P7: Further Physics - Studying the Universe

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  • Created on: 09-03-16 19:44

Looking Into Space

  • The Earth fully rotates on its axis once in just under 24 hours. We cannot feel the Earth spinning, but it us due to this rotation that the stars appear to move east-west across the sky once in just under 24 hours.
  • The Sun and Moon also appear to travel east-west across the sky. Their motion and the time they take to cross the sky are affected by their orbits. In the case of the Sun, it appears to travel across the sky once every 24 hours.
  • The planets also appear to travel east-west across the sky. Their motion and the time they take to cross the sky are affected by their orbits.
  • A sidereal day is the time it takes for the Earth to rotate 360 degrees on its axis.
  • A solar day is the time from noon on one day, to noon on the next day, i.e. 24 hours.
  • As the Earth rotates once on its axis, it is also orbiting the Sun. It is this orbiting movement that results in a sidereal day being shorter than a solar day.
  • A sidereal day is 23 hours 56 minutes - four minutes shorter than a solar day.
  • 1. The Sun is directly overhead the Earth. 2. The Earth has rotated 360 degrees, but as it is also orbiting the Sun, the Sun is no longer directly overhead. 3. The Earth has had time to rotate a bit more so the Sun is now directly overhead, making the solar day longer than the sidereal day.
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Position of the Stars

  • Due to the orbiting movement of the Earth around the Sun, an observer looking at the night sky from the Earth can see different stars at different times of the year, depending on the Earth's position in relation to the Sun's position.
  • When astronomers look into space, they describe the position of objects in terms of the angles of declination and right ascension. These angles describe the positions of the stars relative to the equator.
  • The celestial sphere is an imaginary sphere enclosing the Earth that allows astronomers anywhere in the world to find a particular star or constellation. The celestial sphere can be used to find stars if you know the star's declination and right ascension. Right ascension is measured in hours; the celestial sphere is split up into 24 hours of right ascension as there are 24 hours in a day. A star with a positive declination will be visible from the northern hemisphere. A star with a negative declination will be visible from the southern hemisphere.
  • Mercury, Venus, Mars, Saturn and Jupiter are all planets that can be seen from Earth with the naked eye. These planets look similar to stars, but they change their positions is complicated patterns when compared with the background of fixed stars.
  • It is these complicated patterns that provided some of the first evidence that the planets, including the Earth, orbit the Sun.
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Retrograde Motion

  • We can use the observations of Mars as an example to show how the planets change their position against the background stars.
  • Earth is closer to the Sun than Mars, so Earth's orbit of the Sun takes less time than Mars's orbit of the Sun. If Mars is observed over a long enough period of time, it can be seen to move compared with the background stars.
  • However, approximately once every two years, Mars appears to go back on itself, e.g. it goes east to west rather than west to east. This is called retrograde motion.
  • As the Earth is closer to the Sun than Mars and is travelling faster, it can 'catch up' and undertake Mars as it orbits the Sun. As the Earth goes past Mars, Mars appears to go back on itself compared with the stars in the night sky. Mars appears to 'wander' across the sky and was one of the first pieces of evidence that the planets orbit the Sun and not the Earth, as previously thought. The word 'planet' comes from the ancient Greek 'wanderer'.
  • While the Earth is rotating on its axis, the Moon is orbiting the Earth in the same direction. Due to this orbiting movement, the Moon appears to travel east-west across the sky in a little over 24 hours.
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The Lunar Cycle

  • The lunar cycle describes the Moon's appearance during its 28-day orbit of the Earth.
  • The Moon's shape has nothing to do with the shadow of the Earth, but is due to the part of the Moon that is visible from the Earth.
  • The Moon is visible from Earth because we can see the light from the Sun reflected from it.
  • This means that the side of the Moon facing away from the Sun appears dark.
  • During the Moon's orbit around the Earth, we can see different faces of the Moon: dark face (new Moon), light face (full Moon) and all the points in between the new Moon and full Moon.
  • A lunar eclipse occurs when the Earth is between the Sun and the Moon. This results in the Earth casting a shadow on the Moon.
  • A solar eclipse occurs when the Moon passes between the Earth and the Sun. This can occur during a new Moon and results in the Moon casting a shadow on the Earth.
  • A total solar eclipse occurs when the Moon is directly in front of the Sun and completely obscures the Earth's view of the Sun.
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Eclipses / Waves

  • Eclipses do not occur every month because the Moon does not orbit the earthnin the same plane as the Earth orbits the Sun. The Moon's orbit is inclined 5 degrees to that of the Earth's. Therefore, an eclipse can only occur when the Moon passes through the ecliptic (the apparent path the Sun traces out along the sky).
  • This is more likely to occur when the Moon is to the side of the Earth, rather than between the Earth and the Sun.
  • There are between two and five solar eclipses every year, but a total eclipse will only occur roughly every 18 months.
  • Waves, including light, water and sound waves, can be reflected, refracted or diffracted.
  • Reflection - Waves are reflected when a barrier is placed in their path. This effect can be seen in water waves.
  • Diffraction - When waves move through a narrow gap or past an obstacle, they spread out from the edges. This is diffraction. Diffraction is most obvious in two instances:
  • When the size of the gap is similar to the wavelength of the wave.
  • When the waves that pass obstacles have long wavelengths.
  • Light can be diffracted, but the waves need a small gap. The fact that light and sound can be diffracted provides evidence of their wave natures.
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Refraction/ Diffraction

  • Light changes direction when it crosses an interface, i.e. a boundary between two transparent materials (media) of different densities, unless it meets the boundary at an angle of 90 degrees (i.e. along the normal).
  • When the light passes from one medium to another, such as air to glass (more dense), its speed decreases, and this results in a change in direction by refraction. When the light leaves the glass and re-enters the air (less dense), it speeds up again.
  • Radiation is diffracted by the aperture of a telescope. To produce a sharp image, the aperture must be much larger than the wavelength of the radiation.
  • Large radio telescopes that detect weak radio wave radiations can be built, but because radio waves have a long wavelength they are affected by diffraction. This means that the image produced is not very sharp.
  • Light has a very short wavelength. Optical telescopes have a much larger aperture than the light's wavelength. Therefore, the telescopes are able to produce a sharp image.
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Convex Lenses

  • In a convex lens (also called a converging lens), rays of light are bent inwards as they pass through the lens. If the rays of light entering the lens are parallel, the rays will be brought to a focus at the focal point (F).
  • A lens with a more curved surface is more powerful than a lens with a less curved surface made of the same material.
  • The power of a lens is measured in dioptres and can be calculated using the following formula:
  • Power (dioptres) = 1/ Focal length (metres)
  • Ray diagrams are either magnified or diminished, real or virtual, upright or inverted.
  • With a distant extended source, the images formed are real, inverted and diminished (smaller).
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Telescopes

  • When looking into space, the objects are so far away that rays of light from them are effectively parallel. Therefore, we draw the rays of light entering telescopes as parallel rays.
  • A simple refracting telescope is made from two converging lenses of different powers. The eyepiece lens is a higher power lens than the objective lens.
  • Astronomical telescopes will normally use concave mirrors for the objective lens instead of convex lenses. This allows them to be larger, so that they collect more light. A bigger lens and mirror, therefore, can see more distant objects. This diagram shows how a concave mirror focuses light rays.
  • If a distant object is magnified, the image appears closer than the object. Therefore, the angle made by ray lines entering the eye is greater. This increase in angle is called the angular magnification and makes the image appear bigger/closer.
  • The angular magnification of a telescope can be found if you know the focal length of the two lenses being used. You can use the following formula:
  • Magnification = Focal length of objective lens / Focal length of eyepiece lens.
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Measuring Distance Using Parallax

  • Parallax can be thought of as the apparent motion of an object against a background. However, it is actually the motion of the observer that causes the parallax motion of an object.
  • It looks as though the star has moved, but it is actually the movement of the Earth's orbit around the Sun that causes the observer to see this 'change in position'.
  • The distance to the stars is so great that we cannot observe parallax motion with the naked eye.
  • However, a simple way to observe parallax is if you hold your hand out in front of you with your thumbs sticking up and alternatively close one eye and then the other.
  • Although your thumb appears to move, in reality you are just looking at it from a different angle.
  • The parallax angle of a star is half the angle moved against a background of distant stars in six months.
  • An object that is further away from the Earth will have a similar parallax angle than a closer object.
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Using Parallax

  • Astronomers use parallax to measure interstellar distances within our galaxy using the unit parsec (pc). A parsec is of a similar magnitude to a light-year with 1 parsec equalling roughly 3.25 light-years.
  • Angles are measured in degrees, minutes and seconds. A star that is one parsec away has a parallax angle of one second of an arc.
  • The distance in parsecs can be found by dividing 1 by the parallax angle, as shown in the following formula: Distance (parsecs) = 1 / Parallax angle (arcseconds).
  • Parallax is useful for measuring the distance of relatively close objects. For example, the typical interstellar distance is a few parsecs.
  • Astronomers use the megaparsec (Mpc) to describe much bigger intergalactic distances, even though these objects are so far away that the parallax angle is too small to measure.
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Measure Distance Using Brightness

  • Another method that astronomers use to measure the distance to stars is to observe how bright the stars are. This method sounds very simple, i.e. a close star will appear brighter than a more distant star. Unfortunately, not all stars have the same luminosity. The luminosity is how much energy the star is emitting and it depends on the star's size and temperature.
  • A larger or hotter star will be more luminous than a smaller or cooler star, so it may appear brighter even though it is further away.
  • The star Antares is 500 light-years from the Earth. There are over 100000 stars nearer to Earth than Antares, but Antares has luminosity 10000 times greater than that of the Sun and is the 15th brightest star visible from Earth.
  • The observed intensity of a star depends on its luminosity and its distance from the Earth.
  • A star with a very high luminosity may appear dim if it is very far away.
  • For example, Sirius appears as the brightest star in the night sky. It is relatively close to the Earth and has a luminosity 23 times greater than the Sun.
  • A Cepheid variable star does not have a constant luminosity. It pulses and its luminosity depends on the periods of the pulses. The period is equal to 1 / frequency.
  • By measuring the frequency of the pulses of a Cepheid variable star, astronomers can estimate its luminosity. If we know how bright the star really is and can see how bright it appears, we can work out how far away it is.
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Curtis-Shapley Debate

  • In 1920, a great debate about the scale of the Universe took place between two prominent astronomers - Heber Curtis and Harlow Shapley.
  • Telescopes had revealed that the Milky Way contained lots of stars and this observation led to the realisation that the Sun was a star in the Milky Way galaxy. Telescopes had also revealed many fuzzy objects in the night sky. These objects were originally called nebulae and they played a major role in the debate.
  • Curtis believed that the Universe consisted of many galaxies like our own and the fuzzy objects were distant galaxies. Shapley believed that the Universe contained only one big galaxy and the nebuale were nearby gas clouds within the Milky Way.
  • In the mid-1920s, Edwin Hubble observed Cepheid variables in one nebula and found that the nebula was much further away than any star in the Milky Way. This observation provided the evidence that the observed nebula was a separate galaxy, supporting Curtis' idea that the Universe contained many different galaxies.
  • Observations of many Cepheid variables have shown that most nebulae are distant galaxies and have allowed astronomers to measure the distance to these galaxies, and hence determine the scale of the Universe.
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The Hubble Constant

  • By observing Cepheid variable stars in distant galaxies, Edwin Hubble discovered that the Universe was expanding, i.e. the further away a star was, the faster it was moving away. The speed of recession can be calculated by using the Hubble formula.
  • Speed of recession (km/s) = Hubble constant (s-1) X Distance (km)
  • Speed of recession (km/s) = Hubble constant (km/s per Mpc) X Distance (Mpc)
  • Cepheid variable stars are used to accurately calculate the Hubble constant because we know how far away they are. So we can use redshift to find out how fast they are moving away (their speed of recession).
  • The evidence suggests that the whole Universe is expanding and that it might have started around 14 thousand million years ago, from one point, with a huge explosion, known as the Big Bang.
  • This effect is exaggerated in galaxies that are further away, which means that the further away a galaxy is, the faster it is moving away from us. This suggests that space itself is expanding.
  • Astronomers can now use the Hubble constant and redshift data to calculate the distance to other galaxies.
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Pressure, Volume and Temperature

  • Gas pressure is caused by particles in a gas moving about. When a particle collides with an object it exerts a force. This force is felt as pressure.
  • The amount of pressure depends on the number of collision per second and the momentum of the particles.
  • As the volume is reduced, the particles have less room to move about and so they collide with the piston more often, increasing the pressure.
  • If the gas is heated up, the particles will move around faster. This increases their momentum and the force they exert when they collide with the piston. This could have two effects: 
  • Push the piston back up (increasing the volume).
  • Cause the pressure to increase (if the volume is kept fixed).
  • This effect also works in reverse. So, compressing a gas will cause it to increase in temperature.
  • As the temperature of a gas is reduced, the particles in the gas move more slowly and the pressure falls.
  • The particles eventually stop moving altogether. At this point the particles have no more energy to lose and the temperature cannot go any lower. This occurs at -237 degrees C otherwise known as absolute zero which is measured in Kelvin (K).
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Pressure Laws

  • There are three pressure laws that link the pressure, volume and temperature of a fixed mass of gas in a container. They are as follows:
  • Pressure X Volume = Constant
  • This means if you reduce the volume of a gas, then it will increase the pressure of the gas hitting the sides of the container.
  • Pressure / Temperature = Constant
  • This means if you increase the temperature of a gas inside a container, then it will have more energy and therefore increase the pressure that the gas exerts.
  • Volume / Temperature = Constant
  • This means if you increase the temperature of a gas, then the gas will have more energy and the volume of the gas will increase.
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Structure of a Star

  •  (http://scope.pari.edu/images/starstructure.jpg)
  • The core is the hottest part of the star where fusion takes place. The convective zone is where energy is transported to the surface by convection currents (and by photons of radiation). The photosphere is where energy is radiated into space.
  • Like all hot objects, stars emit a continuous range of electromagnetic radiation.
  • Hotter objects emit radiation of a higher temperature and higher peak frequency (i.e. frequency where most energy is emitted) than colder objects.
  • An object that is red hot emits most of its energy in the red frequency range. The frequency of light given off from a star provides evidence of how hot it is. To analyse the light from stars, we need to look at the frequencies separately.
  • The visible spectrum is produced because white light is made up of many different colours. The colours are refracted by different amounts as they pass through a prism - red light is refracted the least and violet is refracted the most. This is because the different colours have different frequencies and, therefore, different wavelengths.
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Using a Star's Spectra

  • The removal of an electron from an atom is called ionisation.
  • The movement of electrons within the atom causes it to emit radiation of specific frequencies called line spectra. Different elements have characteristic line spectra. This can be seen by placing a prism at the end of a telescope.
  • Due to its high temperature, the spectrum from a star is continuous apart from the spectral lines of the elements it contains (these lines are missing because these frequencies are absorbed).
  • By comparing a star's spectrum to emission spectra from elements, we can find which chemical elements the star contains.
  • The Sun's spectrum is complex, indicating that it contains more than one element. However, by comparing the spectra we can see that the Sun contains hydrogen as well as another element. In the Sun's case we know that this other element is helium.
  • Stars begin as clouds of gas (mainly hydrogen). As gravity brings these gas clouds together, they become denser. The force of gravity pulls the gas inwards, causing the pressure and temperature to increase. As more gas is drawn in, the force of gravity increases. This compresses the gas so that it becomes hotter and denser and forms a protostar.
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Nuclear Fusion

  • At the beginning of the 20th century, discoveries about the nature of the atom and nuclear processes began to help answer the mystery of where the Sun's energy comes from.
  • In 1911, there was a ground-breaking experiment - the Rutherford-Geiger-Marsden alpha particle scattering experiment. In this experiment, a thin gold foil was bombarded with alpha particles. The effect on the alpha particles was recorded and these observations provided the evidence for our current understanding of atoms.
  • Most alpha particles were seen to pass straight through the gold foil. This would indicate that gold atoms were composed of large amounts of open space.
  • However, some particles were deflected slightly and a few were even deflected back towards the source. This would indicate that the alpha particles passed close to something positively charged within the atom and were repelled by it.
  • These observations brought Rutherford and Marsden to conclude that gold atoms, and therefor all atoms, consist largely of empty space with a small dense core (nucleus), the nucleus is positively charged and the electrons are arranged around the nucleus with a great deal of space between them.
  • In a star, the temperature and pressure become so high that the hydrogen nuclei fuse into helium nuclei. This is known as nuclear fusion and is how the star generates its energy.
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Albert Einstein

  • When scientists first started to look at a star's spectra, they found that through nuclear fusion four hydrogen nucleu were fusing into one helium nuclei and releasing vast amounts of energy. Once they were able to measure the mass of nuclei they found something strange. When you add up the mass of four hydrogen nuclei it does not equal the mass of one helium nucleus. The four hydrogen nuclei are heavier - some of the mass is lost in the formation of the helium nucleus. The actual mass that is lost is very smallm but it produces a vast amount of energy in the fusion of elements up to iron.

Nuclear Reactions in Fusion

  • Although four hydrogen nuclei fuse together to form one helium nuclei in stars, the reaction takes place over a number of steps. The first step is shown below:
  • 1H + 1H = 2D + e+ + Energy.
  • D is an isotope of hydrogen called deuterium. The e+ is called a positron. It is exactly the same as an electron except that it is positively charged and is produced in order to conserve charge. A positron is an example of anti-matter. When anti-matter comes into contact with normal matter it annihilates it, producing a lot of energy.
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Life and Death of a Star

  • When a star starts nuclear fusion reactions, it is called a main sequence star and it is stable. Hydrogen is being converted via fusion into helium.
  • Towards the end of a star's life, it's fuel begins to run out (there is insufficient hydrogen remaining in the core for fusion to continue). The star then undergoes several changes, depending on its size.
  • When the core hydrogen has been depleted, the star's photosphere becomes cooler. Small stars like the Sun become red giants, while larger stars become red supergiants.
  • Red giants and red supergiants ontinue to release energy by fusing helium into larger nuclei such as carbon, nitrogen and oxygen.
  • Once the helium has been used up, red giants do not have enough mass to compress the core and continue fusion, so they shrink into hot white dwarfs that gradually cool.
  • Red supergiants have a much greater mass and higher core pressures, so fusion continues to produce larger nuclei such as iron.
  • Once the core is mostly iron, the star explodes in a supernova, leaving behind a dense neutron star or a black hole.
  • Astronomers use a Hertzsprung-Russel diagram to plot the lifetime of a star. From the graph we can identify regions where different stars are located and trace the life of a star.
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Exoplanets

  • Scientists have wondered for years whether there are other planets outside our solar system. These are called exoplanets. Only recently have astronomers been able to provide evidence for the existence of exoplanets orbting around their host star. The easiest way to detect an exoplanet would be to look through an optical telescope, but because the host star is millions of times brighter than the planet, the light reflected from the planet's surface would be drowned out by the star's light. Only planets that are very far away from the star could be seen. In 2004, astronomers using the Hubble Space Telescope discovered a huge planet orbiting a star called Formalhaut. It was only discovered due to its vast distance from the host star. 
  • In order to detect planets much closer to the host star, astronomers have had to come up with ingenious methods that have stretched the limits of scientific discovery.
  • One such way is called the Radial Velocity (RV) method. This relies on the Doppler effect and the shift in the spectral lines of the star. This Doppler shift happens due to the planet exerting a gravitational force on the star, which causes it to 'wobble'. The planet's size and distance from the star can then be calculated. Most exoplanets have been discovered this way.  Another way to detect planets clost to the host star is to measure the amount of light that gets blocked out from the host star when the planet moves in front of the star. This is called the transit method. The next generation of space telescopes have found many exoplanets this way. 
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Ground-based Telescopes

  • Before it was possible to look for exoplanets, scientists monitored certain parts of the electromagnetic spectrum to see if extraterrestrial life was trying to contact us. However, after searching for nearly 40 years, no signals from alien life-forms have been detected.
  • The major optical and infrared astronomical observatories on earth are mostly situated in Chile, Hawaii, Australia and the Canary Islands. The largest optical telescopes in the world are the 10m aperture reflecting Keck Telescopes at the Mauna Kea Observatories, Hawaii.
  • Hawaii has proven an ideal location for ground-based telescopes for several astronomical reasons:
  • Its high altitude means that there is less atmosphere above it to absorb the light from distant objects.
  • The lack of nearby cities means that there is less pollution (light and standard) to interfere with the received signal and the air is drier.
  • Other things that must be considered when deciding where to build an observatory are:
  • cost
  • environmental and social impact near the observatory
  • working conditions for employees
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Space-based Telescopes

  • The most famous space telescope is the Hubble Space Telescope, which was launched in 1990 and designed in collaboration with the European Space Agency and NASA. Its original launch date was delayed by two years because the explosion of the space shuttle Challenger shortly after launch resulted in the shuttle fleet being grounded for two years.
  • After launch it was found that there was a fault on the mirror that required expensive repairs.
  • Despite these problems, the Hubble telescope has been a great success and provided images of the Universe that could not have been obtained in any other way.
  • Advantages - Avoids the absorption and refraction effects of the Earth's atmosphere.
  • Can use parts of the electromagnetic spectrum that the atmosphere absorbs.
  • Disadvantages - Very expensive to set up, maintain and repair.
  • Risk of harm to astronauts from solar radiation and risk of deadly accidents to astronauts while working in space.
  • Uncertainties of the space programme, e.g. launch delays.
  • Radio telescopes use a metal reflector to reflect radio waves onto a receiver. Radio waves are not blocked by clouds so radio telescopes can be sited on the ground. They can detect objects that are too cool to emit much visible or infrared light. However, they have to be much larger than optical telescopes and the images produced are no as clear.
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Developments

  • Infrared telescopes work much like optical telescopes. They have a better resolution than radio telescopes and can observe objects too cool to give off visible light. However, infrared light is easily absorbed by the Earth's atmosphere, so this type of telescope needs to be built at high altitude or be space-based.
  • Most of the big new telescopes are developed through international collaboration. 
  • Advantages  are the cost of manufacturing the telescopes can be shared and astronomers from around the world can book time on telescopes in different countries, allowing them to see the stars from other parts of the Earth.
  • The telescopes can be accessed directly at the site. They can also be accessed through remote computer control, which can be an advantage because astronomers do not have to travel to each telescope to be able to use it. They can also use the telescopes at convenient times.
  • Schools in the UK can access the Royal Observatory over the Internet.
  • This kind of sharing of cost and expertise is essential for many of the big expensive science projects. For example, the Gemini Observatory in Chile, which opened in 2002, was the result of collaboration between Australia and six other countries. The International Space Station has also been jointly funded by the National Aeronautics and Space Administration (NASA).
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