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Nasa ’s Beyond Einstein Program: An Architecture for Implementation H Glossary accretion, accretion disks: The process by which gas flows around and onto a compact gravitating object. Astronomical objects as diverse as protostars and active galaxies may derive their power from the gravitational energy released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk. active galactic nucleus (AGN): Energetic phenomena in the nuclei, or central regions, or galaxies that cannot be attributed clearly and directly to stars. Advanced Satellite for Cosmology and Astrophysics (ASCA): Japan’s fourth cosmic x-ray astronomy mission, and the second mission for which the United States provided part of the scientific payload. It is the first satellite to use charge-coupled devices for x-ray astronomy, and its primary goal is the x-ray spectroscopy of astrophysical plasmas—especially the analysis of discrete features such as emission lines and absorption edges. It was formerly named Astro-D. angular diameter: The diameter of an object as measured as an angle. angular resolution: The resolving power of an optical device such as a telescope. anisotropy: Dependence of the properties of a system on the orientation or the direction of observation. The distribution of galaxies in space is not uniform, whereas the intensity of the cosmic background radiation from the big bang is highly uniform in all directions—that is, it is almost isotropic. Astronomers are using sensitive telescopes to study the small anisotropies in the cosmic background radiation that led to the present nonuniform distribution of galaxies. antimatter: Matter composed of antiparticles (e.g., antiprotons, antineutrons, antielectrons) instead of particles (e.g., protons, neutrons, electrons). antiparticle: Counterpart to a particle, with properties identical to those of the particle, except that the antiparticle’s electrical charge and a few other properties are opposite those of the particle. When a particle and its antiparticle
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation meet, they can annihilate each other and release energy. In the Standard Model of Particle Physics, antiparticles are natural analogues to all the particles. arcminute: A unit of angle corresponding to 1/60th of a degree. The full Moon is 30 arcminutes in diameter. arcsecond: A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a dime viewed from a distance of 1 mile. array: There are two examples of arrays in common use in astronomy: (1) a group, or array, of telescopes combined to simulate a single large telescope, kilometers or even thousands of kilometers across; (2) astronomical instruments composed of detector arrays or charge-coupled devices that consist of thousands of individual detectors constructed on centimeter-sized wafers of silicon, or other materials. astronomical unit (AU): The mean distance between Earth and the Sun. Atacama Large Millimeter Array (ALMA): A single research instrument composed of up to 80 high-precision antennas that will enable research into optically dark regions by probing the millimeter portion of the electromagnetic spectrum, where the cold universe shines brightly. ALMA is designed to probe the first stars and galaxies and directly image the formation of planets. baryon: A subatomic particle with mass and three constituent quarks bound together by the strong force, such as a proton or neutron. Ordinary matter as we know it consists largely of baryons. baryon acoustic oscillations: Cosmological perturbations in the early universe excited sound waves in the photon-baryon fluid. These baryon acoustic oscillations define a standard ruler whose length is the distance that sound can travel, at a speed of ~c/√3, before decoupling. big bang: The theory that the universe began with all matter and energy concentrated to very high density and temperature some 13 billion years ago. The present universe expanded from that era and is still expanding. black hole: A region of space in which the gravitational pull is so strong that, classically, nothing can escape. The boundary of this region is called the black hole’s event horizon. Black holes can form when a massive star undergoes gravitational collapse. blazar: Believed to be an active galactic nucleus (AGN) that has one of its relativistic jets pointed toward Earth, so that the emission we observe is dominated by phenomena occurring in the jet region. Among all AGNs, blazars emit over the widest range of frequencies and are detected from radio to gamma ray. B-mode polarization: A kind of polarization of radiation that is primarily vortex-like and is theorized to be predominantly produced during the inflation period. boson star: A star composed of self-gravitating nonbaryonic matter called bosons. All fundamental particles in nature can be divided into one of two categories: fermions or bosons. Bosons, unlike fermions, do not obey the Pauli exclusion principle. While examples of fermions include electrons, protons, neutrons, quarks, and neutrinos, particles classified as bosons include photons and gluons. brown dwarf: A star-like object that contains less than about 0.08 the mass of the Sun and is thus too small to ignite nuclear fuels and become a normal star. Brown dwarfs emit small amounts of infrared radiation owing to the slow release of gravitational energy and may be a component of dark matter.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Chandra X-ray Observatory: A satellite designed to observe x-rays from high-energy regions of the universe, such as the remnants of exploded stars, black holes, supernovas, and dark matter and to increase the understanding of the origin, evolution, and destiny of the universe. It was launched and deployed by Space Shuttle Columbia on July 23, 1999. charge-coupled device (CCD): An electronic image detector used in modern video cameras and astronomical instruments that uses semiconductor technology to detect incident radiation. comet coma: A cloud, formed as the ice around the comet nucleus evaporates, around the central part of the comet. This cloud, the atmosphere of the comet, can extend for millions of miles. Compton Gamma Ray Observatory (CGRO): The second of NASA’s Great Observatories, launched April 5, 1991. Its mission is to study the high-energy universe, including solar flares, gamma-ray bursts, pulsars, nova and supernova explosions, accreting black holes of stellar mass, quasar emission, and interactions of cosmic rays with the interstellar medium. Cosmic Background Explorer (COBE): A satellite launched in November 1989, it made precision measurements of the spectrum of the microwave background radiation and discovered tiny variations in its intensity across the sky that arose due to small variants in the density of matter. cosmic microwave background (CMB) radiation: The residual light from the big bang. Although the CMB is nearly uniform, there are tiny fluctuations in its temperature owing to variations in the density of the early universe. Some of these tiny fluctuations grew to form galaxies. cosmic nucleosynthesis: Fusion in the early universe when it was very dense and very hot. This fusion of lighter elements resulted in the creation of heavier elements such as deuterium. Cosmic nucleosynthesis is to be compared to stellar nucleosynthesis, which refers to the formation of heavier elements within stars during their fusion life cycles. cosmic rays: Protons, nuclei of heavy atoms, and possibly other particles that have been acclerated to high energies by astrophysical processes in the universe and that impinge on Earth. cosmic string: Theoretical string-like concentrations of matter that could explain the youngest structures seen in the universe. The idea of a cosmic string is based on the idea of boundaries between crystals that form in solidifying liquids. Cosmic Visions 2025: The European Space Agency’s plan for space science until 2025. cosmography: The study and description or mapping of the universe. cosmological constant : The energy density associated with the vacuum (empty space). Recent astronomical observations suggest that there is a net energy associated with the vacuum. If there is a positive vacuum energy, then the expansion of the universe will eventually accelerate and our descendants will find themselves in a nearly empty universe. dark current: The electric current that flows through a detector when it is activated but not receiving any light. dark energy: An as-yet-unknown form of energy that pervades the universe. Its presence was inferred from the discovery that the expansion of the universe is accelerating, and these observations suggest that about 70 percent
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation of the total density of matter plus energy is in this form. One explanation for dark energy is Einstein’s cosmological constant. Dark Energy Survey (DES): A survey to be conducted over the course of 5 years using an extremely red-sensitive 500 megapixel camera mounted on a 2-meter telescope. Proposed by Fermilab in 2004, the project would study the nature of dark energy. Dark Energy Task Force (DETF): A subcommittee of the Astronomy and Astrophysics Advisory Committee chartered to advise NASA, the National Science Foundation, and the Department of Energy on the future of dark energy research. dark matter: Matter that does not emit enough light or other electromagnetic radiation to be observed directly. Most of the matter in the universe is dark. Cold dark matter is made of particles (e.g., axions or neutralinos) that move slowly compared with the speed of light; hot dark matter is made of particles (e.g., neutrinos) that move at nearly the speed of light. Deep Space Network (DSN): International network of antennas that support interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. Eddington limit: The upper limit of a star’s electromagnetic radiation pressure, determined by the limitation that its electromagnetic pressure cannot exceed the force of gravity holding the star together. Einstein Great Observatories: A classification by NASA in its line of astrophysics-based satellites, these are designed to be facility-class missions that can accomplish a broad range of science goals. Laser Interferometer Space Antenna and Constellation-X are Einstein Great Observatories. Einstein X-ray Observatory: Launched in November 1978, it was the first fully imaging x-ray telescope in space, with the specific purpose of enabling x-ray astronomy research. It was also called the High Energy Astrophysics Observatory (HEAO)-2. Its mission ended in April 1981. E-mode: A kind of polarization pattern that can be produced both during inflation and at later times by electron scattering. extended Roentgen Survey with an Imaging Telescope Array (eROSITA): A telescope that will perform the first imaging all-sky survey of medium x-rays, up to 10 keV. facility-class observatory: In addition to its key science projects, a facility-class observatory is meant to contribute to many other astronomical areas based on observations proposed by general observers. flare: See solar flare. frame dragging: An effect predicted by Einstein’s general theory of relativity such that the rotation of an object (such as Earth) twists local spacetime around that object. galaxy: An isolated grouping of tens to hundreds of billions of stars ranging in size from 5,000 to 150,000 light-years across. Spiral galaxies such as our own MilkyWay are flattened disks of stars and often contain large amounts of gas out of which new stars can form. Elliptical galaxies are shaped more like footballs and are usually devoid of significant quantities of gas. galaxy clusters: Groups of hundreds or thousands of galaxies. The nearest galaxy cluster is the Virgo cluster.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation gamma-ray burst: Bursts of gamma rays from cosmic sources observed by detectors on satellites. Several hundred are detected per year. They range in duration from fractions of a second to several seconds. Most gamma-ray bursts come from objects at cosmological distances. Gamma-ray Large Area Space Telescope (GLAST): A next-generation high-energy gamma-ray observatory designed for making observations of celestial gamma-ray sources in the energy band extending from 10 MeV to more than 100 GeV. GLAST is a joint project between NASA, the U.S. Department of Energy, and institutions in France, Germany, Japan, Italy, and Sweden. It is scheduled to launch in late 2007. gas mass fraction: The fraction of material (e.g., in a galaxy) composed of gas, according to mass. general theory of relativity: Einstein’s theory of gravity in which gravity arises from the curved geometry of space and time. gluon: The exchange particles for the strong (or color) force between quarks. As exchange particles, the gluon’s role is analogous to the photon’s role in the electromagnetic force between two charged particles. gravitational lens: An object in which rays of light from a distant astronomical source are deflected by the gravitational pull of an intermediate mass that may be a galaxy or a cluster of galaxies. The deflection causes a distortion in the image of the distant source and sometimes also leads to multiple images. gravitational wave: A traveling perturbation in the gravitational field. As small bodies spiral into massive black holes, they trace tens of thousands of orbits, and emit waves that encode the details of the spacetime structure around the massive black hole. graviton: The putative quantum particle of gravity. Gravity Probe B (GPB): A relativity gyroscope experiment launched in 2004. It was developed by NASA and Stanford University to test unverified predictions of Einstein’s general theory of relativity by measuring how space and time are warped by Earth and the effect of frame dragging. Gravity Recovery and Climate Experiment (GRACE): Twin satellites launched in March 2002, designed to make detailed measurements of Earth’s gravity field. These measurements support discoveries and gravity and Earth’s natural systems. Great Observatories: A NASA program to launch four major observatories to cover the optical (Hubble Space Telescope), gamma-ray (Compton Gamma Ray Observatory), x-ray (Chandra), and infrared (Space Infrared Telescope Facility) portions of the electromagnetic spectrum. half power diameter: The diameter of a circle containing 1/2 of the flux or counts from a point source. hard x-rays: The highest-energy x-rays. Lower-energy x-rays are referred to as soft x-rays. The distinction between the two is not well defined, though hard x-rays are typically those with energies greater than around 10 keV. Herschel Space Observatory: A 3.5-meter space telescope covering a spectral range from the far infrared to the submillimeter. High Energy Focusing Telescope (HEFT): A balloonborne experiment that images astrophysical sources in the hard x-ray (20-100 keV) band.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Hubble parameter: A time-dependent parameter that, when multiplied with the distance to a given galaxy, yields the speed at which that galaxy is receding from Earth. Hubble Space Telescope (HST): A 2.4-m-diameter space telescope designed to study visible, ultraviolet, and infrared radiation; the first of NASA’s Great Observatories. inflation period: An early era when the universe expanded by some 30 orders of magnitude in linear scale, creating nearly all particles and radiation. According to theory, inflation produced gravitational radiation. inflationary universe, inflationary paradigm: An extension of the big bang model characterized by a tremendous burst of expansions. The underlying cause of inflation is not known, although there are many models for it based on particle physics. INTEGRAL mission: A space observatory that simultaneously observes objects in gamma rays, x-rays, and visible light. The mission is sponsored by the European Space Agency and was launched in 2002. interferometer, interferometry: A technique used to combine multiple beams of light that extracts information from differences in characteristics as the light beams arrive at the detectors. A spatial interferometer combines beams of light from different telescopes to synthesize the aperture of a single large telescope; see array. Spatial interferometry is the main technique used by astronomers to map sources at high resolution and to measure their positions with high precision. A different form of a single telescope can be used to separate light into its constituent colors; see spectroscopy. intergalactic medium: Diffuse gas in the regions between galaxies, made up primarily of hydrogen and helium atoms. It is believed to contain most of the atoms of the universe. International Traffic in Arms Regulations (ITAR): A rule administered by the U.S. Department of State that regulates the export and import of defense-related articles. These articles include launch vehicles, spacecraft, and associated equipment. Keck Telescopes: A pair of 10-m-aperture optical and infrared telescopes located on the summit of Mauna Kea, Hawaii. Kepler Telescope: A NASA Discovery mission specifically designed to survey our region of the Milky Way Galaxy to detect and characterize hundreds of Earth-size and smaller planets in or near the habitable zone. The orbiting telescope will have a 0.95-m diameter. It is scheduled to launch in November 2008. Lagrange point: A point where a third mass can orbit in a fixed position, relative to two larger masses (such as Earth and the Sun); that is, the gravitational forces are balanced. Large Synoptic Survey Telescope (LSST): A proposed ground-based telescope that will provide digital imaging of faint astronomical objects. It will track objects that change or move on rapid time scales, and it will also investigate dark matter and dark energy. Laser Interferometer Gravitational-Wave Observatory (LIGO): Interferometric gravitational wave detector built at sites in the states of Washington and Louisiana; initial operation of the detectors began in 2001. LISA Pathfinder (LP): A European Space Agency mission scheduled for launch at the end of 2009. The Laser Interferometer Space Antenna (LISA) Pathfinder will work on developing gravitational-wave measuring technologies that will later be used in the LISA mission.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation lookback time: The apparent time being observed when light is received from distant objects. Due to the time that it takes an object’s light to travel to Earth, the more distant an object being observed, the older the information received from it. Therefore, an object X light-years away is seen as it was X years ago. low Earth orbit (LEO): Altitudes between 250-300 km and 1,000 km above sea level. luminosity distance: The distance to a celestial object calculated using the object’s luminosity (usually determined by using a standard candle). Lunar Reconnaissance Orbiter (LRO): Planned for launch in 2008, this is the first NASA mission in the series of planned missions to the Moon. It would spend at least 1 year in low polar orbit around the Moon as it collected detailed information on the lunar environment. Lyman-alpha forest: The dense abundance of hydrogen absorption lines apparent in the spectra of galaxies and quasars. Lyman-alpha lines are caused by the absorption of light at a particular wavelength by hydrogen ions containing a single electron. The Lyman-alpha forest occurs when a galaxy emits light that must travel through intergalactic gas (commonly occurring in the vast amounts of space between the Milky Way and other galaxies) before reaching telescopes at Earth. Because the universe is expanding, the absorption lines observed on Earth are slightly redshifted (see redshift) from the wavelength that would be observed if the universe were not expanding (1,216 angstrom). By calculating both the redshifted difference and how the universe is expanding, scientists can determine where the intervening hydrogen gas region is located. magnetars: Neutron stars with magnetic-field strengths roughly a thousand trillion times stronger than those of Earth. magnetosphere: The extent of a planet’s magnetic field. Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission: The first spacecraft to travel to Mercury, where it will study the geology and environment of Mercury. Launched in 2004, it will complete its first Mercury flyby in January 2008 and complete Mercury orbit insertion in 2011. metallicity: A star’s abundance of metals. Near Field Infrared Experiment (NFIRE): Sponsored by the Missile Defense Agency and launched in 2007, it will gather high-resolution data that will assist in the development of missile defense applications. neutron star: A star at such high density and pressure that its atoms have been completely crushed, until the nuclei merge and most of the electrons have been squeezed onto the protons, forming neutron-rich material. New X-ray Telescope (NeXT): A Japanese mission that would provide the first imaging spectroscopy in the hard x-ray band above 10 keV and also achieve unprecedented observing capability in the soft x-ray band below 10 keV. It would cover the same x-ray energies as the Constellation-X Hard X-ray Telescope but with an effective area at least an order of magnitude smaller, poorer angular resolution, and a smaller field of view, limiting it to studying only the brightest sources. Its proposed launch date is in 2011. Phase A: NASA terminology for the conceptual design phase. Phase B: NASA terminology for the preliminary design phase. Phase C/D: NASA terminology for the full-scale development and production phase.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Phase E: NASA terminology for the mission operations and data-analysis phase. Planck: A mission implemented by the European Space Agency and designed to measure the cosmic background radiation. With higher resolution and greater sensitivity than the Cosmic Background Explorer, it will test theories of the early universe and the origin of cosmic structure. It is scheduled to launch in July 2008. polarization: A measure of direction of the transverse electric field of light. The electric field associated with polarized light is strong along one of two directions perpendicular to the direction that the light is traveling. Scattered light is always polarized to some degree, and hence the polarization of light can give some information about the path that the light has traveled to reach Earth. The cosmic microwave background is also slightly polarized. probes: Envisioned as medium-scale missions that could be executed much faster, and for considerably less money, than the flagship Laser Interferometer Space Antenna and Constellation-X missions. pulsar: A spinning neutron star that emits radiation in a beam. The sweeping action of the beam causes the object to pulse regularly, like a lighthouse beam, when viewed by an observer. quark: Point-like, elementary constituents of mesons and baryons (e.g., neutrons and protons). quasar: A very compact and extraordinarily luminous source of radiation in the nucleus of a distant galaxy. Quasars are believed to be powered by the accretion of gas onto massive black holes. redshift: Radiation from an approaching object is shifted to higher frequencies (to the blue), while radiation from a receding object is shifted to lower frequencies (to the red). A similar effect raises the pitch of an ambulance siren as it approaches. The expansion of the universe makes objects recede so that the light from distant galaxies is redshifted. The redshift is parameterized by z, where the wavelength shift is given by the factor (1 + z) times the wavelength. reionization: A process by which the predominantly neutral intergalactic medium was ionized by the emergence of the first luminous sources. ringdown waves: Waves emitted by a distorted final black hole as it settles down to a stationary state. Röntgen satellite (ROSAT): Low Earth-orbiting x-ray telescope functioning in the 0.1- to 2.0-keV range. An international mission involving Germany, the United States (NASA), and the United Kingdom, it was launched in 1990. SIGMA telescope: A high-energy observing telescope created by French astronomers. Sloan Digital Sky Survey (SDSS): A full-sky survey being completed by a 2.5-m telescope. When completed, the survey will provide detailed optical images covering more than a quarter of the sky, and a three-dimensional map of about a million galaxies and quasars. As the survey progresses, the data are released to the scientific community and the general public in annual increments. Solar Dynamics Observatory: A NASA spacecraft carrying a suite of instruments designed to investigate the Sun’s influence on Earth and near-Earth space by studying the solar atmosphere on small scales of space and time and in many wavelengths simultaneously. It is scheduled for launch in August 2008. solar flare: Short, intense brightening of the solar photosphere near a sunspot, caused by the release of large amounts of energy from a small area of the Sun’s surface.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation spacetime: The four-dimensional continuum in which we live, consisting of the three dimensions of space and one dimension of time. special relativity: Einstein’s theory of spacetime structure, in which Newton’s notion of absolute time is abandoned to account for the experimental fact that the speed of light is a universal constant and does not depend on the relative motion between the observation and the light source. spectroscopy: A technique whereby the light from astronomical objects is separated by wavelength into its constituent colors. Radiation from the different chemical elements that make up an object can be distinguished, giving information about the abundance of these elements and their physical state. Spitzer Space Telescope: A 0.85-m infrared telescope launched by NASA in August 2003. Square Kilometer Array: A proposed ground-based radio telescope that will be composed of an interferometric array of individual antenna stations with a combined square kilometer of collecting area, synthesizing an aperture with diameter of up to several thousand kilometers. Early science is expected to begin in 2014, and it is scheduled to be fully operational in 2020. standard candle: A celestial object whose intrinsic brightness is known or can be estimated by some physical principle and whose observed brightness is therefore useful as a tool to measure distance. Standard Model of Physics: The theory that summarized the current picture of the field of elementary-particle physics. It includes three generations of quarks and leptons, the electroweak theory of weak and electromagnetic forces, and the quantum chromodynamic theory of the strong force. It does not include answers to some basic questions such as how to unify electroweak forces with the strong or gravitational forces. sterile neutrinos: A hypothetical neutrino that does not interact with other particles. It is also a candidate dark matter particle. supermassive black hole: A black hole that is much more massive than the Sun. Supermassive black holes with masses exceeding a million solar masses are found in the nuclei of most galaxies. supernova remnant (SNR): The remains of supernova explosions, SNRs are extremely important for understanding our Galaxy. They heat up the interstellar medium, distribute heavy elements throughout the Galaxy, and accelerate cosmic rays. Suzaku: A Japanese x-ray satellite, launched in 2005, developed through Japan-U.S. international collaboration. The satellite’s goals are to study hot plasma in the x-ray and gamma-ray wavelengths, study the structure and evolution of the universe, and study black hole candidates and active galactic nuclei. Also called Astro-E2. Swift mission: Launched by NASA in 2004; the goal of the Swift mission is to detect and analyze gamma-ray bursts. Type Ia supernova: The thermonuclear explosion of a white dwarf star caused by the accretion of material from a binary companion. Type Ia supernovas (Snela) can be used as standard candles to chart the universe. Type II supernova: A gigantic explosion that signals the death of a massive star. Often, the explosion leaves behind a neutron star; in other cases it may produce a black hole.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Very Large Telescope (VLT): Organized by the European Southern Observatory, all four telescopes in this array were operational beginning in 2006. The VLT consists of four 8 meter telescopes that can either work independently or in combined mode. Combined, the telescopes provide the light collecting power of a 16 meter telescope. Virgo cluster: An irregular cluster of about 2,500 galaxies. w: Dark energy equation-of-state parameter that indicates how dark energy reacts to varying temperatures and densities. warm dark matter: A theoretical type of dark matter particle with higher temperature and velocity than those of cold dark matter. weak lensing: Gravitational lensing in which the lens is not strong enough to produce multiple images, but merely stretches the image of the background object. Wide-field Infrared Survey Explorer (WISE): An infrared space observatory designed to be an all-sky survey in wavelengths from 3.5 to 23 microns that will be up to 1,000 times more sensitive than the Infrared Astronomical Satellite. Launch is scheduled for November 2009. Wilkinson Microwave Anisotropy Probe (WMAP): Launched by NASA in 2001, the WMAP maps temperature fluctuations in the cosmic microwave background with very high precision; see cosmic microwave background (CMB) radiation. X-ray Evolving Universe Spectroscopy (XEUS): A follow-up to the European Space Agency’s Cornerstone X-Ray Spectroscopy Mission. It is a permanent spaceborne x-ray observatory whose goals are to probe dark matter and dark energy, to study massive blackholes at z ~ 10, to observe the structure of the galaxies near black holes, and to study matter under extreme conditions and the structure of highly collapsed stars. X-ray Multi-Mirror Mission-Newton (XMM-Newton): Launched by the European Space Agency in 1999, the XMM-Newton carries three advanced x-ray telescopes. XMM-Newton has studied x-rays from accretion onto black holes, properties of exploding stars, the nature of exotic matter, and has observed gamma-ray bursts. z: See redshift. zenith: The point on the celestial sphere directly above the observer, that is, opposite to the direction of gravity. zodiacal light: Sunlight scattered by interplanetary dust in our solar system.