist and co-author of a 2005 research paper highlighting this problem.

The basic obstacle for large-scale missions is Mars’ tenuous atmosphere, which is more than 100 times thinner than that of Earth. The pressure of the Martian atmosphere at its surface is equivalent to what someone would experience flying at 100,000 feet on Earth.

“That’s wickedly high — past outer space,” said engineer Robert Manning, the chief flight systems engineer on NASA’s Pathfinder rover and co-author of the research paper. “Landing on Mars is like putting a landing space up on an absurdly high mountain and saying: ‘Land there.’”

But the situation isn’t hopeless. In the years since Braun and Manning’s article appeared, engineers have been coming up with ideas for the next generation of Martian landers. Here are some of the schemes that may one day bring large payloads — and humans — to the surface of Mars.

Above:

Every mission to Mars that NASA has undertaken in the last 40 years has relied on roughly the same technology as the Viking landings in 1976 (top image).

The two Viking landers, which were slightly more than half the weight of MSL, approached Mars inside a rigid aeroshell heat shield that slowed them down in the upper atmosphere. Then around 4 miles above the surface they deployed a parachute to cut down their speed even more. At about 5,000 feet up, they fired three retro-rockets to bring their velocity close to zero and make a soft landing.

Engineers have since been taking the same basic capabilities and incrementally tweaking them, using airbags to land Pathfinder or employing the elaborate sky crane that will lower MSL to the surface.

“We’ve been living off this Viking legacy,” said Braun. “The real challenge is if now we want to do missions more complicated than MSL, we’re going to need entirely new approaches.”

Image: An artist's depiction of a Viking lander inside its aeroshell. Donald Davis.