Ever since the death of the Space Shuttle, NASA has been working on a new launch system to ferry people around the solar system. The Ares program was cancelled due to safety concerns. Ares would have been an incredible rocket, but it used solid fuel for most of the propellant. My understanding is that the solid fuel would transmit vibrations from the engines to the passenger compartment, unlike liquid fuel which absorbs vibrations. These vibrations could have killed the astronauts. That being said, the Ares rocket would have been far and away the most powerful rocket ever built. In my opinion, the program was cancelled too soon. Further tests should have taken place to see if it could be salvaged.
NASA will need this new launch system to fulfill its goals of sending men back to the moon, to Mars, and to the asteroid belt. You can have the best spacecraft ever built, but that is useless without a rocket to take it to its destination. NASA already has the spacecraft, known as the Orion Capsule. It now needs a rocket.
Currently, NASA is reviewing concepts for the next generation rocket, six of them in all. These designs will be liquid fueled. The final design is supposed to be able to lift 143 tonnes to orbit. What bothers me is that the Saturn V rocket was able to lift 118 tonnes to orbit, and that is 50 year old technology. Given the reliability of the Saturn V, spending tens of billions of dollars building a new, unproven system with limited performance gains seems foolish. Call me old fashioned, but I can't see the wisdom in doing something new when you have something old which works.