We know that it is possible for humans to stay physically in shape during long term space missions (see for example Valeri Polyakov who stayed on Mir for more than 14 months for one trip). However, the challenges of manned space exploration are different and would include even longer durations.
Reducing the need of the constant physical training required by the Zero-G environment could help to make manned exploration missions more viable. This could be achieved by artificial gravity.
Is artificial gravity feasible today? Would it indeed help to make manned exploration viable?