def errorScalingMissionLength(extension, p):
return power((_nominalLifeTime+extension)/_nominalLifeTime, p)<EOL>
Calculate the factor by which to scale errors for a given Gaia mission extension.

Parameters
----------

extension - The mission extension in years (a negative extension can be used to make crude
performance predictions part-way through the mission lifetime).
p - The power by which the errors scale with time (error ~ t^p, p=-0.5 for parallax and celestial
position, and -1.5 for proper motion).

Returns
-------

The factor by which to scale the errors