The second flight will have astronauts on board.
Putting crew on the first flight of a new vehicle isn't usually how NASA does things, though.
"After evaluating cost, risk, and technical factors in a project of this magnitude, it is hard to accommodate changes needed for a crewed EM-1 mission at this time", NASA's acting administrator, Robert Lightfoot, sent in an email to agency employees that was obtained by The Verge.
Sending the rocket into space already costs $24 billion, and astronauts would have added $600 million to $900 million to that cost, in addition to delaying the launch until 2020.
WESH 2 Space Expert Dan Billow brought us all the details from Cape Canaveral.More news: Brazil Lifts Public Health Emergency as Zika Cases Drop
NASA did not say if the second test mission, known as EM-2, which will carry crew beyond the moon, will be affected. It also struck some experts as an attempt to please a new president who is eager to see crewed space missions launch from US soil again.
NASA determined it is technically capable of launching crew on EM-1, but after evaluating cost, risk and technical factors in a project of this magnitude, it would be hard to accommodate changes needed to add crew at this point in mission planning. The official launch date will be determined in the coming weeks, NASA said. The first launch had been targeted for next year, but is now off until 2019. "It really reaffirmed the baseline plan we have in place is the best way to go", Lightfoot told the Times.
NASA has yet to formally reschedule the EM-1 flight date, but told the GAO it would do so by September 30, the end of the current USA budget year.
Gerstenmaier and Lightfoot said that the feasibility study is not in a report format and some of the information is ITAR-sensitive, so there will no public release of what they based their decision on.
It would have cost a lot more money and time to add life-support systems and other human-required equipment for the test flight, said Bill Gerstenmaier, head of NASA's human explorations and operations. "We're really building a system", Gerstenmaier said.More news: Half-billion Windows 10 devices, but PC slump stunts growth
The private spaceflight companies SpaceX and Boeing, for example, both hope to begin launching astronauts to the International Space Station in the next couple years under a contract with NASA.
SLS is being developed by the Marshall Space Flight Center in Huntsville, and major components are being built by contractors at the Michoud Assembly Facility outside New Orleans under Marshall's management. Earlier this month, as first reported by the blog NASA Watch, workers at Michoud accidentally damaged beyond fix a large dome that was to be part of a liquid oxygen fuel tank.
In his statement, Lightfoot outlined the progress of SLS and Orion, which he described as "challenging but going well".
A crunch to make the Orion module astronaut-worthy on its first flight could also collide with other White House budget priorities, many of which will cost tens of billions of dollars: a wall on the Mexican border, an infrastructure-renewal plan, and a large increase in US military spending.More news: Wall hits late 3, forces Game 7