WASHINGTON, March 23 (Reuters) - Boeing's first mission carrying astronauts to space aboard its Starliner capsule has been delayed until at least the summer, a NASA official said on Thursday, as people familiar with the matter said last-minute tests and technical debates nixed a plan for an April launch. The Starliner battery concerns and expected upgrades, which had not been previously reported, would add to a growing to-do list of tests and redesigns Boeing has faced before it embarks on the long-awaited operational phase of its NASA contract: six astronaut missions over the next few years. Boeing has now cut Aerojet from the redesign process and is working directly with Aerojet's valve supplier, New Jersey-based company Marotta, said a person involved in the process who asked not to be identified