Mercury to Neptune: the golden age of planetary exploration

From the 1960s to the 21st century, a flotilla of robot spacecraft has explored eight of the currently recognised nine planets. The Voyager missions made a highly successful reconnaissance of the outer planets, using the gravity of Jupiter, Saturn and then Uranus, reaching Neptune in 1989. Pluto remained beyond reach, as it was inconveniently placed, leaving astronomers dependent on the still fuzzy images from the best ground- and space-based telescopes.

Artist's impression of the 'New Horizons' spacecraft. Image: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute In the 1980s engineers planned a mission to the outermost planet. A probe to Pluto could cut down its journey time by first heading for Jupiter and then using the gas giant as a 'gravitational slingshot' to throw it forward. The mission – Pluto-Kuiper Express – was planned for launch early in the 21st century, but NASA budget cuts led to its cancellation.

Following a public campaign (helped by the website set up by 17-year old student Ted Nicholls) the US Congress voted to fund a cheaper but arguably equally as ambitious mission, New Horizons.

Journey to Pluto

The launch of the New Horizons mission to Pluto on 19 January 2006. The spacecraft was at the top of a three-stage rocket and is the fastest ever launched.
Image: NASA / KSC New Horizons launched from Earth on 19 January 2006. The spacecraft used a three-stage rocket and is the fastest ever launched. It crossed the orbit of the Moon just nine hours after take off and will reach Jupiter in February 2007. After the gravitational slingshot the probe will arrive at Pluto in July 2015. Without the Jupiter encounter, it might not have reached its destination until as late as 2020.

The Jupiter encounter also offers an opportunity for scientists to test the instruments on New Horizons. Between 2007 and 2015 New Horizons will be in hibernation, with only occasional communication between the probe and the Earth.

Why go to Pluto?

A true colour image of Pluto, made by monitoring brightness changes when the planet was eclipsed by Charon. Pluto's brown colour is thought to result from frozen methane on its surface. Image: Eliot Young (SWRI) et al, NASA. Pluto is one of the largest 'ice dwarfs' that make up the Edgeworth-Kuiper belt. This is a region beyond Neptune that contains tens of thousands of small bodies similar to the nuclei of comets. These vary in size from less than 50 km across to the large objects like Pluto (2500 km diameter) and the newly discovered 2003 UB313.

They have surfaces of water and methane ice and a coating of organic (carbon-rich) material. At such a great distance from the Sun, its heat hardly affects the ice dwarfs and so they are effectively fossils from the formation of the solar system.

Each Plutonian orbit around the Sun takes 248 years. Pluto's orbital path is highly eccentric with its distance to the Sun varying from 4400 million to 7400 million km. From 1979 to 1999, Pluto was actually closer to the Sun than Neptune. The extra warmth from the Sun evaporated more of the ices to form a thin atmosphere. Some scientists suggest that this may drag along behind Pluto as a giant cometary tail.

But Pluto's great distance and tiny size restricts the view of the planet from the Earth. The very best images from the Hubble Space Telescope reveal a yellow-brown world with prominent light and dark patches but no real detail. New Horizons will send back the first images of any craters, fissures and ice volcanoes on both Pluto and its largest moon, Charon. The probe will also give scientists a spectacular view of the cometary tail.

On to the Edgeworth-Kuiper belt?

The encounter with Pluto will only last a few days. If NASA agrees to fund an extension to the mission until 2020, New Horizons could explore several Edgeworth-Kuiper belt objects. Astronomers can then compare these with with Pluto and its moons.