Humans haven't set foot on the moon in 50 years, but NASA hopes to take one step closer with the launch of a new rocket and space capsule on Monday. Today, science correspondent Nell Greenfieldboyce joins Scientist in Residence Regina G. Barber to talk about what NASA hopes to learn from this test flight and why it might be difficult to justify the program's cost.Planning to tune in for Monday's launch? Email us at shortwave@npr.org.