It's been 25 years since the Space Shuttle Discovery lofted the Hubble Space Telescope into orbit. Yet astronomers were not unanimous in their enthusiasm for the project, as this debate from 1990 recalls.

We sometimes forget how many astronomical discoveries were made before the advent of the Hubble Space Telescope, which left Earth on this date 25 years ago. Likewise, few now recall that the mission ended up costing some $2 billion just to get it to the launch pad, seven years behind schedule and roughly 100% over budget. And then there was the epic fail resulting from its misshapen primary mirror, now a distant memory thanks to the installation of corrective optics by visiting astronauts in late 1993.

As the Space Shuttle Discovery and its crew of five rocketed skyward from Florida on April 24, 1990, we all had high hopes that this go-for-broke spacecraft would become "astronomy's discovery machine." Today, few would argue that the Hubble's observations, which continue unabated thanks to five servicing missions by astronaut crews and sustained funding that's nearing $10 billion all told, have revolutionized astronomy.

Understandably, NASA and the European Space Agency (which shared in HST's development) have much to celebrate.

Yesterday a dazzling cosmic portrait by HST's Wide Field Camera 3 was unveiled by John Grunsfeld, who now serves as associate administrator of NASA's Science Mission Directorate but is better known as the astronomer-astronaut who lovingly repaired and upgraded HST on the servicing missions in 1999, 2002, and 2009. Lots of Hubble highlights appear on the websites of NASA and ESA. In fact, there's even a website devoted to Hubble's silver anniversary.

With all that HST has accomplished, it's worth noting that astronomers were not unanimous in their approval of the project during its long, tortured development. Reproduced below are a pair of Focal Points, written by renowned astronomers of that era, that ran opposite each other in Sky & Telescope's April 1990 issue. These essays offer a window into the soaring hopes and some of the concerns they had — that we all had — at the dawn of the Hubble era. Enjoy!

Is the Space Telescope a Mistake? No!

Some people have suggested that the Hubble Space Telescope (HST) is a mistake. They say that a different approach would have served the astronomical community much better. We disagree. HST will be a powerful, though admittedly costly, new tool for astronomers. Of course, this presupposes that HST works as designed — as we'll learn during the year ahead. If it does, we anticipate that it will make major discoveries and spur important advances in astronomy.

In 1959 one of us (A.B.M.) initiated the Space Division at Kitt Peak National Observatory. At the time NASA planned to launch four small Orbiting Astronomical Observatories. Ultimately, two of these OAO spacecraft failed during or soon after launch. However, OAO 2 (launched in 1968) and Copernicus (in 1972) flew very successful missions.

The division's goal was to look beyond these pioneering craft to the day when astronomers would regularly use large telescopes in space. Since then we have watched NASA's Space Telescope project from a distance, sometimes happy with its progress, sometimes not.

By the end of the 1960s it had become clear that an orbiting telescope with an aperture of 2 to 3 meters was technically feasible and scientifically desirable as a follow-on to the OAOs. Little did we dream that so many years would elapse between the start of the Large Space Telescope project (as it was then called) and the launch of HST. On our bookshelf we have two thick technical reports that show the telescope with essentially the same appearance HST has today - they are dated 1970 and 1972!

At the time, the project had an estimated cost of about $400 million, approximately what the four OAOs together had cost. If anyone had realized that the true price tag would be closer to $2 billion, HST might never have been born.

In retrospect this cost escalation was inevitable, for four reasons. First, NASA added many advanced capabilities to the observatory. Second, the optical system had to be better than any developed for use on the ground yet lightweight enough to be launched into space. Third, there was considerable over-optimism regarding the technical challenges involved in making such a complex spacecraft highly reliable. And finally, HST was tied to the Space Shuttle, leading to a costly three-year delay in the wake of the Challenger disaster.

Despite all that, there are signs that our high expectations for HST are justified. For example, the Space Telescope Science Institute (STScI) is in place with an excellent staff designed both to support the operation of HST and to see that it yields the maximum benefit to the entire astronomical community. Indeed, the institute's efforts have already borne fruit — STScI recently published a star catalogue listing nearly 19 million objects, the most comprehensive such collection ever produced (S&T: December 1989, page 583).

Even marrying HST to the shuttle has its silver lining. Astronauts can revitalize HST every five years or so, amortizing the nation's large investment over perhaps two decades. Already new instruments to be carried aloft for installation into HST are being built. They will provide new capabilities for the mission's second five years. HST's long lifetime may prove to be very important, allowing time for new discoveries to influence the observatory's ultimate capabilities.

A good indicator of the scientific importance of HST is the intense competition by astronomers to place objects of their special interest in the observing schedule (S&T: February, 1989, page 153). During the first year of general, or "guest," observations, HST will far from satisfy the demand for observing time. Yet, many more astronomers will be able to use the data after one year of proprietary ownership by the original observers.

Some astronomers say that the $2 billion cost to put one HST in orbit could have been better spent building several 8-meter telescopes on high mountains. In the mid-1950s, when an advisory committee presented its decision to build Kitt Peak National Observatory, a parallel argument was heard: it would drain off money from university astronomy. Instead Kitt Peak, along with the National Radio Astronomy Observatory, led the way to increasing the National Science Foundation's budget for astronomy, thereby benefiting many university observatories. We have faith that HST will similarly pave the way for a new generation of ground-based telescopes to follow up its discoveries.

Already astronomers are dreaming of the next generation of space telescopes and the quantum leap in capabilities they will bring. Whether or not these come about within the next two decades, HST itself will keep astronomers pushing back the frontiers of astronomy.

— ADEN B. MEINEL and MARJORIE P. MEINEL

Aden Meinel was the first director of Kitt Peak National Observatory. The Meinels later retired from the University of Arizona and the Jet Propulsion Laboratory. Marjorie died in 2008, Aden in 2011.

Is the Space Telescope a Mistake? Yes, but . . .

In my answer to this question, "but" means that the Hubble Space Telescope (HST) will be a magnificent instrument. It also means that Lyman Spitzer, Jr., who began promoting the idea of orbiting telescopes in 1946 and who has been the guiding spirit of space-based astronomy for 44 years, deserves the gratitude and admiration all astronomers feel for him. In 1946 Spitzer wrote of a 3-meter Large Space Telescope (LST): "The chief contribution of such a radically new and more powerful instrument would be, not to supplement our present ideas of the universe we live in, but rather to uncover new phenomena not yet imagined, and perhaps to modify profoundly our basic concepts of space and time." We all share Spitzer's hope that HST will fulfill this promise.

"Yes" means that I remember other words of Lyman Spitzer, words that HST's promoters have forgotten or disregarded: "To provide for a leisurely orbit and thus for relatively constant conditions, such an observatory should preferably be at some distance away from the Earth, probably as far as telemetering techniques and celestial mechanics might allow." In 1968 he added, "Quite apart from the engineering desirability of launching smaller space telescopes before building the large instrument, the astronomical requirement for a continuing series of smaller space telescopes should be an overriding consideration in setting the pace of the LST effort."·

The advantages of a high orbit have been abundantly confirmed by the International Ultraviolet Explorer (lUE), a small space telescope that has been used by more than a thousand observers since it was launched in 1978. IUE sits in a geosynchronous orbit, in constant line-of-sight communication with observers at control centers in Maryland and Spain. As seen from IUE, the Earth obscures only 2 percent of the sky. These advantages were denied HST by the decision to place it in a low-altitude orbit, where Earth blocks roughly half the sky at any time. The difficulties of programming observations from low orbit will limit HST's efficiency to a maximum of about 35 percent, comparable to that of a ground-based telescope, even though it won't have to contend with daylight and clouds.

The scientific output of IUE has also demonstrated the wisdom of Spitzer's call for a continuing series of smaller telescopes as pacesetters for HST. IUE can observe objects typically only down to magnitude 16 and with an angular resolution of 3 arc seconds. In spite of these severe limitations, the satellite has poured out an immense quantity of information about all kinds of celestial objects, from planets to supernovae.

HST will be able to observe objects some 10 magnitudes fainter than IUE with an angular resolution at least 30 times sharper — a great leap forward indeed! Unfortunately, its mirror's collecting area is only 30 times that of IUE, so that the photons from a 26th-magnitude object are collected by HST 300 times slower than those from a 16th-magnitude object are by IUE. Thus, to utilize HST at the limit of its capabilities will be very time-consuming. There are many exciting objects in the sky, and most of them are faint. HST will never have enough time. No single telescope, whether in orbit or on the ground, can provide the world's astronomers with the observing time they need to explore the richness of the universe.

So "yes" does not mean it was a mistake to build the Hubble Space Telescope at all. "Yes" means it was a mistake to sell HST to Congress and to the public as the space telescope for the rest of the 20th century. It was a mistake to push HST ahead of more modest space telescopes that could have been flying earlier. If we had even one imaging telescope in the I-meter class, looking at the sky with 0.l-arc-second resolution, many of the discoveries that HST will make might have been made 10 years sooner. If we had flown a series of I-meter instruments, operating like IUE and paving the way for HST, we would be able to use the precious observing time of HST much more effectively.

But we should not waste time now grieving over our past mistakes. Let us be grateful for what we have, and make the best of it.

— FREEMAN J. DYSON

Dyson, now 91, was a scientist at Princeton's Institute for Advanced Study until his retirement in 1994. He authored several popular books, including Disturbing the Universe (1979), Infinite in All Directions (1988), and A Many-Colored Glass: Reflections on the Place of Life in the Universe (2007).

To learn how Hubble has fundamentally altered our perceptions of the universe, read Govert Schilling's detailed analysis in Sky & Telescope's June issue.