The answer would be 2.2 millimeters because although this is basically impossible, you would use the formulas: F = G M₁ M₂ / R²
G = 6.67 x 10⁻¹¹ m³ / kg - s² to figure out your answer so it would look like this:
(6.67 x 10⁻¹¹) (900) (80) / R²
(6.67 x 10⁻¹¹ x 900 x 80)
4.8 x 10⁻⁶
√(4.8 x 10⁻⁶)
0.0022 meter
2.2 millimeters
Hope this helps :)