International Ultraviolet Explorer
Encyclopedia
The International Ultraviolet Explorer (IUE) was an astronomical observatory
satellite
primarily designed to take ultraviolet
spectra
. The satellite was a collaborative project between NASA
, the UK
Science Research Council
and the European Space Agency (ESA). The mission was first proposed in early 1964, by a group of scientists in the United Kingdom
, and was launched on January 26, 1978 aboard a NASA Delta rocket
. The mission lifetime was initially set for 3 years, but in the end it lasted almost 18 years, with the satellite being shut down in 1996. The switch-off occurred for financial reasons, while the telescope was still functioning at near original efficiency.
It was the first space observatory to be operated in real time by astronomers who visited the groundstations in the United States and Europe
. Astronomers made over 104,000 observations using the IUE, of objects ranging from solar system
bodies to distant quasar
s. Among the significant scientific results from IUE data were the first large scale studies of stellar wind
s, accurate measurements of the way interstellar dust absorbs light, and measurements of the supernova
SN1987a which showed that it defied stellar evolution theories as they then stood. When the mission ended, it was considered the most successful astronomical satellite ever.
can perceive light with wavelengths between roughly 350 (violet) and 700 (red) nanometres. Ultraviolet
light has wavelengths between roughly 10nm and 350nm. UV light can be harmful to human beings, and is strongly absorbed by the ozone layer
. This makes it impossible to observe UV emission from astronomical objects from the ground. Many types of object emit copious quantities of UV radiation, though: the hottest and most massive stars in the universe can have surface temperatures high enough that the vast majority of their light is emitted in the UV. Active Galactic Nuclei, accretion disks, and supernovae all emit UV radiation strongly, and many chemical elements have strong absorption lines in the UV, so that UV absorption by the interstellar medium
provides a powerful tool for studying its composition.
UV astronomy was impossible before the Space Age
, and some of the first space telescopes were UV telescopes designed to observe this previously inaccessible region of the electromagnetic spectrum
. One particular success was the second Orbiting Astronomical Observatory
, which had a number of 20cm UV telescopes on board. It was launched in 1968, and took the first UV observations of 1200 objects, mostly stars. The success of OAO-2 motivated astronomers to consider larger missions.
The orbiting ultraviolet satellite which ultimately became the IUE mission was first proposed in 1964 by British astronomer Robert Wilson
. The European Space Research Organisation was planning a Large Astronomical Satellite, and had sought proposals from the astronomical community for its aims and design. Wilson headed a British team which proposed an ultraviolet spectrograph
, and their design was recommended for acceptance in 1966.
However, management problems and cost overruns led to the cancellation of the LAS program in 1968. Wilson's team scaled down their plans and submitted a more modest proposal to ESRO, but this was also rejected on cost grounds. Rather than give up on the idea of an orbiting UV telescope, they instead sent their plans to NASA
administrator Leo Goldberg
, and in 1973 the plans were approved. The proposed telescope was renamed the International Ultraviolet Explorer.
– that is, one with a period equal to one sidereal day of 23h 56m. A satellite in such an orbit remains visible from a given point on the Earth's surface for many hours at a time, and can thus transmit to a single ground station for a long period of time. Most space observatories in Earth orbit, such as the Hubble Space Telescope
, are in a low orbit in which they spend most of their time operating autonomously because only a small fraction of the Earth's surface can see them at a given time. Hubble, for example, orbits the Earth at an altitude of approximately 600 km, while a geosynchronous orbit has an average altitude of 36,000 km.
As well as allowing continuous communication with ground stations, a geosynchronous orbit also allows a larger portion of the sky to be viewed continuously. Because the distance from Earth is greater, the Earth occupies a much smaller portion of the sky as seen from the satellite than it does from low Earth orbit.
A launch into a geosynchronous orbit requires much more energy for a given weight of payload than a launch into low Earth orbit
. This meant that the telescope had to be relatively small, with a 45cm primary mirror, and a total weight of 312kg. Hubble, in comparison, weighs 11.1 tonnes and has a 2.4 m mirror. The largest ground-based telescope, the Gran Telescopio Canarias
, has a primary mirror 10.4 m across. A smaller mirror means less light-gathering power, and less spatial resolution, compared to a larger mirror.
The stated aims of the telescope at the start of the mission were:
. SERC provided the Vidicon cameras for the spectrographs as well as software for the scientific instruments. ESA provided the solar array
s to power the spacecraft as well as a ground observing facility in Villafranca del Castillo
, Spain
. NASA contributed the telescope, spectrograph, and spacecraft as well as launching facilities and a second ground observatory in Greenbelt, Maryland
at the Goddard Space Flight Center.
According to the agreement setting up the project the observing time would be divided between the contributing agencies with 2/3 to NASA, 1/6 to ESA and 1/6 to the UK's Science Research Council.
primary and secondary mirrors. The primary was 45cm across. The telescope was designed to give high quality images over a 16 arcminute field of view (about half the apparent diameter of the Sun or Moon). The primary mirror was made of beryllium
, and the secondary of fused silica – materials chosen for their light weight, moderate cost, and optical quality.
, and four detectors.
There were two Fine Error Sensors (FES), and their first purpose was to image the field of view of the telescope in visible light. It could detect stars down to 14th magnitude
, about 1500 times fainter than can be seen with the naked eye from Earth. The image was transmitted to the ground station, where the observer would verify that the telescope was pointing at the correct field, and then acquire the exact object to be observed. If the object to be observed was fainter than 14th magnitude, the observer would point the telescope at a star that could be seen, and then apply "blind" offsets, determined from the coordinates of the objects. The accuracy of the pointing was generally better than 2 arcseconds for blind offsets
The FES acquisition images were the telescope's only imaging capability; for UV observations, it only took spectra
. For this, it was equipped with two spectrographs. They were called the Short Wavelength Spectrograph and the Long Wavelength Spectrograph, and covered wavelength ranges of 115 to 200 nanometres and 185 to 330 nm respectively. Each spectrograph had both high and low resolution modes, with spectral resolution
s of 0.02 and 0.6 nm respectively.
The spectrographs could be used with either of two apertures. The larger aperture was a slot with a field of view roughly 10 × 20 arcsec; the smaller aperture was a circle about 3 arcsec in diameter. The quality of the telescope optics was such that point source
s appeared about 3 arcsec across, so use of the smaller aperture required very accurate pointing, and it did not necessarily capture all of the light from the object. The larger aperture was therefore most commonly used, and the smaller aperture only used when the larger field of view would have contained unwanted emission from other objects.
There were two cameras for each spectrograph, one designated the primary and the second being redundant in case of failure of the first. The cameras were named LWP, LWR, SWP and SWR where P stands for prime, R for redundant and LW/SW for long/short wavelength. The cameras were television cameras, sensitive only to visible light, and light gathered by the telescope and spectrographs first fell on a UV-to-visible converter. This was a caesium
-tellurium cathode, which was inert when exposed to visible light, but which gave off electrons when struck by UV photons due to the photoelectric effect
. The electrons were then detected by the TV cameras. The signal could be integrated for up to many hours, before being transmitted to Earth at the end of the exposure.
, Florida
on a Delta rocket
, on 26 January 1978. It was launched into a transfer orbit, from which its on-board rockets fired it into its planned geosynchronous orbit. The orbit was inclined by 28.6° to the Earth's equator, and had an orbital eccentricity
of 0.24, meaning that the satellite's distance from Earth varied between 25,669 km and 45,887 km. The ground track was initially centred at a longitude of approximately 70 degrees W.
, was taken for calibration purposes three days after launch. The first science observations targeted objects including the Moon
, the planets from Mars
to Uranus
, hot stars including Eta Carinae, cool giant stars including Epsilon Eridani
, the black hole
candidate Cygnus X-1
, and galaxies including M81 and M87
.
Then, the spacecraft systems were tested and optimised. The telescope was focussed, and the prime and redundant cameras in both channels were tested. It was found that the SWR camera did not work properly, and so the SWP camera was used throughout the mission. Initially, this camera suffered from significant electronic noise, but this was traced to a sensor used to align the telescope after launch. Once this sensor was switched off, the camera performed as expected. The cameras were then adjusted for best performance, and the slewing and guiding performance of the telescope evaluated and optimised
Finally, image quality and spectral resolution were studied and characterised, and the performance of the telescope, spectrographs and cameras were calibrated using observations of well-known star
s.
After these three phases were completed, the "routine phase" of operations began on 3 April 1978. Optimisation, evaluation and calibration operations were far from complete, but the telescope was understood well enough for routine science observations to begin.
If an astronomer was awarded time, then when their observations were scheduled, they would travel to the ground stations which operated the satellite, so that they could see and evaluate their data as it was taken. This mode of operation was very different to most space facilities, for which data is taken with no real time input from the astronomer concerned, and instead resembled the use of ground-based telescopes.
in Maryland
, and one from the ESA ground station at Villanueva de la Cañada
near Madrid
. Because of its elliptical orbit, the spacecraft spent part of each day in the Van Allen belts, during which time science observations suffered from higher background noise. This time occurred during the second US shift each day, and was generally used for calibration observations and spacecraft 'housekeeping', as well as for science observations that could be done with short exposure times.
The twice-daily transatlantic handovers required telephone contact between Spain and the US to coordinate the switch. Observations were not coordinated between the stations, so that the astronomers taking over after the handover would not know where the telescope would be pointing when their shift started. This sometimes meant that observing shifts started with a lengthy pointing manoeuvre, but allowed maximum flexibility in scheduling of observing blocks.
of 8 bits. The data was then transmitted to Earth via one of six transmitters on the spacecraft; four were S-band transmitters, placed at points around the spacecraft such that no matter what its attitude, one could transmit to the ground, and two were VHF transmitters, which could sustain a lower bandwidth
, but consumed less power, and also transmitted in all directions. The VHF transmitters were used when the spacecraft was in the Earth's shadow and thus reliant on battery power instead of solar power.
In normal operations, observers could hold the telescope in position and wait approximately 20 minutes for the data to be transmitted, if they wanted the option of repeating the observation, or they could slew to the next target and then start the data transmission to Earth while observing the next target.
The data transmitted were used for "quick look" purposes only, and full calibration was carried out by IUE staff later. Astronomers were then sent their data on magnetic tape
by post, about a week after processing. From the date of the observation, the observers had a six month proprietary period during which only they had access to the data. After six months, it became public.
ed scientific articles based on IUE data were published. Nine symposia of the International Astronomical Union
were devoted to discussions of IUE results.
except Mercury
were observed; the telescope could not point at any part of the sky within 45° of the Sun, and Mercury's greatest angular distance from the Sun
is only about 28°. IUE observations of Venus
showed that the amount of sulfur monoxide
and sulfur dioxide
in its atmosphere declined by a large amount during the 1980s. The reason for this decline is not yet fully understood, but one hypothesis is that a large volcanic eruption had injected sulfur compounds into the atmosphere, and that they were declining following the end of the eruption.
Halley's Comet reached perihelion in 1986, and was observed intensively with the IUE, as well as with a large number of other ground-based and satellite missions. UV spectra were used to estimate the rate at which the comet lost dust and gas, and the IUE observations allowed astronomers to estimate that a total of 3×108 ton
s of water
evaporated from the comet during its passage through the inner solar system.
s. A star that is hotter than about 10,000 K emits most of its radiation in the UV, and thus if it can only be studied in visible light, a large amount of information is being lost. The vast majority of all stars are cooler than the Sun
, but the fraction that is hotter includes massive, highly luminous stars which shed enormous quantities of matter into interstellar space, and also white dwarf
stars, which are the end stage of stellar evolution
for the vast majority of all stars and which have temperatures as high as 100,000 K when they first form.
The IUE discovered many instances of white dwarf companions to main sequence
stars. An example of this kind of system is Sirius
, and at visible wavelengths the main sequence star is far brighter than the white dwarf. However, in the UV, the white dwarf can be as bright or brighter, as its higher temperature means it emits most of its radiation at these shorter wavelengths. In these systems, the white dwarf was originally the heavier star, but has shed most of its mass during the later stages of its evolution. Binary stars provide the only direct way to measure the mass
of stars, from observations of their orbital motions. Thus, observations of binary stars where the two components are at such different stages of stellar evolution can be used to determine the relationship between the mass of stars and how they evolve.
Stars with masses of around ten times that of the Sun or higher have powerful stellar wind
s. The Sun loses about 10−14 solar masses per year in its solar wind
, which travels at up to around 750 km/s, but the massive stars can lose as much as a billion times more material each year in winds travelling at several thousand kilometres per second. These stars exist for a few million years, and during this time the stellar wind carries away a significant fraction of their mass, and plays a crucial role in determining whether they explode as supernovae or not. This stellar mass loss was first discovered using rocket-borne telescopes in the 1960s, but the IUE allowed astronomers to observe a very large number of stars, allowing the first proper studies of how stellar mass loss is related to mass and luminosity.
exploded as a supernova
. Designated SN 1987a
, this event was of enormous importance to astronomy, as it was the closest known supernova to Earth, and the first visible to the naked eye
, since Kepler's star in 1604 – before the invention of the telescope
. The opportunity to study a supernova so much more closely than had ever been possible before triggered intense observing campaigns at all major astronomical facilities, and the first IUE observations were made about 14 hours after the discovery of the supernova.
IUE data were used to determine that the progenitor star had been a blue supergiant
, where theory had strongly expected a red supergiant
. Hubble Space Telescope images revealed a nebula
surrounding the progenitor star which consisted of mass lost by the star long before it exploded; IUE studies of this material showed that it was rich in nitrogen
, which is formed in the CNO cycle
– a chain of nuclear reactions which produces most of the energy emitted by stars much more massive than the Sun. Astronomers inferred that the star had been a red supergiant, and had shed a large amount of matter into space, before evolving into a blue supergiant and exploding.
. The ISM is normally observed by looking at background sources such as hot stars or quasar
s; interstellar material absorbs some of the light from the background source and so its composition and velocity can be studied. One of IUE's early discoveries was that the Milky Way
is surrounded by a vast halo of hot gas, known as a galactic corona
. The hot gas, heated by cosmic ray
s and supernova
e, extends several thousand light years above and below the plane of the Milky Way.
IUE data was also crucial in determining how the light from distant sources is affected by dust along the line of sight. Almost all astronomical observations are affected by this interstellar extinction, and correcting for it is the first step in most analyses of astronomical spectra and images. IUE data was used to show that within the galaxy, interstellar extinction can be well described by a few simple equations. The relative variation of extinction with wavelength shows little variation with direction; only the absolute amount of absorption changes. Interstellar absorption in other galaxies can similarly be described by fairly simple 'laws'.
One particular target was NGC 4151
, the brightest Seyfert galaxy
. Starting soon after IUE's launch, a group of European astronomers pooled their observing time to repeatedly observe the galaxy, to measure variations over time of its UV emission. They found that the UV variation was much greater than that seen at optical and infrared wavelengths. IUE observations were used to study the black hole
at the centre of the galaxy, with its mass being estimated at between 50 and 100 million times that of the Sun. The UV emission varied on timescales of a few days, implying that the region of emission was only a few light days across.
Quasar observations were used to probe intergalactic space. Clouds of hydrogen
gas in between the Earth and a given quasar will absorb some of its emission at the wavelength of Lyman alpha. Because the clouds and the quasar are all at different distances from Earth, and moving at different velocities due to the expansion of the universe, the quasar spectrum has a "forest" of absorption features at wavelengths shorter than its own Lyman alpha emission. Before IUE, observations of this so-called Lyman-alpha forest were limited to very distant quasars, for which the redshift
caused by the expansion of the universe brought it into optical wavelengths. IUE allowed nearer quasars to be studied, and astronomers used this data to determine that there are fewer hydrogen clouds in the nearby universe than there are in the distant universe. The implication is that over time, these clouds have formed into galaxies.
s to stabilise the spacecraft. Successive failures of these in 1979, 1982, 1983, 1985 and 1996 ultimately left the spacecraft with a single functional gyro. Telescope control was maintained with two gyros by using the telescope's Sun sensor to determine the spacecraft's attitude, and stabilisation in three axes proved possible even after the fifth failure, by using the Sun sensor, the Fine Error Sensors and the single remaining gyro. Most other parts of the telescope systems remained fully functional throughout the mission.
In 1995, budget concerns at NASA almost led to the termination of the mission, but instead the operations responsibilities were redivided, with ESA taking control for 16 hours a day, and GSFC for the remaining 8 only. The ESA 16 hours was used for science operations, while the GSFC 8 hours was used only for maintenance. In February 1996, further budget cuts led ESA to decide that it would no longer maintain the satellite. Operations ceased in September of that year, and on 30 September all the remaining hydrazine
was discharged, the batteries were drained and switched off, and at 1844 UT, the radio transmitter was shut down and all contact with the spacecraft was lost.
It continues to orbit the Earth in its geosynchronous orbit, and will continue to do so more or less indefinitely as it is far above the upper reaches of the Earth's atmosphere
. Anomalies in the Earth's gravity due to its non-spherical shape meant that the telescope tended to drift west from its original location at approximately 70°W longitude towards approximately 110°W. During the mission, this drift was corrected by occasional rocket firings, but since the end of the mission the satellite has drifted uncontrolled to the west of its former location.
and fast global data transmission links, access to the archive required a visit in person to one of two Regional Data Analysis Facilities (RDAFs), one at the University of Colorado
and the other at GSFC.
In 1987 it became possible to access the archive electronically, by dialling in to a computer at Goddard. The archive, then totalling 23 Gb of data, was connected to the computer on a mass storage device. A single user at a time could dial in, and would be able to retrieve an observation in 10–30 seconds.
As the mission entered its second decade, plans were made for its final archive. Throughout the mission, calibration techniques were improved, and the final software for data reduction yielded significant improvements over earlier calibrations. Eventually, the entire set of available raw data was recalibrated using the final version of the data reduction software, creating a uniform high quality archive. Today, the archive is hosted at the Space Telescope Science Institute
and is available via the World Wide Web.
projects worldwide. Almost 4,000 peer-reviewed papers have now been published based on IUE data, including some of the most cited astronomy papers of all time. The most cited paper based on IUE data is one analysing the nature of interstellar reddening, which has subsequently been cited over 4,000 times. For comparison, the Hubble Space Telescope
has now been in orbit for 21 years (as of 2011) and Hubble data has been used in almost 10,000 peer-reviewed publications.
Observatory
An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
primarily designed to take ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. The satellite was a collaborative project between NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
, the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
Science Research Council
Science and Engineering Research Council
The Science and Engineering Research Council used to be the UK agency in charge of publicly funded scientific and engineering research activities including astronomy, biotechnology and biological sciences, space research and particle physics...
and the European Space Agency (ESA). The mission was first proposed in early 1964, by a group of scientists in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, and was launched on January 26, 1978 aboard a NASA Delta rocket
Delta rocket
Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...
. The mission lifetime was initially set for 3 years, but in the end it lasted almost 18 years, with the satellite being shut down in 1996. The switch-off occurred for financial reasons, while the telescope was still functioning at near original efficiency.
It was the first space observatory to be operated in real time by astronomers who visited the groundstations in the United States and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. Astronomers made over 104,000 observations using the IUE, of objects ranging from solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
bodies to distant quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s. Among the significant scientific results from IUE data were the first large scale studies of stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s, accurate measurements of the way interstellar dust absorbs light, and measurements of the supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
SN1987a which showed that it defied stellar evolution theories as they then stood. When the mission ended, it was considered the most successful astronomical satellite ever.
Motivation
The human eyeHuman eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
can perceive light with wavelengths between roughly 350 (violet) and 700 (red) nanometres. Ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light has wavelengths between roughly 10nm and 350nm. UV light can be harmful to human beings, and is strongly absorbed by the ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
. This makes it impossible to observe UV emission from astronomical objects from the ground. Many types of object emit copious quantities of UV radiation, though: the hottest and most massive stars in the universe can have surface temperatures high enough that the vast majority of their light is emitted in the UV. Active Galactic Nuclei, accretion disks, and supernovae all emit UV radiation strongly, and many chemical elements have strong absorption lines in the UV, so that UV absorption by the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
provides a powerful tool for studying its composition.
UV astronomy was impossible before the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...
, and some of the first space telescopes were UV telescopes designed to observe this previously inaccessible region of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. One particular success was the second Orbiting Astronomical Observatory
Orbiting Astronomical Observatory
The Orbiting Astronomical Observatory satellites were a series of four American space observatories launched by NASA between 1966 and 1972, which provided the first high-quality observations of many objects in ultraviolet light...
, which had a number of 20cm UV telescopes on board. It was launched in 1968, and took the first UV observations of 1200 objects, mostly stars. The success of OAO-2 motivated astronomers to consider larger missions.
Conception
Robert Wilson
Robert Wilson may refer to:In politics:* Rob Wilson , British politician and entrepreneur, MP for Reading East* Robert J. Wilson, candidate in the 1953 Manitoba provincial election* Robert John Wilson, Member of Parliament for Jarrow...
. The European Space Research Organisation was planning a Large Astronomical Satellite, and had sought proposals from the astronomical community for its aims and design. Wilson headed a British team which proposed an ultraviolet spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
, and their design was recommended for acceptance in 1966.
However, management problems and cost overruns led to the cancellation of the LAS program in 1968. Wilson's team scaled down their plans and submitted a more modest proposal to ESRO, but this was also rejected on cost grounds. Rather than give up on the idea of an orbiting UV telescope, they instead sent their plans to NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
administrator Leo Goldberg
Leo Goldberg
Leo Goldberg was an American astronomer who held professorships at Harvard and the University of Michigan and the directorships of several major observatories. He was president of both the International Astronomical Union and the American Astronomical Society...
, and in 1973 the plans were approved. The proposed telescope was renamed the International Ultraviolet Explorer.
Design and aims
The telescope was designed from the start to be operated in real time, rather than by remote control. This required that it would be launched into a geosynchronous orbitGeosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
– that is, one with a period equal to one sidereal day of 23h 56m. A satellite in such an orbit remains visible from a given point on the Earth's surface for many hours at a time, and can thus transmit to a single ground station for a long period of time. Most space observatories in Earth orbit, such as the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, are in a low orbit in which they spend most of their time operating autonomously because only a small fraction of the Earth's surface can see them at a given time. Hubble, for example, orbits the Earth at an altitude of approximately 600 km, while a geosynchronous orbit has an average altitude of 36,000 km.
As well as allowing continuous communication with ground stations, a geosynchronous orbit also allows a larger portion of the sky to be viewed continuously. Because the distance from Earth is greater, the Earth occupies a much smaller portion of the sky as seen from the satellite than it does from low Earth orbit.
A launch into a geosynchronous orbit requires much more energy for a given weight of payload than a launch into low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
. This meant that the telescope had to be relatively small, with a 45cm primary mirror, and a total weight of 312kg. Hubble, in comparison, weighs 11.1 tonnes and has a 2.4 m mirror. The largest ground-based telescope, the Gran Telescopio Canarias
Gran Telescopio Canarias
The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
, has a primary mirror 10.4 m across. A smaller mirror means less light-gathering power, and less spatial resolution, compared to a larger mirror.
The stated aims of the telescope at the start of the mission were:
- To obtain high-resolution spectra of stars of all spectral types to determine their physical characteristics
- To study gas streams in and around binary star system
- To observe faint stars, galaxies and quasars at low resolution, interpreting these spectra by reference to high-resolution spectra
- To observe the spectra of planets and comets
- To make repeated observations of objects with variable spectra
- To study the modification of starlight caused by interstellar dust and gas
Construction and engineering
The telescope was constructed as a joint project between NASA, ESRO (which became ESA in 1975) and the UK Science and Engineering Research CouncilScience and Engineering Research Council
The Science and Engineering Research Council used to be the UK agency in charge of publicly funded scientific and engineering research activities including astronomy, biotechnology and biological sciences, space research and particle physics...
. SERC provided the Vidicon cameras for the spectrographs as well as software for the scientific instruments. ESA provided the solar array
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...
s to power the spacecraft as well as a ground observing facility in Villafranca del Castillo
VILSPA
The European Space Astronomy Centre is the ESA's centre for space science, which means Astronomy as well as Solar System exploration...
, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
. NASA contributed the telescope, spectrograph, and spacecraft as well as launching facilities and a second ground observatory in Greenbelt, Maryland
Greenbelt, Maryland
Greenbelt is a city in Prince George's County, Maryland, United States. Contained within today's City of Greenbelt is the historic planned community now known locally as "Old Greenbelt" and designated as the Greenbelt Historic District...
at the Goddard Space Flight Center.
According to the agreement setting up the project the observing time would be divided between the contributing agencies with 2/3 to NASA, 1/6 to ESA and 1/6 to the UK's Science Research Council.
Mirror
The telescope mirror was a reflector of the Ritchey-Chretien type, which has hyperbolicHyperbolic
Hyperbolic refers to something related to or in shape of hyperbola , or to something employing the literary device of hyperbole .The following topics are based on the hyperbola etymology:...
primary and secondary mirrors. The primary was 45cm across. The telescope was designed to give high quality images over a 16 arcminute field of view (about half the apparent diameter of the Sun or Moon). The primary mirror was made of beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, and the secondary of fused silica – materials chosen for their light weight, moderate cost, and optical quality.
Instruments
The instrumentation on board consisted of the Fine Error Sensors (FES), which were used for pointing and guiding the telescope, a high resolution and a low resolution spectrographSpectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
, and four detectors.
There were two Fine Error Sensors (FES), and their first purpose was to image the field of view of the telescope in visible light. It could detect stars down to 14th magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
, about 1500 times fainter than can be seen with the naked eye from Earth. The image was transmitted to the ground station, where the observer would verify that the telescope was pointing at the correct field, and then acquire the exact object to be observed. If the object to be observed was fainter than 14th magnitude, the observer would point the telescope at a star that could be seen, and then apply "blind" offsets, determined from the coordinates of the objects. The accuracy of the pointing was generally better than 2 arcseconds for blind offsets
The FES acquisition images were the telescope's only imaging capability; for UV observations, it only took spectra
Spectra
Spectra are conditions or values that vary over a continuum.Spectra may also refer to:* Kia Spectra, a car developed by Kia Motors from 2000-present* Optare Spectra, a bus body built by Optare...
. For this, it was equipped with two spectrographs. They were called the Short Wavelength Spectrograph and the Long Wavelength Spectrograph, and covered wavelength ranges of 115 to 200 nanometres and 185 to 330 nm respectively. Each spectrograph had both high and low resolution modes, with spectral resolution
Spectral resolution
The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...
s of 0.02 and 0.6 nm respectively.
The spectrographs could be used with either of two apertures. The larger aperture was a slot with a field of view roughly 10 × 20 arcsec; the smaller aperture was a circle about 3 arcsec in diameter. The quality of the telescope optics was such that point source
Point source
A point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...
s appeared about 3 arcsec across, so use of the smaller aperture required very accurate pointing, and it did not necessarily capture all of the light from the object. The larger aperture was therefore most commonly used, and the smaller aperture only used when the larger field of view would have contained unwanted emission from other objects.
There were two cameras for each spectrograph, one designated the primary and the second being redundant in case of failure of the first. The cameras were named LWP, LWR, SWP and SWR where P stands for prime, R for redundant and LW/SW for long/short wavelength. The cameras were television cameras, sensitive only to visible light, and light gathered by the telescope and spectrographs first fell on a UV-to-visible converter. This was a caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
-tellurium cathode, which was inert when exposed to visible light, but which gave off electrons when struck by UV photons due to the photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
. The electrons were then detected by the TV cameras. The signal could be integrated for up to many hours, before being transmitted to Earth at the end of the exposure.
Mission
Launch
The IUE was launched from Cape CanaveralCape Canaveral
Cape Canaveral, from the Spanish Cabo Cañaveral, is a headland in Brevard County, Florida, United States, near the center of the state's Atlantic coast. Known as Cape Kennedy from 1963 to 1973, it lies east of Merritt Island, separated from it by the Banana River.It is part of a region known as the...
, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
on a Delta rocket
Delta rocket
Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...
, on 26 January 1978. It was launched into a transfer orbit, from which its on-board rockets fired it into its planned geosynchronous orbit. The orbit was inclined by 28.6° to the Earth's equator, and had an orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of 0.24, meaning that the satellite's distance from Earth varied between 25,669 km and 45,887 km. The ground track was initially centred at a longitude of approximately 70 degrees W.
Commissioning
The first 60 days of the mission were designated as the commissioning period. This was divided into three main stage. Firstly, as soon as its instruments were switched on, the IUE observed a small number of high priority objects, to ensure that some data had been taken in the event of an early failure. The first spectrum, of the star Eta Ursae MajorisEta Ursae Majoris
Eta Ursae Majoris is a star in the constellation Ursa Major. It has the traditional names Alkaid and Benetnash ....
, was taken for calibration purposes three days after launch. The first science observations targeted objects including the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, the planets from Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
to Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, hot stars including Eta Carinae, cool giant stars including Epsilon Eridani
Epsilon Eridani
Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73...
, the black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
candidate Cygnus X-1
Cygnus X-1
Cygnus X-1 is a well-known galactic X-ray source in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3 Wm−2Hz−1...
, and galaxies including M81 and M87
M87
M87 or M-87 may refer to:* Messier 87, a giant elliptical galaxy in the Virgo Cluster* M87 machine gun, a Yugoslav copy of the NSVT machine gun* Tumansky M-87, a Soviet aircraft engine* M-87 Orkan, a Yugoslav rocket-artillery vehicle...
.
Then, the spacecraft systems were tested and optimised. The telescope was focussed, and the prime and redundant cameras in both channels were tested. It was found that the SWR camera did not work properly, and so the SWP camera was used throughout the mission. Initially, this camera suffered from significant electronic noise, but this was traced to a sensor used to align the telescope after launch. Once this sensor was switched off, the camera performed as expected. The cameras were then adjusted for best performance, and the slewing and guiding performance of the telescope evaluated and optimised
Finally, image quality and spectral resolution were studied and characterised, and the performance of the telescope, spectrographs and cameras were calibrated using observations of well-known star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s.
After these three phases were completed, the "routine phase" of operations began on 3 April 1978. Optimisation, evaluation and calibration operations were far from complete, but the telescope was understood well enough for routine science observations to begin.
Usage
Use of the telescope was divided between NASA, ESA and SERC in approximate proportion to their relative contributions to the satellite construction: two thirds of the time was available to NASA, and one sixth each to ESA and SERC. Telescope time was obtained by submitting proposals, which were reviewed annually. Each of the three agencies considered applications separately for its allocated observing time. Astronomers of any nationality could apply for telescope time, choosing whichever agency they preferred to apply to.If an astronomer was awarded time, then when their observations were scheduled, they would travel to the ground stations which operated the satellite, so that they could see and evaluate their data as it was taken. This mode of operation was very different to most space facilities, for which data is taken with no real time input from the astronomer concerned, and instead resembled the use of ground-based telescopes.
Ground support
For most of its lifetime, the telescope was operated in three eight-hour shifts each day, two from the US ground station at the Goddard Space Flight CenterGoddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
in Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
, and one from the ESA ground station at Villanueva de la Cañada
Villanueva de la Cañada
Villanueva de la Cañada is a municipality in Spain, located 30 km west of the city of Madrid.Population : 14 809.-General references:*- External links :...
near Madrid
Madrid
Madrid is the capital and largest city of Spain. The population of the city is roughly 3.3 million and the entire population of the Madrid metropolitan area is calculated to be 6.271 million. It is the third largest city in the European Union, after London and Berlin, and its metropolitan...
. Because of its elliptical orbit, the spacecraft spent part of each day in the Van Allen belts, during which time science observations suffered from higher background noise. This time occurred during the second US shift each day, and was generally used for calibration observations and spacecraft 'housekeeping', as well as for science observations that could be done with short exposure times.
The twice-daily transatlantic handovers required telephone contact between Spain and the US to coordinate the switch. Observations were not coordinated between the stations, so that the astronomers taking over after the handover would not know where the telescope would be pointing when their shift started. This sometimes meant that observing shifts started with a lengthy pointing manoeuvre, but allowed maximum flexibility in scheduling of observing blocks.
Data transmission
Data was transmitted to Earth in real time at the end of each science observation. The camera read-out formed an image of 768×768 pixels, and the analogue-to-digital converter resulted in a dynamic rangeDynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of 8 bits. The data was then transmitted to Earth via one of six transmitters on the spacecraft; four were S-band transmitters, placed at points around the spacecraft such that no matter what its attitude, one could transmit to the ground, and two were VHF transmitters, which could sustain a lower bandwidth
Bandwidth
Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context...
, but consumed less power, and also transmitted in all directions. The VHF transmitters were used when the spacecraft was in the Earth's shadow and thus reliant on battery power instead of solar power.
In normal operations, observers could hold the telescope in position and wait approximately 20 minutes for the data to be transmitted, if they wanted the option of repeating the observation, or they could slew to the next target and then start the data transmission to Earth while observing the next target.
The data transmitted were used for "quick look" purposes only, and full calibration was carried out by IUE staff later. Astronomers were then sent their data on magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
by post, about a week after processing. From the date of the observation, the observers had a six month proprietary period during which only they had access to the data. After six months, it became public.
Scientific results
The IUE allowed astronomers their first view of the ultraviolet light from many celestial objects, and was used to study objects ranging from solar system planets to distant quasars. During its lifetime, hundreds of astronomers observed with IUE, and during its first decade of operations, over 1500 peer reviewPeer review
Peer review is a process of self-regulation by a profession or a process of evaluation involving qualified individuals within the relevant field. Peer review methods are employed to maintain standards, improve performance and provide credibility...
ed scientific articles based on IUE data were published. Nine symposia of the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
were devoted to discussions of IUE results.
Solar system
All the planets in the Solar SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
except Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
were observed; the telescope could not point at any part of the sky within 45° of the Sun, and Mercury's greatest angular distance from the Sun
Elongation
In astronomy, a planet's elongation is the angle between the Sun and the planet, as viewed from Earth. Elongations occur when an inner planet’s position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth’s orbits their positions as viewed...
is only about 28°. IUE observations of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
showed that the amount of sulfur monoxide
Sulfur monoxide
Sulfur monoxide is an inorganic compound with formula . It is only found as a dilute gas phase. When concentrated or condensed, it converts to S2O2 . It has been detected in space but is rarely encountered intact otherwise.-Structure and bonding:The SO molecule has a triplet ground state similar...
and sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
in its atmosphere declined by a large amount during the 1980s. The reason for this decline is not yet fully understood, but one hypothesis is that a large volcanic eruption had injected sulfur compounds into the atmosphere, and that they were declining following the end of the eruption.
Halley's Comet reached perihelion in 1986, and was observed intensively with the IUE, as well as with a large number of other ground-based and satellite missions. UV spectra were used to estimate the rate at which the comet lost dust and gas, and the IUE observations allowed astronomers to estimate that a total of 3×108 ton
Ton
The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...
s of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
evaporated from the comet during its passage through the inner solar system.
Stars
Some of the most significant results from IUE came in the studies of hot starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s. A star that is hotter than about 10,000 K emits most of its radiation in the UV, and thus if it can only be studied in visible light, a large amount of information is being lost. The vast majority of all stars are cooler than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, but the fraction that is hotter includes massive, highly luminous stars which shed enormous quantities of matter into interstellar space, and also white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
stars, which are the end stage of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
for the vast majority of all stars and which have temperatures as high as 100,000 K when they first form.
The IUE discovered many instances of white dwarf companions to main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
stars. An example of this kind of system is Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, and at visible wavelengths the main sequence star is far brighter than the white dwarf. However, in the UV, the white dwarf can be as bright or brighter, as its higher temperature means it emits most of its radiation at these shorter wavelengths. In these systems, the white dwarf was originally the heavier star, but has shed most of its mass during the later stages of its evolution. Binary stars provide the only direct way to measure the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of stars, from observations of their orbital motions. Thus, observations of binary stars where the two components are at such different stages of stellar evolution can be used to determine the relationship between the mass of stars and how they evolve.
Stars with masses of around ten times that of the Sun or higher have powerful stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s. The Sun loses about 10−14 solar masses per year in its solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, which travels at up to around 750 km/s, but the massive stars can lose as much as a billion times more material each year in winds travelling at several thousand kilometres per second. These stars exist for a few million years, and during this time the stellar wind carries away a significant fraction of their mass, and plays a crucial role in determining whether they explode as supernovae or not. This stellar mass loss was first discovered using rocket-borne telescopes in the 1960s, but the IUE allowed astronomers to observe a very large number of stars, allowing the first proper studies of how stellar mass loss is related to mass and luminosity.
SN 1987A
In 1987, a star in the Large Magellanic CloudLarge Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
. Designated SN 1987a
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
, this event was of enormous importance to astronomy, as it was the closest known supernova to Earth, and the first visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
, since Kepler's star in 1604 – before the invention of the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
. The opportunity to study a supernova so much more closely than had ever been possible before triggered intense observing campaigns at all major astronomical facilities, and the first IUE observations were made about 14 hours after the discovery of the supernova.
IUE data were used to determine that the progenitor star had been a blue supergiant
Blue supergiant
Blue supergiants are supergiant stars of spectral type O or B.They are extremely hot and bright, with surface temperatures of 30,000-50,000 K. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii up to about 25 solar radii...
, where theory had strongly expected a red supergiant
Red supergiant
Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...
. Hubble Space Telescope images revealed a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
surrounding the progenitor star which consisted of mass lost by the star long before it exploded; IUE studies of this material showed that it was rich in nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, which is formed in the CNO cycle
CNO cycle
The CNO cycle is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars...
– a chain of nuclear reactions which produces most of the energy emitted by stars much more massive than the Sun. Astronomers inferred that the star had been a red supergiant, and had shed a large amount of matter into space, before evolving into a blue supergiant and exploding.
The Interstellar Medium
The IUE was used extensively to investigate the interstellar mediumInterstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. The ISM is normally observed by looking at background sources such as hot stars or quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s; interstellar material absorbs some of the light from the background source and so its composition and velocity can be studied. One of IUE's early discoveries was that the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
is surrounded by a vast halo of hot gas, known as a galactic corona
Galactic corona
The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way...
. The hot gas, heated by cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s and supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e, extends several thousand light years above and below the plane of the Milky Way.
IUE data was also crucial in determining how the light from distant sources is affected by dust along the line of sight. Almost all astronomical observations are affected by this interstellar extinction, and correcting for it is the first step in most analyses of astronomical spectra and images. IUE data was used to show that within the galaxy, interstellar extinction can be well described by a few simple equations. The relative variation of extinction with wavelength shows little variation with direction; only the absolute amount of absorption changes. Interstellar absorption in other galaxies can similarly be described by fairly simple 'laws'.
Active Galactic Nuclei
The IUE vastly increased astronomers' understanding of active galactic nuclei (AGN). Before its launch, 3C 273, the first known quasar, was the only AGN that had ever been observed at UV wavelengths. With IUE, UV spectra of AGN became widely available.One particular target was NGC 4151
NGC 4151
NGC 4151 is an intermediate spiral Seyfert galaxy located 43 million light years from Earth in the constellation Canes Venatici, discovered by Frederick William Herschel on March 17, 1787...
, the brightest Seyfert galaxy
Seyfert galaxy
Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas, named after Carl Keenan Seyfert, the astronomer who first identified the class in 1943...
. Starting soon after IUE's launch, a group of European astronomers pooled their observing time to repeatedly observe the galaxy, to measure variations over time of its UV emission. They found that the UV variation was much greater than that seen at optical and infrared wavelengths. IUE observations were used to study the black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
at the centre of the galaxy, with its mass being estimated at between 50 and 100 million times that of the Sun. The UV emission varied on timescales of a few days, implying that the region of emission was only a few light days across.
Quasar observations were used to probe intergalactic space. Clouds of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas in between the Earth and a given quasar will absorb some of its emission at the wavelength of Lyman alpha. Because the clouds and the quasar are all at different distances from Earth, and moving at different velocities due to the expansion of the universe, the quasar spectrum has a "forest" of absorption features at wavelengths shorter than its own Lyman alpha emission. Before IUE, observations of this so-called Lyman-alpha forest were limited to very distant quasars, for which the redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
caused by the expansion of the universe brought it into optical wavelengths. IUE allowed nearer quasars to be studied, and astronomers used this data to determine that there are fewer hydrogen clouds in the nearby universe than there are in the distant universe. The implication is that over time, these clouds have formed into galaxies.
Mission termination
The IUE was designed to have a minimum lifetime of three years, and carried consumable sufficient for a five year mission. However, it lasted far longer than its design called for. Occasional hardware failures caused difficulties, but innovative techniques were devised to overcome them. For example, the spacecraft was equipped with six gyroGyro
- Science and technology :* An abbreviation for gyroscope, an orientation-stabilizing device* An abbreviation for autogyro, a type of rotary-wing aircraft* The casually used brand name of a detangler mechanism, part of a stunt-adapted BMX bicycle...
s to stabilise the spacecraft. Successive failures of these in 1979, 1982, 1983, 1985 and 1996 ultimately left the spacecraft with a single functional gyro. Telescope control was maintained with two gyros by using the telescope's Sun sensor to determine the spacecraft's attitude, and stabilisation in three axes proved possible even after the fifth failure, by using the Sun sensor, the Fine Error Sensors and the single remaining gyro. Most other parts of the telescope systems remained fully functional throughout the mission.
In 1995, budget concerns at NASA almost led to the termination of the mission, but instead the operations responsibilities were redivided, with ESA taking control for 16 hours a day, and GSFC for the remaining 8 only. The ESA 16 hours was used for science operations, while the GSFC 8 hours was used only for maintenance. In February 1996, further budget cuts led ESA to decide that it would no longer maintain the satellite. Operations ceased in September of that year, and on 30 September all the remaining hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
was discharged, the batteries were drained and switched off, and at 1844 UT, the radio transmitter was shut down and all contact with the spacecraft was lost.
It continues to orbit the Earth in its geosynchronous orbit, and will continue to do so more or less indefinitely as it is far above the upper reaches of the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. Anomalies in the Earth's gravity due to its non-spherical shape meant that the telescope tended to drift west from its original location at approximately 70°W longitude towards approximately 110°W. During the mission, this drift was corrected by occasional rocket firings, but since the end of the mission the satellite has drifted uncontrolled to the west of its former location.
Archives
The IUE archive is one of the most heavily used astronomical archives. Data was archived from the start of the mission, and access to the archive was free to anyone who wished to use it. However, in the early years of the mission, long before the advent of the World Wide WebWorld Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
and fast global data transmission links, access to the archive required a visit in person to one of two Regional Data Analysis Facilities (RDAFs), one at the University of Colorado
University of Colorado
The University of Colorado system is a system of public universities in Colorado consisting of three universities in four campuses: University of Colorado Boulder, University of Colorado Colorado Springs, and University of Colorado Denver in downtown Denver and at the Anschutz Medical Campus in...
and the other at GSFC.
In 1987 it became possible to access the archive electronically, by dialling in to a computer at Goddard. The archive, then totalling 23 Gb of data, was connected to the computer on a mass storage device. A single user at a time could dial in, and would be able to retrieve an observation in 10–30 seconds.
As the mission entered its second decade, plans were made for its final archive. Throughout the mission, calibration techniques were improved, and the final software for data reduction yielded significant improvements over earlier calibrations. Eventually, the entire set of available raw data was recalibrated using the final version of the data reduction software, creating a uniform high quality archive. Today, the archive is hosted at the Space Telescope Science Institute
Space Telescope Science Institute
The Space Telescope Science Institute is the science operations center for the Hubble Space Telescope and for the James Webb Space Telescope...
and is available via the World Wide Web.
Impact on astronomy
The IUE mission, by virtue of its very long duration and the fact that for most of its lifetime it provided astronomers' only access to UV light, had a major impact on astronomy. By the end of its mission it was considered by far the most successful and productive space observatory mission. For many years after the end of the mission, its archive was the most heavily used dataset in astronomy, and IUE data has been used in over 250 PhDPHD
PHD may refer to:*Ph.D., a doctorate of philosophy*Ph.D. , a 1980s British group*PHD finger, a protein sequence*PHD Mountain Software, an outdoor clothing and equipment company*PhD Docbook renderer, an XML renderer...
projects worldwide. Almost 4,000 peer-reviewed papers have now been published based on IUE data, including some of the most cited astronomy papers of all time. The most cited paper based on IUE data is one analysing the nature of interstellar reddening, which has subsequently been cited over 4,000 times. For comparison, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
has now been in orbit for 21 years (as of 2011) and Hubble data has been used in almost 10,000 peer-reviewed publications.