It depends on how you measure it. The fastest-moving spacecraft at launch was the New Horizons probe (pictured), currently hurtling towards Pluto and the icy worlds of the Kuiper Belt. Launched in 2006, it achieved an initial velocity of 16.5 kilometres (ten miles) per second on a trajectory that will allow it to escape from the Solar System completely, and picked up even more speed in a gravity-assisted manoeuvre as it swung past Jupiter in 2007. However, the fastest-moving space probes of all time are the Helios probes, launched towards the Sun in the Seventies. They reached a staggering 70.2 kilometres (44 miles) per second.
Answered by Giles Sparrow