The return of U.S. lawmakers from their annual August recess has brought a resumption of the long-running debate over the future of NASA’s human spaceflight program, with the focus now on a pair of authorization bills that reject different elements of U.S. President Barack Obama’s plan to rely on privately operated vehicles to transport astronauts to the international space station while deferring work on a heavy-lift vehicle needed for deep space exploration.

The bill passed by the U.S. Senate directs NASA to begin work next year on a heavy-lifter that utilizes space shuttle-heritage technology, whereas the president’s proposal calls for selecting a vehicle design by 2015 — after new technologies that could affect that design are evaluated. The Senate bill also recommends spending $1.3 billion over the next three years to develop commercial crew transportation systems, thus lending some support to the centerpiece of President Obama’s policy. The White House proposes allocating $3.3 billion for that purpose over the same time period.

The House bill effectively guts funding for commercial crew services, providing just $450 million through 2013. Instead, House lawmakers want NASA to develop a government-owned crew transportation system featuring a launch vehicle — possibly the shuttle-derived Ares 1 already in development — that could be the foundation of a heavy-lifter that would begin operations by the end of the decade. In short, this version of the bill, which has yet to come to the House floor, would result in something that resembles the Moon-bound Constellation program the president is determined to cancel.

Not surprisingly, those who oppose abandoning Constellation, notably former NASA Administrator Mike Griffin, have come out in support of the House bill. The Senate legislation, widely viewed as a compromise, has won backing from some members of the Augustine commission, whose review of NASA’s human spaceflight program last year served as the basis for the new direction charted by the White House in February. Former Lockheed Martin chief executive Norm Augustine, who chaired that panel, found the Senate bill more palatable than the House version, but nonetheless raised questions about its affordability.

One of the strengths of the Vision for Space Exploration espoused by former U.S. President George W. Bush is that it outlined specific exploration goals and destinations, and the Constellation program logically flowed from that. According to the Augustine panel, however, Constellation was unsustainable under any plausible budgetary scenario for NASA. In fact, the panel concluded, any sort of meaningful human exploration beyond Earth orbit is not in the cards without a gradual increase to NASA’s budget over the next few years to $3 billion above the current annual level of $19 billion.

The president’s latest five-year budget projections do not meet that growth threshold, which makes it difficult to see how either the House or Senate bill, both of which aim to put NASA on a path to send astronauts to deep space destinations within the next two decades, are grounded in long-term fiscal reality.

The fundamental problem with President Obama’s plan is that it lacks a driving mandate in the form of a credible deep space exploration destination and schedule; the White House’s notional goal of visiting a near-Earth asteroid by 2025 using as-yet-unidentified hardware and technology doesn’t stand up to scrutiny. Indeed, a NASA team tasked with matching hardware to a relatively near-term asteroid mission came up with an architecture that most closely resembles what’s in the Senate bill. According to the Human Exploration Framework Team, even if NASA were to immediately begin work on a shuttle-derived heavy-lifter — shuttle-derived being the quickest path to that capability — sending astronauts to an asteroid probably isn’t feasible until several years after 2025.

While the president’s call for developing technologies with the potential to open up new exploration possibilities has appeal, particularly given NASA’s research and development charter, the reality is that these kinds of programs are politically vulnerable, especially in tough budget environments. Moreover, demonstrating technologies with the potential to change the requirements for a heavy-lift rocket, such as on-orbit fuel depots, is bound to be expensive and time consuming. It seems highly unlikely that such demonstrations will be completed by 2015, by which time the administration has pledged to settle on a heavy-lift rocket design.

The House and Senate bills both reflect skepticism — albeit to different degrees — about the president’s plan to rely exclusively on commercially operated space taxis for astronaut transport. This is not unreasonable: The commercial model now being applied to space station resupply through contracts with Orbital Sciences Corp. and Space Exploration Technologies has yet to prove itself, and transporting humans is a vastly more difficult prospect.

But the House bill would turn skepticism into a self-fulfilling prophesy. Estimates of what it will cost to develop a commercial crew vehicle vary widely, but $450 million spread over three years is not nearly enough to get this fledgling industry off the ground.

The Senate bill would give commercial crew services a much better chance of materializing toward the end of the decade. At the same time, by directing NASA to begin serious work next year on a heavy-lift vehicle, the bill would help position the agency to undertake deep space exploration missions within the next two decades. However, this bill also supports flagship-class technology demonstrations, again raising the affordability question, particularly given the fact that operations of the space shuttle — previously slated to retire at the end of this year — are likely to be extended until the middle of 2011.

In short, none of the options on the table right now — the House and Senate bills or the president’s plan — answers the question that’s been hanging over the U.S. human spaceflight program since February: Where is it going, and why? With the White House and Congress fundamentally at odds over key issues such as commercial crew and heavy-lift, it seems unlikely that anything will be resolved anytime soon, and the extended stalemate all but assures that billions of U.S. taxpayer dollars will continue to be spent on programs that may or may not have a future.

Spending money will help preserve jobs in the near term — always a key consideration but especially in an election year. But for NASA’s human spaceflight program, the long-term future has never looked more uncertain. It is time to hit the reset button yet again. The White House, in close consultation with Congress as well as prospective international partners whose participation is the only answer to the affordability question, should begin work on a new plan that offers at least some hope that humans will begin exploring deep space destinations within the next 20 years.