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Jim writes:

"The observer in the lab will experience a resistance to her gravitational acceleration toward the earth’s center of mass."

The problem is that you are assuming that the Earth's center of mass is a privileged reference frame.

Besides, who says the lab is on Earth? All the observer sees is an object (the astronaut) following a parabolic path with a relative acceleration of "g" with respect to the lab.

The drawing might help. The laboratory observer has a window, and all that is visible from that window is the astronaut.

The astronaut is not in a windowless craft but is in open space. All the astronaut sees is darkness except for the window through which the observer is looking.

In other words, their circumstances are equivalent. They can each determine their relative acceleration, and if they had scales, they could determine the apparent gravity in their frame of reference. But there is neither absolute acceleration nor absolute gravity.

Hope that clarifies things.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)