Visiting the planets

Only one spacecraft has ever flown past Uranus and Neptune: Voyager 2, launched on 20 August 1977.

From telescopes to spacecraft

For most of history people knew six planets, the ones bright enough to see without a telescope: Mercury, Venus, Earth, Mars, Jupiter and Saturn. The telescope changed that. In January 1610 Galileo Galilei pointed one at Jupiter and saw four points of light that stayed close to the planet and moved with it. They were moons, the first ever found beyond Earth, and today they are called Io, Europa, Ganymede and Callisto.

On 13 March 1781 William Herschel noticed a faint object in the constellation Gemini that moved slowly against the stars. He first thought it was a comet, but it turned out to be the seventh planet, Uranus. Neptune followed in 1846, found where calculations said it should be, and Pluto in 1930.

The Grand Tour

In the 1960s engineers at NASA’s Jet Propulsion Laboratory noticed that the outer planets would line up in the late 1970s, an arrangement that comes around only once every 175 years. A spacecraft could fly past Jupiter and use its gravity to bend its path toward Saturn, then repeat the trick at each planet. This is called a gravity assist. It saves fuel and years of travel time.

The Voyagers were built to last five years and to visit only Jupiter and Saturn. Voyager 2 kept working, so NASA sent it on to Uranus in January 1986 and Neptune in August 1989. At Uranus it returned more than 7,000 photographs and found 11 new moons. At Neptune the Sun is so faint that the light level was only 3% of what the cameras had at Jupiter, and the team changed how they took pictures to cope.

After Voyager

Later missions stayed longer instead of flying past. Cassini entered orbit around Saturn in 2004 and studied it for 13 years. In 2017 it was steered into Saturn on purpose, so that it could never crash into moons such as Enceladus and Titan, which may hold places where life could exist. Juno reached Jupiter on 4 July 2016. New Horizons launched in January 2006, borrowed speed from Jupiter in 2007, and reached Pluto on 14 July 2015, after about nine and a half years in flight.

The two Voyagers are still sending data. Voyager 1 left the heliosphere, the bubble of solar wind around the Sun, in August 2012, and Voyager 2 followed on 5 November 2018. Both are now in interstellar space, still carrying the instruments that first showed us the outer planets up close.

Talking to a far spacecraft

Radio signals travel at the speed of light, about 300,000 km per second. That sounds instant, but at 30 AU a message from Neptune needs about 4 hours and 10 minutes to arrive. Engineers cannot steer a spacecraft live from that far away. They send commands hours ahead and the spacecraft carries them out by itself.