Planetary Society Weblog: Landing ellipses
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To begin with, the lander is, in fact, targeting a specific latitude and longitude on Mars. But, [Phoenix landing guy Rob Manning] said, "Before landing we are not perfectly confident as to what square kilometer on Mars the lander will land in, because there are lots of variables that would cause it to move 'off target'. Uncertainty about the density of the atmosphere is among the biggest contributors to our uncertainty. And the ability of our spacecraft and navigation teams to precisely target a point above the atmosphere (at a grazing angle, no less) is another source of uncertainty. [...]"
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(Prediction, if history repeats: when Phoenix doesn't hit the exact center of the landing ellipse, some BAUT members will argue that the overlooked key variable is the all-important fudge-factor their own personal non-mainstream theory of gravity demands.)