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During World War II, the British at Bletchley Park (40 miles north of London) achieved a number of successes at breaking encrypted German military communications. The German encryption machine, Enigma, was first attacked with the help of electro-mechanical bombes. [67] They ruled out possible Enigma settings by performing chains of logical deductions implemented electrically. Most possibilities led to a contradiction, and the few remaining could be tested by hand.

The Germans also developed a series of teleprinter encryption systems, quite different from Enigma. The Lorenz SZ 40/42 machine was used for high-level Army communications, termed “Tunny” by the British. The first intercepts of Lorenz messages began in 1941. As part of an attack on Tunny, Max Newman and his colleagues helped specify the Colossus. [68]

Tommy Flowers, still a senior engineer at the Post Office Research Station [69] was recommended to Max Newman by Alan Turing [70] and spent eleven months from early February 1943 designing and building the first Colossus. [71] [72] After a functional test in December 1943, Colossus was shipped to Bletchley Park, where it was delivered on 18 January 1944 [73] and attacked its first message on 5 February.

Colossus was the world’s first programmable electronic digital computer. [43] It used a large number of valves (vacuum tubes). It had paper-tape input and was capable of being configured to perform a variety of boolean logical operations on its data, but it was not Turing-complete. Nine Mk II Colossi were built (The Mk I was converted to a Mk II making ten machines in total). Colossus Mark I contained 1,500 thermionic valves (tubes), but Mark II with 2,400 valves, was both 5 times faster and simpler to operate than Mark 1, greatly speeding the decoding process. Mark 2 was designed while Mark 1 was being constructed. Allen Coombs took over leadership of the Colossus Mark 2 project when Tommy Flowers moved on to other projects. [74]

Colossus was able to process 5,000 characters per second with the paper tape moving at 40 ft / s (2.12 m / s; 27.3 mph). Sometimes, two or more Colossus computers tried different possibilities simultaneously in what now is called parallel computing, speeding the decoding process by perhaps as much as double the rate of comparison.

Colossus included the first ever use of shift registers and systolic arrays, enabling five simultaneous tests, each involving up to 100 Boolean calculations, on each of the five channels on the punched tape (although in normal operation only one or two channels were examined in any run). Initially Colossus was only used to determine the initial wheel positions used for a particular message (termed wheel setting). The Mark 2 included mechanisms intended to help determine pin patterns (wheel breaking). Both models were programmable using switches and plug panels in a way their predecessors had not been.
Without the use of these machines, the Allies would have been deprived of the very valuable intelligence that was obtained from reading the vast quantity of encrypted high-level telegraphic messages between the German High Command (OKW) and their army commands throughout occupied Europe. Details of their existence, design, and use were kept secret well into the 1970s. Winston Churchill personally issued an order for their destruction into pieces no larger than a man’s hand, to keep secret that the British were capable of cracking Lorenz SZ cyphers (from German rotor stream cipher machines) during the oncoming cold war. Two of the machines were transferred to the newly formed GCHQ and the others were destroyed. As a result, the machines were not included in many histories of computing. [76] A reconstructed working copy of one of the Colossus machines is now on display at Bletchley Park.

The US-built ENIAC (Electronic Numerical Integrator and Computer) was the first electronic programmable computer built in the US. Although the ENIAC was similar to the Colossus it was much faster and more flexible. It was unambiguously a Turing-complete device and could compute any problem that would fit into its memory. Like the Colossus, a “program” on the ENIAC was defined by the states of its patch cables and switches, a far cry from the stored program electronic machines that came later. Once a program was written, it had to be mechanically set into the machine with manual resetting of plugs and switches.

It combined the high speed of electronics with the ability to be programmed for many complex problems. It could add or subtract 5000 times a second, a thousand times faster than any other machine. It also had modules to multiply, divide, and square root. High speed memory was limited to 20 words (about 80 bytes). Built under the direction of John Mauchly and J. Presper Eckert at the University of Pennsylvania, ENIAC’s development and construction lasted from 1943 to full operation at the end of 1945. The machine was huge, weighing 30 tons, using 200 kilowatts of electric power and contained over 18,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, inductors and. [77] One of its major engineering feats was to minimize the effects of tube burnout, which was a common problem and machine reliability at that time. The machine was in almost constant use for the next ten years.

Mercury is the smallest and innermost planet in the Solar System.[a] Its orbital period (about 88 Earth days) is less than any other planet in the Solar System. Seen from Earth, it appears to move around its orbit in about 116 days. It has no known natural satellites. It is named after the Roman deity Mercury, the messenger to the gods.

Partly because it has almost no atmosphere to retain heat, Mercury’s surface temperature varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day in some equatorial regions. The poles are constantly below 180 K (−93 °C; −136 °F). Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about 1⁄30 degree), and its orbital eccentricity is the largest of all known planets in the Solar System.[a] At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury’s surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.

Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance,[14] and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[b][15] As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

Because Mercury orbits the Sun within Earth’s orbit (as does Venus), it can appear in Earth’s sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth. Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years, before exhausting its fuel and crashing into the planet’s surface on April 30, 2015
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi).[2] Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.[19] Mercury’s density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth’s density of 5.515 g/cm3.[2] If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth’s 4.4 g/cm3.[20]

Mercury’s density can be used to infer details of its inner structure. Although Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.[21]

Geologists estimate that Mercury’s core occupies about 55% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.[22][23] Surrounding the core is a 500–700 km mantle consisting of silicates.[24][25] Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is estimated to be 35 km thick.[26] One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.[27]

Mercury’s core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System’s rocky matter, and a mass approximately 2.25 times its current mass.[28] Early in the Solar System’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several thousand kilometers across.[28] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[28] A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.[28]

Alternatively, Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K.[29] Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of “rock vapor” that could have been carried away by the solar wind.[29]

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.[30] Each hypothesis predicts a different surface composition, and two space missions, MESSENGER and BepiColombo, will make observations to test them.[31][32] MESSENGER has found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off by the extreme heat of these events. The findings would seem to favor the third hypothesis; however, further analysis of the data is needed

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