Asked by Ross Hartley
The Helios probes hold the esteemed title of being the fastest satellites in space. They reached a fantastic 253,000 kilometers (157,000 miles) per hour as they raced around the Sun measuring the solar wind and environment near to the Sun.
The main reason these probes went so quickly is because their trajectory had them plummet towards the Sun. The Sun is the dominant source of gravity in our solar system, holding all the planets, asteroids and other bits in orbit. The gravitational force of our local star is incredibly strong and the force increases as you get closer to the source. As the Helios probes got closer and closer to the centre of our Solar System their gravitational attraction to the Sun would have increased causing them to accelerate more.
The designs and missions of these probes meant that they passed fairly close to the Sun, which is why they were able to reach some impressive speeds.
Interestingly, due to orbital mechanics, when the spacecraft left Earth they had to slow down. This reduction in speed caused the craft to start falling towards the Sun.
Answered by Josh Barker from the National Space Centre
Image courtesy of NASA