Is critical mass of fissile material the same in gravitational fields with different force? Let's take plutonium-239 for example. $10\, Kg$ (sphere $9.9\, cm$) need for a nuclear reaction.
Are we talking about these numbers in Earth environment with $\approx 9.8\, m/s^2$ gravitational acceleration value? What if our environment is a high Earth orbit?
UPD:
I think more correct comparison in this question - an impact of gravity of heavy object as a neutron star and an impact of gravity of Earth.
 A: Actually, the critical mass is not affected by external gravitational fields. Gravity does not effect nuclear reactions. And, for the most part, gravity does not effect even chemical reactions either. Intermolecular forces (electromagnetic in nature) are vastly stronger than gravitational forces.  
Rather, the critical mass has to do with the effective cross section of all the surrounding nuclei to the neutrons from natural radioactive decay. This is proportional to the density of the material. The parameter is stated as "critical mass" because that's the easiest way of taking into account the density/cross section. A more precise statement might be "critical cross section", but that isn't something you can easily measure in the laboratory. If you put a huge compression force on a sub-critical sample, you can increase its density (cross section) and make an otherwise non-critical chunk of radioactive Uranium or Plutonium critical. 
Nuclear bombs made of Plutonium, like the first one exploded at Trinity in NM back in 1945, worked this way. The bomb itself looks large and impressive (hence the nickname "fat man"), but that's mostly high explosive surrounding a sub-critical 'pit' of Plutonium. When the explosive goes off, it compresses the Plutonium and the nuclear reaction is triggered. 
I hope this helps!
PS: Well ... I guess if you drop a sub-critical sample of Plutonium on to the surface of a neutron star (where the gravitational fields are immense) it would explode. But that's again due to compression. The gravity doesn't effect the nuclear physics. :-)
A: The mass is the same to a very good approximation: gravity is absurdly weak compared to the factors that influence this.
One way that it might influence things was if the shape of the mass was significantly macroscopically distorted by gravity (ie a sphere might become flattened and you might therefore need slightly more mass).  However metals are quite rigid: I have no idea what the deformation of a sphere of some suitable material would be, if resting on a plate under Earth's gravity, but it's very small.  In addition of course, the critical mass in a weapon is assembled using high explosive lenses and these are causing accelerations hugely greater than Earth's gravity.
