Between July 29 and August 6, 1985, an extraordinary research opportunity occurred for the Space Science Division of the Naval Research Laboratory in Washington, DC. Orbiting the Earth aboard the space shuttle Challenger were two NRL instruments observing the Sun, accompanied by NRL scientist and payload specialist Dr. John-David Bartoe. Meanwhile, alternate payload specialist Dr. Dianne Prinz and a team of other Space Science Division members headed by Principal Investigator Dr. Guenter Brueckner worked around the clock at the control center in Houston. The two instruments, HRTS (High Resolution Telescope and Spectrograph) and SUSIM (Solar Ultraviolet Spectral Irradiance Monitor), represent new advances in techniques to observe the mechanisms at work near and above the Sun's surface. These experiments were part of the Spacelab 2 mission that carried 12 scientific instruments. The mission was originated and funded by NASA and managed by NASA's Marshall Space Flight Center.
This publication describes the series of observations that resulted from the lengthy process
of experimental conception, design, and development involving a large number of scientists and
engineers. This process is placed in the context of NRL's history as a leader in space research and
as an institution with a significant program for future studies of the Sun and stars. These studies
continue a long tradition of NRL solar research, based on the recognition that the Sun has a major
influence on the sky and sea-the operating environment of the U.S. Navy.
Spacelab 2 crew in orbit. Clockwise from upper left: Dr. John-David Bartoe, payload specialist, NRL; Colonel C. Gordon Fullerton, mission commander, USAF; Dr. Tony England, mission specialist, NASA; Dr. Karl Henize, mission specialist, NASA; Colonel Roy Bridges, pilot, USAF; Dr. Loren Acton, payload specialist, Lockheed. Center: Dr. Story Musgrave, mission specialist, NASA (NASA photograph).
Space Research: The Past
As it did for much of American science, the path to space at NRL led from an interest in the workings of the ionosphere to the influence of extraterrestrial forces on the atmosphere-especially solar effects. The availability of sounding rockets after World War II found NRL scientists ready to use research rockets to extend ground-based observing techniques to altitudes that had until then only been probed with radio waves and then to extend their observations into space.
When E.O. Hulburt arrived at NRL in 1924 to study physical optics, he found a growing interest in the properties of Earth's reflecting layer because of the Laboratory's promotion of shortwave radio for Navy communications. An able theorist with a keen interest in the behavior of all kinds of radiation in nature, Hulburt fashioned a series of mathematical descriptions of the ionosphere during the 1920s and 1930s. In 1927, Hulburt and associates at the Carnegie Institution of Washington - Gregory Breit and Merle Tuve - explored the possibility of equipping Robert Goddard's rockets to explore the upper atmosphere. Two years later, he proposed an experimental program in which a rocket might be instrumented to explore the upper atmosphere, including detection of ultraviolet radiation and X rays at high altitudes.
By war's end, NRL radio scientists had learned about telemetry, radio guidance, and
tracking for guided missiles. Ernst Krause, who had been the wartime coordinator of missile
work at the Laboratory, met with his staff and kept a list of possible future projects on his office
chalkboard, and a consensus favorite emerged: rocket research. Krause approached the U.S.
Army, which was directing Project Hermes - the test firing of captured German V-2 rockets in the
New Mexico desert - and secured for NRL a leading role in coordinating the experimental
program in which scientists from several agencies and universities provided instruments to be
carried aloft on V-2s.
NRL scientists J. D. Purcell, C. Y. Johnson, and Dr. F. S. Johnson among those recovering instruments from a V-2 used for upper atmospheric research above the New Mexico desert. This is V-2 number 54, launched January 18, 1951 (photo by Dr. Richard Tousey, NRL).
Hulburt's optical scientists were among the first to use the V-2s. Richard Tousey began to design spectroscopes to measure the Sun's ultraviolet radiation above the atmosphere's screening effects. His first effort, in 1946, went unrewarded when the rocket returned to New Mexico in a screaming dive, ending up as a crater and a bucketful of debris. However, the second flight, in October 1946, yielded the first solar spectrum in the far ultraviolet. Tousey's work continued into the era of satellite research and manned space flight; in 1973 he was principal investigator for a solar instrument package that orbited on Skylab, producing solar data that are still being processed and digested by theorists.
In addition to ultraviolet studies, researchers pursued radio astronomy and cosmic-ray studies - both ground- and rocket-based - and engineers designed and supervised the development of the Viking rocket to replace the V-2 for high-altitude research. NRL's Herbert Friedman began X-ray solar studies in 1949 and soon reported that the energy of "the solar X-ray spectrum ... is adequate to account for all of E-layer ionization." Thus one of Hulburt's original questions, the source and behavior of the radio-reflecting layer, began to find its answer in space research.
Going back to interest in the transmission characteristics of the upper atmosphere, NRL scientists have pursued the interactions between Sun and Earth. Their fascination, which began with the mechanisms of the ionosphere and the influence of solar radiation, extended to the production of that radiation in the Sun itself. They have pursued these questions by means of instruments placed in rockets, satellites, Skylab, and most recently Spacelab 2. This last mission, then, continued NRL's long-standing leadership in this field.
8 Years of Planning-8 Days of Exhilaration
Eight months after the return of Spacelab 2, Dr. John-David Bartoe, payload specialist, and Dr. Dianne Prinz, alternate payload specialist, discussed the process of preparing for and flying the mission and retrieving the scientific results from NRL instruments on board. Their comments are given below as they describe NRL's participation and cooperation with other agencies in Spacelab 2.
Dr. Bartoe: HRTS, a high power telescope that sees in the ultraviolet, has the ability to
zoom in on very small features on the surface of the Sun. The primary goal here is to try to
understand how the Sun makes the solar wind. Some interesting things happen right on the
surface of the Sun - for instance, the temperature goes up very dramatically as you go just above
the surface. So we're trying to look at that region right there where that sudden transition of temperature takes place; most of the light emitted there is in the ultraviolet. So far we don't have the
answer; the theories have changed over the years as a result of our observations, but the right clue
hasn't turned up yet.
Engineers Donald N. Lilley (left) and James K. Smith work with the HRTS instrument in the laboratory at NRL
Dr. Prinz: SUSIM is essentially a sophisticated light meter. It looks at the Sun from the
ultraviolet up through the visible. Our intent is to fly SUSIM many times over the course of a
solar cycle, and that should tell us just how the Sun really varies in this wavelength region over a
solar cycle. The reason we go all the way to the visible is that from the ground you can see those
wavelengths, which SUSIM also sees, and you can match your measurements from space to those
on the ground and have a unifying point. We expect SUSIM will answer some questions about
variability in the ultraviolet, which is believed to start a chain of events in Earth's atmosphere that
eventually leads to global weather phenomena. The high-tech feature of the SUSIM is its
precision; the atmospheric physicists say they need to know precisely how much ultraviolet is
striking the top of the atmosphere.
The SUSIM instrument shown without its external case. The red laser beam shows the path through which the Sun's rays travel.
Dr. Prinz: The crew made a special trip with the HRTS instrument to White Sands
Missile Range (WSMR) where we put the instrument on a sun-tracker so we could actually look
at the Sun and the crew could see how the image they'd be seeing in space really looked. It's
always exciting to see the real target through a telescope - the crew spends a lot of time in
simulators, imagining things. We did similar tests with a lot of the instruments; we went to all the
institutions, and they did their best to set their instruments up so we could pretend we were flying
them. White Sands is always like a camping trip, so we had an interesting time.
Dr. John Bartoe operates the ground-based Sun tracker during training at WSMR. Clockwise from upper left: Dr. Dennis Socker, HRTS project scientist, NRL; Dr .Dianne Prinz, alternate payload specialist, NRL; Dr. Tony England, mission specialist, NASA; Dr. Loren Acton, payload specialist, Lockheed (NASA photograph).
Dr. Prinz: All of the NASA crew was of a like mind when it came to the purpose of the
mission, which was to get the science done. Everybody knew what everybody else was doing,
and there were certain fatiguing tasks they'd do in order to get better results-for instance, the crew
controlled free drift to maintain pointing stability on the Sun. It was that kind of understanding of
what the intent of the mission was that made the crew such an outstanding group. We had a
fantastic relationship with each other - very, very close.
Drs. John Bartoe (left) and Loren Acton (center) listen to emergency egress advice from mission specialist, Dr. Karl Henize (NASA photograph)
Drs. Bartoe and Prinz: The mission commander often acted as pilot on the 707 we rode
to experience weightlessness-it's a ten thousand foot roller-coaster ride. The sensations you feel
are so strange that you have to give your body time to figure out what's going on-and there are
only about 20 seconds of weightlessness before you feel 2-Gs as the plane climbs to the top of the
Drs. Bartoe and Prinz experience weightlessness on NASA's O-G aircraft (NASA photograph)
Dr. Bartoe dons his flight suit in preparation for reentry at the end of the mission(NASA photograph)
Drs. Bartoe and Acton work with solar instruments in the aft flight deck. Dr. Bartoe is holding
the hand controller for pointing the telescopes. Television pictures of the Sun generated from
within the telescopes are displayed on the two monitors at the center (NASA photograph).
Dr. Bartoe: I'm holding a joystick control in my hand, like on a video game, that permitted us to move the solar pointing telescope around to point at particular features on the Sun. We would have a conference call - a solar conference - just before sunrise each orbit. This gave us a chance to talk to the investigators so we'd know what we were trying to do on that orbit. We also received about 20 feet of typed messages over the teleprinter every orbit. This was the first mission where we had to replace the teleprinter paper, because we had to totally replan the mission.
One of the advantages of the fact that this mission took 8 years from formation to launch
was that there was a long time to fine tune each instrument's observing plan. And that was good
because a lot of things went wrong - we didn't get into the right orbit, for instance, and that had a
major impact on all the instruments. But because we'd planned so carefully, we really understood
how the various instruments' observations fit together, and it was easy to make quick changes.
It's like once you learn how to drive a car very well, you know how to react in an unusual
Dr. Bartoe: The position of alternate payload specialist during the mission is at the top of a pyramid over all the experimenters who are trying to get information up to the crew. During this mission Dianne had to listen to five or six or seven different telephone conversations at a time, listen to the crew, and then try to sort it all out. In my opinion, this is the toughest job, much more difficult than flying-we only had seven people and two telephone lines up there.
Dr. Prinz: I suppose the crew was just as tired as we were on the ground, but it was
really fatiguing, trying to keep track of everything at once. Each experimenter was very interested
in getting dedicated information about his experiment, but I'd try to prioritize what we could ask
of the crew without overtaxing them. I was the interface for all the experiments not just the solar
ones - and I'd be called into back rooms to iron out problems that occurred. There was almost no
time to think.
Dr. Prinz talks to the Spacelab 2 crew over the air-to-ground link during the mission; crew coordinator, Mr. William Bock, listens in (NASA photograph)
HRTS and SUSIM Scientific Results
Spacelab 2 was a laboratory and observatory for 13 investigations in 7 scientific
disciplines: solar physics, atmospheric physics, plasma physics, high-energy astrophysics, infrared
astronomy, technology research, and life sciences. NRL flew two instruments on Spacelab 2 - the
HRTS and the SUSIM. The purpose of the HRTS experiment was to study the fine scale
structure of the Sun's outer layers: the chromosphere, the corona, and the transition zone between
them. The objective of the SUSIM investigation was to determine both long term and short-term
variations of the total ultraviolet flux emitted by the Sun. Both instruments collected new and
valuable scientific data. A brief description of the instruments, their importance, and some
scientific results from the mission follow.
Something like Ernst Krause's office in 1945, the chalkboard in Guenter Brueckner's office today
is filled with possibilities for future research. NRL's Solar Physics Branch, headed by Dr.
Brueckner, has several programs: to make use of the data collected on Spacelab 2 by HRTS and
SUSIM, to prepare HRTS and SUSIM to fly again when the shuttle program continues, to build a
new SUSIM instrument for a long flight on the Upper Atmosphere Research Satellite, and to
resume sounding-rocket flights. All of these projects center around an effort to use ultraviolet
and X-radiation as indicators of solar activities not yet fully understood and which, as Hulburt
knew, influence terrestrial conditions.
HRTS ultraviolet image of explosive events (EE) in a spicule on the solar limb. Protrusions toward the legend "blue" are Doppler shifted into shorter wavelengths-approaching the observing instrument; those pointing to the "red" label are Doppler shifted into longer wavelengths-moving away from the instrument. NRL scientists expect such photos to help them understand these physical mechanisms at work near the Sun's surface and perhaps ultimately to explain the production of the solar wind.
Getting full value out of the images collected by SUSIM and HRTS will require the most sophisticated computerized equipment available to extract all possible data from the films. Research on image processing itself is a major future commitment. The power of space-based instruments, given the chance to view celestial objects from above the atmosphere, is such that even a few days' deployment returns an avalanche of data and provides material for years of analysis and theorizing as a return for years of designing and building the instrument.
Both HRTS and SUSIM are the products of new designs that give their measurements great precision. The idea behind SUSIM, in particular, rests on repeated flights to ensure proper calibration and to compare aspects of activity at different points in the solar cycle.
HRTS represents an advance in instrumental design, allowing very high resolution and a close examination of the smallest apparent features on the Sun's surface.
Beyond the ongoing programs, the Solar Physics Branch is actively pursuing many plans for future research. Sounding rockets are suddenly very attractive again because periods between shuttle flights will be longer. In addition, they offer thrift and flexibility and are the proving ground for new experiments that are then flown on satellites, the shuttle, or on the future space station. Research on the still-mysterious origin of the solar wind is paramount to the efforts of the Solar Physics Branch. New instrumentation will emphasize coronal research.
Sounding rockets, however, have only very short flight duration above the atmosphere, and they severely limit the size of instruments they can accommodate. The Solar Physics Branch is actively working on plans for a large solar telescope.
Space research is now 40 years old. However, it is still in its infancy, and there are no
limitations to the possibilities for the future.