PARIS The U.S. and Germany will jointly undertake a long-planned $600-million aerial infrared astronomy mission, following the conclusion of a new Memorandum of Understanding. The MOU, signed at the end of 2006 but only revealed last week by German aerospace center DLR, will allow the two countries to move ahead with the Stratospheric Observatory for Infrared Astronomy (Sofia) program, which has been plagued by large schedule and cost overruns.