• New Horizons clocked the fastest launch ever recorded when it blasted off in January 2006, exceeding 36,000 miles per hour. The spacecraft passed the Moon after just nine hours and reached Jupiter the following year. It took just three minutes to cross the diameter of Pluto.
• The first images beamed back show that, at 2,370km (1,473 miles) in diameter, Pluto is slightly larger than previously believed. This makes it undisputedly the largest dwarf planet in the solar system. For the past decade astronomers had been undecided about whether this title belonged to Pluto or Eris, another planetary object beyond Neptune, although Eris still has a bigger mass. The extra volume means that Pluto must be less dense than thought, meaning it probably contains more ice beneath the surface.
• Sensors on New Horizons have detected a thin nitrogen atmosphere extending far out into space, and the surface colouring suggests it is a patchwork of different concentrations of frozen methane and nitrogen. These probably sublime into the atmosphere when the planet is closer to the sun and drift back to the ground as snowfall during colder periods.
• Nasa mission scientists have started giving unofficial names to some of the features. A dark patch near the south pole, initially called the whale, is being referred to as Cthulhu, after the octopus-dragon hybrid deity featured in one of HP Lovecraft’s stories. Other patches have been named Meng-p’o, after the Buddhist goddess of forgetfulness, and Balrog – a menacing creature featured in JRR Tolkien’s The Lord of the Rings. For the names to become official, though, they would need to be approved by the International Astronomical Union (IAU).
• The mission is now speeding onwards into the Kuiper belt where it will examine one or two of the ancient icy objects in the vast region. Potential Kuiper belt objects that it might visit include Quaoar, Eris, Makemake, Haumea and Sedna. In coming months, scientists will decide the spacecraft’s next flyby target and send signals from Earth to New Horizons to thrust its rockets to tweak its trajectory.
• Most spacecraft depend on solar energy to power their on-board systems, but at 4bn miles from the sun, the faint light reaching a spacecraft at Pluto makes this impractical. Instead, the mission is powered by nuclear fuel – aptly, plutonium – that gives off heat as it decays. The fuel is designed to last until the late 2020s or even beyond.
• Pluto was downgraded to a “dwarf planet” just months after the launch of New Horizons, following a vote by astronomers at the IAU to change the definition of the word “planet”. The flyby has resurrected the debate, however, and Charles Bolden, Nasa’s chief administrator, said he hoped the official classification would be reconsidered. “I call it a planet, but I’m not the rule maker,” he said.
• New Horizons has spent much of its journey in hibernation mode, but it made some closeup observations of Jupiter and its moons when it completed a slingshot flyby of the planet in 2007. These included a time-lapse video of a volcanic eruption on Jupiter’s moon Io, the first detailed recording of a volcanic event outside Earth.
• When New Horizons was planned, astronomers thought it would just be observing Pluto and its largest moon, Charon. Seven months before launch, two other moons, Nix and Hydra, were discovered and then, once the mission was under way, astronomers spotted two more – Styx and Kerberos – increasing the number of objects that the spacecraft would be able to observe and need to avoid colliding with.
• The mission is carrying the ashes of Clyde Tombaugh, who discovered Pluto in 1930. Before dying in 1997, he requested that his ashes be sent to space. Nasa obliged, affixing a small container with his remains to the inside upper deck of the probe. It bears the inscription: “Interred herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system’s ‘third zone’, Adelle and Muron’s boy, Patricia’s husband, Annette and Alden’s father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997).”
