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Part of the problem of landing a probe on Mars is that mission telemetry data takes 4-21 seconds (depending on the two planets relative position) to be transmitted from Mars to Earth (at the speed of light). Any corrections that must be made take an additional 4-21 seconds to be sent from Earth back to Mars. This delay of 8-42 seconds is too long to make any "fine tuned" mid-course corrections, so the flight parameters have to be pre-loaded into the probe's computer - and you cannot adjust it in real time.

A manned spacecraft however, could make these adjustments in real time - just as Neil Armstrong did in changing the landing site in Apollo 11, when the original site was found to be too rocky to make a safe landing.