In short, its gravitational potential was high and the ship's Calculator was a run-of-the-mill model not designed to plot landing trajectories at that potential range.
The ship's owners could not afford a better calculator. 
mnli hypothesis: The ship's owners could not afford a better calculator.  premise: In short, its gravitational potential was high and the ship's Calculator was a run-of-the-mill model not designed to plot landing trajectories at that potential range.