A line from the astronaut and tangent to the Earth's surface will be at right angles to a line from the center of the Earth to its surface at the point of tangency.
So, you have a right triangle with one leg of 4000 miles and a hypotenuse of 4050 miles. Determine the other leg.
4050^2 = 4000^2 + x^2
16,402,500 = 16,000,000
√402500
=634.43m
