Some clarifications: Orion is the spacecraft (crew modules, service module, launch abort system, miscellaneous sundries) that is delivered to orbit the the Ares I space launch vehicle (rocket, a five-segment version of the Shuttle Solid Rocket Booster with a second stage using an upper stage based on the J-2X liquid engine which is a modified and uprated version of the engine use on the Saturn S-IVB). Other propulsive components of the Constellation exploration system are the Earth Departure Stage to inject the Orion into transLunar injection orbit (the last concept used a J-2X, making it basically a bigger version of the S-IVB), the Altair Lunar Exploration Module (a larger Apollo LM), and the heavy lift Ares V rocket (based on Shuttle-derived hardware) which would launch the EDS and the Altair into orbit for rendezvous with the Orion.
Officially, the Constellation program has been placed on hold, with no funding for the program itself other than a small amount of sustainment funding. There is still a commitment to developing the Orion spacecraft and the Ares I vehicle, and tests of both systems have and will continue to occur, although there is a significant amount of discussion about using an existing heavy lift booster like the Delta IV or Atlas V in place of the Ares I for numerous technical reasons that are not germane to the discussion. This would require man-rating these boosters, which is not an insignificant effort but which may ultimately be more cost effective. The Ares V development is currently on standby.
Alternatives to the current Constellation program components have been suggested, including the Jupiter rocket system proposed by a group of NASA Marshall engineers and managers as the launch vehicle component of the DIRECT proposal, now in its third version. The Jupiter is a modular heavy-to-super heavy lift rocket based upon Shuttle-derived hardware which is claimed to be superior to the Ares V in that it can use the existing Shuttle assembly and launch facilities with minimal modification. It would use the existing Shuttle RSRBs as parallel Stage 0 boosters attached to a “common core” cryogenic Stage 1 using a tank structure similar to the Shuttle External Tank (ET) with three or four Shuttle Main Engines (SSME) for primary propulsion. The second stage of the superheavy lift version would be a cryogenic upper stage using six of the RL10 engine from the Saturn S-IV and Centaur (also currently used on the Delta IV Upper Stage). Post-boost departure stages would be part of the payload, although the RL10 is technically restartable and could be used as a post-boost stage as well. Apparently these engineers have formed a company or consortium known as C-Star Aerospace, LLC. (C-star is the abbreviation for the a quality known as the characteristic exhaust velocity, used to compare the relative performance of different chemical rocket systems and propellants.)
Other Shuttle-drived proposals have been drawn in the past. In addition, there are the commercial rocket vendors; in addition to the Lockheed Martin Atlas V and the Boeing Delta IV Heavy there is also the Space Exploration Falcon-9 and -9H (albeit still having teething problems into their sixth flight of the Falcon system) and the Orbital Sciences Taurus II (using two refurbished NK-33 engines on Stage 1 and a commercial Castor-30 solid motor for Stage 2, as yet unflown). Both of these are primarily intended to provide Commercial Orbital Transportation System program (COTS) for continued access to the ISS, though they could be adapted for use with a revamped Constellation program.
As for commitments, plans, and visions for a manned mission to Mars: there are none that are fixed, nor have there been in the history of the American space program, all banner waving aside. There are three reasons for this: the first is cost; the cost of such a program would eclipse Apollo (adjusted for inflation) by at least an order of magnitude. There are new technologies to be developed, and such a program is logistically more complex and hazardous than any Lunar mission. An Apollo XIII type failure during a Mars manned mission would be a fatal loss of billions of dollars.
The second reason is a lack of public interest. The public interest during the Gemini/Apollo program was waning, and that was a program that was able to come to fruition in less than a decade. A manned mission to Mars is easily twenty years away from inception, regardless of the amount of money or manpower thrown at it. The necessary development hurdles cannot be swept away with bulk cash or enthusiasm.
The third reason is necessity, or rather the lack of it. Current robotic missions to Mars (both orbiters and mobile landers) have provided a wealth of information for a mission period that is several times what would be possible with a manned mission, and at a cost that is literally fractions of pennies to what it would cost for a manned mission. And of course, we don’t have to return the rovers, or mourn their loss once the mission is complete. The Mars Exploration Rovers have been extraordinarily successful, lasting far beyond their planned 90 day missions, and the Mars Science Laboratory, if it is deployed without failure, will be able to perform exploration and scientific examination far beyond what could be expected of any manned mission for a planned duration of nearly two years. While a manned mission to Mars would provide a great flag and footprints moment, a failure (and lost of crew) would be both devastating and extremely costly, while the lost of a rover or probe, while embarrassing, will not spell an end to exploration.
In general, manned space exploration beyond Earth orbit is neither productive or, at this point of technological maturity, adequately safe to be worthy of serious discussion. While I believe that humanity will at some point begin to colonize orbital and interplanetary space, I suspect that the first of such ventures will be to mine the Near Earth Asteroids and then the asteroid belt for resources, then moving to the outer planets. There is little on Mars that appears to be of great value either for practical research (beyond a better understanding of the development of the solar system and perhaps the possibility of extraterrestrial life) or resource exploitation, which is necessary to put space habitation on a sound fiscal basis.
As for ion engines, a low but high specific impulse constant thrust engine would definitely be a significant improvement over chemical engines with a specific impulse of a few hundred seconds, and a thrust period of tens or hundreds of seconds. However, the energy efficiency of the engines that we can currently build is very low (on the order of 1-2%) and the level of thrust is only suited to stationkeeping and attitude control systems on orbiting satellites or very small probes. There are a number of different systems for generation ion thrust for propulsion (the Wikipedia page has a decent summary of these); in addition to the efficiency problem, many of them suffer from problems with erosion or heating which make it difficult to scale them up to a usable thrust for large spacecraft propulsion.
There are other systems your son might research, like nuclear pulse propulsion (Project ORION, not to be confused with the Constellation Orion spacecraft) and Prometheus, nuclear thermal rockets (NERVA), and fission fragment or nuclear fission salt rocket propulsion that is suitable for interplanetary propulsion. All of these systems are nascent, though, and none are being actively developed for use.
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