Astronomy & Space | Pluto Out of the Darkness Running time | 37m 30s Good evening. Excitement is growing among planetary scientists and amateur astronomers, as we wait to see the first ever close up images of Pluto at the edge of our solar system. Later this month the mystery of Pluto and its five known moons will be historically unveiled by the NASA New Horizons spacecraft. New Horizons was launched on January 19, 2006, from Cape Canaveral, directly into an Earth-and-Sun-escape path with an Earth-relative speed of about 16 km/s. It set the record for the highest launch speed of a man-made object from Earth. New Horizons travelled to Jupiter, making its closest approach on 28 February 2007 at a distance of 2.3 million km. The Jupiter flyby provided a gravity assist that increased New Horizons speed by 4 km/s (14,000 km/h). The encounter was also used as a general test of New Horizons scientific capabilities. Most of the post-Jupiter voyage was spent in hibernation to preserve on-board systems, except for brief yearly checkouts. On 6 December 2014, New Horizons was brought back on-line and the instruments were checked and found to be working fine. On 15 January 2015, the New Horizons spacecraft began its approach to Pluto. Video: New Horizons Description. 3m 20s Travelling at a speed of 14 Km/s New Horizons is intended to pass within 12,000 km of Pluto on 14 July. These images show Pluto in the latest series of New Horizons Long Range Reconnaissance photos, taken between 8 May and 12 May, 2015, compared to images taken one month earlier. In the month between these images, New Horizons' distance to Pluto decreased from 110 to 75 million km, as the spacecraft speeds toward a close encounter with the Pluto. The April images are shown on the left, with the May images on the right. Between April and May, Pluto appears to get larger as the spacecraft gets closer, with Pluto's apparent size increasing by about 50 percent. A day on Pluto is 6.4 Earth days in length, and these images show the variations in Pluto's surface features during its rotation. The discovery of Pluto 85 years ago seems so far away now, while it's reclassification to a dwarf planet by so few members of the International Astronomical Union was, in my opinion, an injustice; I will always regard Pluto as a planet. Clyde Tombaugh was actually born on a farm near Streater, Illinois, on 4 February 1906 and was the youngest of 6 children. He had a good upbringing and soon took an interest in the stars visible most clear evenings. In 1922 the family moved to a farm near Burnett, Kansas, where he went to grade school. Uncle lee, who was an amateur astronomer, lived on a farm nearby where Clyde often visited at weekends. He was equipped with a 75mm refractor telescope, which was not of good quality since it showed spurious colour around the moon and planets, however, it proved to be useful to Tombaugh, as he used it to study the night sky and learned his way around the constellations. In 1925 Clyde Tombaugh graduated from the small high school in Burnett, Kansas, and there was no prospect of him going onto college because times were hard. However, his interest in astronomy was enhanced enough for Clyde to have a go at constructing his own telescope, and in 1926 he set up a grind stone on a sturdy post and began to grind his own 20cm mirror. Upon completion he sent it off to a French company for silvering while he built the telescope tube, and equatorial mounting. When the mirror was

returned the French company Napoleon Carreau, advised that it was not of good quality. However, with its maximum power of 84 it was sufficient to show some of the deep sky objects along with detail on the main planets. While Clyde was able to see Messier 13 in Hercules, along with a number of other deep sky objects, he was disappointed that he was unable to see any of the marking on Mars during the planet's opposition in November 1926. He therefore determined to build a much better 17½ cm reflector for his uncle lee, and Tombaugh learned from his mistakes. Upon completion, he had a pleasing view of comet Pons–Winnecke which passed within 6,000 km of the Earth in June 1927. This comet is believed to be the parent body of the June Bootide meteor shower occurring roughly between 26th June and 2nd July each year. With the money his uncle lee gave him for this telescope, Clyde Tombaugh began constructing yet another which was a 22½ cm reflector. He soon put the telescope to good use making astronomical drawings of the Moon and planets which proved to be a turning point in his career. Previously, the Lowell Observatory in Flagstaff Arizona had been established in 1894, placing it among the oldest observatories in the United States. The Observatory's original 60cm Alvin Clark Telescope is still in use today for public education. The observatory was founded by astronomer Percival Lowell of Boston's well-known Lowell family, and is overseen by a sole trustee, a position historically handed down through the family. Percival Lowell used the Alvin Clark refractor to study the planet Mars, and made numerous drawings of the planet's system of what he termed Martian channels. For fifteen years Percival Lowell studied Mars extensively, and made intricate drawings of the surface markings as he perceived them. While this idea excited the public, the astronomical community was sceptical. Many astronomers could not see these markings, and few believed that they were as extensive as Lowell claimed. As a result, Lowell and his observatory were largely excluded by the astronomical community. In 1909 the 1.6m Mount Wilson Observatory telescope in Southern California allowed closer observation of the structures Lowell had interpreted as canals, and revealed irregular geological features, probably the result of natural erosion. Another telescope at the observatory is the 32½ cm Cassegrain telescope which was a new addition to the observatory in February 1929. It so happened that the Observatory's Director, Dr. Vesto Slipher, was looking for a suitable amateur astronomer to work with this newly acquired telescope. Clyde Tombaugh responded by sending in his drawings of Mars and Jupiter with a covering letter. After further correspondence Dr. Slipher invited Clyde Tombaugh to Flagstaff for a few month's trial beginning in mid January 1929. Previously, the prestige of the Lowell observatory had been damaged by Percival Lowell's obsession with the Martian canals. Another hobby that interested him, that of discovering a trans Neptune planet which he referred to as planet X was brought forward. John Coach Adams who was a British amateur astronomer and mathematician born at Liddcot in Cornwall in 1819, had carefully worked out the position of the planet Neptune a few months before it's actual discovery using a similar prediction by Urbain Le Verrier. Telescopic observations at the New Berlin Observatory confirmed the existence of a major planet which was made on the night of September 23–24, 1846. Not long afterwards, John Coach Adams was given equal recognition for the discovery of Neptune. As observations of Neptune began to accumulate, Lowell's greatest contribution to planetary studies came during the last decade of his life, which he devoted to the search for Planet X, a hypothetical planet beyond Neptune. Lowell believed that the planets Uranus and Neptune were displaced from their predicted

positions by the gravity of the unseen Planet. Lowell's search had begun in 1905 using a special 12½ cm telescope which was specially mounted to allow 3 hour time exposures to be made on to large glass photographic plates, which he carefully examined using an instrument called a Blink Microscope Comparator. The Blink comparator allowed Percival Lovell to place two plates side by side, and a mechanism allowed a mirror to flip, so that first one then the other plate could be observed, one after the other. If planet X was in the field, the star-like object would appear to hop between positions. Sadly there had been no success for Percival Lowell until his death in November 1916, or during subsequent searches until the winter of 1928. Soon after his arrival at the Lowell observatory, Dr Slipher put Clyde Tombaugh to work using the 32½ cm Cassegrain telescope to search for planet X. Plate number one was taken on the night of April 6th 1929. Hundreds of plates were taken as Clyde worked his way along the plane of the Ecliptic, until he came to the region of sky containing the Mag +3.3 star Delta Gemini, or Wasat to give it its proper name. The star lies close to the Milky Way so that each photographic plate contained thousands of stars, and even recording each image proved troublesome. Clyde Tombaugh had to make 4 plates, each of which recorded Pluto, however, he did not know this until 10 months later. On 23 January 1930 Clyde Tombaugh photographed the Delta Gemini region again, and took a further plate on the night of 29 January. On the morning of 18 February Clyde placed both plates on the Blink Comparator, and was fortunate to start looking for planet X on the eastern half. By around 4pm Tombaugh had covered about a 4th of the plate, when he suddenly seen a magnitude +15 'star' popping out and disappearing in the alternating views. Over the next 45 minutes Clyde Tombaugh carefully checked everything to make sure there was no mistake. Clyde later recalled “A terrific thrill came over me”. Then he walked down the hall to Dr Vesto Slipher's office trying to control himself. He explained “I stepped into his office as nonchalantly as possible. He looked up from his desk. 'Dr Slipher, I have found your planet X.' “He rose right up from his chair with an expression on his face of both elation and reservation.” The two men immediately hurried down the hall to the blink comparator room, and Clyde shown Dr Slipher the evidence. “Looks pretty good to me,” he said. It was from this point a new addition to the solar system came to the attention of the astronomical world. The name Pluto, after the god of the underworld, was proposed by Venetia Burney, an eleven-year-old schoolgirl who lived in Oxford, here in the UK. Venetia Burney interview: 4m 14s Pluto is a mysterious world, it lies at a distance of 5.9 billion km from the Sun and takes 248 Earth years to travel once around its orbital path. Its orbit is remarkable, when at perihelion Pluto is actually closer in to the Sun than Neptune can ever be. It will next be at perihelion on 15 September 2237 when it will be 257m km closer to us than it is today. Pluto was moving inside Neptune's orbit between 21 January 1979 and March 14 1999 so that it is now very remote. Pluto will be at aphelion, its most distant position from the Sun on 19 February 2114. As the planets move around the Sun there orbits are inclined to the plane of the Ecliptic, astronomers call

this Inclination. Most of the planets move in about the same orbital plane with inclinations ranging from 3˚ for Venus, and less than 2˚ for Mars, Jupiter, Saturn, Uranus and Neptune; while Mercury has an inclination of 7˚ , which is greater than its planetary neighbours. Pluto however is exceptional, it has an inclination of 17˚ degrees which makes it stand out from the rest. The diameter of Pluto has been difficult to determine because the planet is so remote, at the moment our best estimate is 2,390 km making it almost 1,086 km larger than our Moon. Back in 1936 astronomers did not have the powerful telescopes we have today, and were using various methods to work it out. The astronomer Andrew Claude Crommelin of Greenwich suggested the theory of specula reflection. If Pluto was highly reflective because it was covered with ice, the bright light of the Sun might falsify the diameter estimates, so that Pluto could be much larger and more massive that it seemed. Also in 1936 astronomer Raymond Lyttleton of Cambridge suggested that the planet Pluto might be an escaped satellite of Neptune. The largest moon of Neptune is Triton, which is 2,706 km in diameter, and is the only Moon with a retrograde orbit, that is to say it moves in the opposite direction to all the rest, so the idea of great disturbances in the remote past was logical. The first measurement of Pluto's diameter was made by astronomer Gerard Kuiper using the 2m reflector at the McDonald Observatory in Texas, in 1949. The diameter was given as 10,197 km. The following year Gerard Kuiper made new measurements with the Palomar 5m reflector, and the diameter was reduced to 5,792 km, making Pluto smaller than Mars and Neptune's Moon Triton. The diameter was further refined in 1965 when Pluto partially occulted a Mag +15 star in the constellation of Gemini (The twins), which was observed using the 14½ cm refractor at the Lick Observatory. The next problem for astronomers was to work out the rotation period of Pluto. The planet is so remote that its angular diameter is tiny, about 0.1 seconds of arc, so that observing any surface features was an impossible task. The alternative was to carefully measure the planet's magnitude over time to see if there was a regular variation. In 1955 astronomers M Walker and R Hardie found that the brightness of Pluto varied by 20% in 6.39 days which most astronomers today believe is due to the planet's rotation. This tells us that there are 14,100 Plutonian days in each Plutonian year. While we're on the subject, during the opposition of 1982 Richard Binzel and J. Derral Mulholland at the McDonald Observatory made observations of Pluto's brightness over 18 nights using the 2m and 91-cm reflectors at Mt. Locke. Their photometric observations suggest that the polar regions must be brighter than the equatorial zone, which indicate that Pluto may have polar ice caps. Of course if Pluto happened to have a Moon, observations of the motion of the satellite could be used to accurately work out the Mass of Pluto. The planet had already partially eclipsed a star in 1965, and astronomers at the US Naval Observatory in Arizona was busy taking photographs of the planet using the 24½ cm reflector to predict future occultations. Nothing unusual shown up until astronomer James Christy took photographs of Pluto in April and May 1978. When he examined the plates on 22 June Pluto's image appeared to be elongated, he therefore rechecked the plates taken in previous years, and the same effect was seen. There was an air of excitement, did Pluto have a satellite? Astronomer James Graham confirmed the discovery on 6 July 1978 using the 4m reflector of the Cerro Tolodo observatory in Chilli. The moon has since been named Chiron. The Earth has one natural satellite, the Moon, which is 3,476 km in diameter and orbits our planet once every 27 days. The diameter of the Earth is 12,756 km so that our planet is 3 ½ times bigger than the Moon, therefore most astronomers prefer to consider the Earth Moon system as a double planet, which is a

unique arrangement, or so we believed up until the discovery of Chiron. Chiron has been measured to be 1,200 km in diameter and takes 6 days 9 hours to orbit Pluto at a mean distance of 19,570 km. So that Pluto is almost twice the size of Chiron, and the system can again best be described as a double planet. Also, as with the case with the Earth Moon system, Chiron is locked in synchronous rotation with its parent planet. The atmosphere of Pluto was discovered in 1980, it's density is very low with an atmospheric pressure 100,000 times less than the Earth's at sea level. It is very extensive, at least as deep as the planet's diameter of 5,792 km. The main constituent is thought to be Methane, and there may be some Carbon Monoxide and even Argon. Pluto is so remote from the Sun that it receives very little of the Sun's warmth, so that its surface temperature is around -220 degrees C. When Pluto is in the far part of its orbit, as it will be in the year 2114, the temperature is so low that Methane may be expected to be frozen out, so that it is possible that for part of its year Pluto has no atmosphere at all. The close approach of New Horizon to Pluto will take place on 14 July, and instruments on board the spacecraft will study the atmosphere in detail. Video: Horizons Close approach 2m 10s The Hubble Space telescope was launched into orbit on 24 April 1990. It has a 2.4m mirror and is ideally placed to make observations above the Earth's atmosphere. Two additional moons were imaged by astronomers working with the Hubble Space Telescope on 15 May 2005. The International Astronomical Union officially named Pluto's newest moons Nix, the inner of the two, and Hydra the outer moon on 21 June, 2006. These small moons orbit Pluto at approximately two, and three times, the distance of Charon: Nix at 12,600 km and Hydra at 12,614 km. They have nearly circular orbits in the same orbital plane as Charon. As I explained earlier, astronomer Andrew Claude Crommelin of Greenwich suggested the theory of specula reflection, and this is the main difficulty when it comes to trying to measure the diameter of these new satellites of Pluto. If Nix and Hydra was highly reflective because they are covered with ice, the bright light of the Sun might falsify the diameter estimates, so that the Moons could be much larger and more massive than they seemed. Albedo, from the Latin albedo, which means "whiteness," is the diffuse reflectivity or reflecting power of a surface. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Their sizes are estimated from their Albedo. If their Albedo is similar to that of Charon, which is 0.35, then their diameters are 120 km for Nix and 157 km for Hydra. A fourth moon, Kerberos, was announced on 20 July 2011. It was detected using NASA's Hubble Space Telescope during a survey searching for rings around Pluto. It has an estimated diameter of 32 km and is located between the orbits of Nix and Hydra. Kerberos was first seen in a photo taken with Hubble's Wide Field Camera on 28 June 2011. It was confirmed in subsequent Hubble pictures taken on 3 July and 18 July. A fifth moon, Styx, was announced on 7 July 2012 while looking for potential hazards for NASA's New Horizons spacecraft. Styx is believed to have a diameter of between 34 and 65 km, and to orbit Pluto at a distance between Charon and Nix.