Expert: Kenneth Renshaw - 9/22/2009

Question
i had a general question about the trip to the moon. if a spacecraft has the ability to move at a speed of around 25,000 miles per hour and the average distance to the moon is 252,088 miles... than why did the moon mission take around 5 days round trip? now i know you have to allow time compensation for acceleration and deceleration upon departure and approach... but say by the math, if you started and ended traveling the same consistent speed of 25,000 mph, it should have only taken around 10 hours to reach the Moon from the Earth, then you add your time compensations to the 10 hours and it comes to around a five day trip? do you know how long it took for them to get from point "A" to point "B" if point A was the Earth and Point B was the Moon? not including landing? just a straight shot from the earth to the moon?

Answer
Hi, thanks for writing.  It took longer since the Apollo craft orbited the earth 1 1/2 times before leaving it for the moon.  It left at an initial speed of 25,000 MPH (escape velocity), but with the earth's pull, it slowed down a lot below the 25,000 MPH as it went toward the moon.  Escaping earth's gravity, it sped up again as it approached the moon, and orbited it 20 times before making the separation for a landing.  With the orbits and speed changes, it took 4 1/2 days to get to the moon.  For more info, see http://en.wikipedia.org/wiki/Apollo_11  Write anytime.