Galaxy Evolution Explorer (GALEX or Explorer 83 or SMEX-7) was a NASA orbiting space telescope designed to observe the universe in ultraviolet wavelengths to measure the history of

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

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 hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

s and dark matter. 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, with power coming from four fixed

solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

s. The satellite bus is from Orbital Sciences Corporation based on OrbView 4. The telescope was a Modified Ritchey–Chrétien with a rotating

grism A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for imag ...

. GALEX used the first ever UV light dichroic beam-splitter flown in space to direct photons to the

Near UV NEAR or Near may refer to: People * Thomas J. Near, US evolutionary ichthyologist * Near, a developer who created the higan emulator Science, mathematics, technology, biology, and medicine * National Emergency Alarm Repeater (NEAR), a former ...

(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 Incline, inclined, inclining, or inclination may refer to: *Grade (slope), the tilt, steepness, or angle from horizontal of a topographic feature (hillside, meadow, etc.) or constructed element (road, railway, field, etc.) *Slope, the tilt, steepn ...

at 29.00° to the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

Launch

An air launched

Pegasus Pegasus ( grc-gre, Πήγασος, Pḗgasos; la, Pegasus, Pegasos) is one of the best known creatures in Greek mythology. He is a winged divine stallion usually depicted as pure white in color. He was sired by Poseidon, in his role as hor ...

launch vehicle, launched on 28 April 2003 at 11:59:57 UTC, placed the craft into a nearly circular orbit at an altitude of and an orbital inclination to the Earth's

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 In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

range of Z between 0 and 2. GALEX conducted an all-sky imaging survey, a deep imaging survey, and a survey of 200

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

nearest to the Milky Way galaxy. As well, GALEX performed three spectroscopic surveys over the 135-300

nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

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, and was images in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

of Hercules 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, Houston, Texas. 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 (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 The federal government of the United States (U.S. federal government or U.S. government) is the national government of the United States, a federal republic located primarily in North America, composed of 50 states, a city within a fede ...

can be transferred to educational institutions and non-profit organizations. 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 A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

formation in the universe 80% of the way back to the

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

. 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 and the rate of

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

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 (JPL) on the mission were the California Institute of Technology, Orbital Sciences Corporation, University of California, Berkeley, Yonsei University, Johns Hopkins University, Columbia University, 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 ...

, France. The observatory participated in GOALS with

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...

, Chandra X-ray Observatory, and Hubble Space Telescope.GOALS: The Great Observatories All-Sky LIRG Survey
/ref> GOALS stands for ''Great Observatories All-sky LIRG Survey'', and Luminous Infrared Galaxies 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 A Ritchey–Chrétien telescope (RCT or simply RC) is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma). The RCT has a wi ...

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 Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...

(GaAs) solar cells which supply nearly 300 watts 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 impact, by the time delay of each pulse at the two ends. The telescope has a field of view (FoV) of 1.2°, and a resolution of five arcseconds, and enables either imaging or spectral composition of a single star/galaxy, by a rotatable wheel containing a clear window and a

Grism A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for imag ...

(Grism is a cross between a grating and a prism).

Pre-launch images

References

Wikisky.org Sky-Map.org (or WikiSky.org) is a wiki and interactive sky map that covers more than half a billion celestial objects. Users can view the whole star sky at once and zoom in to view areas in greater detail. WikiSky includes many stars, galaxies, ...

{{Orbital launches in 2003 Satellites orbiting Earth Explorers Program Space telescopes Ultraviolet telescopes Spacecraft launched in 2003 Spacecraft launched by Pegasus rockets