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We have produced 22 VLBI images of the TeV blazar Markarian 421 at 11 epochs, including a Space VLBI observation with the HALCA satellite. We measure the speeds of the three innermost jet components to be 0.64+/-0.33, 0.48+/-0.09, and 0.06+/-0.09c (H 0 =65 km s -1 Mpc -1 ). Interpretation of these subluminal speeds in terms of the high Doppler factor demanded by the...