Russian space officials proposed an ambitious project on Friday to send a six-person team to Mars by the year 2015, a trip that would mark a milestone in space travel and international space cooperation. Russia’s space program hopes to work closely with the American agency NASA and the European Space Agency to build two spaceships capable of transporting the crew to Mars, supporting them on the planet for up to two months and safely bringing them home, said Nikolai Anfimov, head of the Central Research Institute of Machine-Building.