NASA’s New Horizons space probe has been on its way to Pluto since its launch in January 2006, and NASA has recently announced that it has started its “approach phase”. As part of testing its instruments, and in order to make any final corrections before it flies past Pluto in July, the probe is already taking images even though it is still some 200 million km from Pluto (which is more than the distance from the Earth to the Sun).
Pluto, discovered by Clyde Tombaugh in 1930, was listed as one of the nine planets in our Solar System. But, in 2006, the International Astronomical Union (IAU) demoted it to the status of “dwarf planet”; a status it shares with a few other objects like Ceres (the largest object in the asteroid belt between Mars and Jupiter) and Eris, which is in the Kuiper belt. In fact, it was the discovery of objects in the Kuiper belt in the 1990s which led to Pluto being demoted to a dwarf planet status, as it was realised that it just happened to be the largest of the known Kuiper belt objects, and the first to be discovered.
Although we have sent space probes to all the other “classical” planets, and in fact to some asteroids, this will be the first time a space probe has visited Pluto. Because of this, we hope to learn a lot more about Pluto as the probe flies past it. Because of the speed of the probe, and the relatively small mass of Pluto, the probe will not be able to go into orbit about Pluto, but instead will pass about 14,000 km from its surface (roughly the same as the diameter of the Earth). As it whizzes by the various instruments will make their measurements, and they will only get one chance at it.
It was discovered in 1978 that Pluto had a satellite, Charon, the discovery being made from ground-based telescope images of Pluto. Since then, four other satellites have been discovered; Nix and Hydra were discovered in 2006, Kerberos in 2011 and Styx in 2012. So, as things stand, we now know of five moons orbiting Pluto, although New Horizons may discover more. We also hope to learn more about Pluto and Charon’s surface features, their composition, and whether Charon has an atmosphere. I will write future blogs on this as NASA releases these results.