I just read, with a mixture of fascination and embarrassment, writer Nancy Atkins' compelling article — posted on the Universe Today web site — about the realities of landing humans on Mars. The article asks the simple question of how we would land a crewed spacecraft on Mars. She does a beautiful job with it, and you should stop right here and read the article for yourself.
My embarrassment is in regard to the fact that the whole question never even occurred to me, after all these years of talking about Mars landings in planetaria, magazines, and public talks. As Atkins' article points out, we just assume it can be done. But how?
The basic problem is that Mars is big enough to have significant gravity (about one-third of Earth's), but its atmosphere is too thin to enable a large, heavy lander to slow down fast enough to prevent the astronauts from ending up as moist stains at the bottom of a deep crater.
"Heat shield!" you are probably thinking. "Parachutes! Air bags!"
Sure, if you are a small robotic rover weighing less than a metric ton — 1,000 kilograms, or 2,200 pounds — chutes and bags are fine. That's how we got the Mars Exploration Rovers, weighing in at a mere 175.4 kg (387 lbs) each to Mars' surface.
In other words, if you want to land a Sumo wrestler on Mars, no problem. But if your space ship weighs more than 1 metric ton, Houston, we've got a problem. The crew would have about 90 seconds to slow from Mach 5 to Mach 1, get into position to land, and then use chutes and thrusters to slow down for a soft landing.
A crewed lander, as currently envisioned by proponents of Mars exploration, could weigh up to 60 metric tons (133,000 lbs). At this mass, parachutes and airbags, even in combination, are insufficient to prevent the mission from splatting on the surface at the speed of sound.
Fine, then how about thrusters, like the ones Apollo astronauts used to land on the Moon? The problem is that for every pound of spacecraft and astronaut, you would need 6 pounds of rocket fuel to slow down for a soft landing. So that's about 800,000 pounds of fuel, give or take, to land a 133,000-pound craft. And you still have to bring enough fuel to get to Mars, and then to lift off and get back to Earth. And you thought it was expensive to fill up your car's gas tank.
Another dilemma: Unlike the Moon, which is relatively airless, Mars has an atmosphere, however thin (1% of Earth's). As the craft fires thrusters, it descends at supersonic speeds into a maelstrom of exhaust and turbulent atmosphere. This situation is unstable, and if the craft loses its balance for a moment, it could be torn apart by aerodynamic forces.
But all is not lost. NASA engineers have some clever ideas for how to get to Mars' surface, like deploying a huge, inflatable "hypercone" around the craft to increase its surface area enough to slow down. Read Atkins' excellent article for the details.
And next time a science writer blithely refers to a future planetary mission as if it were as straightforward as, say, designing a new living-room recliner, start asking the tough questions. Starting with: How?