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After months of study, NASA has decided that it won't try to send astronauts to space aboard the first flight of its huge Space Launch System (SLS) rocket, designed to eventually bring people and payloads to destinations like Mars or the moon.

While acting NASA administrator Robert Lightfoot said that it would be technically feasible to add crew to the first SLS mission (called Exploration Mission-1 or EM-1), the risk would be too great when compared to the possible benefits.

"... After evaluating cost, risk, and technical factors in a project of this magnitude, it is difficult to accommodate changes needed for a crewed EM-1 mission at this time," Lightfoot said in a memo sent to NASA employees Friday. The memo also indicated the first SLS mission would not occur in 2018, as anticipated, but rather sometime in 2019.

After the feasibility study wrapped up, NASA officials presented the results to the White House. Lightfoot told reporters on Friday afternoon that the space agency and the Trump administration made the decision not to pursue a crewed mission for EM-1 together.

So, the humans are staying home, at least at first.

The whole idea of sending people to space aboard the first flight of the SLS was a little ridiculous to begin with.

NASA has always been working toward an uncrewed first mission for the SLS, so inserting a crew at this stage in the game seemed at best, slightly reckless. It also struck some experts as an attempt to please a new president who is eager to see crewed space missions launch from U.S. soil again.

The SLS program in general has been replete with delays, and that's no different now.

While NASA has been working toward a launch date in 2018 for EM-1, the agency now admits that it won't be able to hit that deadline. For now, the launch data is slipping to 2019, though it's unclear when it will fly that year, and more delays are possible.

The latest delay was predicted in a July 2016 report released by the independent Government Accountability Office. Development of the Orion spacecraft -- designed to fly with the SLS on EM-1 -- has also encountered technical issues that have delayed that program.

If NASA did decide to fly crew on EM-1, it would have also delayed the mission until at least 2020, Lightfoot added.

Aside from limiting further delays in the schedule, keeping EM-1 uncrewed will also allow NASA to do a full shakedown on a relatively untested system.

The 21 to 25 day EM-1 mission will bring the Orion spacecraft to a retrograde orbit around the moon, where mission managers can really stress its engines and flight systems to see how it does under extreme circumstances in deep space.

For example, in such an orbit, the craft will be exposes to more cosmic radiation, and is a difficult part of space to navigate in.

If NASA decided to put humans on that mission, it would have forced it to change that mission to make it far less risky.

NASA is still planning to send people to space for its EM-2 mission, expected to fly at some point in the early 2020s... at least for now.

While this delay means that the next-generation NASA rocket won't fly with humans until the next decade, it does not mean there will be no crewed launches from U.S. facilities before then.

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.