Galaxy Evolution Explorer (GALEX or Explorer 83 or SMEX-7) was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

orbiting space telescope designed to observe the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

s to measure the history of star formation in the universe. In addition to paving the way for future ultraviolet missions, the space telescope allowed astronomers to uncover mysteries about the early universe and how it evolved, as well as better characterize phenomena like black holes and

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

. The mission was extended three times over a period of 10 years before it was decommissioned in June 2013. GALEX was launched on 28 April 2003 and decommissioned in June 2013.

Spacecraft

The spacecraft was

three-axis stabilized Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle/satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, et ...

, with power coming from four fixed solar panels. The

satellite bus A satellite bus (or spacecraft bus) is the main body and structural component of a satellite or spacecraft, in which the payload and all scientific instruments are held. Bus-derived satellites are opposed to specially produced satellites. Bus- ...

is from

Orbital Sciences Corporation Orbital Sciences Corporation (commonly referred to as Orbital) was an American company specializing in the design, manufacture, and launch of small- and medium- class space and launch vehicle systems for commercial, military and other governmen ...

based on OrbView 4. The telescope was a Modified Ritchey–Chrétien with a rotating grism. GALEX used the first ever UV light dichroic beam-splitter flown in space to direct

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

s to the Near UV (175-280 nanometers) and Far UV (135-174 nanometers) microchannel plate detectors. Each of the two detectors has a diameter. The target orbit is circular and inclined at 29.00° to the equator.

Launch

An air launched Pegasus

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload ( spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and ...

, launched on 28 April 2003 at 11:59:57 UTC, placed the craft into a nearly circular orbit at an

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

of and an

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

to the Earth's equator of 29.00°.

Mission

The Galaxy Evolution Explorer (GALEX) which explored the origin and evolution of galaxies, and the origins of stars and heavy elements over the redshift range of Z between 0 and 2. GALEX conducted an all-sky imaging survey, a deep imaging survey, and a survey of 200 galaxies nearest to the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

galaxy. As well, GALEX performed three spectroscopic surveys over the 135-300 nanometre band. GALEX had a planned 29 month mission, and is a part of the Small Explorer (SMEX) program. The first observation was dedicated to the crew of the

Space Shuttle Columbia Space Shuttle ''Columbia'' (OV-102) was a Space Shuttle orbiter manufactured by Rockwell International and operated by NASA. Named after the first American ship to circumnavigate the upper North American Pacific coast and the female personif ...

, and was images in the constellation of

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the ...

taken on 21 May 2003. This region was selected because it had been directly overhead the shuttle at the time of its last contact with the

NASA Mission Control Center NASA's Christopher C. Kraft Jr. Mission Control Center (MCC-H, initially called Integrated Mission Control Center, or IMCC), also known by its radio callsign, Houston, is the facility at the Lyndon B. Johnson Space Center in Houston, Texas, ...

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. After its primary mission of 29 months, observation operations were extended. In 2009, one of its detectors, which observed in far-ultraviolet light, stopped functioning. Late in the mission, observations of more intense UV sources were allowed, including the Kepler field. Observation operations were extended to almost 9 years, with NASA placing it into standby mode on 7 February 2012.''GALEX Mission Comes to an End'' 7 February 2012 Universe Today
/ref> NASA cut off financial support for operations of GALEX in early February 2011 as it was ranked lower than other projects which were seeking a limited supply of funding. The mission's life-cycle cost to NASA was US$150.6 million. The

California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

(Caltech) negotiated to transfer control of GALEX and its associated ground control equipment to the California Institute of Technology in keeping with the Stevenson-Wydler Technology Innovation Act. Under this Act, excess research equipment owned by the U.S. government can be transferred to educational institutions and

non-profit organizations A nonprofit organization (NPO) or non-profit organisation, also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized and operated for a collective, public or social benefit, in co ...

. On 17 May 2012, GALEX operations were transferred to Caltech. On 28 June 2013, NASA decommissioned GALEX. It is expected that the spacecraft will remain in orbit for at least 65 years before it will re-enter the atmosphere.

Science mission

The telescope made observations in ultraviolet wavelengths to measure the history of star formation in the

80% of the way back to the Big Bang. Since scientists believe the Universe to be about 13.8 billion years old, the mission studied galaxies and stars across about 10 billion years of cosmic history. The spacecraft's mission was to observe hundreds of thousands of galaxies, with the goal of determining the distance of each galaxy from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and the rate of star formation in each galaxy. Near-UV (NUV) and Far-UV (FUV) emissions as measured by GALEX can indicate the presence of young stars, but may also originate from old stellar populations (e.g. sdB stars). Partnering with the NASA

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

(JPL) on the mission were the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...

Yonsei University Yonsei University (; ) is a private research university in Seoul, South Korea. As a member of the " SKY" universities, Yonsei University is deemed one of the three most prestigious institutions in the country. It is particularly respected in th ...

Johns Hopkins University Johns Hopkins University (Johns Hopkins, Hopkins, or JHU) is a private research university in Baltimore, Maryland. Founded in 1876, Johns Hopkins is the oldest research university in the United States and in the western hemisphere. It consi ...

Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...

, and

Laboratoire d'Astrophysique de Marseille Marseille Observatory (french: Observatoire de Marseille) is an astronomical observatory located in Marseille, France, with a history that goes back to the early 18th century. In its 1877 incarnation, it was the discovery site of a group of gala ...

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. The observatory participated in GOALS with Spitzer Space Telescope,

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, and

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

.GOALS: The Great Observatories All-Sky LIRG Survey
/ref> GOALS stands for ''Great Observatories All-sky LIRG Survey'', and

Luminous Infrared Galaxies Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above . They are also referred to as submillimeter galaxies (SMGs) through their normal method of detection. LIRGs are more abundant than starburst ...

were studied at the multiple wavelengths allowed by the telescopes.

Science objectives

The primary objective of the Galaxy Evolution Explorer was to learn what factors trigger star formation inside galaxies; how quickly stars form, evolve and die; and how heavy chemical elements form in stars. Additional goals include: * Determining how fast stars are forming inside each galaxy * Determining when and how the stars we see today formed * Creating the first map of the ultraviolet universe * Helping scientists find and understand ultraviolet bright quasars. These objects can serve as background sources for the Hubble Space Telescope and FUSE as it probes the gases from which galaxies form stars To accomplish its objectives, the Galaxy Evolution Explorer will conduct eight surveys, grouped into two broad categories - a local universe investigation and a star formation history investigation. The local universe investigation includes the following four surveys: * All-sky imaging survey - will look at the entire sky and develop a comprehensive catalogue of ultraviolet galaxy images, useful to map the distribution of star formation within the local universe * Nearby galaxy survey - will study about 150 nearby galaxies that are familiar to scientists to understand how stars formed in individual galaxies * Wide-field spectroscopic survey - will analyze the light wavelengths of galaxies in a wide swath of the sky * Medium spectroscopic survey - will examine the light properties of galaxies within a narrower portion of the sky The star formation history investigation will take information gathered by the local universe investigation and apply it to more distant galaxies by looking further back in time. It includes the following four surveys: * Deep imaging survey - will look at a portion of the sky to study the distribution of star formation in the deep universe * Deep spectroscopic survey - will look for the most distant galaxies * Ultra-deep imaging survey - will look as deep as possible at a very small portion of the sky * Medium imaging survey - will study star formation in galaxies beyond our local cosmic neighborhood, but not as deep as the deep imaging survey

Telescope specifications

The telescope had a diameter aperture primary, in a Ritchey–Chrétien telescope f/6.0 configuration. It can see light wavelengths from 135 nanometres to 280-nm, with a field of view of 1.2° wide (larger than a full Moon). It had gallium arsenide (GaAs)

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s which supply nearly 300

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s to the spacecraft.Encyclopedia Astronautica - GALEX

Experiment

Ultraviolet telescope

GALEX carries a single f/6.0, Ritchey–Chrétien telescope, with a diameter primary, and a secondary mirror. Beam-splitters direct the Near UV (NUV) and Far UV (FUV) components to separate photoelectric detectors of diameter . In each, the photoelectrons are multiplied by a microchannel plate, and detected by the anode grid. The grid enables determination of the exact position of

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

impact, by the time delay of each pulse at the two ends. The telescope has a

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...

(FoV) of 1.2°, and a resolution of five

arcseconds A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The ...

, and enables either imaging or spectral composition of a single star/galaxy, by a rotatable wheel containing a clear window and a Grism (Grism is a cross between a grating and a prism).

Pre-launch images

References

External links

GALEX website
by the

GALEX data archive
by the

STScI The Space Telescope Science Institute (STScI) is the science operations center for the Hubble Space Telescope (HST), science operations and mission operations center for the James Webb Space Telescope (JWST), and science operations center for the ...

MAST
GALEXView Search Tool
by the STScIMAST
GALEX Ultraviolet Sky Survey
at Wikisky.org {{Orbital launches in 2003 Satellites orbiting Earth Explorers Program Space telescopes Ultraviolet telescopes Spacecraft launched in 2003 Spacecraft launched by Pegasus rockets