International Ultraviolet Explorer

IUE firsts

Resources

The International Ultraviolet Explorer satellite (IUE) was a joint project of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Planetary Plasma and Atmospheric Research Center (PPARC) in the United Kingdom. It was originally proposed in 1964 by a group of British scientists within the European Space Research Organization (ESRO), an organization that turned into the ESA. Beyond their technological capabilities, NASA scientist Robert Wilson developed the project as Small Astronomy Satellite-D (SAS-D).

NASA provided the spacecraft, telescope, spectrographs, and one ground observatory facility. ESA created the solar panels for powering the craft in orbit, and the second ground observatory site. The PPARC provided the four spectrographic detectors. In addition to controlling the satellite, the ground sites acted as typical astronomical observatories, except that instead of using telescopes at their locations, their direct participation was by a link to a telescope orbiting far out in space. IUE was the longest lasting and most productive orbiting astronomical observatory up to its time (it lasted 19 years after initially set for a three-year mission). It was also the first orbiting ultraviolet observatory available to general users, and the first orbiting astronomical observatory in high Earth orbit. Because ultraviolet light from space is largely absorbed by Earth’s atmosphere, observations by IUE provided a whole new range of information not readily available from the ground. Only a small number of high-altitude observatories on Earth can be used with limited effectiveness for ultraviolet studies.

IUE was launched into geosynchronous orbit on January 26, 1978, and remained there until 1996. During these nearly 19 years of operation, it sent to the Earth 104,470 images of 9,600 astronomical objects, ranging from comets in the inner solar system to quasars at the edge of the known universe. IUE was the first scientific satellite that allowed visiting astronomers to make real-time observations of ultraviolet spectra with a response time of less than one hour. This ability provided great flexibility in scheduling observation targets for the satellite. In conjunction with the IUE, simultaneous ground-based observations were performed in wavelengths other than ultraviolet in order to provide measurements of the same objects over a wide range of the electromagnetic spectrum. This provided astrophysicists with a new multi-wavelength method of looking at objects. The end result was a vast archive of new and more complete information than ever before made available to the scientific community worldwide.

IUE greatly surpassed its expected lifetime and the original science goals set for the mission. These included:

• Obtaining high-resolution spectra of stars of all types in order to determine their physical characteristics. The IUE extended the range of observations available from ground-based observatories into the ultraviolet region.
• Studying streams of gas in and around binary star systems, which are difficult to observe from the ground or with standard optical telescopes even from space.

KEY TERMS

Geosynchronous orbit —When placed in orbit at an altitude of 22,241 mi (35,786 km) above the surface of Earth, a satellite orbits Earth once each day. This means it remains stationary over a specific location on Earth and is said to be synchronized with Earth. Communications satellites can be found in geosynchronous (also called geostationary) orbit above the equator.

Gyroscope —A device similar to a top, which maintains rotation about an axis while maintaining a constant orientation of that axis in space. The child’s toy gyroscope is a very simple version of the gyros used to provide a frame of reference for guidance and attitude control systems in spacecraft.

High Earth orbit —The region around Earth above 500 mi (380 km) from the surface. This is where the communications and many other satellites are found. For example, the Space Shuttle orbits the Earth in low Earth orbit, about 300 mi (460 km) above the surface of Earth.

Magellanic Clouds —Two small irregular galaxies that are relatively close to Earth’s own galaxy. They can be seen in the sky from low northern and all southern latitudes as small fuzzy patches of light.

Spectrum —A display of the intensity of radiation versus wavelength.

• Observing faint stars, galaxies, and quasars at low resolution, and comparing these spectra to high-resolution spectra of the same objects.
• Obtaining ultraviolet spectra of planets and comets, again extending scientific knowledge by looking at them in new ways. Such spectra help determine the composition of the atmospheres of planets and gas content of comets.
• Making repeated observations of objects with spectra that change over time in order to reveal new information about them. The long duration of IUE allowed several long-term studies to be performed on objects in areas never before possible.
• Studying the changes of observed starlight passing through interstellar dust and gas. This can reveal how much and what type of gas and dust exists between Earth and the objects from which the light originated.

IUE firsts

IUE contributed to a number of studies and made discoveries that might not have been possible without the long-term availability of a successfully working satellite. One was the discovery of short-term variations in the auroras in the atmosphere of Jupiter (which were initially discovered by IUE). Since auroras are caused by the interaction between the upper atmosphere of a planet and particles radiated from the sun, and the emission of these particles increases as the sun becomes more active, the long life of IUE allowed unique studies associating Jovian aurora activity with the solar sunspot cycle. IUE was the first instrument to provide a systematic study of the distribution of different species of comets in space. The long life of IUE also enabled the monitoring of variations in the occurrence of different types of comets, the discovery of new material within them, and the classification of comets into groups as a function of age. The behavior and distribution of stellar particle radiation (stellar winds) is now beginning to become clearer, and there is hope of understanding the underlying mechanisms driving the stellar winds because of the observations performed with IUE.

IUE spectra combined with optical observations have allowed distances to the Magellanic Clouds, the closest galaxies to the Milky Way, to be determined. Many other studies within and outside the Milky Way galaxy were also conducted adding significant data to the science of astrophysics. Volumes of scientific data and information have already been published on these and many other topics. With the importance of the IUE observations and the concurrent development of the Internet while the data was being received and analyzed, the IUE data archive has become the most heavily used astronomical archive in existence.

Possible future needs were identified during and after the IUE mission, which brought about the concept of creating a World Space Observatory that could provide flexible access to space-based observatories and observation times for astrophysicists world-wide. A working group was formed to further study the associated problems and opportunities.

With all its success, IUE had a few serious problems during its very long mission. All of these came from the fact that five of the six gyroscopes in its attitude control system failed over the years. After the fourth one failed in 1985, IUE continued operations because of the use of its fine sun sensor as a substitute to controlling the attitude of the spacecraft. Even when another gyro was lost in the final year, IUE could still be stabilized in three-axes, with only one remaining gyroscope, by adding star tracker measurements to other guidance parameters. Until October 1995, IUE was in continuous operation, controlled 16 hours a day from the Goddard Space Flight Center in Greenbelt, Maryland, and eight hours from ESA’s Villafranca Satellite Tracking Station (VILSPA) west of Madrid, Spain. After that, ESA took on the role of redesigning control schemes to make it feasible to cover the science operations fully controlled from VILSPA. However, then, only 16 hours were used for scientific operations, with eight hours used for spacecraft housekeeping. IUE remained operational until its attitude control fuel was deliberately released into space, its batteries drained and its transmitter turned off on September 30, 1996.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Nature’s Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

OTHER

European Space Agency (ESA). “International Ultraviolet Explorer (IUE).” <http://www.vilspa.esa.es/iue/iue.html> (accessed October 13, 2006).

Starchild Project Team. IUE Home Page. (October 13, 2006). http://starchild.gsfc.nasa.gov/docs/StarChild/space_level2/iue.html (accessed October 13, 2006).

Clint Hatchett

International Ultraviolet Explorer

The International Ultraviolet Explorer satellite (IUE) was a joint project of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Planetary Plasma and Atmospheric Research Center (PPARC) in the United Kingdom. NASA provided the spacecraft, telescope , spectrographs, and one ground observatory facility. ESA created the solar panels for powering the craft in orbit , and the second ground observatory site. The PPARC provided the four spectrographic detectors. In addition to controlling the satellite, the ground sites acted as typical astronomical observatories, except that instead of using telescopes at their locations, their direct participation was by a link to a telescope orbiting far out in space. IUE was the longest lasting and most productive orbiting astronomical observatory up to its time. It was also the first orbiting ultraviolet observatory available to general users, and the first orbiting astronomical observatory in high Earth orbit. Because ultraviolet light from space is largely absorbed by our atmosphere, observations by IUE provided a whole new range of information not readily available from the ground. Only a small number of high-altitude observatories on Earth can be used with limited effectiveness for ultraviolet studies.

IUE was launched into geosynchronous orbit on January 26, 1978 and remained there until 1996. During these nearly 19 years of operation, it sent to Earth 104,470 images of 9,600 astronomical objects, ranging from comets in the inner solar system to quasars at the edge of the known universe. IUE was the first scientific satellite that allowed "visiting" astronomers to make real-time observations of ultraviolet spectra with a response time of less than one hour. This provided great flexibility in scheduling observation targets for the satellite. In conjunction with the IUE, simultaneous ground-based observations were performed in wavelengths other than ultraviolet in order to provide measurements of the same objects over a wide range of the electromagnetic spectrum . This provided astrophysicists with a new "multi-wavelength" method of looking at objects. The end result was a vast archive of new and more complete information than ever before made available to the scientific community worldwide.

IUE greatly surpassed its expected lifetime and the original science goals set for the mission. These included:

• Obtaining high-resolution spectra of stars of all types in order to determine their physical characteristics. The IUE extended the range of observations available from ground-based observatories into the ultraviolet region.
• Studying streams of gas in and around binary star systems, which are difficult to observe from the ground or with standard optical telescopes even from space.
• Observing faint stars, galaxies, and quasars at low resolution, and comparing these spectra to high-resolution spectra of the same objects.
• Obtaining ultraviolet spectra of planets and comets, again extending our knowledge by looking at them in new ways. Such spectra help determine the composition of the atmospheres of planets and gas content of comets.
• Making repeated observations of objects with spectra that change over time in order to reveal new information about them. The long duration of IUE allowed several long-term studies to be performed on objects in areas never before possible.
• Studying the changes of observed starlight passing through interstellar dust and gas. This can reveal how much and what type of gas and dust exists between Earth and the objects from which the light originated.

IUE firsts

IUE contributed to a number of studies and made discoveries that might not have been possible without the long-term availability of a successfully working satellite. One was the discovery of short-term variations in the auroras in the atmosphere of Jupiter (which were initially discovered by IUE). Since auroras are caused by the interaction between the upper atmosphere of a planet and particles radiated from the sun , and the emission of these particles increases as the sun becomes more active, the long life of IUE allowed unique studies associating Jovian aurora activity with the solar sunspot cycle. IUE was the first instrument to provide a systematic study of the distribution of different species of comets in space. The long life of IUE also enabled the monitoring of variations in the occurrence of different types of comets, the discovery of new material within them, and the classification of comets into groups as a function of age. The behavior and distribution of stellar particle radiation (stellar winds) is now beginning to become more clear, and there is hope of understanding the underlying mechanisms driving the stellar winds because of the observations performed with IUE. IUE spectra combined with optical observations have allowed distances to the Magellanic Clouds, the closest galaxies to the Milky Way , to be determined. Many other studies within and outside our galaxy were also conducted adding significant data to the science of astrophysics . Volumes have already been published on these and many other topics. With the importance of the IUE observations and the concurrent development of the Internet while the data was being received and analyzed, the IUE data archive has become the most heavily used astronomical archive in existence.

Possible future needs were identified during and after the IUE mission, which brought about the concept of creating a World Space Observatory that could provide flexible access to space-based observatories and observation times for astrophysicists world-wide. A working group was formed to further study the associated problems and opportunities.

With all its success, IUE had a few serious problems during its very long mission. All of these came from the fact that five of the six gyroscopes in its attitude control system failed over the years. After the fourth one failed in 1985, IUE continued operations because of the use of its fine sun sensor as a substitute to controlling the attitude of the spacecraft. Even when another gyro was lost in the final year, IUE could still be stabilized in 3-axes, with only one remaining gyroscope , by adding star tracker measurements to other guidance parameters. Until October 1995, IUE was in continuous operation, controlled 16 hours a day from the Goddard Space Flight Center in Greenbelt, Maryland, and eight hours from ESA's Villafranca Satellite Tracking Station (VILSPA) west of Madrid, Spain. After that, ESA took on the role of redesigning control schemes to make it feasible to cover the science operations fully controlled from VILSPA. But then, only 16 hours were used for scientific operations, with eight hours used for spacecraft housekeeping. IUE remained operational until its attitude control fuel was deliberately released into space, its batteries drained and its transmitter turned off on September 30, 1996.

Resources

books

Pasachoff, Jay M. Contemporary Astronomy. Saunders College Publishing, 1989.

other

Starchild Project Team. IUE Home Page. [cited 2003]. <http://starchild.gsfc.nasa.gov/docs/StarChild/space_level2/iue.html>

Clint Hatchett

KEY TERMS

Geosynchronous orbit

—When placed in orbit at an altitude of 22,241 mi (35,786 km) above the surface of Earth, a satellite orbits the earth once each day. This means it remains stationary over a specific location on Earth and is said to be synchronized with Earth. Communications satellites can be found in geosynchronous (also called geo-stationary) orbit above the equator.

Gyroscope

—A device similar to a top, which maintains rotation about an axis while maintaining a constant orientation of that axis in space. The child's toy gyroscope is a very simple version of the gyros used to provide a frame of reference for guidance and attitude control systems in spacecraft.

High Earth orbit

—The region around Earth above 500 mi (380 km) from the surface. This is where the communications and many other satellites are found. The Space Shuttle orbits Earth in low Earth orbit, about 300 mi (460 km) above the surface of Earth.

Magellanic Clouds

—Two small irregular galaxies that are relatively close to our own. They can be seen in the sky from low northern and all southern latitudes as small fuzzy patches of light.

Spectrum

—A display of the intensity of radiation versus wavelength.

International Ultraviolet Explorer

Developed during the 1970s, the International Ultraviolet Explorer (IUE) was a joint project between the National Aeronautics and Space Administration (NASA); Particle Physics and Astronomy Research Council (PPARC), formerly known as the Science and Engineering Research Council of the United Kingdom (SERC); and the European Space Agency (ESA). The IUE was built to explore astronomical objects such as stars, comets, galaxies, and supernovae that exist in the ultraviolet portion of space. The IUE was defined as an "explorer class" mission. These missions are smaller in scope and the objective is a particular task, such as the study of ultraviolet radiation. Ultraviolet radiation is electromagnetic radiation (radiation that transmits energy through the interaction of electricity and magnetism) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

IUE explores ultraviolet radiation

The Earth's ozone layer blocks ultraviolet radiation—which is harmful to humans—from penetrating the atmosphere. But the blockage makes it difficult to study ultraviolet radiation from the Earth's surface. In order to better understand ultraviolet radiation, an observatory must be created and sent beyond the Earth's atmosphere where the ozone layer does not interfere with the ultraviolet radiation. The IUE was such an observatory; it observed astronomical objects from space and relayed the information back to scientists on Earth.

The IUE was launched in January 1978 aboard a Delta rocket and put into a geosynchronous orbit (an orbit that is fixed with respect to Earth). Weighing 1,420 pounds (645 kilograms), the IUE measured 14 feet by 5 feet by 5 feet (4.3 meters by 1.5 meters by 1.5 meters) and was powered by 2 solar panels. The IUE was equipped with a 17.7-inch (45-centimeter) telescope hooked up with two spectrographs (instruments that photograph spectra) that could record ultraviolet wavelengths and transmit the information back to observatories on Earth. At the time, the IUE was the only satellite observatory that worked continually 24 hours a day.

While the IUE was orbiting Earth, astronomers monitored the information that the IUE was transmitting. Scientists at the Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, handled IUE operations for sixteen hours a day and scientists at the Villafranca Satellite Tracking Station (VILSPA) in Spain operated the IUE for the other eight hours of the day. The ultraviolet telescope mounted on the IUE continually gathered information on astronomical objects.

Words to Know

Geosynchronous orbit: When placed in orbit at an altitude of 22,241 miles (35,786 kilometers) above the surface of Earth, a satellite completes one orbit around Earth at the same time Earth completes one revolution on its axis. This means the satellite remains stationary over a specific location on Earth and is said to be synchronized with Earth.

Ozone layer: The atmospheric layer of approximately 15 to 30 miles (24 to 48 kilometers) above Earth's surface in which the concentration of ozone is significantly higher than in other parts of the atmosphere and that protects the lower atmosphere from harmful solar radiation.

Spectrum: Range of individual wavelengths of radiation produced when white light is broken down into its component colors when it passes through a prism or is broken apart by some other means. (Plural: spectra.)

Ultraviolet radiation: Electromagnetic radiation (energy) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum and thus with higher energy than visible light.

By studying the light that is either emitted (thrown off) or absorbed by a celestial body (an object in the sky, such as a star, the Moon, or the Sun), scientists can learn about the activities that occur in space. They do this through the field of ultraviolet astronomy, the study of the dark absorption lines or bright emission lines of a spectrum. A spectrum comprises the colors of red, orange, yellow, green, blue, indigo, and violet. These colors travel at different wavelengths, decreasing in length from red (the longest) to violet (the shortest). When sunlight enters the atmosphere, materials present there break up sunlight into its component colors through reflection (bouncing off an object), refraction (bending through an object), or diffraction (bending around the edge of an object). It is from the individual spectrum lines that astronomers can understand the makeup of stars, galaxies, and other astronomical objects. For example, the more energy a star emits, the brighter it appears and the more ultraviolet wavelengths it sends off. The IUE allowed astronomers to better understand why the atmospheres of some stars are so hot and burn so brightly.

IUE highlights

The IUE made history when it helped make the first identification of an exploding star, named Supernova 1978A. In March 1996, the IUE observed the nucleus of the Comet Hyakutake as it underwent chemical changes during its five-day breakup. As the ultraviolet telescope continually sent back pictures to Earth, scientists learned that every time the comet passed the Sun, it ejected ten tons of water every second and that the eventual breakup of the comet involved only a very small piece of the comet. Other major milestones that the IUE aided in the study of are stellar winds (charged particles ejected from a star's surface); hot gas around the Milky Way (a galaxy that includes a few hundred billion stars, the Sun, and our solar system); the size of active galaxies; and stars with magnetic fields and surface activity.

Originally built to last five years, the IUE lasted almost nineteen years. During its lengthy service to the astronomy community, it did suffer some minor mechanical problems. Although engineers were

able to keep the IUE functioning at various capacity levels, the final shutdown occurred on September 30, 1996, after a joint decision by NASA and ESA.

The IUE was awarded the U.S. Presidential Award for Design Excellence. It is considered one of the great success stories of astronomy as it made observations of over 100,000 astronomical objects during its use. Scientists from all over the world have enjoyed the information the IUE was able to collect. Over 3,500 scientific articles have been generated from this information, which is the most productive for any observatory satellite to date. Because of its endurance of almost nineteen years, the IUE was able to help astronomers gain a better understanding of ultraviolet astronomy.

On June 7, 1992, NASA launched the successor to the IUE, another Explorer-class mission, called the Extreme Ultraviolet Explorer (EUVE). This satellite went beyond the coverage of the IUE due its more powerful telescope.

[See also Ultraviolet astronomy ]