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| International Extreme Ultraviolet Hitchhiker (IEH-3) |
| Payload Bay |
| 5,672 lb lbs. |
| Prime: Stephen Robinson |  |
Principal Investigator: Dr. Don McMullin |
| Backup: Scott Parazynski | |
| Overview |
 The primary purpose of the IEH is to investigate the magnitude of the absolute solar extreme ultraviolet (EUV) flux and EUV emitted by the plasma torus system around Jupiter and stellar objects. It will also study the Earth's thermosphere, ionosphere, and mesophere.SOLAR EXTREME ULTRAVIOLET HITCHHIKER Developed by the University of Southern California for NASA's Solar System Exploration Division, the SEH will measure the wavelengths of nonvisible light with an extreme ultraviolet (EUV) solar spectrometer and photograph the EUV region of the solar spectrum between 250 and 1,700 angstroms. (For comparison, visible light is between 4,000 and 7,000 angstroms. An angstrom is one ten-billionth of a meter.) SEH also will use helium and neon instruments to provide extremely accurate measurements of the brightness of solar EUV radiation. On IEH-03, the solar system response to the solar input will be observed by a complementary set of instruments, UVSTAR and SEH. Both are international cooperative experiments. The UVSTAR instrumentation will provide Jovian system extreme ultraviolet/far ultraviolet data, and SEH will provide the required solar flux data for proper interpretation. Such missions will continue to provide the planetary community with the highest quality solar EUV data available. One type of detector SEH is investigating is a helium double ionization cell. This detector measures the photoionization rate of helium, from which the total number of photons in emission lines between 50 and 504 angstroms can be calculated. Commands sent from the ground trigger the release of a precise amount of helium into an ionization cell. Photons entering the cell collide with the helium atoms, which absorb the photons in the emission line being investigated. Electrons created in this process flow through electrometers to two collectors. The photoionization rate of the helium is calculated by comparing the currents of the two electrometers that serve each collector. The neon rare-gas ionization cell is similar to the helium cell, but it provides the actual number of photons between 50 and 575 angstroms. In this detector, photons colliding with neon atoms create electrons and ions of neon. Electrons flowing through an ion current pico ammeter to a collector produce a current proportional to the photon count. The silicon photodiode is a highly stable detector that measures the solar flux at EUV wavelengths. Solar photons enter the detector's 5mm aperture and pass through an aluminum filter. The light flux that is not filtered out (170 to 800 angstroms) strikes the aluminum-coated detector, creating a photocurrent that is measured by an electrometer. The SEH EUV spectrometer measures the relative distribution of emission lines from 250 to 1,750 angstroms rather than the number of photons in the solar radiation. Radiation entering the instrument is broken into its component wavelengths. The photons then pass through a filter into a charged microchannel plate, where they crash into the glass surface of the plate. The collision produces electrons that impinge on a resistive anode detector, generating a current that is proportional to the intensity of the light. Emission lines may be recorded with a spectrometer, an instrument that measures the intensity of radiation at particular wavelengths. The product of these measurements is a spectrograph, which reveals the chemical composition of the object being observed. The data from these four instruments will be combined to reveal the absolute solar flux. SEH will also measure changes in the Earth's atmosphere caused by solar EUV and daytime temperatures. Its observations will also be coordinated with those of two identical on-orbit instruments--a payload to be launched on a sounding rocket and the European Space Agency-NASA Solar and Heliospheric Observatory--to provide tight cross-calibration of the three instruments. Dr. Don McMullin is the principal investigator. ULTRAVIOLET SPECTROGRAPH TELESCOPE FOR ASTRONOMICAL RESEARCH The UVSTAR instrument complement consists of two telescopes with imaging spectrographs that cover overlapping spectral regions of 500 to 900 angstroms and 850 to 1,250 angstroms. The telescopes are capable of spectral imaging of extended plasma sources. Internal gimbals allow the entire UVSTAR telescope/spectrograph assembly to move several degrees about its azimuth and elevation axes. A newly developed code autonomously determines the pointing direction of the telescope. Two smaller telescopes on the front of the UVSTAR instruments acquire and track targets after the orbiter is oriented to position the 6-by-8-degree field of view of UVSTAR's finder on the objects. UVSTAR will obtain and spectrally resolve images of extended plasma sources. Key targets are planetary, such as the high-temperature plasma confined in a toroidal ring around Io's orbit at Jupiter, remnants of supernovae with their expanding envelopes, and the hot blue star content of the globular clusters (i.e., very dense stellar aggregates that give clues about stellar evolution). Targets of opportunity, such as comets or special sudden events occurring in the sky, also will be observed. The volcanic Io spews volcanic gasses and materials that are trapped in Jupiter's magnetic field, forming a torus (a donut shape with Jupiter in the middle). The UV emissions from the torus will reveal the nature of the Ionian material and Jupiter's energy output. UVSTAR is carrying an instrument called the extreme ultraviolet imager, which will measure the Earth's atmosphere in EUV wavelengths. The EUVI has two imagers that will map the intensity of helium and oxygen ions in the atmosphere by scanning along the Earth's shadow line. The EUVI will allow scientists to obtain precise measurements of the Earth's ionosphere and plasmosphere. Since the crucial lines for interpreting the properties of the upper atmospheres and magnetospheric plasmas of planets and the structure of the interplanetary medium are situated in the 500- to 900-angstrom wave band, UVSTAR's extreme ultraviolet channel is perfect for conducting solar system research. Space-based measurements in the EUV spectrum offer scientists a more direct means of studying certain torus processes than ground-based observations of torus emissions in the visual wavelength. The principal investigators are Dr. Lyle Broadfoot of the University of Arizona and Dr. Roberto Stalio of the University of Trieste. SOLAR CONSTANT EXPERIMENT SOLCON's measurements will be used to calibrate instruments on satellites that are continuously monitoring the total solar irradiance. Since SOLCON returns to Earth in the shuttle for recalibration, its TSI measurements are not subjected to possible degradation from solar radiation that other orbiting instruments may suffer. Ultimately, SOLCON gives researchers a "quality control" capability to check orbiting TSI instruments continuously. The radiometer consists of two channels through which solar radiation may be sensed. Each channel contains a radiation sensor and has two apertures. The first aperture of each channel is protected by independent shutters that seal out any solar radiation from the radiation sensor when they are closed and allow the sensor to receive solar radiation when they are open. The SOLCON flight operators will work with the Belgian Space Remote Operation Center at the Royal Meteorological Institute of Belgium and perform experiments in preparation for scientific research on board the International Space Station. Dr. Dominique Crommelynck of the Royal Meteorological Institute is the principal investigator. SPECTROGRAPH/TELESCOPE FOR ASTRONOMICAL RESEARCH STAR-LITE is a telescope and imaging spectrograph that will study astronomical targets in the ultraviolet. Targets of scientific investigation include diffuse sky background emissions, scattered dust, and recombination emission lines from the hot and interstellar medium, supernova remnants, planetary and reflecting nebulae, star-forming regions in external galaxies and the torus formed around Jupiter by volcanic emissions of its moon Io. Dr. Jay Holberg of the University of Arizona is the principal investigator. PETITE AMATEUR NAVAL SATELLITE PANSAT, a nonrecoverable satellite developed by the Naval Postgraduate School (NPS) in Monterey, Calif., is basically a small telecommunications satellite. It will be used to enhance the education of military officers at NPS by giving them hands-on experience in developing and operating a small satellite. It will also serve as a space-based laboratory for officers. PANSAT is a spread-spectrum satellite. Spread spectrum allows communication satellites to capture and transmit signals that normally would be lost because the original signals were too weak or had too much interference. Normal radio frequencies use about 3 kilohertz to 1 megahertz of bandwidth, but the spread spectrum is about a thousand times wider and difficult to intercept. The low probability of interception would be important to the military in situations like rescues of downed pilots. A downed pilot could obtain his location from the Global Positioning System and uplink the data to an orbiting spread-spectrum satellite with minimum risk of interception by an enemy. Civilians could use this type of communication during emergency rescues and as a basis for establishing communication in remote areas. PANSAT will demonstrate the capabilities of low-cost spread spectrum on a small satellite. It will provide store-and-forward digital communication using direct sequence, spread-spectrum modulation. Store-and-forward digital communication allows the PANSAT ground station to send data to the satellite. PANSAT will operate in the amateur radio frequency range to allow ham operators to evaluate the satellite's performance. The PANSAT principal investigator is Rudolf Panholzer of the NPS. COSMIC DUST AGGREGATION (G-764 GAS EXPERIMENT) The CODAG experiment is designed to simulate the aggregation of dust particles and dynamics of dust clouds that occurred in the early stages of the formation of our solar system. Scientists hope that understanding the dust growth process in the early solar system will enable them to answer questions about planet formation. The experiment apparatus consists of a vacuum chamber equipped with windows and sensors. Small glass particles will be injected into the chamber, and digital cameras will record the dust's motion in a small control area through microscopes. Sensors will measure the scattering characteristics of the dust cloud for comparison with astronomical measurements. Ten experiment runs are planned. During each run, the dust will be observed for 15 minutes to five hours. CODAG is sponsored by the University of Bremen, Germany, and ZARM (Zentrum fur Angewandte Raumfahrttechnologie und Mikrogravitation). The principal investigator is Hans J. Koenighsmann. ROACH EXPERIMENT (G-238 GAS EXPERIMENT) The only biological experiment on STS-95, G-238 will study the effects of space on the life cycle of the American cockroach. The payload is sponsored by the American Institute of Aeronautics and Astronautics and managed by students at DuVal High School in Lanham, Md. The roach experiment apparatus consists of a habitat that has been divided into three sections: one for young adults, one for nymphs, and one for eggs. Air, heat, water, and food will be supplied. Battery-powered heaters will maintain a comfortable temperature in the habitat. An 8-mm camcorder will record activity inside the habitat at regular intervals. David Eakman of The Boeing Company in Seabrook, Md., is the payload manager. |
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| History/Background |
 This is the third of five planned IEH shuttle missions to monitor long-term variations in the sun's EUV irradiance. The first flight was STS-69 in 1995. IEH consists of two cooperative experiments--the Solar Extreme Ultraviolet Hitchhiker (SEH) and the Ultraviolet Spectrograph Telescope for Astronomical Research, or UVSTAR. The SEH and UVSTAR experiments are carried on a Hitchhiker cross-bay bridge in Discovery's cargo bay. SEH instrumentation is contained in a canister on the side of the Hitchhiker cross-bay bridge, and UVSTAR is mounted on a pallet on top of the bridge. Five other payloads are hitching a ride with the IEH. The Spectrograph/Telescope for Astronomical Research payload will study astronomical targets in the ultraviolet. The Solar Constant experiment is designed to accurately measure the solar constant and identify variations in its value during a solar cycle. The Petite Amateur Naval Satellite is a small spread-spectrum communications satellite that will operate in the amateur radio ultrahigh frequency range. Two getaway specials are also part of the Hitchhiker payload complement. IEH-03 is managed by the Goddard Space Flight Center. The Hitchhiker avionics provide power to the payload and commanding capabilities from Goddard Payload Operations Control Center. Some of the payloads will be controlled from the POCC; others will be run automatically by preprogrammed commands. The crew will support the IEH-03 payload by activating and deactivating it and performing attitude maneuvers in support of experiment observations. The UVSTAR experiment, which was jointly developed by NASA and the Italian Space Agency, is a collaboration between the Universities of Arizona and Trieste. UVSTAR will use measurements of the solar flux recorded by the SEH to observe the response of the solar system to the solar input. The UVSTAR instruments will measure EUV emissions (500 to 900 angstroms) and far ultraviolet emissions (800 to 1,250 angstroms) from stellar objects such as hot stars and nebulae and the Jovian system. The UVSTAR scientific program is an offshoot of the astronomy research conducted by the Voyager 1 and 2 spacecraft, which are sending back low-resolution spectra of faint stellar and nonstellar objects. UVSTAR's enhanced sensitivity and greater spectral resolution will allow it to study fainter objects and a greater variety of objects than the Voyager spectrometers, and its spectral imaging capabilities will enable it to study a wider range of objects. The Belgian SOLCON is a unique instrument that is designed to ensure that spaceborne measurements of the absolute value of the solar constant (the total solar radiative power absorbed by one square meter at the outer layer of the Earth's atmosphere) remain accurate and precise. With its differential absolute radiometer, SOLCON will try to pinpoint the absolute value of the total solar irradiance with an accuracy of better than 0.01% and a precision better than 0.005%. |
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| Benefits |
| SOLAR EXTREME ULTRAVIOLET HITCHHIKER Since EUV flux, or radiation, cannot penetrate the Earth's atmosphere, scientists who want to learn more about this important energy source from the sun must conduct their experiments in space. Scientists need accurate measurements of the sun's absolute EUV emissions to develop models of the scattering, ionization, and heating of planetary atmospheres (including our own), moons, and comets. Data from early attempts to measure EUV radiation exhibited rather large uncertainties caused primarily by instrument calibration uncertainties and difficulty separating changes in the sensitivity of instruments from the variability of solar EUV emissions. Because of the stability of its three EUV detectors, the SEH can surmount these obstacles. When the SEH flew on STS-69, it produced the most reliable absolute solar EUV data so far. Scientists hope that these experiments will provide data that will help them improve their global solar atmospheric models, which will lead to a better understanding of solar variability. SOLAR CONSTANT EXPERIMENT Solar energy, Earth's only external source of energy, is a primary natural driver of climate changes. The measurement of total solar irradiance is an important tool for researchers who are studying the effects of global warming. |
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