WASHINGTON (Reuters) - The multibillion-dollar U.S. ballistic missile shield due to start operating by Sept. 30 appears incapable of shooting down any incoming warheads, an independent scientists' group said on Thursday. A technical analysis found "no basis for believing the system will have any capability to defend against a real attack," the Union of Concerned Scientists said in a 76-page report titled Technical Realities. The Pentagon (news - web sites)'s Missile Defense Agency rejected the report. "It will provide a defense against incoming missiles for the first time in this country's history," said Richard Lehner, an agency spokesman. The Pentagon's...