I read the letter from James F. Jackson with mixed feelings (C&EN, April 12, page 4). I agree that microgravity is a poor term, but weightless is even worse. Since weight is usually presented as the pull of gravity, weightless implies no gravity. Saying that weight is the force you can measure and you can't measure gravity in orbit doesn't make things much clearer.
The simplest explanation is the best. A satellite is in orbit because one unbalanced force, gravity, causes the satellite to fall. Because of the initial velocity, the satellite falls in a curve around Earth. Objects in orbit are best described as being in free fall; that is, only gravity is acting.
A centrifugal force balancing out gravity is totally wrong. First, no source of such a force can be found. Second, "equal but opposite" forces (Newton's third law) act on two different objects, not on the same object. Third, if there were to be a force balancing gravity, the net force on the satellite would be zero, and by Newton's first law, the satellite would be moving in a straight line.
The apparent weightlessness of objects in orbit is simply looking at objects falling with the same acceleration when gravity is the only force acting. This can be seen by dropping a book and a crumpled piece of paper from the same height. They reach the floor together. If you picture what the book sees, it is a hand releasing the paper and the paper staying unsupported in front of the book for the entire fall.
Wesley A. McCullough