In the paper, the problems of controllability and approximate controllability are studied for the control system wt = wxx, wx(0, •) = u, x > 0, t ∈ (0, T ), where u ∈ L ∞ (0, T ) is a control. It is proved that each initial state of the system is approximately controllable to each target state in a given time T . A necessary and sufficient condition for controllability in a given time T is obtained in terms of solvability of a Markov power moment problem. It is also shown that there is no initial state which is null-controllable in a given time T . Orthogonal bases are constructed in H 1 and H 1 . Using these bases, numerical solutions to the approximate controllability problem are obtained. The results are illustrated by examples.