If we ever do get to Mars, getting home might prove to be as difficult. NASA today selected three companies -- Alliant Techsystems, Lockheed Martin and Northrop Grumman - to being the task of defining the spacecraft that will leave Mars, presumably at first loaded with red planet rock samples, then later possibly humans - for a safe trip back to Earth.

The engineering challenges those three companies face are immense.

For more on Mars: 15 reasons why Mars is one hot, hot, hot planet

From a NASA whitepaper on the challenges of a return trip from Mars: "Lifting geology samples off of Mars is both a daunting technical problem for propulsion experts and a cultural challenge for the entire community that plans and implements planetary science missions. The vast majority of science spacecraft require propulsive maneuvers that are similar to what is done routinely with communication satellites, so most needs have been met by adapting hardware and methods from the satellite industry. While it is even possible to reach Earth from the surface of the moon using such traditional technology, ascending from the surface of Mars is beyond proven capability for either solid or liquid propellant rocket technology. Miniature rocket stages for a Mars ascent vehicle would need to be over 80 percent propellant by mass. It is argued that the planetary community faces a steep learning curve toward nontraditional propulsion expertise, in order to successfully accomplish a Mars sample return mission. A cultural shift may be needed to accommodate more technical risk acceptance during the technology development phase. Achieving a small size and mass for the Mars Ascent Vehicle (MAV) is critical to mission affordability, because program budgets have been 1-2 million dollars per kilogram of useful mass landed on Mars."

NASA said of the MAV in the past, "of particular interest is a MAV design/architecture or supporting technologies that reduces the system mass as compared to the previous studies. Technologies should be applicable for, but are not limited to either a two-stage solid (primary interest) or a liquid propulsion system with three-axis stabilization."

Such a spacecraft is not part of NASA's upcoming Mars Science Laboratory mission, which is scheduled to launch this year. On this mission NASA is expected to send its most advanced rover or rolling lab called Curiosity to the surface of the red planet. During the 23 months after landing, Curiosity will analyze dozens of samples drilled from rocks or scooped from the ground as it explores the planet and looks for evidence of life.

The MAV research could be used as part of the technology that will be part of the NASA and the European Space Agency have picked the all-important instruments for their joint Mars mission set to blast off in 2016. NASA and ESA plan launches opportunities in 2018 and 2020 as well, with landers and orbiters conducting astrobiological, geological, geophysical and other high-priority investigations. They plan to return of samples from Mars in the 2020's, but how that would be done remains ambiguous.

Whether or not any of these technologies actually gets funded is of course the next challenge (not to mention the heavy lift rocket and system that would get the spacecraft to Mars) . New obstacles to NASA's plans seem to present themselves every day. Still President Obama has challenged NASA to get a manned spaceflight to Mars by 2030 via a number of steps that include robotic flights to the Moon, Mars and other planets as well as a host of other technological missions.

You may recall to that the Review of United States Human Space Flight Plan Committee report in 2009 said: "Mars is the ultimate destination for human exploration of the inner solar system; but it is not the best first destination. If humans are ever to live for long periods on another planetary surface, it is likely to be on Mars. But Mars is not an easy place to visit with existing technology and without a substantial investment of resources."

Follow Michael Cooney on Twitter: nwwlayer8

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