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Home , Atmosphere of Pluto, Blink comparator, Clyde Tombaugh, Hydra (moon), Kerberos (moon), Lowell Observatory

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Journey to the edge of the

History of a space discovery The existence of a beyond the orbit of had been predicted since the 1840s. of the time, in fact, thought there was another large planet as yet unknown, situated on the edge of the Solar System, responsible for the disturbances and changes to the orbits of and Neptune. Complex mathematical calculations based on the known of Neptune showed, in fact, that its orbit, as well as that of nearby Uranus, did not perfectly correspond to the predictions on the motion of bodies in the Solar System. The search for the ninth planet started seriously in the twentieth century. Percival , founder (1894) and director of the in Flagstaff (), Arizona, dedicated the last eight years of his life to the search for Planet X, a phrase used to indicate a planet beyond Neptune. Lowell died in 1916, without being able to prove the existence of the missing planet. We had to wait another 14 years and more precisely 18 February 1930 to demonstrate the existence of Planet X. On that , the twenty four year old was intent on observing celestial bodies with a blink comparator, an instrument that allows images of the sky obtained at different times to be compared. Clyde Tombaugh, a Kansas farmer with a great passion for , while never having carried out any formal studies on the subject, had got a job at the Lowell Observatory, thanks to drawings of and , which he had made using a he had built with pieces of old farm machinery.

Clyde Tombaugh with the telescope that he had built with pieces of old farm machinery. Credits: NASA - Via Wikimedia Commons

The task of Tombaugh was to find moving objects by comparing the different images: in fact, a celestial body, such as a planet, which has travelled a certain distance in the time between the capture of the various images, will appear to move with respect to the background of fixed stars, moving from one photograph to another. Nothing could be simpler! Instead, the task of Tombaugh was anything but easy. To discover he wore out his eyes for more than six months to scan the photographs of the sky in search of a moving dot among the millions of celestial bodies. After a few weeks of

Home / Space/ Special Reports verification, on 13 March, the discovery of Planet X was officially announced, Only the name was missing: the Observatory received numerous letters from fans of the new planet and initially Minerva seemed to be the candidate name. Then, eleven year old suggested to call it Pluto, as the Roman god of the underworld. A name perhaps a little obscure and shrouded in mystery, but which was certainly appropriate for a planet on the edge of the Solar System.

Pluto and (the smaller celestial body) in comparison with the . Credits: Nasa

The kingdom of the underworld of Pluto has been enriched over the years with the discovery of new celestial bodies that orbit around it. On 22 June 1978, James Christy and Robert Harrington, from the Flagstaff Station US Naval Observatory, discovered a small lump on the disk of Pluto, periodically present in the photographic plates showing it: it was Charon (name of the ferryman of the souls of the dead in Hades), one of the of Pluto. In October 2005, scientists at the Johns Hopkins University in Baltimore (USA) announced the discovery of two new : and . In 2011 and 2012, another two satellites, and , were identified, thanks to the careful observations of Hubble. All the names chosen have an affinity with the kingdom of the dead: Hydra is the many-headed serpent guardian of the underworld, along with the three-headed dog Kerberos; Nix is an ancient deity who dwelt in Hades; Styx is one of the five rivers of hell.

The passage from planet to On 29 July 2005, the position of the planet Pluto began to creak. In fact, Mike Brown from the California Institute of Technology discovered (named after the Greek goddess of discord), a frozen dwarf planet situated in the , a region beyond the orbit of Neptune, made up of thousands of small icy particles, residues of the formation of the Solar System. Eris is an object with dimensions similar to Pluto, indeed in the beginning it appeared to be bigger, so much so that the astronomer began to doubt whether Pluto was really a planet like the others. In reality, this dilemma dates back to the seventies, when more sophisticated techniques to estimate the mass and size of celestial bodies were developed. With each new measurement, Pluto was found to be less and less like a standard planet, each time becoming smaller. In 1992, David Jewitt and J. Luu from the University of Hawaii discovered a strange object, dubbed

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1992 QB1, a small frozen celestial body similar in size to an , in orbit around the at a distance of one and half times that of Neptune. QB1 was the first clue that other objects similar to Pluto (with regard to the composition of ice and rock) populated the outermost areas of the Solar System. Since 1992 to date, nearly one hundred objects like QB1 have been found: this swarm of celestial bodies is now known as the Kuiper Belt, named after , who first proposed the existence of this belt. Coming back to Eris, it was clear that either it had to be classified as a planet, or Pluto had be reclassified. Astronomers were also convinced to be able to discover other objects with the dimensions of the ninth planet and, therefore, the number of with characteristics similar to Pluto would have grown rapidly. The issue was the subject of intense debate during the XXVI General Assembly of the International Astronomical Union in August 2006 that led to the definition of a new category of celestial bodies, dwarf planets, specifically created by astronomers to take account of what the systematic and continuous exploration of the Solar System seemed to highlight, namely that certain characteristics of Pluto are not exclusive to it, but also belong to other bodies that orbit around the Sun. Starting from 24 August 2006, the image we had of our Solar System, made up of nine planets, changed and Pluto was "downgraded" to a dwarf planet. But what are then the differences between a planet and a dwarf planet? To define a planet, the fact that gravity, being a central force (central forces are defined as those forces acting on a body such that their line of action is always directed toward a well defined point in space called "centre" of the force), tends to constitute bodies of a spherical shape is used. Astronomers have decided to call planets those objects orbiting the Sun that have sufficient mass, and therefore sufficient gravity, to have a spherical shape and to have "cleared" the neighbourhood around their orbit of all the small bodies. Pluto always came under this category with difficulty because it did not satisfy the last requirement, being within the Kuiper belt. To date, the International Astronomical Union recognises five dwarf planets: Pluto, , , and Eris.

A mission to the edge of the Solar System Since the day of its discovery in 1930, the dwarf planet Pluto has always remained outside our reach, its surface being shrouded by a mystery which is impenetrable even by the most powerful . And it is precisely to unravel these mysteries and learn more about the boundaries of our Solar System that, on 19 January 2006, NASA launched the spacecraft towards Pluto from Cape Canaveral, Florida. Objective of the mission: perform a flyby of Pluto and reach the Kuiper belt. Let's go back to 2006 and follow the voyage of New Horizons before reaching Pluto. On 7 April 2006, the probe flew by Mars, in May it reached the asteroid belt, from which it exited in October of the same year. It then headed for Jupiter. The NASA astronomers had calculated that to reach Jupiter the probe had to leave within the first 23 days of January. The objective was to use the powerful gravitational field of the giant gaseous planet as a gravitational sling, a space flight technique that utilises the gravity of a planet to alter the path and speed of a spacecraft. The close encounter with Jupiter boosted the speed of New Horizons by approx. 4 m/s, inserting the probe into a faster trajectory to Pluto and shortening its journey by approx. four-years. After passing Jupiter, New Horizons entered a state of hibernation, in which it remained for most of the remaining journey time. All the instruments were turned off to save power, the probe only sent a weekly report on the general conditions and a fortnightly report with telemetry data. After having passed the orbit of in 2008, of Uranus in 2011 and of Neptune in 2013, on 6 December 2014, New Horizons was woken up from the last hibernation and started approach operations to Pluto. Due to the speed of arrival and the low gravitational pull of the dwarf planet, the mission did not envisage entry into orbit ("braking" would have been required that the engine would not have been able to perform), but a simple flyby, which took place successfully on 14 July 2015 at 13: 49.57 Italian time. After passing Pluto, the probe will continue its journey to the Kuiper Belt. What does New Horizons look like? New Horizons is the size of a piano and weighs 478 kg. It has a number of sophisticated instruments that can study the composition of the of Pluto, take high-resolution images via which we will be able to reconstruct the geology

Home / Space/ Special Reports and morphology of Pluto and Charon, analyse the ionosphere and the charged particles , observe the four minor satellites Styx, Nix, Kerberos and Hydra and much more

The seven instruments of the New Horizons spacecraft. Credits: NASA

Let's find out about them in detail:

• LORRI (Long Range Reconnaissance Imager ) is a high resolution digital camera working in the visible field used to obtain a detailed mapping of the surface of Pluto and Charon.

• SWAP (Solar Wind Around Pluto ) has the task of collecting information on the interaction of Pluto with the solar wind, the stream of charged particles emitted by the Sun.

• PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation ) is a spectrometer that has the task of measuring the composition and density of the plasma (ions) around the .

• Alice , an spectrometer designed to capture the characteristic spectral signatures (Each surface reflects radiation in a different way as a function of the wavelength. This radiation is called the spectral signature of the surface) of the different elements present and to measure temperature and pressure according to altitude. It has the task of characterising the atmosphere of Pluto and and to find out whether Charon has one.

• SDC (Student Dust Counter ) has the task of searching for dust, i.e. the microscopic grains of material projected into space as a result of collisions between , and also Kuiper Belt objects during New Horizons' long journey.

• Rex (Radio Science Experiment ) is a passive radiometer that has the task of collecting the radio emission of the surface of Pluto in order to derive information on the differences in temperature and atmospheric composition.

• is aspectrometer working in the visible and near infrared field, representing the "eyes" of New Horizons.