1. The problem statement, all variables and given/known data Consider a spacecraft in an elliptical orbit around the earth. At the low point, or perigee, of its orbit, it is 400 km above the earth's surface; at the high point, or apogee, it is 4000 km above the earth's surface. 1. What is the period of the spacecraft's orbit? 2. Relevant equations Kepler's 3rd Law: T=(2*pi*a3/2)/ sqrt(GME) where a=semi-major axis 3. The attempt at a solution So the first thing I did was find the semi-major axis (value a of the eqn above): (1/2)*(4000+400)=2200 km or 2.2*106 m Then I plugged it into the equation along with the following constants: G=6.67*10-11 m2/kg2, ME=5.97*1024kg T=(2*pi*2.2*10(6)3/2)/ sqrt(6.67*10-11*5.97*1024)= 1027 seconds I checked the back of the book and the answer is wrong. I have no clue what I am doing wrong....:( Any help would be greatly appreciated.