(adsbygoogle = window.adsbygoogle || []).push({}); I have a problem where it ask to find the period/time(using Kepler's 3rd law) of an unpowered space craft to move from earth's orbit to mars orbit using the transfer orbit approach.

I found P/T using kepler's 3rd law but the second part ask to find how fast the spacecraft need to be moving at the beginning of its trip i.e while in earth's orbit using

V^{2}= GM(2/r-1/a) where a is the semi-major axis of the orbit, r distance from the sun.

Now the units of a is in astronomical units and r can also be AU, what units would V^2 be if I use AU for both r and a in the Vis Viva equation above?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Escape velocity when in earth's orbit

**Physics Forums | Science Articles, Homework Help, Discussion**