Can private companies replace space shuttle? Congress warms to the idea.

From shop floors to launch pads at Cape Canaveral, momentum is building toward lofting the first commercial services to carry humans to and from the space station by the end of 2017.

The two companies NASA has selected for the job – Boeing and Space Exploration Technologies – are clearing initial, crucial milestones, and to keep the program moving, President Obama has proposed that the government spend $1.24 billion on the effort in fiscal 2016, which begins Oct. 1. That's up from $805 million the program received this year.

During the past five fiscal years, Congress repeatedly has provided less money for the program than the White House sought, although the gap has narrowed significantly. In the eyes of some analysts, Congress is increasingly warming to the program.

The 2017 target "will be here before we know it," and all indications are that the two companies are on pace to launch, says Eric Stallmer, president of the Commercial Spaceflight Federation, a Washington-based organization promoting commercial human spaceflight.

Indeed, the United States finds itself in a unique position with regard to human-spaceflight technology, according to John Elbon, vice president and general manager for space exploration at Boeing

"Never before in the history of human spaceflight has there been so much going on all at once," he observed during a recent briefing on the status of NASA's commercial-crew program.

Beyond the work his company and SpaceX are performing to build a commercial capability for human spaceflight, Sierra Nevada Corporation is working on its Dream Chaser craft for carrying humans into low-Earth orbit.

In addition, companies such as Blue Origin, Virgin Galactic, and Bigelow Aerospace are developing hardware to commercialize various aspects of human spaceflight. And NASA is pressing ahead with its own Space Launch System rocket and Orion crew capsule for deep-space exploration.

NASA's bid to turn over resupply of the space station to two commercial contractors began in 2010, when five companies received seed money. After whittling the number of companies to four and then three during the next two years, NASA picked Boeing and SpaceX as its two providers last September. The last contender to be eliminated, Sierra Nevada Corporation, filed a protest with the Government Accountability Office, but the GAO ruled in NASA's favor in January.

Boeing and SpaceX already have passed one of NASA's five milestones: approval of their long-term plans for earning the agency's OK for carrying astronauts. Meanwhile, the companies have been busy.

Boeing. The company is adding a crew-access tower to the launch pad it uses at the Cape Canaveral Air Force Station in Florida. The company also has cleared a key review of its plans for ground-based systems supporting its taxi missions to the space station. And it is in the midst of refurbishing a building at the Kennedy Space Center where Boeing will build and test its crew capsule – the CTS-100. The company has three test flights planned between February and July of 2017, culminating in the first crewed mission to the station at the end of that year.

SpaceX. SpaceX plans a test of the Dragon cargo capsule's launch-abort system at the pad in a couple of months and later during a full-scale launch and ascent. The system is crucial to labeling the craft fit for human transportation. It also is crucial to the company's goal of giving the capsule the ability to land under its own power. Other upgrades are planned for the capsule and for the Falcon 9 rocket that will carry it into space. The company plans to conduct a crew-less flight to the space station in late 2016 and the first crewed mission in early 2017, according to Gwynne Shotwell, SpaceX's president and chief operating officer.

Still, the program's officials have some important fences to mend. On Jan. 14, NASA's Aerospace Safety Advisory Panel sharply criticized NASA's Commercial Crew program for a lack of transparency on its certification plans for the two companies' systems.

The panel, set up after the Apollo 1 capsule fire that killed three astronauts in 1967, notes that this has been a problem for several years. NASA has argued that, because of its selection process for the two companies, as well as resulting challenge to its decision, it wasn't at liberty to share the kind of information the panel sought.

The panel acknowledged that in December, NASA began to send it some of the information it wanted – after being alerted that the lack of cooperation was to appear in its latest report. But the information arrived too late to be evaluated for the panel's 2014 report.

The lack of openness and transparency alone has the potential to increase the program's risk, the panel said – noting that similar failings contributed to the Challenger and Columbia accidents, according to investigations.