NASA Decision On Crewed EM-1 Feasibility, NASA

"We are grateful for the near-term flexibility offered by the FY 2017 Consolidated Appropriations Act, and we are confident that we remain technically capable of launching crew on EM-1. However, 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."

NASA pays the price of being subjected to a massive, expensive rocket, Ars Technica

"In the face of political pressure, then, NASA chose prudence. Yet the evidently shrewd short-term decision to hold crew off the maiden flight of the SLS rocket could not mask a larger political problem that NASA has grappled with for nearly this entire decade. Simply, it has been tasked with building a massive, complicated rocket that it can't really afford."

NASA delays debut launch of $23 billion moon rocket and capsule, Reuters

"By the end of the next fiscal year on September 30, 2018, NASA will have spent $23 billion on the rocket, capsule, launch site and support systems, according to an audit by NASA's Office of Inspector General. That excludes $9 billion spent on the mothballed Constellation lunar exploration program, which included initial development of the Orion and a second heavy-lift rocket."

NASA won't put astronauts on first flight of new rocket, Washington Post

"We're essentially building a multi-decadal infrastructure that allows us to move the human presence into the solar system," [William] Gerstenmaier said. The commercial space sector - including SpaceX, founded by Elon Musk, and Blue Origin, owned by Jeffrey P. Bezos (who also owns The Washington Post) - is racing to develop its own rockets that are comparable in scale to the SLS. "This really isn't about NASA vs. SpaceX, or NASA vs. Blue Origin, it's more about the past way of doing business vs. how do we run the government more like a business," [Phil] Larson said."

Keith's 11 May note: This is not going to position NASA in a good place. Safety issues not withstanding, NASA has big, chronic delays with the overall SLS program. Then the SLS software program is shown to be bogus. Then they have welding issues. Then they drop the LOX dome. Now there is no way to do EM-1 without money that will never be there. And even if the money was there the existing slip would simply be magnified by attempts to human-rate the SLS to be used for EM-1, bring Orion systems forward etc. There are not enough people or capacity to take the extra money even if it was there.

Slipping to late 2019 is just going to give fuel to those who think that there are SLS alternatives i.e. commercial. SLS is not necessarily dead, but the chance that humans will ever fly on it are becoming increasingly remote. At best, one might expect a batch order for a half dozen or so planetary missions or large cargo launches so as to save face and do something with the immense investment made in the whole AresV/SLS thing. If Trump wants NASA to go to Mars while he is still in office he is going to need a Plan B. But Boeing and SpaceX would have to show that they are up to the task since they have been slipping launch dates too.

Keith's 12 May additional note: At the end of the day this is all about risk - engineering, programatic, and human. Dealing with risk takes time and time = money. There comes a point when the time/money you need to retire added risk exceeds the benefits that come with adding that new risk. That seems to be the case with the EM-1 crew exercise. Trying to retire risk atop a program that is already struggling to reduce risk is just asking for trouble - and the unlearning of lessons learned with great difficulty. It would seem that NASA has come to this conclusion - and they would not be making this announcement without coordination with the White House. One would assume that the White House understands the rationale behind this decision to decline putting a crew on EM-1.