This past week has marked a rather poignant anniversary in the history of humanity's space efforts. Thirty years ago this past Thursday, on December 19, 1972, the Apollo 17 command module America splashed down into the Pacific Ocean, carrying astronauts Eugene Cernan, Harrison "Jack" Schmitt, and Ronald Evans. The first two of these had spent a little over three days near the Taurus Mountains on the lunar surface, with Evans remaining in lunar orbit during that time. The return of Apollo 17 brought to a close the first era of humanity's exploration of the moon and the solar system; there has, however, been no second era, and at this time there are no concrete plans to begin such anytime within the foreseeable future. For the time being, we appear to have turned our backs on reaching out into the solar system.

Not that we've been spending all the intervening time standing still, certainly. The American Space Shuttle fleet, comprising four orbiters, has now been operational for over two decades. The former Soviet Union, meanwhile, continued with a series of orbiting space stations it had begun in the early 1970s, culminating with the launch of the space station Mir in 1986 which remained operational and almost continuously occupied until it was de-orbited into Earth's atmosphere last year. More recently, the U.S., Russia, and several other nations have collaborated in building a larger International Space Station, the in-orbit construction of which began in 1998, and which has been continuously occupied since late 2000.

A pair of "tourist" visits to the International Space Station during the past two years notwithstanding, for the overwhelmingly large part human spaceflight is currently limited to a handful of specially-designated astronauts who have undergone extensive training. If we are truly to move out into space, however, and recapture the vision that was presented by Apollo three decades ago, space must become open to a much larger fraction of humanity.

While there are numerous challenges that are currently creating significant obstacles to this, one of the more prominent involves the enormous expenses necessary in launching anyone, or anything, to orbit. Even if space travel were more "open," the current launch costs (which include fuel, construction of launch vehicles, support services, etc.) of approximately $10,000 per pound would make space travel prohibitive for all but the very wealthiest people. For humanity to expand into space these costs must be reduced by at least a factor of a hundred to a thousand.

The first steps toward this end were taken a decade ago, with the initiation of the Delta Clipper-Experimental (DC-X) program, initially under the Department of Defense's Strategic Defense Initiative (and later under NASA). The DC-X was an experimental prototype Single-Stage-to-Orbit Reusable Launch Vehicle (RLV) which, while never designed to reach orbit, successfully performed a series of test flights at White Sands Missile Range in southern New Mexico during the mid-1990s.

The DC-X was followed by NASA's X-33 program, an effort to develop a prototype sub-orbital RLV which, if successful, was envisioned as leading to development of a fully reusable commercially operational spacecraft. The X-33 was originally planned to have its first demonstration launches in 1999, however several problems, most notably a rupture in its fuel tank, delayed this. Before the X-33 could be fully completed the program was cancelled by the incoming Bush administration in March 2001.

In place of the X-33 NASA instituted the Space Launch Initiative (SLI) program. While a necessary component of SLI involved modifications and upgrades to the Space Shuttle fleet, the majority emphasis was on a continuation of the original goal of X-33, i.e., the development of a fully reusable vehicle to, among other things, replace the Space Shuttle. The planned SLI timetable called for several launch companies to submit designs for an RLV, with a downselect to two (or possibly three) designs for further development to be made around 2004, and with a final selection of one of these designs to be made around 2006. Once this selection was made SLI's goal was to have an operational vehicle by the early 2010s.

This timetable now appears no longer to hold, however. Last month NASA Administrator Sean O'Keefe announced plans to restructure the SLI program so that the majority of its funding is directed toward development of a so-called Orbital Space Plane -- to be launched aboard expendable launch vehicles -- that would hopefully become operational by 2010. According to this plan, some SLI funding would still be directed towards research on RLV technology, but development of such a vehicle would be delayed indefinitely, and would probably not take place until 2020 or later.

It is true that the technological challenges involved in developing a fully reusable launch vehicle have proven to be enormous. It is likewise true that the Space Shuttle fleet is aging, and especially in an era of limited budgets the development of some kind of interim replacement vehicle is not a bad idea. At the same time, however, it can be argued that, without a sustained and concerted effort to develop an operational RLV (and, consequently, drive down the launch costs to an acceptable level) we will continue, as a race, to remain stuck on Earth without being able to move out into the solar system. While it is within the realm of possibility that a purely privately-funded effort may succeed in developing a valid RLV, a reshifted emphasis for SLI away from this endeavor would not appear to bode well for any large-scale future human space activity. In the apparently prophetic words of DC-X program manager William Gaubatz, "space will not be opened by always leaving it to another generation."

The full moon currently shining in our nighttime skies reminds us of the three decades that have elapsed since the promise of Apollo beckoned us. How many more decades will pass before we again seek to fulfill that promise?