Out the Window
New telescope turns the space shuttle into low-cost observatory
Planetary exploration using mass spectroscopy and measurement of plasmas and magnetic fields has long been a technical strength of SwRI's Instrumentation and Space Research Division. These in situ techniques are extremely powerful tools for studying planetary atmospheres, ionospheres, and magnetospheres. However, by definition, these instruments must be brought to the place they are investigating.
By contrast, space remote sensing, using telescopes and photo detectors to study asteroids, comets, planets, and other phenomena, is not constrained by proximity because those instruments can view objects across vast distances. Because of its promise for opening new avenues of scientific research, the Institute made a strategic decision in 1991 to gain experience and become a center of excellence in the field of space remote sensing.
With those goals in mind, a project was funded internally to develop a versatile, low-cost ultraviolet (UV)/visible imager that could serve as a prototype miniature astronomical laboratory onboard the space shuttle.
Within a little over a year, a working version of the concept, named SWUIS (Southwest Ultraviolet Imaging System), was designed and assembled. The instrument's capabilities were proven first in laboratory tests and later with a field trip in 1992 to the McDonald Observatory in West Texas.
SWUIS was then accepted by NASA to fly on a pair of high-altitude demonstration missions aboard SR-71 Blackbird aircraft in early 1993. These missions took SWUIS to altitudes of almost 90,000 feet and speeds exceeding Mach 3.2.
With experience gained from the internally funded laboratory demonstration and the high-performance aircraft flights, it was felt that there was a good basis to propose to build and fly a SWUIS instrument onboard a shuttle mission, using it to tackle some difficult remote sensing observations in space.
Fortunately, the universe soon cooperated by providing Hale-Bopp, one of the most spectacular comets of the century. Comets are relics from the formation epoch of the solar system, and planetary scientists relish the opportunity to study a bright one. So when Hale-Bopp was discovered in mid-1995 and it was recognized that when it approached the sun in early 1997 it would become one of the brightest comets ever recorded, NASA requested proposals to conduct space observations of this unique celestial body.
Thus it was proposed that SWUIS fly aboard the space shuttle in order to image the comet from space when the comet was too close to the sun to risk the more powerful, but much more light-sensitive Hubble Space Telescope (HST). The goal was to obtain a long, time-lapse series of wide-field images of the comet so its behavior and morphology could be studied in detail.
Maiden voyage: imaging Hale-Bopp
When in mid-1996 NASA announced its selection of payloads to study comet Hale-Bopp, the roster included a host of suborbital rocket missions, high-altitude aircraft flights, and one shuttle experiment: SWUIS. Between that point and the June 1997 delivery of SWUIS to NASA's Johnson Space Center before launch, all of the flight interface hardware for SWUIS was designed, fabricated, and certified to fly on the shuttle. In addition, all pre-mission shuttle crew training was completed and the necessary documentation, safety certification, and hardware tests required to fly on the shuttle were developed. SwRI scientists and technicians calibrated the instrument both in the lab and in the field, using star fields and comet Hale-Bopp itself. After instrument delivery, the ground software needed for quick-look image quality analysis was developed, and SwRI personnel participated in mission simulations in Houston.
SWUIS was assigned to space shuttle mission STS-85, Discovery, which was launched August 8, 1997. The mission lasted 10 days. SWUIS was operated on nine separate orbits, one more than had been planned prior to the flight. The instrument performed well, and some nine hours of data were recorded, including more than 410,000 images of the comet in a variety of key emission bands.
The data collected by SWUIS provide the only wide-field UV images of Hale-Bopp, the first-ever UV time-lapse series of a comet, tens of thousands of images of broadband UV and OH (oxygen-hydrogen) fluorescence, as well as images in the visible and the near-UV using the standard, NASA-provided set of Hale-Bopp watch filters.
These data contain valuable new insights into Hale-Bopp's coma and tail morphology on three fundamental timescales: minutes (during a given filter image sequence), hours (on successive orbits on a given date), and days (over the six-day span of the data set). Further, the SWUIS data set contains the only OH (and therefore water-production) rate data scientists are aware of prior to the HST observations in September 1997, the only UV dust/continuum images of the whole molecular coma, and high-frequency photometry of Hale-Bopp, making it possible to search for pulsations and other phenomena of scientific interest.
Detailed analysis of the SWUIS Hale-Bopp images is currently under way. The bulk of the work that has taken place so far has been in extracting the video images from the data tapes, digitizing and co-registering each image to remove unwanted image jitter, and then co-adding image frames to boost the signal-to-noise ratio and allow subtle features in the images to surface. In addition, each set of processed images is then converted to brightness units using the SwRI calibration database.
Preliminary results show that a variety of molecular species were detected in the coma and tail regions of Hale-Bopp including OH. The images are now being carefully studied to determine and quantify the production rates from the coma of these identified species. In addition, the SwRI team hopes to put together a time history of the changes in appearance of these species to determine where these emissions originated from on the comet nucleus and how they are influenced by and coupled to the solar wind.
The SWUIS shuttle instrument
SWUIS presently has two hardware configurations for space shuttle missions: a telescope science mode (TSM) and a camera science mode (CSM).
TSM uses a telescope for high-spatial resolution imaging of faint object targets such as planets, comets, and space debris, while CSM uses a wide-field camera lens for imaging bright targets that occupy larger swaths of the sky, such as aurora and lightning sprites. Both TSM and CSM hardware are sensitive to UV, visible (VIS), and infrared (IR) wavelengths.
The SWUIS TSM hardware is composed of three major elements: the telescope, the intensified charge-coupled device (ICCD) camera, and the electronics that power and control the ICCD camera. In addition to these major components, SWUIS uses a custom-built mounting bracket that couples the telescope to the space shuttle side-hatch window for UV observations, a telescope optical coupling assembly (TOCA) that physically and optically couples the ICCD camera to the telescope and which can hold up to three imaging filters in the optical path, a filter caddy that holds the filters and lenses used in the TOCA, and associated power and data cables.
Data from the ICCD camera are recorded onto a portable camcorder as an analog video signal that can be downlinked from the shuttle to the ground for real-time assessment. A third mode and new capabilities are being planned for SWUIS, with a spectrograph now in design for flight in 1999.
The SWUIS TSM provides astronomers and planetary scientists with a small, but highly capable space telescope. Although far less sensitive than the HST, SWUIS has its own advantages. These include a far wider field of view and a capability to study objects that are much closer to the sun, such as the inner planets and comets.
The SWUIS CSM configuration is very similar to the TSM except the ICCD camera is used with a UV transmissive wide-field lens instead of the main telescope. A mini-TOCA is used to hold filter combinations. The CSM can be mounted to any of the shuttle windows including the side-hatch window and the nine flight deck windows. The wide-field lens assembly provides a field of view of approximately 12.5 degrees (full cone). The SWUIS CSM allows SWUIS capabilities to extend to studies of the Earth's atmosphere, aurora, and ozone layer; it is also useful for certain types of stellar astronomy and even for studies of orbital debris.
Moving forward: flights 2 and 3
SWUIS is scheduled to fly its second shuttle mission in December 1998, onboard the Space Shuttle Columbia (STS-93). This mission will focus on obtaining UV imagery of an array of planetary and astrophysical targets. Specific objectives of SWUIS during this flight are to obtain the mid-UV albedo of the planet Mercury for the first time and to search for spatial variations across the planet; obtain mid-UV dynamic movies of the upper atmospheres of the planets Venus and Jupiter; establish the morphological appearance and phase curve of the moon in the mid-UV for the first time; and obtain mid-UV dynamic movies of the airglow along the Earth's limb (edge).
For a third SWUIS flight slated for 1999, a spectrograph will be added to the SWUIS telescope. This spectrograph will allow SWUIS to pursue mid-UV spectroscopy of Mercury and Venus, a capability that no other facility presently can offer. The spectrograph will also provide a valuable capability for cometary and asteroidal studies in the mid-UV as well as the VIS and near IR wavelengths.
With these new capabilities, SWUIS will become a very versatile tool. This unique suite of instrumentation has grown from a camera flying aboard high- altitude aircraft to a sophisticated and highly reconfigurable astronomical and Earth-remote sensing laboratory that has applications ranging from studies of celestial objects, to the ozone hole, to space debris. SwRI scientists are hopeful that SWUIS will be selected for flight as a remote sensing facility aboard the International Space Station when it becomes operational.
SWUIS's maiden flight aboard Discovery last summer was a rewarding capstone to the first phase of this project and an affirmation of the ideas that spawned the original internal research effort. Now, with two more flights on the books and plans for more being formulated, the SWUIS team is entering a vigorous operational phase for this small, but versatile space observatory. Already on the horizon is the expansion of SWUIS capabilities into the infrared and the application of SWUIS to a wide variety of terrestrial applications, including lightning and aurora studies.
In future years, more SWUIS flights on the shuttle are expected, as well as an expanding array of airborne applications (see sidebar above), and the potential to make SWUIS a Space Station facility for remote sensing. None of this would have been possible without the internal research funding provided at the project's outset.
For more information about SWUIS, visit www.boulder.swri.edu/swuis.
Published in the Summer 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.