The space shuttle program has been an incredible achievement and demonstration of U.S. technical expertise. The shuttle also has served many useful civil, commercial and defense space purposes, inspired a populace, and was in many ways ahead of its time in terms of its range of capabilities.

But the shuttle program’s record after 133 launches also includes two tragic failures, each of which resulted in the loss of shuttle orbiters and their entire crews. These accidents taught us many lessons that the shuttle program subsequently used to reduced its risks. Regrettably, however, these measures also contributed to higher costs, making progress in human space exploration ever more difficult.

Fortunately, concurrent with the shuttle’s retirement, several commercial companies have the ability to launch payloads — and, with relatively modest modifications, even human-rated vehicles — into low Earth orbit (LEO). These include Boeing and Lockheed Martin through their United Launch Alliance joint venture, Space Exploration Technologies and Orbital Sciences, and one day not very far off might also include companies such as ATK, Sierra Nevada and Blue Origin.

These firms argue that their experience in delivering payloads to the international space station using proven launch vehicles like Atlas 5 and Falcon 9 can be extended in a timely way into a human transport capability to LEO at reasonable cost. We think they are right.

We also think that commercial crew LEO transport has the potential (and, many believe, high probability) of providing crew transport at a far lower cost. And not only should these alternatives be cheaper than a full NASA development program, they also should come on line more quickly, relieving our country of its soon-to-be complete dependence on Russia for access to LEO and the international space station.

As a result of these factors, there is now a growing consensus that our path into the future should lie in promoting a commercial human space launch program in LEO, and that such systems will be substantially safer and less expensive than the shuttle system they will replace.

Concerning mission and crew risk, all of the new proposals for launch vehicles have some kind of integral launch abort/escape capability, just as our Apollo program had, and which the Russian Soyuz has now had for decades. As a result, in most launch and even re-entry failure scenarios, a second independent failure is required before there will be a loss of crew. Even if, for example, the launch vehicle has a 1:50 risk of catastrophic failure, but the abort system has an independent 1:100 risk of failure, the consequent risk of crew loss would be only 1:5,000, which is certainly satisfactory and far superior to the shuttle system.

We conclude that the enhanced crew safety, lower costs, faster time to initial operational capability and resultant greater independence of the U.S. for meeting its human spaceflight needs mean that these vehicles offer a highly attractive option for our space program. In fact, we believe that the funding of such vehicles is imperative for the future of human spaceflight in the United States.

Owen Garriott is a former NASA Apollo, Skylab and shuttle astronaut, and a former member of the NASA Advisory Council. Alan Stern is the previous NASA associate administrator for science, and also a former member of the NASA Advisory Council.

Alan Stern is a planetary scientist and the chief executive of Golden Spike. He is a former associate administrator in charge of all of NASA’s science efforts, and he serves as the chairman of the Commercial Spaceflight Federation’s Suborbital Applications...