Key Terms

section : A
Auroras are the emission of different types of light (infrared, visible, ultraviolet, or x-ray) from the upper atmosphere of a planet that is caused by electrically charged particles striking atoms of gases in the atmosphere from above. On Earth, we often call auroras the northern and southern lights. Jupiter has auroras that are many times more brilliant and powerful than Earth’s. Auroras are a consequence of a planet having a magnetic field, and Jupiter’s are influenced greatly by the planet’s fast rotation.
A spacecraft’s orientation, meaning the direction in which it is pointed. Juno has an attitude control system that consists of four sets of thrusters that allow the spacecraft to change its orientation in space.
Atomic particles
Particles that make up atoms, such as electrons, neutrons, and protons.
The layer of gases surrounding a planet or moon. Jupiter’s atmosphere is mostly hydrogen and helium gas.
Atlas V Rocket
The Juno spacecraft was launched aboard an Atlas V 551 rocket on Aug. 5, 2011. Atlas refers to a family of expendable launch vehicles that has played a major role in U.S. space history and dates back to the 1950s. The Atlas V (in service since 2002) uses a core booster; zero-to-five, strap-on, solid rocket boosters (SRBs); a Centaur upper stage; and one of several payload fairings.
Large, high-pressure systems in planetary atmospheres are often called anticyclones. An anticyclone exists where cooler (and therefore heavier) air at higher altitudes sinks in spiraling motions to reach lower altitudes. The circular motion of winds in large air masses like cyclones and anticyclones results from a planet’s rotation. On Earth and Jupiter (and other planets that rotate in the same direction), anticyclones rotate clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. Jupiter’s Great Red Spot is a large example of an anticylonic feature.
Ammonium Perchlorate
A chemical compound composed of the elements oxygen, hydrogen, nitrogen, and chlorine that is used in solid-rocket motors, like the solid-rocket boosters on Juno’s Atlas V launch vehicle. Ammonium perchlorate is a powerful oxidizer, which means it allows rocket fuel to burn very quickly and efficiently, producing a great deal of thrust.
Ammonium Hydrosulfide
Ammonium hydrosulfide is a chemical compound made from the elements nitrogen, hydrogen and sulfur with the chemical formula NH4SH. On Jupiter, below an upper layer of clouds made of ammonia ice crystals, there is thought to be a layer of ammonium hydrosulfide clouds.
Ammonia clouds
The bright, usually white, uppermost layer of clouds on Jupiter is thought to consist of ammonia ice. On Earth, white wispy cirrus clouds are made of ice crystals. On Jupiter, the same sort of clouds can form, but the crystals are made of ammonia (NH3) instead of water (H20). Scientists think it is possible that the formation of ammonia cirrus clouds at high altitude may have caused one of Jupiter’s belts (the darker southern equatorial belt) to seemingly disappear in 2010.
Ammonia is a strong-smelling chemical compound made from the elements nitrogen and hydrogen. It has the chemical formula NH3. Along with water and methane, ammonia is one of the chief compounds that make up the ices found in the outer solar system. The bright, usually white, uppermost layer of clouds on Jupiter is thought to consist of ammonia ice. On earth, ammonia is a common ingredient in products like window cleaner and smelling salts.
Aft Low-Gain Antenna (A-LGA)
Juno has three low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). [The forward deck is where the large, saucer-shaped high-gain antenna (HGA) is mounted; the aft deck is where the main engine is located.] The aft LGA is used during the period in 2013 when the spacecraft is inside of Earth’s orbit and the angle formed by the sun, Juno, and Earth is greater than 110 degrees.
Advanced Stellar Compass
The Advanced Stellar Compass, or ASC, is used to provide accurate information about how the Juno spacecraft is oriented in space. It does this by comparing an onboard star map with images of the sky taken by two CCD cameras. The ASC was built by the Technical University of Denmark.
section : B
Biomarkers, or biosignatures, are evidence for life on a planetary body. Ideally, a biomarker would provide a chemical signature indicating that a distinct biological process is occurring on a world. For example, such a signature might be recognizable in the spectrum of a planet’s light.
Bar (pressure)
A bar is a unit for measuring atmospheric pressure. One bar (also called one atmosphere) of pressure is roughly equal to the air pressure on Earth at sea level. The pressure at the center of Jupiter is estimated to be roughly 50 million bars.
section : C
Within an atmosphere, convection is a type of circulation in which warm air from deeper in the atmosphere rises while cooler air from higher altitudes sinks. Jupiter is still hot on the inside from its formation 4.6 billion years ago, and convection is the main way the heat is able to get out from the planet’s interior. Convection can also take place within liquids (for example a pot of soup or hot caramel boiling on a stove top) and in plasmas (as on the Sun).
Conducting Fluid
Liquid, gas or plasma material in which electrons are able to flow, thus making the fluid able to conduct electricity. When an electrically conductive fluid circulates, it generates a magnetic field. Earth’s outer core is an ocean of hot, liquid iron in constant motion; its circulation generates our planet’s magnetic field. Within Jupiter, scientists think its hydrogen is compressed by gravity into a liquid form that can conduct electricity; thus it is a conducting fluid that generates the planet’s magnetic field.
Common Core Booster
The Common Core Booster is the large, first stage of the Atlas V rocket that launched Juno into space.
A comet is an like icy clod of dirt made of frozen gases, rock, and dust, leftovers from the early formation of our solar system. Many comet-like bodies may have collided and stuck together to form the ancient core from which the giant planet Jupiter grew. In size, comets are often roughly the size of a small town. When a comet’s orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of kilometers.
Cloud tops
The visible tops of clouds in a planet’s atmosphere. When we look at Jupiter in visible light (for example, with our eyes through a telescope), we are only able to see the tops of the clouds. The clouds are not flat, but towering, three-dimensional structures. In some places the tops of the visible clouds are at lower altitudes, allowing us to see deeper into the planet’s atmosphere. Juno will pass over Jupiter’s uppermost cloud tops at an altitude of only 3,100 miles (5000 kilometers) every 11 days.
Cloud bands
Broad, parallel bands of clouds are Jupiter’s most distinguishing feature. Winds in adjacent bands generally flow in opposite directions, either east-to-west or west-to-east. The bands are the result of the planet’s fast rotation combined with convection in which warm air from deeper in the atmosphere rises and cooler air from higher altitudes sinks.
Charged Particle
Electrons or atomic nuclei (called ions) that have an electric charge.
Centaur Upper Stage
The Centaur rocket served as the upper stage of Juno’s launch vehicle. This upper stage also gave the spacecraft its initial spin. The Centaur was put into a slightly different orbit around the Sun so it would not follow Juno. Centaur has a long history of launching U.S. spacecraft, and dates back to the 1950s.
Cassini Mission
Cassini is the first spacecraft to orbit the planet Saturn. Launched in 1997 and arriving at its destination in 2004, Cassini has since carried out an in-depth, multi-year tour of Saturn, its rings, and moons. Cassini flew past Jupiter on the way to Saturn, picking up a gravity assist as Juno will do at Earth. In addition, Cassini will transfer to a nearly polar orbit at the end of its mission. This will allow it to get very close to Saturn and conduct similar observations of that planet at the same time Juno is orbiting Jupiter. Cassini is slated to plunge into Saturn in 2017, about one month before Juno is de-orbited into Jupiter.
Cape Canaveral
Located on the eastern central coast of Florida, Cape Canaveral is the historic site of most launches in the U.S. space program. Juno was launched from Cape Canaveral Air Force Station in Aug. 2011. NASA’s Kennedy Space Center is located directly adjacent to Cape Canaveral Air Force Station.
section : D
Doppler shift
A common example of a Doppler shift is the change in the pitch of a train’s whistle as it approaches and then recedes. When the train is moving toward you, the pitch gets higher (the wavelength of the sound waves it produces are getting shorter); once the train passes the pitch gets lower (the wavelength of the sound waves it produces are getting longer). The Doppler effect applies to light waves and radio waves as well as sound waves. When an object (Juno, for example, emitting its radio signal) is moving toward Earth, its signal is shifted to shorter wavelengths. When it moves away from Earth, the signal is shifted to longer wavelengths. This effect can be measured very precisely to detect small changes in the motions of objects (including spacecraft).
Descent Camera
Some spacecraft that land on planets or other bodies carry a camera called a descent camera (or descent imager) to take photographs of the terrain as they approach landing sites. NASA’s Mars Science Laboratory rover, scheduled to land on Mars in late 2012, will carry the Mars Descent Imager or (MARDI). Juno’s visible light camera, JunoCam, was adapted from the design of MARDI.
Deep Space Network
The NASA Deep Space Network, or DSN, is a global network of giant radio antennas that supports space missions. The DSN consists of three facilities spaced approximately 120 degrees apart around the world, in the U.S. (California), Spain, and Australia. This strategic placement permits constant observation of spacecraft as Earth rotates; and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world. The DSN is managed and operated by NASA’s Jet Propulsion Laboratory (JPL).
Deep Space Maneuver
The use of a spacecraft’s propulsion system to change its orbital path, or trajectory, when it is far from Earth. Juno performs two large deep-space maneuvers, or DSMs, using its main engine about a year after launch. The DSMs occur a bit beyond the orbit of Mars, when Juno is at the farthest point in its orbit around the Sun. This is when the spacecraft is moving the slowest, making it less costly in terms of rocket propellant required to change Juno’s orbit. The DSMs will direct Juno back toward the Earth for its gravity-assist flyby in October 2013.
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Toward the equator of a planet. The opposite direction would be poleward, or toward the pole or poles. The equator is an imaginary line that encircles a planet’s ‘middle’ (it is perpendicular to the planet’s rotational axis).
Enrichment, enriched
Enrichment refers to the chemical composition of a planet or other body in our solar system having a greater abundance of certain elements than that of the sun. Most relevant to Juno, most elements (other than hydrogen and helium) measured by the Galileo atmospheric probe were found to be enriched by two to four times, compared to the sun. How these materials came to be enriched in Jupiter is a great mystery that Juno will help us understand.
Electronics Vault
To protect its most sensitive and vital electronic components from the searing radiation that encircles Jupiter, the Juno spacecraft carries a radiation-shielded electronics vault. The vault is made of half-inch thick titanium and has a volume about the size of a car’s trunk.
Earth flyby
Juno will fly past Earth, performing a gravity assist maneuver, once on its way to Jupiter, about two years after launch. Other spacecraft have previously used Earth flybys to reach their destinations as well. The Earth flyby will provide Juno about half of the change in velocity required to reach Jupiter; the other half was provided by its launch vehicle.
Third planet from the Sun. A world of rock and metal with oceans of liquid water on its surface. Our home planet. The only planet known to harbor life.
section : F
Forward Low-Gain Antenna (F-LGA)
Juno has thee low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). [The forward deck is where the large, saucer-shaped high-gain antenna (HGA) is mounted; the aft deck is where the main engine is located.] The forward LGA points in the same direction as the medium- and high-gain antennas, but can send and receive radio signals in a much broader beam.
Fluxgate Magnetometer
The two Flux Gate Magnetometers (FGM) are part of Juno’s magnetometer (MAG) experiment and will measure the strength and direction of Jupiter’s magnetic field.
Failed star
Objects made of mostly hydrogen and helium gas, having much more mass than Jupiter, but much less mass than the sun are sometimes called ‘failed stars.’ Below a certain amount of mass, perhaps 60 times the amount of material in Jupiter, objects do not achieve pressures and temperatures high enough within their cores that can sustain the nuclear fusion reactions that cause stars to shine. Thus their ‘failure’ in this context refers to having insufficient mass to allow for fusion. They are also sometimes referred to as sub-stellar objects or brown dwarfs.
section : G
Gyroscopes are devices on a spacecraft that sense the slightest change in the rate of rotation in one or multiple directions. They are reference devices that keep the spacecraft stable. Reference gyros measure the forces acting on them. In deep space, the force will be proportional to the ship’s velocity (speed). In a spacecraft, gyroscopes tell the onboard computer when the craft has changed its orientation in space. The computer then sends the information to the spacecraft’s stabilization device, which can make corrections.
Great Red Spot
A giant, oval-shaped feature in Jupiter’s atmosphere that is more than twice as wide as Earth. It has been present for at least 250 years and could be much older. Scientists do not understand very well what causes the spot’s reddish color; how deeply it is rooted to the interior is also unknown.
Gravity field
All objects with mass have gravity, which is a force of attraction between the object and all others in the universe. The more mass, or material, an object has, the more powerful its gravity. The gravity field of an object (Jupiter, for example) is the three-dimensional region of space around it in which the force of its gravity has a measurable influence. Unless an object is perfectly spherical, its gravity field will have variations in strength at different places that are related to how material within it is arranged.
Gravity assist maneuver
To save on rocket fuel and/or launch vehicle size, spacecraft traveling to other worlds sometimes make use of maneuvers in space called gravity assists. These maneuvers use the gravity of one massive body (a planet or moon) to change the spacecraft’s orbit. The approaching spacecraft actually steals a tiny amount of momentum from the body it flies past, adding that momentum to its own. Juno’s October 2013 Earth flyby is an example of a gravity assist maneuver. The Cassini spacecraft uses gravity assist flybys of Saturn’s moon Titan to adjust its orbital path and reach various destinations in the Saturn system.
Gravity Science pass
One of two types of orbits Juno will use during its science mission. During a Gravity Science pass, Juno’s high-gain antenna will be pointed at Earth so that it can receive and retransmit a precise radio signal. Variations in Jupiter’s gravity field will affect the spacecraft’s motion, speeding it up in some places and slowing it down in others. These changes in motion due to the shape of gravity field will be imprinted upon the radio signal as a Doppler shift that can be measured and made a 3D map of the planet’s gravity.
Gravity Science Experiment (GSE)
The Juno Gravity Science experiment will enable Juno to measure Jupiter’s gravitational field and reveal the planet’s deep internal structure. Instead of a dedicated instrument mounted on the spacecraft, the gravity science experiment uses radio signals sent between the spacecraft and the antennas of the Deep Space Network.
Gossamer ring
Jupiter’s gossamer rings are diffuse rings of fine dust particles outside the planet’s main ring. Observations by NASA’s Galileo spacecraft indicated that the rings coincide with the orbits of two small moons, Amalthea and Thebe. Jupiter’s rings are formed from dust particles hurled up by micro-meteor impacts on Jupiter’s small inner moons and captured into orbit. If the impacts on the moons were any larger, then the larger dust thrown up would be pulled back down to the moon’s surface by gravity. The rings must constantly be replenished with new dust from the moons to exist.
Galileo mission
NASA’s Galileo mission sent the first spacecraft to orbit Jupiter. Galileo was launched from the Space Shuttle Atlantis in 1989 and arrived at Jupiter in 1995. The mission was designed to be an extensive up-close survey of the Jovian system – the planet, its rings, and especially its moons. In 2003, after nearly eight years in orbit in which the spacecraft provided a treasure trove of amazing discoveries, Galileo was purposely deorbited (meaning a controlled crash) into the atmosphere of Jupiter.
Galileo Probe
The Galileo Probe was an atmospheric entry probe carried by NASA’s Galileo spacecraft. The probe separated from Galileo shortly before arrival at Jupiter and went on to parachute into the planet’s clouds. The probe survived for about one hour, reaching a depth of about 120 miles (200 km) below the cloud tops, where the pressure was about 24 times that at sea level on Earth.
Galileo Galilei is perhaps the best known astronomer in history. He is usually credited as the first scientist to use the telescope (in 1610, at the time a cutting edge military technology) to view the heavens. His most famous discoveries include the four large moons of Jupiter, mountains and craters on the Moon, sunspots, the phases of Venus, and the rings of Saturn. Galileo lived in Italy from 1564 to 1642 and is also notable for his work in mathematics and physics. A Lego® minifigure of Galileo is mounted onboard the Juno spacecraft, along with a plaque that celebrates the great astronomer.
An enormous collection of stars, usually numbering in the billions to hundreds of billions. Our sun and its family of planets (our solar system) is just one of perhaps 400 billion stars in the Milky Way Galaxy. Galaxies come in a variety of shapes and sizes. Ours is shaped like a spiral disk with a bright bulge in the center and is so vast that it takes light (the fastest thing we know of) 100,000 years to cross from one side to the other.
section : H
Hydrogen is a lightweight gas, the first element on the periodic table, and the most abundant element in the universe. Jupiter and Saturn, the gas-giant planets, are composed mainly of hydrogen and helium, much like stars. As a lightweight gas, it takes a massive body with strong gravity to hold onto hydrogen in its atmosphere and keep it from escaping to space.
Hydrazine is a chemical used as fuel in many spacecraft, including Juno, for maneuvering in space. Hydrazine is sometimes used by itself (as a monopropellant) in maneuvering thrusters, and sometimes with liquid oxygen (combined to form a bi-propellant) in rocket engines. Hydrazine is a hazardous chemical and is treated with great care by technicians when fueling a spacecraft in preparation for launch.
Hubble Space Telescope
The Hubble Space Telescope (launched 1990) and designed to be serviceable by astronauts, is one of NASA’s most successful and long-lasting science missions. Located above the Earth’s atmosphere, which distorts and blocks the light that reaches our planet, it gives a view of the universe that typically far surpasses that of ground-based telescopes. Famous for its dazzling pictures of galaxies and nebulae, the Hubble Space Telescope has also been an important tool for planet hunters.
Hotspots refer to places in Jupiter’s atmosphere that appear bright when viewed in infrared light that represents heat (thermal infrared at a wavelength of 5 microns). These features are places in which the uppermost cloud layers have cleared or descended, allowing heat from deeper in the planet to escape directly to space. In visible light images, the hotspots look dark and grayish or slightly bluish, allowing a glimpse slightly deeper into Jupiter’s murky depths. Thanks to the Galileo probe, which is thought to have parachuted into a hotspot, we think these areas are dryer, like deserts in the atmosphere with little water vapor.
High Gain Antenna (HGA)
The High-Gain Antenna (HGA) is a 2.5-meter (8-foot) wide, saucer shaped radio antenna that serves as Juno’s main communications link with Earth. It will be the primary antenna used during Juno’s time at Jupiter, both for sending science data back to Earth and for transmitting information about the health of the spacecraft. The HGA has the strongest signal of the spacecraft’s five antennas, which enables Juno to transmit data at a much higher rate than the others. The HGA is covered with insulating blankets for protection from heat produced by the sun’s harsh light while Juno is in the inner solar system. In addition to its communications role, the HGA also functions as part of Juno’s Gravity Science system. The antenna requires extremely accurate pointing because it sends and receives radio signals in the form of a tight beam.
Helium is a lightweight gas, the second element on the periodic table, and the second most abundant element in the universe. Jupiter and Saturn are gas-giant planets, composed mostly of hydrogen and helium, much like stars. Since helium is a lightweight gas, it takes a massive body with strong gravity to hold onto helium in its atmosphere and keep it from escaping to space.
The halo is part of Jupiter’s faint and dusty ring system. First seen by the Galileo spacecraft, it is a broad, faint torus (or donut-shaped region) of material about 6,000 miles thick and extending halfway from the main ring down to the planet’s cloud tops. Halo particles are very small - perhaps 100 times smaller than the width of a human hair. Particles this small are believed to survive only for years, and so must somehow be replenished to Jupiter’s ancient ring system. One possible explanation for this unusual halo is that electromagnetic fields around Jupiter gently push small charged particles out of the ring plane.
section : I
Ions are atoms that have lost or gained electrons, giving them a positive or negative electric charge. Electrically charged atomic particles (both ions and electrons) feel the force of magnetic fields and move in response. The magnetosphere of Jupiter is filled with ions and electrons.
Infrared radiation
Another name for infrared light.
Infrared light
Light that is “infrared” features a longer wavelength than human eyes can detect (between red light and microwave radiation in the electromagnetic spectrum). Some of Jupiter’s auroral emission is in the infrared, and the JIRAM instrument will observe these auroras. Infrared light is also emitted from the warmer depths of the planet. JIRAM will observe the heat from within with cooler clouds silhouetted against the warm interior, and detect the chemical fingerprints of gases in the atmosphere.
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Jupiter Orbit Insertion (JOI)
Jupiter orbit insertion refers to the period of time during which the Juno spacecraft will arrive at the planet Jupiter and be captured by its gravity. The JOI maneuver is accomplished by approaching Jupiter over the north pole and then firing the spacecraft’s main engine for about 30 minutes. This is a critical event in the mission that slows Juno enough to become bound to Jupiter, like an artificial satellite.
The fifth planet outward from the sun, Jupiter is the exploration target of the Juno mission. Jupiter has more than twice the mass of all the other planets in the solar system combined. A gas-giant planet, Jupiter is composed like a star. It did not, however, grow big enough to ignite the core nuclear fusion that makes stars shine. With an enormous magnetic field, the planet has a kind of miniature solar system with dozens moons. Its swirling cloud stripes are punctuated by massive storms such as the Great Red Spot, which has raged for hundreds of years.
JunoCam is a color, visible-light camera designed to capture remarkable pictures of Jupiter’s clouds. The camera’s main purpose is for education and public outreach; still, scientists will be very interested in its images.
Juno Energetic Particle Detector Instrument (JEDI)
The Jupiter Energetic Particle Detector Instrument (JEDI) will measure the energetic particles that stream through space, and study how they interact with Jupiter’s magnetic field.
Jovian Infrared Auroral Mapper (JIRAM)
The Jovian Infrared Auroral Mapper (JIRAM) will study Jupiter’s atmosphere in and around the auroras, learning more about the interactions of the auroras, the magnetic field, and the magnetosphere. JIRAM will be able to probe the atmosphere down to 50 to 70 kilometers (30 to 45 miles) below the cloud tops, where the pressure is five to seven times greater than at Earth’s sea level.
Jovian Auroral Distribution Experiment (JADE)
The Jovian Auroral Distributions Experiment (JADE) will work with some of Juno’s other instruments to identify the particles and processes that produce Jupiter’s stunning auroras. JADE will also help create a three-dimensional map of the planet’s magnetosphere.
Jet streams
Jet streams are like flowing rivers of air. As motions in an atmosphere, jet streams carry high-altitude clouds rapidly eastward or westward. Jupiter has very prominent bands in its atmosphere that are driven by east- and west-flowing jets. On Earth, jet streams can reach speeds approaching 200 miles per hour; on Jupiter they have been measured to travel at around 400 miles per hour.
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Kuiper Belt
The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune – billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of most comets that orbit our sun.
Kennedy Space Center
NASA’s main launch operations center, located on Florida’s eastern central coast, Kennedy Space Center (KSC) has been the point of origin for most of NASA’s planetary exploration missions and all of its manned launches. The Juno spacecraft was launched from KSC on Aug. 5, 2011.
The Kelvin temperature scale measures an object’s temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale the freezing point of water is 273 Kelvin (0 degrees Celsius, 32 degrees Fahrenheit). It is often used in astronomy and other sciences.
Ka-band is a new radio frequency developed to enhance performance in spacecraft communications by using significantly less power. The long-time standard in deep-space communication is a section of the radio frequency spectrum known as X-band. Operating four times higher than X-band (32 gigahertz compared to 8 gigahertz), Ka-band frequency enables transmission of much higher data rates using less power. Juno makes use of both the X- and Ka-bands for its gravity science investigation.
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Low-Gain Antenna (LGA)
Juno has three low-gain antennas, one located on its forward deck (F-LGA), and two located on its aft deck (A-LGA and T-LGA). Its high-gain and medium-gain antennas send and receive radio signals in much narrower, or more tightly focused, beams. LGAs were designed with a sufficiently broad beam width so that even when Juno is pointed at the sun and not earth, the LGA can still receive radio signals from the earth. The LGA also helps mission controllers to command the Juno spacecraft.
Lockheed Martin
The company that built the Juno spacecraft under contract with NASA. Juno was built at Lockheed Martin Space Systems near Denver, Colo.
Liquid oxygen
Oxygen (like other gases) can become a liquid when cooled sufficiently or placed enough pressure. Liquid oxygen is used by spacecraft and rockets in propellants; it reacts with fuel (causing the fuel to burn), which creates thrust.
Liquid metallic hydrogen
A form of liquid hydrogen thought to exist in the inner half of Jupiter, where pressures are so great that the electrons are squeezed off the hydrogen atoms, allowing the fluid to conduct electricity. Since being good conductors of electricity is a property usually ascribed to metals, this strange material is called liquid metallic hydrogen.
Liquid hydrogen
Hydrogen gas can become a liquid when it is cooled sufficiently or placed under enough pressure. The pressure within Jupiter’s gaseous hydrogen atmosphere creates sufficient pressure to force the hydrogen to liquefy.
Light gases
Hydrogen and helium are light gases, meaning they do not have much mass. It takes a planet with powerful gravity to hold onto such light gases.
Launch Vehicle
Another name for the rocket used to launch a spacecraft. There are many different types and configurations of launch vehicle. Juno’s launch vehicle was the Atlas V 551.
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Microwave Radiometer (MWR)
Juno will use its Microwave Radiometer (MWR) instrument to probe deep into Jupiter’s atmosphere, learning about its structure and chemical composition. To see what’s under the planet’s cloud tops, the MWR will measure the microwave radiation emitted from inside the planet. This radiation also carries imprinted upon it information about the amount of water in the atmosphere – a key piece of missing information about Jupiter.
Microwaves are a type of electromagnetic radiation that has a longer wavelength than visible light and infrared light. Microwaves can be used to study the Universe, communicate with satellites in space, and cook foods. On Earth, microwaves are good for transmitting information because microwave energy can penetrate haze, light rain and snow, clouds, and smoke. For the same reason, microwaves coming from inside Jupiter provide a great way to see what’s going on deep below the planet’s cloud tops.
A micron, or micrometer, is a millionth of a meter, or a thousandth of a millimeter. As a reference, the diameter of a human hair is about 100 micrometers. Wavelengths of infrared radiation are typically expressed in micrometers. A thousandth of a micrometer is called a nanometer.
Micrometeors, micrometeoroids
A meteoroid is usually a fragment of space debris consisting of rock, ice and/or metal. It can be very large with a mass of several hundred tons, or it can be very small. A micrometeoroid is a very small meteoroid – a particle smaller than a grain of sand. Micrometeors are a hazard for spacecraft because their impacts can cause damage. Spacecraft get some protection from these impacts thanks to the multi-layered blankets that cover most of their external surfaces.
Methane, chemical formula CH4, is a compound containing the elements carbon and hydrogen. Along with water and ammonia, methane is one of the chief compounds that make up the ices found in the outer solar system.
Medium-Gain Antenna (MGA)
The Medium-Gain Antenna (MGA) is one of two small antennas located next to Juno’s High-Gain Antenna. It has wider field of view than the HGA, which means it doesn’t have to be pointed quite as precisely at the Earth to transmit and receive signals, but its signal is not as powerful. The MGA will be used during the cruise phase of the mission at times when it would be difficult to keep the high-gain antenna locked onto Earth while pointing the solar rays more or less toward the sun. If the spacecraft enters safe mode due to a problem, it will switch to the MGA in most cases.
Fourth planet from the sun. A rocky planet similar to Earth, but about half its size. It is thought that because it is smaller than Earth, Mars cooled much more quickly, allowing its inner core to solidify. The lack of a churning liquid metal core shut down the Red Planet’s magnetic field and allowed much of its atmosphere to be stripped away by the solar wind.
Main ring
The main ring is about km 4000 miles wide and has an abrupt outer boundary about km 80,000 miles from the center of the planet. The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring. At its inner edge, the main ring merges gradually into the vertically thick, but more diffuse, ring region called the halo.
A magnetosphere is the area of space around a planet that is controlled by the planet’s magnetic field. Jupiter’s magnetosphere is the largest structure in the solar system and is a primary target of investigation for the Juno mission.
Magnetometer (MAG)
Using its magnetometer (MAG), Juno will create an extremely accurate and detailed three-dimensional map of Jupiter’s magnetic field. This unprecedented study will allow us to understand Jupiter’s internal structure and how the magnetic field is generated by the dynamo action inside – the churning of electrically charged material deep below the surface. MAG will also monitor the magnetic field for long-term variations, helping to determine the depth of the region where the field is generated.
Magnetic field
A magnetic field is the invisible region of influence around a magnet in which other magnets, things made of metal, or things with an electric charge feel its magnetic force. Some objects in space, like the planets earth and Jupiter, as well as the sun, act like magnets and thus have their own magnetic fields. By studying their magnetic fields, scientists can learn what these objects are like deep inside.
Magnetic braking
Magnetic braking is a process by which a spinning object in space that has a magnetic field can have its rotation slowed as the magnetic field drags though a disk of material that surrounds the object. Young stars are thought to be affected by magnetic braking as their powerful magnetic fields drag though the disk of excited gas that surrounds them. Jupiter’s rotation might have been similarly affected by this process, and studying its magnetosphere can teach us how the process affects many similarly magnetic objects in the universe.
section : N
Nitrogen is a chemical element . It is the most abundant gas in Earth’s atmosphere, and is important to the chemical reactions that power life as we know it. Nitrogen is present in various forms of ices in the outer solar system, especially those involving ammonia. The atmosphere of Saturn’s moon Titan is mostly nitrogen, and Neptune’s moon Triton has geysers of liquid nitrogen on its surface.
New Horizons mission
NASA’s New Horizons spacecraft is designed to make the first close-up study of Pluto and its moons and other icy worlds in the distant Kuiper Belt. The spacecraft has seven scientific instruments to study the atmospheres, surfaces, interiors, and intriguing environments of Pluto and its distant neighbors. New Horizons flew past Jupiter in 2007, receiving a gravitational assist from the giant planet.
Neptune, the eighth-farthest planet from the sun, is a giant planet made of mostly gas and the ‘ices’ of the outer solar system: water, ammonia, and methane. Dark, cold and whipped by supersonic winds, Neptune is more than 30 times as far from the sun as Earth. The planet takes almost 165 years to orbit our sun.
A nebula is a vast cloud of gas and dust in space, often light years wide, and usually illuminated by radiation from stars within or nearby. Nebulae are places where stars are born, as the gravitational collapse of some of their material forms new systems of stars and planets. Some nebulae are the remnants of stars that have reached the end of their lives – stars that have either thrown off their outer layers or exploded as supernovae.
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Oort Cloud
The Oort Cloud is a vast spherical shell (or bubble-shaped region) of icy, comet-like bodies that surround our sun and extends from about 5000 times Earth’s distance from the sun to perhaps halfway to the nearest star. Most of the comets in the Oort Cloud are thought to have been sent there by close gravitational encounters with giant Jupiter, which kicked the comets into wildly long and random orbits.
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Propellant refers to the material(s) used to move or propel rockets or spacecraft. A propellant can be a single material (called a monopropellant), like the hydrazine used for Juno’s attitude control thrusters; they can also be composed of two materials (called a bi-propellant) like the ones that that ignite in Juno’s main engine to produce thrust. In either case, the continuous ejection of a stream of expanding gases in one direction causes a steady motion of the spacecraft in the opposite direction. This is the basic principle of a rocket.
Polar orbit
An orbit that passes over the north and south poles of a planet, compared to an equatorial orbit, which passes over the planet’s equator, or middle. Polar orbits are excellent for spacecraft on missions to map planets, because they cover all latitudes from one pole to the other on each orbit.
Plasma waves
Plasma waves occur within the plasma of a magnetosphere and are created by the movement of electrically charged particles that fill the magnetosphere. They can be similar to sound waves (electrostatic waves) or they can be radio waves (electromagnetic waves). Spacecraft can detect a wide range of phenomena that occur on and around planets can create plasma waves from lightning and auroras, to particles streaming along the contours of a planet’s magnetic field. For this reason, the study of plasma waves can reveal much about processes occurring in the space around a planet that would otherwise be undetectable.
Plantesimals represent one of the small bodies that formed from the original disk of gas and dust from which our solar system formed (the solar nebula). Most planetesimals stuck or collided, eventually growing into more massive objects that became the planets.
Planetary Protection
Planetary protection is a practice of protecting solar system bodies (i.e., planets, moons, comets, and asteroids) from contamination by Earth life. It also protects Earth from possible life forms that may be returned from other solar system bodies. Planetary protection is essential to: preserve our ability to study other worlds as they exist in their natural states; avoid contamination that would obscure our ability to find life elsewhere – if it exists; and ensure that we take prudent precautions to protect Earth’s biosphere in case it does.
Phosphine, a chemical compound, is composed of the elements phosphorous and hydrogen. Phosphine is found in small amounts in the atmospheres of Jupiter and Saturn. On Earth, phosphine is flammable and toxic to humans.
Payload fairing
The fairing of a launch vehicle is the cone-shaped section at the front end (the nosecone). The payload fairing protects the spacecraft within (the rocket’s payload) from weather and contamination on the ground prior to launch and from friction with the atmosphere during flight. Once the rocket has reached a height above most of Earth’s atmosphere, the payload fairing splits in half and falls away, landing in the ocean.
Payload refers to the important material being carried into space by a rocket or spacecraft – the things it was intended to carry to its destination. For missions like Juno that explore the solar system, their payload is the suite of scientific instruments and experiments.
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Rotations Per Minute (RPM)
Rotations per Minute (RPM) refers to the number of times a spinning object rotates in one minute. The Juno spacecraft rotates at about 2 RPM for much of its cruise to Jupiter.
Rocket fuel
Rocket fuel refers to a variety of chemical compounds that release large amounts of stored energy when burned in a rocket engine. Most rocket fuels are combined in an engine with an oxidizer, such as liquid oxygen, that allows them to burn very efficiently both within earth’s atmosphere and in space where there is no atmospheric oxygen. The reaction of a fuel with an oxidizer creates an explosive chemical reaction that produces a rocket engine’s thrust.
Radiometry pass
Radiometry pass refers to one of two orbit types the Juno science mission will use. To explore Jupiter’s inner clouds, Juno turns on its microwave radiometer (MWR) and orients itself so that the MWR antennas point at Jupiter. Mounted on two of Juno’s six sides, the MWR antennas take continuous measurements while Juno spins. These so-called radiometry passes occur during orbits 3 and 5 through 8, out of a total 33 planned orbits.
Radio frequency
Radio frequency represents the number of radio waves passing an antenna per second. The more waves that pass by each second, the higher the frequency. One wave, or “cycle,” per second is called one Hertz. The radio communications equipment used by spacecraft are tuned to send and receive specific radio frequencies in the range of billions of cycles per second (or Gigahertz). Juno carries an antenna (WAVES) sensitive to a range of radio frequencies produced by phenomena in the Jupiter system.
Radiation belt
A radiation belt is a donut-shaped region of charged-particle radiation that surrounds a planet. Both Jupiter and Earth have radiation belts. Particles in these belts carry a great deal of energy and pose a hazard for spacecraft (and any organisms) traveling through them.
Radiation describes a variety of waves or particles that carry energy through space. Electromagnetic radiation includes visible, infrared and ultraviolet light; radio waves; x-rays and gamma rays. Particle radiation refers to atomic-scale particles – from whole atomic nuclei to electrons, protons and neutrons – that move through space, carrying energy with them. These particles can be the product of radioactive decay, in which a material emits particle radiation as it turns into another material. Particle radiation can also result from the acceleration of particles within a powerful magnetic or electric field – as is the case with the charged particle radiation within Jupiter’s magnetosphere.
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Subatomic particles
Smaller than atoms, subatomic particles refer to typically constituent particles such as protons, neutrons electrons, or quarks that make up atoms.
A star is huge, spherical mass of mostly hydrogen gas that shines brilliantly due to nuclear fusion occurring at its core. A star represents a fine balance between the crushing weight of its outer material and the outward pressure produced by the nuclear reactions in its center. In fusion reactions, atoms of one element are squeezed together to form a heavier element, a process that releases enormous amounts of energy. Stars come in a range of sizes and colors (from white to red to yellow to blue) that depend on the amount of mass they contain and their age. More massive stars live for only a few million years, while smaller, less massive stars (like our sun) can live for billions of years.
Solid rocket booster
A solid rocket booster is a type of rocket, packed with a solid form of rocket fuel, that gets attached to a launch vehicle to provide extra boost in velocity in its way to orbit. Solid rocket boosters are jettisoned a couple of minutes into the flight of a rocket.
Solar wind
Solar wind refers to the escape of rapidly moving atoms and ions from the sun’s outer atmosphere (the corona). Blowing outward in a million mile-per-hour gale, solar wind fills the solar system and interacts with all of the planets.
Solar eclipse
A solar eclipse is observed when a planet or moon completely blocks an observer’s view of the sun. The Juno spacecraft will experience only one solar eclipse during its cruise to Jupiter, for about 20 minutes during its earth flyby gravity assist maneuver in October 2013.
Single fault tolerance
Single fault tolerance is a mission design principle that seeks to ensure mission success. This allows spacecraft to continue functioning in the event that one or another systems experiences a problem, or “fault”. In this way, no one particular fault should result in a total failure of the spacecraft.”
Saturn is the sixth planet outward from the sun and distinguished by a distinctive ring system. All four gas-giant planets have rings – made of bits of ice and rock – but none are as spectacular or as complicated as Saturn’s. Like the other gas giants, Saturn is a massive ball of mostly hydrogen and helium.
Safe mode
Safe mode is a pre-programmed condition that a spacecraft enters when encountering a problem it does not know how to correct on its own. Safe mode carries out a specific set of actions that are intended to prevent the loss of the mission. Safe mode is different for each spacecraft, but it typically involves the spacecraft reorienting itself so that a particular antenna is pointing toward earth, calling home, and waiting for mission controllers to diagnose what caused it to enter the safe condition.
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Tungsten is a hard metallic element with a high melting point that is often used in fabricating spacecraft components.
A torus is a shape like a ring, but with substantial thickness. A round life preserver and a donut are both shaped like a torus.
Toroidal Low-Gain Antenna (TLGA)
The Toroidal Low-Gain Antenna (TLGA) fills the gap between low-gain antennas by sending its signal out of the sides of the spacecraft. This antenna comes in handy during maneuvers when the other antennas must be pointed far away from the direction of Earth. Juno will rely on its TLGA during a couple of critical moments: the Deep Space Maneuvers, which adjust the spacecraft’s path on the way to Jupiter, and the Jupiter Orbit Insertion burn that will be executed upon arrival at the giant planet.
Tones refer to special sets of radio signals that some modern planetary exploration missions use to indicate specific events that have occurred onboard a spacecraft, such as an engine being fired or a parachute being deployed. The Juno tones are a collection of more than 100 radio signals the spacecraft can produce to indicate its status when it is operating autonomously and cannot send richer telemetry data. Tones will be the only indicators of spacecraft status during the two critical deep space maneuvers and Jupiter orbit insertion, plus the final main engine burn that reduces Juno’s orbital period to its desired 11-day cadence.
Titanium is a strong metallic element with many uses, especially in the space and aerospace industries. The box that shields Juno’s most sensitive electronics from Jupiter’s searing radiation has half-inch-thick walls of titanium.
Thermal blankets
Thermal blankets are used to protect instruments and spacecraft in the space environment (helping to disperse the energy of micrometeoroid strikes) and help maintain the temperature range required to ensure a spacecraft’s operational performance. The blankets are typically made of multiple, very thin layers of plastic and cloth materials. Different materials are used depending on the spacecraft’s operating environment. Juno’s thermal blankets are doped with the conductive metal germanium, which helps electrons in Jupiter’s magnetosphere to flow over the spacecraft, rather than building up an electrical charge that could cause a dangerous discharge or spark.
Telecommunications subsystem
A spacecraft’s communications equipment is collectively referred to as its telecommunications (or telecom) subsystem. Key components of telecom include antennas and radio receivers. Telecommunications subsystem components are chosen for a particular spacecraft in response to the requirements of the mission. Anticipated maximum distances, planned frequency bands, desired data rates, available on-board electrical power (especially for a transmitter), and mass limitations, are all taken into account.
Tantalum is a strong multi-purpose metallic element. Engineers originally considered tantalum for the material to construct Juno’s radiation shielding vault, but eventually chose titanium instead.
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Uranus is the seventh planet outward from the sun and is especially notable for its unique tilt, which has left the planet rotating on its side. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its bluish tint – not because methane is blue, but because it scatters red light out of sunlight leaving mostly bluish light to be reflected by the planet.
The universe is a vast expanse of space that contains all of the matter and energy in existence. The universe contains all of the galaxies, stars, and planets. The exact size of the universe is unknown and may be infinite. Scientists believe the universe is still expanding outward due to a violent, powerful explosion that occurred about 13.7 billion years ago.
Ultraviolet emission
An ultraviolet emission refers to light with a shorter wavelength than human eyes can detect (between violet light and x-rays in the electromagnetic spectrum). Some gases in Jupiter’s upper atmosphere give off, or emit, ultraviolet light when they absorb energy from the hail of charged particles raining down on the planet’s polar regions from the magnetosphere. Juno carries a detector (UVS) designed to study this emission from Jupiter’s auroras.
Ultraviolet Imaging Spectrograph (UVS)
The Ultraviolet Imaging Spectrograph (UVS) will take pictures of Jupiter’s auroras in ultraviolet light. Working with Juno’s JADE and JEDI instruments, which measure the particles that create the auroras, UVS will help us understand the relationship between the auroras, the streaming particles and the magnetosphere as a whole.
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Venus, the second planet outward from the sun is about the same size as Earth. A very dense atmosphere of carbon dioxide, with thick clouds of sulfuric acid, engulfs this rocky world. This thick blanket of atmosphere holds in heat, creating a surface temperature of nearly 900 degrees.
Van Allen Belts
Van Allen Belts refer to Earth’s radiation belts. This region of charged particles trapped by our planet’s magnetic field is similar to but much less intense than Jupiter’s radiation belts. The Van Allen belts were one of the first discoveries of the space age, detected in 1958 when the U.S. launched its first satellite, Explorer 1.