The International Ultraviolet Explorer (IUE)

The International Ultraviolet Explorer (IUE) satellite is a collaborative project between the National Aeronautics and Space Administration (NASA), the European Space Agency, and the United Kingdom. The satellite, launched in January 1978 with a five-year mission goal, is still healthy and functioning more than 15 years later. The satellite carries a telescope and instruments for astronomical research. The instruments are used to obtain ultraviolet (UV) spectra of a wide variety of astronomical objects.

UV radiation is light created by processes more energetic than those that produce visible light. For example, the light one sees from the Sun is produced at the solar surface, at a temperature of about 10,340 degrees Fahrenheit (6000 degrees Kelvin).The sun also produces ultraviolet light, from the much hotter gases that lie above the surface, at temperatures of 17,540 to 179,540 degrees Fahrenheit (10,000 to 100,000 degrees Kelvin).

Why Build A Satellite Instead Of Using Telescopes On Earth?

The same ozone layer that protects life on Earth from the harmful effects of solar ultraviolet radiation also makes observing other sources of UV radiation nearly impossible from the ground. Therefore, the only way to efficiently observe UV radiation from astronomical objects is to put the detectors above the Earth's atmosphere.

Until the early 1930s when radio signals first were detected from cosmic objects, only objects detectable in the visible region of the spectrum were known. The advent of the space age in the 1960s enabled astronomers to make observations at a whole new range of wavelengths. Progress was rapid, and much new data were obtained, particularly at ultraviolet, X-ray and gamma-ray wavelengths .

The very early scientific experiments were carried out using rockets which stayed above the Earth for only a few minutes. In spite of this, the results were encouraging and new and unexpected phenomena were identified. The next step was to place observatories above the atmosphere; in other words, to use spacecraft as observing platforms.

Why Study Objects In The Ultraviolet Part Of The Spectrum?

IUE is a means of finding out about activities in the Universe which only can be detected at ultraviolet wavelengths and by broadening our knowledge of events which also can be detected by other methods.

Astronomers can't place their subjects in a laboratory for testing, so they must learn all they can from the light emitted or absorbed by an astronomical object. Many techniques are used to squeeze information from a spectrum.

The spectrum contains features, either dark absorption lines or bright emission lines, characteristic of various atoms, ions, and molecules. By identifying and measuring individual lines, an astronomer can determine the composition of a star, planet, nebula, or galaxy.

What Is Studied By Ultraviolet Astronomy?

All objects known to exist in our universe, from the nearest planet and most tenuous comet in our solar system to the farthest and most massive of the enigmatic quasars, which are probably the most distant and the oldest extragalactic objects we have observed to date, can be studied effectively in the ultraviolet region of the spectrum.

Ultraviolet astronomy also provides interesting information about the interstellar medium --the space between the stars. Although one tends to think of this space as a sort of vacuum, it contains the material from which all stars are formed.

Observations at ultraviolet wavelengths have shown that the very low-density material found in the interstellar medium is made in fact of rather similar composition throughout the galaxy, but that its distribution is far from homogenous. In fact, big gas bubbles have been blown into it by very strong solar winds emanating from the hot stars to be found in groups of recently formed stars.

In the same way as our sun is losing matter from its atmosphere through the solar wind, the atmosphere of younger and more massive stars is blown away by similar solar winds.

Knowledge of these winds is very important in developing our ideas of stellar evolution. Observations in the ultraviolet show that material from some stars are blown off in irregular bursts and not in a steady flow as was first imagined. Observations of the shift of spectral lines relative to the lines emitted by atoms at rest permit astronomers to determine the speed at which atoms are arriving or receding. This technique has been used to show that in some cases matter is first blown away from a star and later some of the matter returns to the star's surface. This outward and return flow is very similar to that seen in solar flares on our own sun.

It has been found that the ultraviolet wavelength region is the most appropriate for studying many phenomena occurring in distant galaxies. Galaxies are known to be made up of millions of stars. However, some galaxies harbor in their nucleus objects that emit more energy than all the stars in the rest of the galaxy together. Very little is known about those objects at present. Observations in the ultraviolet wavelength indicate that such objects are rather small and the fact that they generate such enormous quantities of energy puzzles astronomers. These objects are some of the strangest phenomena observed to date in the universe and probably are closely related to the quasars, which are considered to represent the light beacons for the very early stages of the formation of the universe.

What Is The IUE Mission, And How Did It Originate?

The International Ultraviolet Explorers a joint project of NASA, ESA and the UK Science and Engineering Research Council (SERC), is an example of international scientific collaboration. SERC provided the on-board cameras with associated software and design inputs for the scientific instrumentation and sunshade. NASA provided the spacecraft, the rest of the scientific instrumentation, the launch vehicle, and the ground observatory at the Goddard Space Flight Center (GSFC), Greenbelt, Md. ESA was responsible for the solar panels and the ground observatory at Villafranca, Spain, near Madrid.

The IUE mission provides an observation facility for ultraviolet spectrometry of the celestial objects for astronomers from all countries. The IUE satellite, which carries a 45-centimeter diameter telescope, was launched into a geosynchronous orbit in January 1978 and, after a check-out and calibration period of two months, was opened as an astronomical facility for scientists worldwide. Even after 15 years in operation, the demand for observation time far exceeds the time available.

As a space astronomy facility, IUE is unique in that it is accessible to the general scientific community without training in space techniques. Because of its geosynchronous orbit, the spacecraft is visible for many hours to a single ground station and complicated scheduling of overhead passes is avoided. Thus a ground-station facility permits satellite-based observations to be made in much the same manner as those from a ground-based observatory. Guest astronomers coming to NASA's Goddard or to the ESA Villafranca Satellite Tracking Station (VILSPA) find themselves in a familiar environment where they are able to make real time decisions about their observing programs and thus can be sure of maximum scientific return from their observing time.

Who Has Access To The Satellite, And What Have Been The Results?

Scientific operations are performed for 16 hours a day from GSFC and eight hours a day from VILSPA. There are teams of resident astronomers and telescope operators at both centers assisting the guest observers to obtain their observations in real time.

Guest observers are selected on the basis of their response to annual call for proposals. These proposals are descriptions of observations planned and contain an explanation of the importance of the expected results for astrophysics. Normally, some 600 proposals are submitted to NASA and ESA by astronomers from all over the world. The granting of time on the telescope is done solely on the basis of scientific merit, as judged by two independent peer review panels. More than 2,000 astronomers have used the NASA observatory at GSFC and the ESA observatory at VILSPA as guest observers over the past decade and a half. The results of these observations have been described in more than 2,500 scientific papers in the major refereed astronomical journals at the time of the fifteenth anniversary of the operational life of the IUE. This makes IUE the most productive and successful telescope of all time.

Some of lUE's discoveries and research results are:

• Comparative studies from aurorae from Jupiter, Saturn, and Uranus.

• Discovery of molecular sulfur emission from the nucleus of a comet.

• Identification of many stars with magnetic fields and surface activity.

• Identification of previously unknown star companions to cool stars.

• Measurements of stellar "winds."

• Mapping of low-density gas bubbles around the Sun and nearby stars.

• Supernova observations in the Large Magellanic Cloud (a nearby galaxy).

• Measurements of the composition of planetary nebulae.

• Discovery of hot (179,540 degrees Fahrenheit) (100,000 degrees Kelvin) gas surrounding the Milky Way.

• Estimates of active galaxy sizes.

  IUE FACTS

  • Weight: 1,420 pounds (644 kilograms) before launch, 1,020 pounds (462 kilograms) in orbit.

  • Dimensions: 14 feet by 5 feet by 5 feet (4 meters by 1.5 meters by 1.5 meters).

  • Power: Two solar panels, which currently provide sufficient power to point at angles of roughly 70 to 150 degrees from the sun, and two batteries, which provide power during biannual earth shadow seasons.

  • Slewing: Three reaction wheels (one backup, never used).

  • Attitude Sensors: Two gyros (out of original six) and one Fine Sun Sensor (one backup).

  • Computers: One eight kilobyte computer on board (one backup, never used).

  • Orbit: Synchronous, with semi-major axis of 26,199 miles (42,164 kilometers), inclination of 34 degrees, and eccentricity of 0.31.

  • Telemetry: Uplink at 148.98 megahertz (VHF), downlink at 2249.80 megahertz (S-band), at a rate of 20 to 1.25 kilobits per second.

  • Ground Stations: NASA Goddard and ESA, Villafranca observatory.

  • Data in Archives: Approximately 46,000 short-wavelength prime camera images, 23,000 long-wavelength prime camera images, and 17,000 long-wavelength redundant camera images as of May 1993.