Following the publishing of this article, it was brought to our attention that a few inaccuracies occurred in our reporting. Due to an error in our database, the record we awarded to Juno was for geocentric speed, while the record set by Helios 1 in 1976 was for heliocentric speed.
Guinness World Records can confirm at this time that the record for the Fastest spacecraft speed, defined by its heliocentric velocity (i.e., relative to our Sun), is held by Helios 2.
As Harvard astrophysicist Jonathan McDowell confirmed, Helios 2 achieved geocentric speeds significantly faster than Juno during the late 1980s, when it was travelling almost directly away from the Earth as it moved around the Sun. At its fastest in January 1989, Helios 2 was moving away from Earth at a speed of 356,040 km/h (221,232 mph).

To further complicate matters, by the time Helios 2 set this speed record it was an inert brick of dead electronics, having been shut down almost 10 years earlier, and so was arguably not a spacecraft anymore (still the fastest man-made object though). The Fastest geocentric speed achieved by an active spacecraft is around 346,320 km/h (215,193 mph), set by Helios 1 in December 1980.
Geocentric speed isn’t really the best way of measuring these things, however. Once you get out beyond Earth’s immediate vicinity, our planet just becomes another object hurtling around the Sun. Measuring velocity relative to Earth is like measuring the speed of moving cars from inside another moving car – the measurements you get are more dependent on which direction the cars are going than how fast they're moving. In terms of its speed relative to the Sun, Juno was moving at a top speed of 213,480 km/h (132,650 mph) – very fast, but still not as fast as Helios.
We have taken immediate action to rectify this situation by correcting the information in our database and have posted the accurate record information online. Guinness World Records is reviewing its research and editorial procedures to ensure this doesn’t happen again and we thank our readers who provided feedback.
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NASA’s Juno probe is now safely parked in orbit around Jupiter, ready to begin its historic survey of our largest neighbour. This marks the end of journey that has seen the probe travel 2.7 billion km (1.7 billion miles), setting two new world records on the way. 
Juno’s first record was set back in January 2016, when it became the Most distant solar-powered spacecraft. Juno passed the mark of 791 million km (492 million mi) set by the European Space Agency’s Rosetta probe in October 2012. Probes like Voyager and New Horizons have gone farther, but they are powered by nuclear generators. 
Juno’s second record was set on 4 July 2016 as it plunged into Jupiter’s massive gravity well. The gas giant’s pull accelerated the already fast-moving probe to a speed of 265,000 km/h (165,000 mph), making it the Fastest spacecraft (and arguably the fastest man-made object) ever. This breaks a record that has stood for more than 40 years. It was last broken on 17 April 1976 by the Helios 1 probe when it plunged past the sun in a highly elliptical orbit.
The Juno team celebrates after receiving confirmation from the spacecraft that it had successfully completed the engine burn and entered orbit of Jupiter

Juno’s speed-record run was ended at 23:18 EDT (04:18 BST, 5 July), when the command was sent to hit the brakes. The probe’s powerful rocket (which was built in Buckinghamshire, UK, by Moog-ISP) ignited and burned at full power for 35 minutes, knocking 1,212 km/h (753 mph) off the spacecraft’s speed. If the rocket hadn’t ignited, or if it had failed to burn for long enough, then Juno would have zoomed straight through Jupiter’s gravity well and been sent tumbling off into deep space.
“The spacecraft worked perfectly, which is always nice when you’re driving a vehicle with 1.7 billion miles on the odometer,” said Rick Nybakken, Juno project manager from NASA’s Jet Propulsion Laboratory. 
Jupiter’s gravitational field pulls on all objects within about 180,000 km of its surface. The closer an object gets to Jupiter, the stronger that gravitational pull becomes. This creates an effect similar to rolling down an increasingly steep slope, hence the term “gravity well”. Approaching objects (like Juno) are accelerated to incredibly high speeds as they “descend” into the well and then slowed back down as they “climb” back out on the other side.
The record-holding spacecraft will now spend the next 20 months orbiting Jupiter, collecting data on the planet’s mysterious weather and internal composition. Among all the cameras and scientific instruments it carries, Juno also holds three LEGO Minifigures – representing the Roman god Jupiter, his wife Juno and the astronomer Galileo Galilei – who have boldly gone farther than any Minifigures in history.