As the 40th anniversary celebrations of the first manned moon landing end, a human voyage to Mars remains a holy grail for NASA.

"We're still looking at human exploration of Mars as one of the goals of the future at the top level," said NASA researcher Bret Drake with Lunar and Mars Integration at Johnson Space Center in Houston. "Having a human actually set foot on another planet would be one of the greatest adventures possible, one of the greatest monuments to history."

A crewed mission to the red planet is a daunting challenge that lies at the edge of current technological capabilities and possibly beyond. Still, NASA keeps a strategy to go there and constantly keeps up to date with new ideas.

"Mars is one of those targets of fascination that has been around a long time," Drake said.

How to get there

A voyage to Mars would take a crew about 180 days. So far NASA is exploring two options for propulsion there — a nuclear thermal rocket and a chemical engine.

A nuclear thermal rocket, based off designs from the '60s and '70s, would use a nuclear reactor to super-heat a gas and blast it out the nozzle to generate thrust. "It's a very high-performance vehicle, and we think it's very safe, not radioactive at launch, but it is a nuclear system," Drake said. "The idea for the chemical engine is similar to that used on the space shuttle, liquid oxygen and liquid hydrogen. It's a fairly well-known technology, but it's not as efficient as nuclear thermal."

To reach the Martian surface, NASA envisions an aerodynamic lander that flies down with thrusters to help it descend. The ascent vehicle that takes the crew back into space for the six-month trip home will likely rely on a combination of methane and liquid oxygen. "Oxygen is present in the Martian atmosphere in the carbon dioxide, so you can use resources on Mars to make it," Drake said.

Before the crew even gets to Mars, the plan is to send as much cargo there ahead of time as possible.

"That way we can know it's operating right before we ever commit the crew," Drake said. "A Mars mission is not like a lunar mission where you can come home at any time — once they're committed, a crew is out there for years."

By current NASA estimates, a crewed mission to Mars needs to lift about twice the mass of the International Space Station into space — roughly 1.76 million lbs. (800 metric tons) of technology. To launch the equipment, NASA plans on using the Ares V rocket, designed to be the most powerful rocket ever built and capable of carrying about 414,000 lbs. (188 metric tons) to low Earth orbit at one time.

"We're going to try to minimize the amount of assembly needed," Drake said. "The heavy lift capacity we'll have with the Ares V will allow for simple automatic rendezvous in orbit and docking of components."

The crew would ride up in one of the upcoming Ares I rockets before starting the voyage to Mars.

"Having humans in place could bring a wealth of experience and training and the ability to put into context what they see and to make real-time decisions, all things difficult to do with robots," Drake said.

The very habitat the crew stays at on the Martian surface would be sent ahead of time. "You can also do things like produce and store oxygen from resources at Mars beforehand for the crew and the ascent vehicle. You could generate water as well."

Big crew, long stay

NASA envisions a crew of six astronauts for a Mars mission. "That's about what's required for the skills needed — a commander, scientist, engineer, medical officer, things like that, as well as cross-training," Drake said. "They'll need expertise in a wide range of disciplines."

Currently NASA envisions a long stay for a crew at Mars, about 500 days.

"Crew autonomy is vital, because there's an up to 40 minute time delay in communication between Earth and the crew because of the distance," Drake said. "And the crew doesn't have a capability for re-supply — they'll just have what they send ahead or what they bring with them — so when things fail, they'll have to be able to repair them. They must be self-sufficient."

To survive the voyage, air and water need to be completely recycled regularly.

"We're learning a lot on the International Space Station right now on air revitalization and water recovery," Drake said. "What's nice about Mars is that there's carbon dioxide in the atmosphere, so that can help get us oxygen and water for the crew. In terms of food, we're looking at smaller systems, 'salad machines,' to grow food for the crew. Fresh food is not only good for nutrition, but good for the mind as well. A fresh tomato can really boost psychology."

Mental and physical challenges

The long hardship of roughly two-and-a-half years in space with only a few people in a potentially lethal environment will undoubtedly challenge the psyches of Mars explorers.

"The Russians are conducting a test right now that hopefully will shed light on the behavioral sciences aspect of a Mars mission," Drake said. "Looking at other remote exploration endeavors is helpful as well — Antarctica, or submarines — all that feeds into the human behavioral aspects of crew selection."

A key concern for astronauts as well as during the stay on Mars is dangerous radiation in the form of storms of high-energy particles from the sun as well as cosmic rays from deep space. "The best radiation protection material is hydrogen, or water, which is rich in hydrogen," Drake said.

On the surface of Mars, NASA envisions that cargo deployed ahead of time can produce water before the crew arrives to use as a shield during the crew's stay there. On the way to and from Mars, the ship could be configured so that water and food surround areas where crew spend most of their time, but "a 'storm shelter' aboard the ship will be an integral part for short events of radiation that can be lethal," Drake said.

No firm date has been set for any potential Mars mission, but it remains of keen interest not just to NASA, but also others, such as China.

"It's humanity's next step to understanding and expanding our presence outward," Drake said. "We view human exploration of Mars as being an international endeavor, most likely not limited to just one country, but probably of global scale.