COLORADO SPRINGS, Colo. — The former head of NASA’s space shuttle program, now an executive with Boeing Space Exploration, defended the heavy-lift Space Launch System (SLS) the Houston-based company is helping NASA build as the only credible way to mount crewed missions beyond low Earth orbit using today’s technology.

“If you look at a wide range of missions — anything really beyond low Earth orbit — you have to have more lift capability than we have commercially available right now, and SLS provides that,” John Shannon, international space station program manager at Boeing, said April 10 during a press conference at the 29th National Space Symposium. “People that say there are other options, or other ways to get beyond low Earth orbit — it’s just not a fact, it’s just not true. There are technologies you could develop that would be years and years in the future … but SLS gives you the capability to do that much, much quicker.”

Shannon, who spent 25 years at NASA before joining Boeing in January, pointedly dismissed the idea that NASA has to identify a specific destination and mission for SLS to make the big rocket worthwhile.

“This ‘SLS doesn’t have a mission’ is a smokescreen that’s been put out there by people who would like to see that [program’s] budget go to their own pet projects,” Shannon said. “SLS is every mission beyond low Earth orbit. The fact that NASA has not picked one single mission is kind of irrelevant.”

NASA is spending roughly $2.7 billion a year on SLS and its companion deep-space crew carrier, the four-seat Orion capsule being built by Lockheed Martin Space Systems of Denver. Orion will first fly to space in 2014 aboard a United Launch Alliance Delta 4 rocket for an Earth orbit and re-entry mission to test the capsule’s heat shield. SLS will first launch Orion in 2017, when it will send the craft to lunar space without a crew. NASA will repeat that mission with a crew in 2021.

NASA’s 2014 budget request, which was rolled out by the administration of U.S. President Barack Obama only hours after Shannon spoke here, contained no alterations to these plans. The White House requested about $1.7 billion for SLS alone, about $300 million less than the program has this year under a $1 trillion spending bill signed March 26. This funding would be spent on vehicle development at the Marshall Space Flight Center in Huntsville, Ala., launch infrastructure at the Kennedy Space Center in Florida, and support from NASA’s national network of field centers.

An administration official said the funding request for SLS, though lower than the 2013 level, is sufficient to keep the program on track.

“SLS and Orion remain fully funded and on track for 2014 and 2017 tests,” this source said. “More money than we request will not move the dates forward and would be a waste.”

Meanwhile, with Orion and SLS still in development, it remains unclear whether astronaut crews will have a destination to visit in low Earth orbit after 2020, which is still the official end-of-mission date for the space station.

Shannon said Boeing is evaluating whether the station is structurally sound enough to fly beyond 2020, and also whether NASA and Boeing have enough spare parts to support an extended mission. Boeing executives have previously said the station might be able to operate until 2028.

Shannon said Boeing will know for sure once it completes a space station life-extension study for NASA.

“We expect to provide that data to NASA at the end of this year, and then they can go out to engage with the international partners to start discussing what lifetime of the [station] should be,” Shannon said.

NASA’s top human spaceflight official, William Gerstenmaier, has already said NASA must soon start talking about extending station operations beyond 2020.

“I believe this is the year we need to start talking about extending beyond 2020,” Gerstenmaier told members of the National Academies’ Space Studies Board April 4 in Washington.

Gerstenmaier said NASA has found strong support from its Russian partner for keeping the space station aloft for almost 15 more years. The 20-nation European Space Agency, which has not fully committed to funding station activities through 2020, thought such a commitment is expected. Gerstenmaier said the Europeans have given what he characterized as “soft” support for an even longer extension.

Another major partner, Japan, “is struggling with their budget” and has not yet decided whether it can contribute beyond the next decade, Gerstenmaier said.

Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...