9UNP1747HD_Jordan_Marchè

- Welcome to Space Place, and thank you for coming tonight. Tonight we have a talk of special significance, because we just passed the 50th anniversary of the launch of the Orbiting Astronomical Observatory 2, the first successful space observatory. And it had very important connections and roots here in Wisconsin, and so I've asked a historian of astronomy, who's actually done research on the OAO program and published on that topic, to give the talk here tonight. That's Jordan Marchè, and Jordan got his PhD in the History of Science from Indiana University, and then has been working here in Madison, Wisconsin for a few years, and I imposed on him to give us a talk tonight about the history of the OAO project. So, let's welcome Jordan up here to the front. 
(audience applauding)

Thank you, Jim, for that very kind introduction.

And good evening, ladies and gentlemen. So, welcome to this public celebration of the 50th anniversary of the launching of the Orbiting Astronomical Observatory or OAO-2 satellite. And the reason why we are hosting this celebration is again because the principle and longest-lasting component of this device, termed the Wisconsin Experiment Package or WEP, which is what I'll be referring to it as, was designed and constructed by a team of researchers from the astronomy department here, although they couldn't do it entirely along, of course.

Now, OAO-2 was the first true stellar space observatory. And in its day it represented a greater technological step forward than did the much larger and more sophisticated Hubble Space Telescope that was launched in 1990. And explaining the hows and whys behind that claim will take up perhaps the majority of our time here this evening. But that's not all that we have to share with you tonight.

The WEP which occupied one-half of the OAO-2 satellite, and which is highlighted in white there,lasted for 50 months until February 14th of 1973, when its power supply finally failed.

Yet, during that time it managed to observe more than a thousand celestial objects of many kinds.

There were chiefly stars, including one nova, plus nebulae and galaxies that lay well beyond our solar system,

along with certain planets and active comets within it.

So, we will highlight some of the scientific discoveries toward the end of this talk.

Now, we will see that OAO, and especially the WEP, marked a significant turning point in the way that astronomical research was conducted up to that time.

And without the usage of high-speed, digital computers, shown there on the right for example, then virtually none of these observations and discoveries with OAO-2 would have been possible. Now, please note here that we're talking about room-sized mainframe computers, and/or refrigerator-sized minicomputers,

not personal or desktop computers, which hadn't been invented yet.

And as we'll see, these digital computing means were absolutely necessary to handle the data gathering and storage and transmission and analysis that was conducted with OAO-2, so this too is one of the principle legacies of that observatory and the WEP.

Now, we're extremely fortunate to have here, just beyond that wall, in this very building a full-scale engineering model of the OAO-2, here at Space Place. And if you've seen it or will, this will make it much easier for you to visualize, perhaps, the scale and complexity of the various devices and things that we are going to be talking about tonight.

In fact, there is no other full-scale model of the OAO anywhere on earth, correct, Jim? 
(audience member chattering faintly)
 Not even at the Smithsonian Air and Space Museum. So, we have the one and only example of its kind left in the world.

But now let us pretend to travel even a little further back in time, by about another decade or so, to around the beginning of the space age, and to look into the origins of the this remarkable Orbiting Astronomical Observatory.

Now, to most people the space age began with a jolt on the evening of October 4th, 1957, when they heard the surprise news that the Soviet Union had launched the world's first artificial satellite named Sputnik.

But in fact both the United States and the Soviet Union had announced plans several years earlier that they would try to launch such a satellite in conjunction with the International Geophysical Year, or IGY for short, which was a cooperative scientific venture that began on July 1st, 1957. And this involved a group of around 60 nations altogether that performed experiments in concert over the coming 18-month period.

Thus, the US was really in no hurry to try and launch the first IGY satellite, which was named Vanguard, and most officials strongly doubted that the Soviet Union even had the launch capabilities to perform such a feat.

Although those experts were mistaken, and the launch of Sputnik touched off what some historians have called a crisis of confidence here in the US.

Now, this development strongly hastened the subsequent creation of NASA, the National Aeronautics and Space Administration, roughly one year later in October of 1958.

But the desire to launch a new breed of scientific satellites actually preceded NASA's creation, as we'll see, and this is where we must look for the origins of the OAO.

Now, in June of 1958 a 15-member panel, which called itself the Space Science Board, or SSB, was created by the US National Academy of Sciences.

And the board was headed by a geophysicist named Lloyd Berkner, who shortly afterwards sent telegrams, and not email messages (snickering) to a number of leading US astronomers, asking for suggestions about scientific experiments that could be performed aboard future orbiting satellites. And one of those who received such a telegram was Dr. Arthur D. Code, the newly appointed director of the Washburn Observatory here at UW-Madison, but first we must digress a bit.

Professor Code was a native of Brooklyn, New York, who had earned his PhD in Theoretical Astrophysics from the University of Chicago, in 1950.

Yet, he was that rare sort of individual who was both an outstanding theorist, and here on the process of radiative transfer in Stellar Interiors, as well as in equally talented experimentalist, where he had gained vast experience with electronic equipment in the US Navy during World War II.

Now, Code was first hired on to the faculty of the Department of Astronomy at UW-Madison, in 1951, and he remained until 1956.

Now, he specialized in the photoelectric photometry of stars, but in that year he was hired away by the California Institute of Technology, or Caltech in Pasadena, which awarded him a tenured position.

But the launching of Sputnik, as we'll see, was to change all of that.

Now, when Code and a colleague there, Professor Donald Osterbrock, another former Wisconsin astronomer, observed the passage of Sputnik over the Caltech campus in the fall of 1957, Code realized that it should now be possible to measure the ultraviolet energies of the stars from an orbiting satellite.

And why this should be such a big deal is what will require and get a little more of an explanation.

Now, first of all, the hottest stars emit most of their energy in the ultraviolet portion of the spectrum. And they appear bluish-white to the unaided eye.

But while the earth's atmosphere is transparent to visible light and longer radio waves, it is essentially opaque to all other forms of energy that are shorter in wavelength than visible light. So, those to the right side of that diagram up there.

So that is the atmosphere blocks and absorbs most ultraviolet and then all X-ray and gamma ray energies that come to us from astronomical objects. Now, if that were not the case, these ionizing forms of radiation would make life on earth's surface nearly impossible. And we would likely not be here. But to a ground-based astronomer, the inability to examine and measure those other shorter wavelengths made a more detailed understanding of the properties of stars pretty much impossible. So, only by getting above the atmosphere where all of those wavelengths can be observed could significant progress then be made along those lines.

And as far back as 1960, Code had written there that carrying out such observations from above the atmosphere, "Cannot fail to have a tremendous "impact on the future course of stellar astronomy."

And he was certainly right.

Now, what happened next for Professor Code's career path may be regarded as mixture of fortunate timing and opportunity, but also entailed a somewhat unusual and potentially risk-filled decision on his part, namely he resigned from his tenured position at Caltech.

And effective on July 1st, 1958, returned here to UW-Madison as a full tenured professor, as head of the Astronomy Department and director of the Washburn Observatory.

Now, the Wisconsin position offered him the autonomy to pursue research in space astronomy that almost certainly would have been lacking had he remained at Caltech, which housed the 200-inch Mount Palomar Reflector shown there in the lower left, which was the then largest ground-based optical telescope in the world, but he knew that the 200-inch telescope could never see into the ultraviolet, and that it therefore remained a somewhat limited instrument in his judgment.

Now, also, critical to his decision to return to Wisconsin was the commitment expressed by the UW administration to rapidly enlarge the Department of Astronomy, and to establish a full graduate-level program here. These were steps that had not yet been taken by Code's predecessor, Professor Albert Whitford, but which had been recommended by him. And Whitford went on to become the director of the Lick Observatory in California.

So, Code goes back-and-forth returning to Wisconsin.

Now, the upshot of all of this is that by the end of 1958 Code was appointed as a member of the Space Science Board, which after the formation of NASA evolved into that organization's Space Science Working Group, or SSWG, shown here, which consisted of four or five key teams and institutions who then carved out niches for themselves, starting in 1959. Now, the first four teams shown here all shared an interest in pursuing ultraviolet astronomy from space, while the fifth group went on to create the first Orbiting Solar Observatories, or OSOs, some of which preceded the OAOs.

So that was the breakdown.

In 1959, Professor Code, along with Professor Theodore Houck here, founded the Space Astronomy Laboratory or SAL as it is known here at UW-Madison.

Sal was the first of several notable startups in Professor Code's career, which even came to include the later Space Telescope Science Institute.

And Professor Houck was a recent PhD graduate of the astronomy department. Incidentally, the order of these four individuals from left to right, Code, Houck, McNall, whom I'll get to, and Professor Bless there on the right.

They were joined by Professor Robert C. Bless, a recent PhD graduate from the University of Michigan, who began as a postdoctoral researcher in 1958, before becoming a permanent member of the Astronomy Department.

Both Houck and Bless had done their research on the photoelectric photometry of so-called early-type stars, that is OB and A Stars.

And then the fourth original member of SAL was Professor John McNall, standing there in the back, whose expertise lay in electrical engineering and computer science most especially. And this, as we will see, played a crucial role in the success of OAO-2.

Now, SAL was originally housed in Sterling Hall, but this soon became overcrowded and

among several temporary relocations, including buildings such as this on South Park Street, that were far from glorious, it finally acquired a permanent home in adjoining Chamberlain Hall.

In spite of the somewhat adverse working conditions early on, the four researchers developed a strong camaraderie among themselves and formed a rather loose-knit organization which seemingly enjoyed an absence of bureaucratic red tape.

(audience members chuckling)
 Now, well before OAO was launched, the team received financial support from NASA, and constructed some early prototypes of their photometers, and so on, and their worthiness under near space-flight conditions was first tested aboard high-altitude weather balloons, shown there in the middle, although this is not their balloon, it's just a balloon, starting in 1961.

Then aboard Aerobee sounding rockets, shown there on the lower-left, that were launched from White Sands, New Mexico, starting in 1962, and also on several flights of NASA's X-15 rocket plane, shown there in the lower-right. And one departmental PhD graduate Charles or Chuck Lily performed research on the interstellar radiation field, using an Aerobee sounding rocket, in the mid-1960s.

Now, when the Space Science Working Group was first organized, a satellite payload of around 100 to maybe 150 pounds was considered the maximum possible weight that could be lifted into space using existing rocket boosters of the project Vanguard class.

And so this is an early concept of one of those possible satellites, but by 1960 with the development of much larger boosters in preparation for the manned project Mercury series, the payload capacity was then raised to around 5,000 pounds. And this, of course, had enormous implications for the planning of individual experiment packages by the various teams.

And based on this newer weight capacity, NASA also announced a crucial decision that would impact all of the teams in the working group.

And this was the choice to adopt a so-called modular approach that would enable all of the experiment packages to be flown inside a standardized platform or bus that would be carried into orbit.

Now, on the one hand, that decision made perfect sense, because all of the experiment packages would share common pointing requirements, and they would all derive their electrical power from a pair of solar panels that were attached on the outside of the platform. And standardization is usually a good thing and is followed.

Now, the module itself was to be a single tube roughly 10 feet long and around 40 inches in diameter.

Now, NASA also felt that perhaps two or more of the experiment packages should be combined and operated jointly within the same module.

But now how is this to be done? And here there was an interesting twist or outcome to the story.

When this requirement was announced, Professors Code and Houck suggested that two of the packages, namely those from Wisconsin and the Smithsonian's Project Telescope, which we'll describe more coming up, could be placed at opposite ends of the same tube.

But NASA engineers immediately countered that such a configuration would be impossible to execute in actual operation.

However, about two weeks later they changed their minds, and then agreed that the Wisconsin team's suggestion posed an optimal solution to that problem.

Thus, we have this arrangement.

And with that said, we can now turn towards some of the challenges that were to be overcome in operating a full-fledged astronomical observatory in space. And this is where we can begin to demonstrate why the OAO-2 satellite represented a greater technological step forward in its day than the launching of the Hubble Space Telescope, more than two decades later.

Now, the OAOs would be placed into an orbit some three to 400 miles above the earth's surface, and they would circle the earth roughly every 90 to 100 minutes or so. And of course for each half of the orbit the OAO was in sunlight, then during the other half it was in earth's shadow.

But now the sun and earth and moon are constantly changing their positions, and so everything must be recalculated in advance to know where they are, and thus the OAOs demanded a "Complex stabilization and control system," in order to make all of this happen.

So, essentially there are four principle tasks that need to be done in order to operate such a telescope in space, and I'll go over each of these briefly to help us understand how each one was to be solved. And when they're considered as a whole, I think you'll see why high-speed, digital computers are absolutely essential for performing these tasks.

So, number one, you always need to know what the telescope's orientation is at all times, and this is accomplished with six star trackers. Two, I assume on the end of each of three axes that look out in opposite directions, and so that will tell you where you are at the moment. But then when you get ready to observe something else, you have to generate slewing commands that will move the telescope from one position or one object to another. And this also needs to be done in an efficient manner in order to save fuel in these maneuvers. And I might also mention both one and two there require a pointing accuracy of better than one arc minute. Now that's less than 1/60th of a degree in angle on the sky, so this is a high precision operation.

You also need to know when the celestial objects might appear too close to the sun or moon or wind up behind the earth, and where they could not be observed.

So, you have these safety checks. And then every so often, at least, you also have to reposition the spacecraft as a whole to allow the solar panels then to recharge its batteries.

And those require additional slewing commands.

So, all of these things must be accomplished. The OAOs like any satellite or spacecraft use an independent coordinate system with three axes of rotation that are known as roll, pitch and yaw, respectively.

So to solve all four of those principle tasks required the writing of a complex computer program, that would enable it to perform all of the calculations to make the many logical decisions needed to carry out the slewing commands, and perform the safety checks to know whether you could or could not observe a celestial object at a particular time, and no other system of this complexity had ever been created before.

Now, the computer program which was written in FORTRAN was developed by UW graduate student, Harry or Curt Peacocks, the name he goes by, which earned him a master's degree from the Department of Astronomy, and there is the title page of his thesis there on the left.

The program was named HARUSPEX, an acronym I won't explain here.

But Healow’s advisor was SAL Scientist John McNall.

And Peacocks is there on the right in that image, beside him is Kathy Spittlebug who began as an assistant and ended up staying a long time with the program, and becoming associate director of SAL after something like 20 to 30 years. So, that as far as I know is the only image we have of Peacocks from that time frame.

Now, this program was then run on an IBM 360 mainframe computer at the Goddard Spaceflight Center there in the Greenbelt, Maryland, which was the overall manager of the OAO program.

But the program also provided an interface to the Wisconsin team's ground operating station, shown with people around it here, which ran on a smaller Digital Equipment Corporation minicomputer. This machine had only 32 kilobytes, not megabytes of memory, and was operated by a teletype.

How things have changed. The minicomputer displayed the status of the Wisconsin team's equipment, such as the filter positions, gains, voltages, and temperatures of all of the instruments. And if an emergency arose, it could communicate directly with OAO assuming that the ground station was in contact with the satellite.

And the program HARUSPEX was also used to prepare future observing schedules, which might extend for about a week's time.

Well, it's now time to talk about the three kinds of scientific instruments that went into the Wisconsin Experiment Package, or WEP, and how they were assembled into a single unit.

Now, NASA had previously selected the Grumman Corporation from Long Island, New York as the prime contractor of the OAO's standard platform, shown there.

And that alone had a weight of roughly 3,400 pounds.

Each team's payload was then restricted to a weight of around 500 pounds.

Now, to be clear the Wisconsin and Smithsonian packages were to be united in the first OAO satellite, although each team had significantly different goals, as well as working styles.

Neither team however had the full in-house resources needed to build the packages themselves, and so both teams solicited bids from outside contractors for their respective packages.

But the Wisconsin teams' reluctance to place its investment in the hands of a large bureaucratic organization that was located perhaps one or two thousand miles away.

So, instead they selected the much smaller and nearer Cook Technological Center. It was located at Morton Grove, Illinois, outside of Chicago. And this regional company more nearly matched their own less formal organizational style, and perhaps most important of all, they could drive down there in about two hours and see it.

So, Cook thus designed and tested all of the electronic circuitry used in the WEP, although the SAL team performed the actual assembly. And the chief electronics engineer there at Cook was Curtis Bend ell.

Now, for reasons of redundancy as well as to maximize the UV spectral ranges that could be observed in the WEP, they consisted of seven co-aligned scientific instruments [inaudible] and the longest one shown here which took up the central portion of the cylinder was a 16-inch diameter reflecting telescope that was equipped with a photoelectric photometer.

And with it having the largest light grasp of the OAO telescopes, it was used to examine the ultraviolet spectrum of nebulae and galaxies, and other extended objects.

Now, it was surrounded by four identical eight-inch, off-axis telescopes that were also equipped with photoelectric photometers. And these were used you upon stellar or other point-source targets, and had narrow fields of view, and thus requiring the high pointing accuracy.

Now, between the eight and the 16-inch telescopes, they gave SAL scientists a range of sensitivity of somewhere between about four and six orders of magnitude, that is powers of 10, in their observations.

Now, the final two instruments were nearly identical scanning spectrometers, that also employed eight-inch diameter optical systems, but what made them different was the incoming UV light was first directed onto a plane grating, which dispersed that light into its component colors.

And the light was then directed into a telescope and finally focused onto a photometer.

But now during an observation, the grating was slowly tilted by a drive motor, and which then allowed the brightness measurements to be made in 100 equal steps along the spectrum.

And I'd like to emphasize that these spectrometers produced very accurate readings of the ultraviolet energies of stars, and were what Professor Code had envisioned in the beginning in the wake of Sputnik.

Now, in the light of what was to come, I also need to briefly describe the other instrument package aboard the OAO-2 satellite, and this was the one constructed by the Smithsonian Astrophysical Observatory, or SAO for short.

And it was named Project Telescope, short for celestial telescope.

And they planned to install four roughly identical, ultraviolet-sensitive television cameras into the other end of the module.

And their goal was to photograph long-exposure, two-degree-by-two-degree images of the sky whose characteristics could then be examined afterwards.

This also meant that the pointing accuracy needed for Telescope was actually far less critical than that needed for the narrow-angle instruments aboard the WEP.

Now, by 1966, the WEP had passed all of its pre-flight inspections, and was declared ready for launch.

However, Project Telescope remained incomplete from their inability to secure the UV-sensitive cameras from its contractor.

And I guess the expected delays were thought to maybe last another year or so, before they could be resolved, and thus NASA was forced to come up with a last-minute substitution for the Telescope experiment.

And then in its place, two MIT physicists named William Kraushaar and George Clark were made principle investigators on a gamma ray detector, while two other physicists, Phillip Fisher and Kenneth Frost, were the principle investigators on two soft X-ray experiments.

Now, Professor Kraushaar later joined the faculty of the Physics Department here at Madison.

And by April of 1966 these two substitute experiments were also deemed ready to launch, although in retrospect this may have been a somewhat poor management decision on NASA's part, because the rush nature of these latter two experiments may well have led to their failures.

Now, when the first five or six launch attempts of OAO-1 were scrubbed for various reasons, the Wisconsin team had to go on virtually no sleep to prepare for the next attempt, because they've got to reprogram the computers and everything to be ready. But at last on April 8th, 1966, OAO-1 was successfully launched from the Kennedy Space Center in Florida.

But major problems soon developed with the satellite's power and guidance systems, and it's thought that one of its batteries may have exploded from some high-voltage arcing.

And after two days all contact with the satellite was permanently lost, so not a single astronomical observation had been performed with either instrument package.

And this naturally was a devastating blow to the Wisconsin team and others whose timing it was later realized was marked by a perverse symbolic coincidence, because OAO-1 had been launched on Good Friday and died on Easter Sunday. 
(audience members laughing)
 Read into that what you may, if you wish. 
(audience member laughing)

But not all was lost, however, because another full-scale prototype of the OAO had already been built and could be adapted to make it space worthy. It meant, however, that the Wisconsin team would have to completely rebuild their experiment package, which they did. NASA thus resorted to its original flight schedule, as if OAO-1 had never happened, and this finally united the new WEP with the Smithsonian's Telescope experiment. And it also kept the remaining Goddard and Princeton teams and experiments in their original pipeline, so to speak.

So, after another two and one-half years, these teams were ready to try their luck again.

Fingers crossed. Now, this time the OAO-2 satellite was successfully launched by this Atlas-Centaur Rocket on December 7th, 1968. And it was placed into a nearly circular orbit some 480 nautical miles high, and was inclined some 35 degrees to the earth's equator.

Now, the new mission's expected lifetime though had been reduced to only year. Yet, the satellite lasted more than four times that long, making observations over a 50-month period that again reached over a thousand celestial objects.

Project Telescope, by contrast, only exceeded its expected lifetime by four months.

Now, there was, however, one notable instrument failure on the WEP that occurred roughly two and a half months into the mission. And there the circuitry that controlled a stepper motor, which turned a filter wheel on the 16-inch telescope stopped working, and the calibration source was permanently positioned in front of the light path.

Oops. That unfortunately eliminated any more data gathering with the WEP's largest instrument.

But that's why you chose redundancy in your design.

Now, also troubling to the Wisconsin team was a planned financial move on NASA's part to shut down the OAO-2 at the end of June 1971, because the satellite had then exceeded, I'm sorry, its expected lifetime by two and a half times.

But the Wisconsin astronomers mounted an intensive lobbying effort that caused the space agency to rethink its decision, and as a result kept funding for the still active satellite intact, until its eventual demise.

Now, the scientific results from OAO-2 have been published in a number of places, and a majority of those findings were first reported in a series of 41 successively numbered research papers, that appeared in the Astrophysical Journal, between 1970 and 1982, that were authored by Professor Code and his colleagues. This is number one in that series.

And a symposium was also convened by NASA, and the American Astronomical Society at Amherst, Massachusetts in August of 1971. And its proceedings, shown there on the right, have formed the principle monograph of the OAO-2 results, and which was also edited by Professor Code.

And various catalogs of the OAO's data have likewise appeared through the years, at least through 1999.

Now, in the limited time that I have left, I'll try to present a very brief summary of the most important findings that came from OAO-2, but with an emphasis on those acquired by the WEP. And at the head of that list, WEP gathered the first true measures of the spectral energies of hot stars in the ultraviolet, as Professor Code had long sought to do. And this data led to more precise determinations of stellar temperatures and their chemical compositions. Now, the previous temperature scale that had been extrapolated from visual light observations was found in fact to be too low.

And chiefly among O and B-type stars, which were the hottest, the absolute luminosities of around 150 of those stars were determined to an accuracy of around 10%.

Now, around a quarter of the O and B stars also lent themselves to an independent measurement or derivation of the so-called interstellar extinction or reddening factor within the Milky Way Galaxy. And these results offered a confirmation of the densities of interstellar hydrogen gas that were certainly not at variance from other measurements taken from radio astronomy.

The spectra of a number of peculiar stars were observed, and the strengths their ultraviolet emission lines were measured, and that evidence suggested that these stars were surrounded by expanding shells of gas.

And this led toward improved models of the processes of mass loss in such highly evolved stars.

Now, a fortunate occurrence was the chance to monitor the behavior of a nova, that is Nova Serpentis, 1970, following its outburst.

And OAO-2 first showed that the UV brightness continued to increase even while the visual brightness was decreasing. And this finding supported the notion of an optically thick shell of material around the star, that may have behaved analogously to that of so-called planetary nebulae seen around lower mass stars.

Now, most of these so-called dark nebulae appeared brighter at UV wavelengths than did ordinary diffused or gaseous nebulae when seen in visible light.

And this finding implied that interstellar dust grains, whatever they were, had much higher UV albedos, or reflectivities, than was expected. And this evidence also suggested at that time that these grains might consist of graphite particles, although it is now considered that other kinds of hydrocarbon compounds may in fact be the more likely sources or makeups of those kinds of particles.

And it was found that galaxies on average are brighter in the ultraviolet than was expected. Just from the known main sequence stars that make them up.

And therefore, the populations of O and B stars in these galaxies' discs evidently turned out to be larger or higher than had been expected.

Now, some important discoveries were also made in our own solar system. For instance, OAO detected vast clouds of hydrogen gas around the nuclei of two different comets.

And the hydrogen clouds were measured to extend some 100,000 kilometers away from the comet's nucleus, that is they were even larger than the sun.

Now, these observations actually confirmed an earlier prediction that such clouds might exist, and they provided evidence that a comet's nucleus is largely made of water ice, which breaks down into its atomic components under the action of intense sunlight.

And finally, OAO-2 detected the presence of ozone gas in the atmosphere of Mars.

And this finding was later confirmed by the orbiting Mariner spacecraft, and is judged to have offered some important clues to the chemistry and evolution of the Martian atmosphere.

Now, if time permits, I'd just like to provide a brief set of conclusions about the importance, again, of OAO-2 on the 50th anniversary of its launch.

That is that OAO-2 may be credited with a number of firsts in the field of astronomy. And for example, it was the first true stellar space observatory, having been launched, again, more than two decades before the Hubble Space Telescope.

And from its computer-operated control systems, OAO-2 represented a greater technological step forward in its day than did the Hubble itself.

'Cause now all of the standard control techniques that require knowledge of the telescope's orientation and allow efficiently moving the telescope from one object to another, and all of the safety checks needed to prevent potentially harmful observations, these were all first performed successfully on OAO-2.

And as well, it marked the first significant opening of the electromagnetic spectrum to wavelengths shorter than visible light, and let me just give you an analogy here that Professor Bless often used to describe this innovation. The electromagnetic spectrum may be likened to the totality of sounds or musical notes produced, say, by a symphony orchestra, yet visible light corresponds to a very narrow range of perhaps only a few notes on that composite sound spectrum.

But that is all that optical astronomers had been able to hear for centuries.

And thus, over the past 50 years or more, astronomy has opened similar windows into just about every part of the electromagnetic spectrum: again, from gamma rays and X-rays and ultraviolet up to infrared and microwave and radio waves. So, we now pretty much covered it all.

And then perhaps most important of all, OAO-2 marked this other significant milestone in astronomical research, and this was the growing dependence placed upon digital data gathering, storage, transmission, and analysis that is practiced throughout astronomy, and I would say virtually all other scientific disciplines, today. And to quote Professor Code here from a 1969 article about OAO-2,

"High-speed computers are essential to the astronomer" for carrying out these many tasks.

At that time, however, digital computing as we've shown meant using mainframes, like the IBM and smaller minicomputers in the Wisconsin ground operating station.

Thus, it should be remembered that this transition to digital computing took place well before the introduction of desktop or personal computers, starting in the late 1970s and before the so-called charge-coupled device or CCD became another universal piece of astronomical equipment.

So by and large, apart from the telescope, or the spacecraft used itself, a computer is perhaps the single most important tool that an astronomer uses today.

So, I hope that we've been able to give you here a greater appreciation for the significance of OAO-2's launch, which is now 50 years behind us. So, thank you for your attention this evening, and if you have any questions, I will try to answer them now, but let me just throw in one more thing. If you'd like some more information about OAO-2, then you might find it within this paper that I published on its history, in reference given there. Thank you very much. 
(audience applauding)