|SLS courtesy NASA|
So far, the major commercial players to launch NASA assets into orbit are ULA, Orbital Sciences, and SpaceX. These guys represent the current launch might of the United States. The rockets are:
If NASA never gets SLS, how could it explore asteroids and Mars? SLS represents an approach that was used for Apollo. That approach was to put all your assets on one BIG ROCKET and go for your target. Man has never been manlier (that's a joke). There is another theoretical approach but it's never been tried. That approach requires the use of medium size rockets (Atlas V, Delta IV, Falcon 9). The idea is to use more than one launch to Earth orbit and put together your stack that goes to your target in orbit. That stack would have a rocket big enough to leave Earth orbit, carry all the assets needed for the mission including a capsule to return the crew back to earth. This would give commercial companies contracts and responsibility for launches. Plus the old axiom is put to use that says "don't put all your eggs in one basket". Launch from Earth represents the most risk to hardware and crew on any mission apart from landing. Does this seem inefficient? Perhaps to a rocket scientist, but it just may very well be more economically feasible than the alternative.
What about Orion? The Orion capsule is the only asset to come out of the Constellation program to survive, albeit modified. It is slated to launch on a Delta IV Heavy for its first space test run and return at high speed through the atmosphere. That tells me it doesn't need SLS to get to orbit. Delta IV or Falcon Heavy are capable of handling the job.
What could this stack look like? If you look at SLS, the stack could be the upper stage that uses the J-2X engine, whatever space module needed form the mission, and the Orion Command Module. 2 or 3 launches should do the trick. Later, they could make the J-2X booster into a reusable booster that comes back to Earth orbit ready to be refueled for another mission.
SLS is based on old idea of space operations. This is a new century. NASA has bigger plans for exploration than is SLS can economically provide. SLS is really putting money in a rocket that is seldom going to be used. That makes no economic sense. NASA should stop reinventing the rocket.
NASA without its own rocket could open up the architecture of a mission to become more economically feasible, more reliable, and more evolvable. I say let commercial companies lift NASA to orbit. Then NASA can take care of exploration from there.