A PASSION FOR
The Sky at Night presenter looks back at Pluto’s past ahead of its meeting with the New Horizons probe
With New Horizons rapidly reaching its point of closest approach to Pluto, it seems a good time for an overview of the dwarf planet’s fascinating history to date. First known as the hypothetical Planet X, Pluto’s existence was originally inferred from gravitational perturbations on the orbits of Neptune and Uranus. It was discovered, and hailed as a planet, in 1930 by Clyde Tombaugh. However, estimates of Pluto’s proportions were too high; they have been plummeting over the century and have now settled at around one-sixth the mass of the Moon and one-third its volume. At this mass it is unlikely that any orbital effects would have been observed, so it seems that Pluto was actually discovered by accident.
After discovery came naming, and a competition was set up. It was won by 11-year-old Venetia Burney, who suggested it should be named after the king of the underworld in classical mythology. The discovery generated much public interest and it is claimed that Disney named Mickey Mouse’s canine companion Pluto after it.
Observations of Pluto have been few and far between. This world can lie 7.5 billion km from Earth, and the alignment didn’t work for a visit from either of the Voyager spacecraft. In 1978 photographic plates revealed the presence of Charon, Pluto’s largest moon; a companion so large that the pair could be considered a binary system. It was not until March 1996, some 66 years after the planet’s discovery, that the Hubble Space Telescope was able to take a semi-detailed image of Pluto’s surface.
Details trickle in
Hubble’s imaging campaign was conducted over 6.4 days – a full rotation of the Pluto. The images revealed polar caps and dark patches over the surface. Pluto’s moons Nix and Hydra were both discovered in 2005, also using the Hubble Space Telescope. In January 2006 the New Horizons space probe was launched to fly past Pluto, then the only world in our Solar System that had yet to be have a robotic visitor from Earth. In the same year, Pluto was demoted from planet to dwarf planet – a decision that has stuck, though some debate still continues.
The objectives of the New Horizons mission remained the same: to characterise and analyse the surface, geology and atmosphere of both Pluto and Charon. It will then look for additional satellites beyond the five we now know about before preforming similar tests on other bodies in the Kuiper Belt. But as well as looking forward, New Horizons has paid homage to the past. On board the spacecraft is an ounce of the ashes of Pluto’s discoverer Clyde Tombaugh.
In the near future we will have a much better understanding of Pluto and its companions, but for now I’m looking forward to sharing the excitement of this historic moment in an hour-long Sky at Night special report (BBC Four, 20 July, 10pm) direct from New Horizons mission control during the flyby.