This July, a NASA space probe completed our set of images of the planets, at least as I knew them growing up. New Horizons, a probe that launched back in 2006, arrived at Pluto and its moons, and over a very brief encounter, started to send back thousands of images of this hitherto barely known place.
Pluto, a world smaller than our Moon, was discovered 85 years ago, during a search for an unseen object, the so-called Planet X, thought to be disturbing the outer planets. There was a precedent for this – in 1846, Neptune was found through its gravitational influence on Uranus and in the early 20th century astronomers thought they were seeing the same effect again.
Amateur astronomer turned professional researcher Clyde Tombaugh started work at the Lowell Observatory in Arizona in 1929, where he searched for Planet X. He compared thousands of photographic images of the skies, looking for a shift from one night to the next in the positions of objects on the plates that would suggest a planet rather than a distant and apparently unmoving star.
On 18 February 1930 he looked at plates taken a few days apart in January, spotting a “star” that moved in the constellation of Gemini, but only a bit, suggesting it was a long way from the Sun. More analysis confirmed it to be a full 6 billion kilometres (3.8 billion miles) away, well beyond Neptune.
The discovery became public in March that year. Lowell Observatory had the right to name the planet, and in the end took the suggestion of an eleven-year-old schoolgirl, Venetia Burney (later Venetia Phair), who passed it to astronomer Herbert Hall Turner and then to Lowell. The new planet became Pluto on 1 May. (Venetia finally got to see Pluto for herself in 2008, saying “It’s not much to look at, is it!”)
For almost half a century, even the best telescopes in the world showed Pluto to be nothing more than a single starlike point. Though estimates for Pluto began with it around the same size and mass as the Earth, astronomers successively shrunk these values. Today’s measurements give Pluto’s mass as about 1/450th of our own world – rather smaller than our Moon – and a diameter of just 2,400 kilometres, meaning it would more or less fit on top of the contiguous United States.
In 1978 astronomer James Christy found the largest of Pluto’s moons, Charon, and in the 1980s Earth-based scientists watched the two worlds pass in front of each other. These occultation events led to a dip in the light coming from the system as each object was blocked in turn, and helped scientists to make crude maps of their surfaces. 1990 saw the launch of the Hubble Space Telescope which later in the same decade made the best – though still crude – maps yet, with obvious light and dark regions.
On the ground, instruments like the James Clerk Maxwell Telescope on Hawaii analysed Pluto’s light, revealing a thin atmosphere of nitrogen, methane, and carbon dioxide, which at the time was thought to freeze out when the planet was furthest from the Sun, though more recent results suggest otherwise.
Since the late 1950s, space probes – robots of increasing sophistication – have explored the main planets of the Solar System, and visited comets and asteroids too. The Pioneer, Voyager, Galileo, and Cassini missions travelled beyond the asteroid belt – visiting the gas giants Jupiter, Saturn, Uranus, and Neptune – and giving us images and data that are simply impossible to get from a telescope here on Earth.
But Pluto was too far away to fit in neatly to those other itineraries, so it needed a separate plan. After an earlier mission was cancelled, New Horizons was built and sent to Pluto on a fast (though still nine year) journey. En route, its target was controversially demoted by the International Astronomical Union to a “dwarf planet”, and my school textbooks became even more out of date. During the journey, astronomers also found four new moons, making this tiny world a more intriguing target.
New Horizons made its closest approach on 14 July 2015, passing 12,500 kilometres from Pluto’s surface. So far from the Earth, the weak signal strength means a slow bandwidth for data transmission, significantly slower than dial-up home internet connections were in the late 1990s. This means that the probe sends preview images from its cameras, before the sharpest pictures arrive over the weeks and months that follow. As a result, the science team expect to be getting new pictures of Pluto until well into 2016.
It’s fair to say that no one was disappointed by the results from New Horizons. So far we’ve seen one of the most varied surfaces imaginable, with geology shaping a black, orange, and white landscape. Despite being cold (this far from the sun the temperature doesn’t get much above -220 degrees Celsius), Pluto shows signs of major – and recent – activity, with jagged 3,500 meter-high mountains of ice, smooth bright plains including the heart-shaped “Tombaugh Regio”, and glaciers.
New Horizons has completed our first survey of the (original) nine planets and turned the dot of my childhood into a real world. Like Mars, we can now imagine setting foot there some day. Future astronauts may one day get to look up at a piercingly bright point (our sun), see Charon hanging in the sky above their heads, and even make a bid to climb those mountains.
Featured image credit: Pluto’s Majestic Mountains, Frozen Plains and Foggy Hazes. Public domain via NASA/JHUAPL/SwRI.