Days after the US President Donald Trump promised to "unlock the mysteries of space", NASA has kicked off a study to assess the feasibility of adding a two-member crew to the first unmanned flight of the agency's Space Launch System (SLS) rocket and Orion spacecraft called Exploration Mission-1 (EM-1).

The study, that NASA acting administrator Robert Lightfoot has asked William Gerstenmaier, associate administrator for NASA's Human Exploration and Operations Mission Directorate, to conduct is expected to be completed in early spring.

The assessment will review the technical feasibility, risks, benefits, additional work required, resources needed and any associated schedule impacts to add crew to the first mission, NASA said in a statement on Saturday.

Orion's first flight atop the SLS was earlier decided to be an unmanned mission.

"Our priority is to ensure the safe and effective execution of all our planned exploration missions with the Orion spacecraft and Space Launch System rocket," said Gerstenmaier, adding, "This is an assessment and not a decision as the primary mission for EM-1 remains an uncrewed flight test."

The study will assume launching two crew members in mid-2019 and consider adjustments to the current EM-1 mission profile.

In this mission, NASA is planning to send the spacecraft into a distant lunar retrograde orbit, which will require additional propulsion moves, a flyby of the moon and return trajectory burns.

NASA is also investigating hardware changes associated with the system that will be needed if crew are to be added to EM-1.

Regardless of the outcome for the study, the feasibility assessment will not conflict with NASA's ongoing work schedules for the first two missions.

Hardware for the first flight has already started arriving at NASA's Kennedy Space Centre in Florida, where the missions will launch from the agency's Pad 39B.