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What was the FUSE Project?

For hundreds of years astronomers observed the Universe using only the visible light our eyes can see. However, visible light is a tiny portion of a much broader range of light energy known as the electromagnetic spectrum, which includes everything from energetic X-rays and gamma rays to infrared radiation and radio waves. Much of this "invisible" light gets blocked by the Earth's atmosphere, but since the late-1960's astronomers have been using telescopes above the atmosphere to obtain entirely different perspectives on the Universe. A new perspective, one that has only been glimpsed a few times before, was provided by a telescope known as the Far Ultraviolet Spectroscopic Explorer, or FUSE. Funded by NASA as part of what was called its Origins program, FUSE was launched into orbit aboard a Delta II rocket on June 24, 1999 for a nominal three years of science operations. In reality, FUSE lasted over eight years on orbit, before finally succombing to pointing system failures that made it unable to perform pointed science operations. The satellite was decommissioned on October 18, 2007, and is no longer active, although it may take decades before its orbit decays and it re-enters earth's atmosphere and burns up.

FUSE was developed and operated for NASA by the Johns Hopkins University. FUSE was developed in collaboration with the space agencies of Canada and France, who shared in the observing time over the first three years. This is the first time that a mission of this scope was developed and operated entirely by a university.

FUSE was designed for a very specialized and unique task that is complementary to other NASA missions. FUSE looked at light in the far ultraviolet portion of the electromagnetic spectrum (approximately 90 to 120 nanometers), which is unobservable with other telescopes. FUSE observed these wavelengths with much greater sensitivity and resolving power than previous instruments used to study light in this wavelength range.

The FUSE satellite consisted of two primary sections, the spacecraft and the science instrument. The spacecraft contained all of the elements necessary for powering and pointing the satellite: the attitude control system, the solar panels, communications electronics, and antennas. The science instrument collected the light of distant objects and contained the equipment necessary to disperse and record the light: the telescope mirrors, the spectrograph (and its electronic detectors), and an electronic guide camera called the Fine Error Sensor (or FES). The spacecraft and the science instrument each had their own computers, which together coordinated the activities of the satellite.

Astronomers viewed the Universe in a whole new light using the unique data obtained with FUSE. In particular, they sought answers to long-standing questions such as: "What were the conditions like in the first few minutes after the Big Bang?" ,"How are the chemical elements dispersed throughout galaxies, and how does this affect the way galaxies evolve?", and "What are the properties of the interstellar gas clouds out of which stars and solar systems form?" All of these questions, and many others, can be addressed by observing the far ultraviolet light from stars, interstellar gas, and distant galaxies with FUSE.

The scientific approach of the FUSE mission was special because a science team had been charged by NASA with providing answers, or at least partial answers, to intriguing questions like those posed above. Toward this end, the FUSE science team undertook a comprehensive study of the cosmic abundance of deuterium, a rare form of "heavy hydrogen" formed only in the Big Bang. The team also studied the hot gas content of our galaxy, the Milky Way, and its nearest neighboring galaxies, the Magellanic Clouds. To conduct these large studies, the FUSE science team observed hundreds of astronomical objects, using about half of the observing time during the three-year mission. The remaining observing timein the first three years, and all of the observing time in what was called the Extended Mission, was devoted to a Guest Investigator program where NASA selected scientific investigations proposed by astronomers from around the world.

What Did FUSE Explore?

FUSE was selected by NASA based on a proposal to perform several specific high-profile science goals. However, over time the mission developed into a much broader, general purpose observatory that carried out hundreds of investigations for scientists all over the world. Many of these science projects were not even conceived of at the time FUSE was being built and launched, and yet became important parts of the FUSE science legacy. FUSE investigators have published many hundreds of astronomy research papers based on FUSE observations, and we expect many more to be published in the future using the FUSE data archive.

Below, we describe briefly a couple of the initial major science goals of FUSE, but point the reader to the FUSE Science Summaries page for a representative selection of other science results.

Deuterium in the Local Universe

In the infancy of the Universe, the extreme conditions present everywhere gave rise to the creation of simple chemical elements out of which all matter was made. The simplest element, hydrogen, consists of a positively charged nucleus containing a single proton orbited by a negatively charged particle known as an electron. In some instances, these hydrogen atoms also have a second particle called a neutron in the nucleus accompanying the proton; this type of hydrogen has its own name and is called deuterium. More complicated elements consist of atoms having larger numbers of protons and neutrons in their nuclei surrounded by correspondingly higher numbers of electrons.

When atomic nuclei formed in the early Universe, the conditions were so severe that electrons were unbound to the nuclei and moved about freely. Gas with this property is known as plasma. In this plasma, some of the hydrogen was converted to deuterium, and some of the deuterium was converted to helium. The relative amounts of each element produced by this nuclear fusion of protons and neutrons were very sensitive to the temperature, density, and number of the particles in the plasma at that early time. As the Universe expanded, the plasma cooled, the creation of elements ceased, and the free electrons and nuclei combined to form complete atoms.

It is the sensitivity of the nuclear reactions in the primordial plasma to the initial conditions in the Universe that makes astronomers interested in studying the simple elements today. By measuring the relative amounts of each element, it is possible to infer the conditions present at a time before complete atoms existed! In particular, knowing the ratio of deuterium atoms to hydrogen atoms left over from the Big Bang would allow astronomers to place a strong constraint on how much observable matter there is in the Universe.

Alas, Nature does not reveal secrets such as these easily; the abundances of some elements have changed over time. The interior cores of stars are hot enough (tens of billions of degrees) to mimic those conditions in the first few minutes of the Universe and convert deuterium into helium by the addition of another proton to the deuterium nucleus. Unlike the early Universe, however, the nuclear reactions in stars are sustained over very long periods of time, which means that fragile light elements like deuterium can be readily converted into much heavier elements. For this reason, astronomers believe that the total amount of deuterium in the Universe is decreasing as matter gets cycled through stars, but they do not know how fast it is decreasing or how much deuterium has already been destroyed.

This is where FUSE entered the quest to understand our cosmic origins. Astronomers used FUSE to search for deuterium in the interstellar medium near the Sun, in gas clouds in the far reaches of the Milky Way, and even in distant intergalactic clouds between galaxies. What researchers found was a new puzzle: the amount of deuterium (relative to normal hydrogen) appears to vary over a much broader range of values than expected. The best current interpretation of the FUSE results is that deuterium can be incorporated preferentially into interstellar dust grains, giving low gas-phase abundances (what FUSE measured). In places where supernova shock waves have heated this dustgrains, deuterium has been released back into the gaseous form where FUSE could observe it. Thus, the highest measured values are what we should compare to estimate how toal value to estimate how dmuch deuterium has been destroyed since the Big Bang. This amount is significantly less than many astronomers have assumed to be the case.

As is often the case in science, the solving of one mystery opens to door to new questions that must be asked and understood.

The Chemical Evolution of Galaxies

Galaxies like our own are massive collections of stars, gas, and dust. Matter and energy are exchanged between these various components in a grand cycle that changes the chemical and physical properties of galaxies. Stars form from the interstellar material, synthesize chemical elements in their interiors, and return their products to the interstellar gas during their lives and in their death throes. All naturally occurring elements heavier than lithium are produced by these cycles. The carbon atoms that form the basis of life, the oxygen we breathe, and the silicon in the sand on our beaches were all formed deep inside some previous generations of stars. The calcium in our teeth, the copper in our coins, and the iron in the steel frames of our cars are formed in massive stellar explosions called supernovae that occur as stars exhaust their nuclear fuel, collapse under their immense weight, explode, and reseed the interstellar gas for a new generation of stars.

The beautiful Horsehead Nebula in Orion dramatically demonstrates the presence of gas and dust in the vast regions of space between the stars. (Image © Anglo-Australian Observatory.)

Understanding how stars and the interstellar medium interact with each other is a major concern of astronomers. The energy produced by stars is shared with the interstellar medium as stellar winds sweep up gas and dust, and stellar explosions vacate large cavities and create "bubbles" filled with very tenuous, hot gas. This stellar activity can trigger interstellar gas clouds to collapse and form new stars and solar systems, or it can disrupt the very same processes and prevent them from occurring.

One of the major predictions of theories for these interactions is that some portion of the interstellar medium should be heated to very high temperatures by all this activity. In the hot gas, atoms are ionized ­ that is, the electrons that normally surround the atomic nuclei are stripped off the atoms. As the gas cools, some of the electrons reattach to the positively charged ions. One of the most important ions that astronomers can observe is oxygen that has had five of its eight electrons removed; this form of oxygen is called O VI ("oxygen six"). It is a very good indicator of gas that has been heated to temperatures of one million degrees or more and is cooling as the recombining electrons and ions emit or absorb light.

The graceful arcs of the Vela supernova remnant are seen against the rich star field of the Milky Way. These gaseous filaments arise where the 10,000 year old supernova blast wave has swept up and heated the tenuous interstellar gas. (Image © Anglo-Australian Observatory.)

FUSE was designed to make very sensitive measurements of O VI in the interstellar medium and the remnants of supernova explosions. One of the primary scientific objectives of the FUSE mission was to determine whether a large halo of hot gas surrounds our galaxy. By studying the distributions of O VI and many other atoms and ions, astronomers were able to determine the composition of the interstellar gas, how well it is mixed, and which processes have been effective in heating the gas. Not only did FUSE confirm the presence of a hot halo of gas, but it found an even more tenuous, expended `corona' of hot gas that extends out even further than the halo. All of this new information must now be incorporated into theoretical models to help us understand how galaxies evolve and form new generations of stars and planets.

For more on FUSE Science:
go to the
FUSE Science Summaries page.

How Did FUSE Work?

To accomplish its task, FUSE incorporated a number of unique design features. For instance, instead of a single mirror FUSE used four mirror segments to reflect the light to focus. Two mirror segments were coated with a material (silicon carbide) that has superior reflectivity at the shortest ultraviolet wavelengths, and two mirror segments were coated with a different material (aluminum and lithium fluoride) that reflects better at longer wavelengths. This optimizes performance over the entire spectral range. FUSE also used two sophisticated electronic detectors to "see" the incoming ultraviolet light and record it digitally for downlink to the ground.

The ultraviolet light seen by FUSE was dispersed (or broken up) into a spectrum by four special optical components called gratings (one for the light from each of the four mirror segments). The FUSE gratings were quite large, and were etched with a very large number of fine, parallel grooves. The grooves dispersed the light into a spectrum, and the large number of grooves packed closely together provided the high resolving power (ability to see details) that allowed FUSE to do its job. The FUSE gratings were curved instead of flat, which made their manufacture very complex and difficult.

The Fine Error Sensor (or FES) cameras were the "eyes" of the satellite. The FES worked in visible light, and imaged a region about a third of a degree in size in the direction that the telescope was pointing. (For comparison, the moon is about half a degree across.) The FES could see stars down to about 14th magnitude, which is about 5,000 to 10,000 times fainter than you can see on a typical clear night! The FES produced the only "pictures" that came from FUSE; but the real job of FUSE was to observe the spectrum of astronomical objects in far-ultraviolet light invisible to ground-based telescopes. Analysis of these spectra provided a wealth of information about each object being observed and any gas or dust along the line-of-sight that may have absorbed some of the light along the way.

FUSE Operations

FUSE was controlled through a primary ground station antenna located at the University of Puerto Rico, Mayaguez. The satellite's circular 775 kilometer (480 mile) orbit, which took about 100 minutes for a single revolution, brought it over the ground station for less than 10 minutes at a time (on average) for about six or seven orbits in a row, followed by roughly seven or eight orbits without contact. Hence, the satellite had to operate on its own most of the time, moving from target to target, identifying star fields, centering objects in the spectrograph apertures, and performing the observations. The scientific data, which were stored in digital form, were radioed to the ground during contacts with the ground station.

All of the instructions the satellite needed to perform its tasks were pre-planned and uplinked to the onboard computer during contacts with the ground station. Preparation of these instructions occurred in the Satellite Control Center located in the Bloomberg Center for Physics and Astronomy at The Johns Hopkins University in Baltimore, MD. Potential observations were scheduled based on predicted viewing intervals, spacecraft positioning constraints, and the needs of each science program. These schedules, or timelines of activities, were then turned into detailed instrument instructions and uplinked to the satellite by a team of engineers. The observations normally took place without direct interaction by ground controllers.

FUSE Primary and Extended Missions

The first 3-1/2 years of FUSE operations were dubbed the Primary Mission. During this period, the observing time on FUSE was shared roughly 50-50 between the FUSE science team and a host of Guest Investigators, astronomers from around the world selected by NASA to participate in the FUSE program. Starting April 1, 2003, the FUSE project wntered an extended phase of operations which lasted until mid-2007. With funding from NASA, the FUSE satellite continued to be operated as an observatory for the broad astronomical community, with 100% of on-orbit observing time selected by NASA peer review. Some 29 million seconds of science data were obtained during the Primary Mission phase, and a total of nearly 65 million seconds of data were archived from the entire mission.

The Extended Mission period presented a number of challenges, especially for satellite operations. Many procedures had to be automated, allowing the project to cut back on staffing and minimize operations costs. As one example, the Satellite Control Center was staffed around the clock during the Prime Mission, but transitioned to a 16 hour per day, Monday through Friday staffing profile in the Extended Mission. Less redundancy and less access to ongoing engineering support was consistent with NASA policy for missions in their extended phase, where a higher level of risk is allowed.

FUSE's Principal Investigator

Dr. Warren Moos is Professor of Physics and Astronomy at the Johns Hopkins University. He is a specialist in space optics and ultraviolet instrumentation. In addition to being the Principal Investigator of the FUSE mission, he was a co-investigator for the Space Telescope Imaging Spectrograph installed on the Hubble Space Telescope in 1997 and a co-investigator for the Hopkins Ultraviolet Telescope, flown on Space Shuttle flights in 1990 and 1995. Dr. Moos was also a co-investigator for the Voyager UVS and the Apollo 17 UVS experiments and has been an extensive user of space telescopes. Dr. Moos has served previously as Director of the Center for Astrophysical Sciences and as Chair of the Department of Physics and Astronomy at the Johns Hopkins University.

FUSE Partners

FUSE is a joint project of the National Aeronautics and Space Administration and the Johns Hopkins University in collaboration with:

Centre National d'Etudes Spatiales (France), the Canadian Space Agency, the University of Colorado, and the University of California, Berkeley.