The European Space Agency (ESA) and its industrial partners need to reduce the weight of a service module they are developing to fly on NASA’s Orion multipurpose crew exploration vehicle in 2017, a hurdle that will delay preliminary design review of the project by a little more than three months.

Based on Europe’s Automated Transfer Vehicle (ATV), the service module is to be led by Astrium Space Transportation, the prime contractor on the ATV. Astrium has been working on the effort since shortly before ESA’s 20 member states approved a first tranche of around €250 million ($330 million) in funding for the service module at a ministerial budget meeting last November.

Thomas Reiter, ESA director of human spaceflight, says the service module is roughly 0.5 metric tons overweight, but that the agency is working closely with NASA and Orion prime contractor Lockheed Martin to reduce mass ahead of a preliminary design review now slated for October or November.

ESA Director General Jean-Jacques Dordain says program funding is being released to Astrium in slices, but that project development is currently covered through this summer. At that point, he expects to award a contract to Astrium to continue work into 2014, when the agency’s member states will again meet to approve a second tranche of about €200 million for the €455 million development.

“It’s not the service module itself that is the bottleneck of the mass problems,” Dordain said during a press conference at the 50th international Paris air show here today. “This is something we need to do with NASA; we can’t do it on our own.”