Probably no one in the space community has missed the renewed and often disorderly debate over whether to retire the space shuttle at the end of 2010 as planned. We are well down the path toward that retirement, to be followed by a five-year gap while the new Orion crew vehicle and Ares launcher are being developed. During this time, we will depend – also as planned – upon Russia and its Soyuz system for crew transportation and, if needed, crew rescue for the international space station (ISS).

It is worth recalling that this sequence of events is not an accident, not a bug in the system, not a deviation from the plan. It is the plan, advocated by the administration since 2004 and, so far, ratified by the Congress through each year’s budget. It is, therefore, national policy, decided at government levels above NASA and provided as a ground rule to the agency.

Many believe the decision to retire the shuttle and accept a period of dependence on Russia for ISS logistics is ill-advised. Because the present administration cannot force the retirement of the shuttle in 2010, it was always predictable that the decision would be revisited in the next administration or the next Congress. However, Moscow’s recent military action in Georgia has suddenly intensified the debate over the wisdom of our current plan.

In my view, this is a side issue – merely a particular example of a more general case. Any decision to accept dependence upon another nation for a strategic need brings with it the risk of compromise by other events affecting the parties’ relationship. That is why great nations do not like to be dependent upon others in strategic matters. The highly visible activity of human spaceflight is, I believe, strategic for the United States, and I think most policymakers would agree.

Thus, the question of shuttle retirement goes above and beyond the self-interest of particular contractors, or the economic and personal dislocations that must occur when long-established operations are brought to a close.

The core issue is this: are we willing to depend for an extended period upon another power, one not always aligned with our interests, for access to the international space station, the centerpiece of the U.S. human spaceflight program for the last two decades? And, if not, are we willing to pay the costs – higher risk, more money, and lost opportunities – of mitigating that dependence? If the debate over shuttle retirement is to be renewed as a new president and a new Congress come to Washington, then I believe these are the proper grounds for the discussion.

To assess the risk, it must be understood that any decision to extend shuttle operations to obviate dependence on Russia requires that we plan for an extension of about five years, at a minimum flight rate of two per year. In light of the knowledge gained since the loss of Columbia, we believe we have about one chance in 80 of losing a crew on any single shuttle launch. If we were to conduct 10 additional launches prior to retiring the shuttle, we would incur a risk of about one chance in eight that another shuttle crew would be lost at some point in the sequence. These are sobering odds, one reason the Columbia Accident Investigation Board recommended replacing the shuttle as soon as possible.

The additional risk does not end with the shuttle itself. The shuttle cannot remain in space for more than a few weeks. This precludes its use as a lifeboat, a continuously available rescue vehicle in the event of a severe technical malfunction or, more likely, a medical emergency involving a single crew member. This is one reason we are dependent upon the Russian Soyuz even now, although shuttle is still flying. A decision to eliminate all dependence upon Russia would require that we follow one of three paths: we can reduce U.S. presence aboard the ISS to periods of a couple of weeks for a few times each year, we can develop and build a new special-purpose crew rescue vehicle or we can ask our astronauts to assume the risk of remaining aboard station without the possibility of rescue in the event of an emergency.

Our astronauts are extraordinarily dedicated, and many would volunteer to accept the additional risk of flying without a guaranteed rescue capability. It has been done before. Crew rescue was not available for the Apollo lunar astronauts, it will not be when we return to the Moon and it will be out of the question for future voyages to Mars. But it has been a ground rule for the international space station, a written commitment by the United States to our own astronauts and those of our international partners. To fly without it would be to change the game in a very significant way.

If additional shuttle operations past 2010 were mandated, additional funds would be required, as they are not in the present NASA budget. Today’s shuttle program could certainly be scaled back somewhat if reduced to a minimal two-per-year flight rate to sustain the ISS. Even so, several billion dollars would be required for each year of extended operations. If new money is appropriated for NASA for such a purpose, then presently authorized and funded programs could stay on track. If the cost of extended shuttle operations must be absorbed essentially within NASA’s present top line, then existing programs must be curtailed or deferred.

The only realistic internal source for additional shuttle operations money is the Constellation Program, which is presently developing the new Ares and Orion vehicles to replace the shuttle. A reduction of several billion dollars per year in the budget allocated to these systems means that, as a practical matter, the shuttle will not be replaced. Unless additional funds are made available, it is not possible to develop a replacement for the shuttle while still flying it.

Orion and Ares are the first two pieces of a new space transportation architecture designed to return our astronauts to the Moon, this time to stay. From there, we will continue on to Mars. If shuttle operations continue past 2010 without additional funding, those plans will be stillborn. The U.S. space program will again be mired in Earth orbit, as it has been for the 35 years since Apollo, with no American option to define and extend the human frontier in space. As time goes by, that frontier will be seized by others, while we watch it on television.

None of this is to argue that it is wise to depend upon others for a strategic capability such as access to space. I believe that our present circumstances offer a valuable lesson for the future in that regard. But we are where we are. A conscious decision was made to depend upon Russia for crew transportation to and from the international space station, for the limited period between the retirement and replacement of the shuttle. At this point, reversing that decision will be very costly, in many different ways. If money is no object, we can have it all. But with present funding, we can either hold on to the past, or purchase the future. We cannot do both.

I know what I would choose.

Michael D. Griffin is NASA Adminstrator.