Nick,

I am going to assume you want the simple deceleration at a given radial distance from the Sun, and not the time rate of change of acceleration. I am also going to assume you are not asking for the objects velocity at effectively infinite distance from the Sun. If my assumptions are wrong, please clarify what it is you are seeking.

For an object moving radially away from the Sun, the force of gravitation, F, at any point would be:

F = (GMm)/r^2

where:

G = Gravitational constant

M = Mass of Sun

m = mass of your object

r = distance from Sun

Since F = ma, simple substitution gives:

ma = (GMm)/r^2 and dividing both sides by m gives the acceleration at that point, thus:

a = (GM)/r^2

Since this is directed opposite to the path of you object, it would normally be expressed as a negative value, which is the deceleration of the object at that distance from the Sun. I hope this is the value you are seeking.

Brett