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: Old Technology 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.

The Problem With Parachutes Astronauts coming back to Earth from space have a thick, fluffy atmosphere to help slow them down. The Apollo-style landings, where parachutes are used to reduce speed, or the airplane-like landing of the Space Shuttle, benefit from this dense cushion of air to provide friction or lift during descent. “Mars has a poor excuse for an atmosphere,” said engineer Neil Cheatwood of NASA’s Langley Research Center. “If a planet had less than Mars, it basically wouldn’t have an atmosphere.” The typical approach to landing on Mars has been to use a parachute to ease down toward the ground. Yet even after parachute deployment, a vehicle retains a great deal of speed. Compare a skydiver on Earth, whose speed changes from 120 mph to less than 10 mph once the parachute is open to one on Mars (now traveling 1,000 mph due to the diminished atmosphere), who would only slow down to 200 mph after pulling the ripcord. “That’s not good enough for your legs or your landing gear,” said NASA engineer Robert Manning. Past Mars landers have used either rockets or giant Kevlar airbags to reduce this final speed. When it lands, the Mars Science Laboratory will employ a 51-foot, supersonic parachute — the largest ever used on an interplanetary mission — and will still require an elaborate maneuver to land. A UFO-like platform will fire rockets to hover 25 feet above the ground and then gingerly lower the rover down on wires. Unfortunately, this technology can’t be scaled up. The parachute needed to slow down a 40-ton manned lander in the Martian atmosphere would have to be the size of the Rose Bowl and would take so long to inflate that the vehicle would hit the ground before it was fully open. “MSL is a dead end,” said engineer Michael Wright, NASA’s co-chair of entry, descent, and landing technology. “None of the technology will get us to 40 tons. We have to step back and look at new ideas.” Image: 1) The Mars Science Laboratory floating down with its parachute. NASA/JPL-Caltech 2) MSL being lowered on the "sky crane" NASA/JPL-Caltech.

Inflatables in Space Spacecraft approach Mars at more than 15,000 mph but placing a large, flat object — the wider, the better — underneath a lander could reduce much of its speed when it hits the upper Martian atmosphere. MSL's heat shield will slow it down this way, but at 15 feet in diameter, the shield is as wide as can be made while still fitting inside one of NASA’s launch vehicles. So the agency is looking into Inflatable Aerodynamic Decelerator (IAD) technology. Unlike a flimsy parachute, these devices would be rigid when inflated and be able to operate at speeds that would tear a parachute to shreds. For instance, a HIAD (the H stands for hypersonic) could be folded and packed inside a rocket for take-off but then inflated just before entering the Martian atmosphere, growing from 15 feet to more than 80 in a few seconds. It would likely be made of lightweight materials that are rugged and rigid enough to survive heating up in the atmosphere, such as Kevlar. NASA currently has plans to test this technology early next year, said engineer Ian Clark of the agency’s Jet Propulsion Laboratory. In January, engineers plan to send a hot-air balloon into Earth's upper atmosphere to drop a 15-foot-diameter, doughnut-shaped object that inflates to 20 feet, to simulate Martian conditions. Clark is confident that building a 20-foot IAD will be fairly straightforward. But one of the worries with inflatables is that they will have a hard time staying completely rigid. Even slight wobbliness could affect how they come down through the atmosphere. “Smaller is better,” said NASA engineer Michael Wright. “The bigger it is, the more floppy and complicated it gets.” Image: NASA

Hitting the Atmosphere As it falls toward the Martian surface, a large vehicle could reduce its speed with a Supersonic Inflatable Aerodynamic Decelerator (SIAD). “A SIAD is like a parachute except it can open higher, faster, and do a better job,” said NASA engineer Robert Manning. “It’s scalable to much larger sizes, too. We can imagine a 100-foot or even larger inflatable that might do the job.” Because it is rigid, this technology would be able to inflate while a lander is traveling at four to five times the speed of sound in the Martian atmosphere — a velocity that parachutes can’t withstand. The SIAD would use its large surface area to increase drag on a vehicle and reduce its speed. Unlike a parachute, a SIAD wouldn’t trail behind a lander, instead opening up like a gigantic umbrella to cover the front of a spacecraft. Engineers estimate that a human-scale mission would need a SIAD in the range of 75 to 160 feet across. One thing that would need to be taken into consideration is the large amount of heat that the inflatable will be subject to as it tears through the Martian atmosphere. Advanced materials or some sort of heat shielding might be able to take care of this problem. But a larger obstacle is how to create a SIAD system that is large enough yet deploys quickly. “At some point, it’s too big to inflate instantaneously,” said NASA engineers Neil Cheatwood. Cheatwood said he wouldn’t be comfortable building a SIAD larger than 50 feet, and that he’s not even positive that 30 feet is a sure bet. “If you start going to really high masses — more than two or three times Mars Science Laboratory — a SIAD isn’t reasonable,” he said. Image: NASA

Supersonic Retropropulsion The final technology that could bring larger payloads to the surface of Mars is called supersonic retropropulsion. This method involves a vehicle firing rockets forward in the direction it's traveling in order to slow it down. Most landings on Mars have made use of retro-rockets, but not until the vehicle was already below the speed of sound. Bringing down masses larger than the Mars Science Laboratory will require engineers to create a rocket that can fire while the vehicle is still moving faster than the speed of sound, a hazardous undertaking in the presence of an atmosphere. “Turning our propulsion on at those speeds creates all kinds of complicated shock waves that can go back onto a vehicle,” said NASA engineer Michael Wright. NASA currently has no experience and very little data on supersonic retropropulsion, so predicting how this shock wave would affect a lander is very difficult. In the past two years, the agency has conducted tests inside a wind tunnel on Earth to observe the flow field of a jet fired into a supersonic stream. But those experiments were axed along with much of the Bush Administration-era Constellation program that would have taken humans to the moon and Mars. This is problematic because a NASA study identified supersonic retropropulsion as a must-have for a human mission to Mars. Without this technology, landing humans on the Red Planet could prove impossible. This pessimistic view has its detractors, though. It might alternatively be possible to use an Inflatable Aerodynamic Decelerator and then a parachute to bring a lander down to subsonic speeds, and still be able to use traditional retro-rockets, said NASA engineer Neil Cheatwood. Image: A prototype supersonic retropropulsion rocket during wind tunnel testing. George Homich/NASA Langley.