The European Space Agency (ESA) Planck satellite conducted a critical mid-course correction to place the satellite on its final trajectory for arrival at L2, the second Lagrange point of the Sun-Earth system, early next month.
That correction took several hours, in which Planck main thrusters conducted repeated pulse burns, switching on, then off, for 6 seconds every minute.
This pulse-burn technique is necessary because Planck is slowly spinning as it travels through space, rotating at 1 rpm. The thrusters, which are fixed to the spacecraft and are not steerable, can only burn when they are oriented in the correct direction, which occurs for 6 seconds during each 60-second rotation.