OK TC lets see if I can`t help you out here....
How can heat move from one place to another?
1)Conduction.
2)Convection.
3)Radiation.
Since 1) and 2) require matter they are hardly likely to fit the bill for near vacuum which leaves us with radiation...
So we look in our handy dandy astrophysics textbook (in this case Modern Astrophysics by Carroll and Ostlie - the one I recomended to you a while back) and look up the Stefan-Boltzmann equation (derived by Ludwig Boltzmann from thermodynamics and Maxwell`s work on radiation pressure) which is....
L = 4.pi.r
2.sigma.T
4
where sigma = 5.67 times 10
-8 [W.m
-2.K
-4]
Now this is only an approximation assuming the body in question is a black body but its probably close enough for what you want....
Told you you should have bought that book
.....