The advantages of placing a telescope above Earth’s turbulent, absorbent atmosphere were apparent well before German V-2 rockets reached space in the 1940s. But what were the main technological hurdles to be surmounted? In an 11-page report for the think tank RAND, astronomer Lyman Spitzer Jr, then at Yale University, laid them out.1 He also outlined what research might be accomplished with two sizes of extraterrestrial orbiting telescope: a 10-inch reflector and a 200-inch reflector.

On the need to steadily point instruments at stars and other targets, Spitzer was optimistically brief: “Orientation might be accomplished in principle by reducing the angular momentum of the satellite to zero by means of external jets; thereafter the satellite could be rotated by internal means to any particular direction.” On the need to telemeter data back to Earth, he acknowledged that it “would involve many problems.” Undaunted, Spitzer predicted that a large space telescope would not so much supplement ground-based instruments. Rather, it would “uncover new phenomena not yet imagined, and perhaps to modify profoundly our basic concepts of space and time.”

In 1990, the year NASA launched the Hubble Space Telescope, Spitzer added a postscript to his 1946 RAND report.1 He noted that the report did not appear in the astronomical literature and it had not been widely distributed as a preprint. Its influence on astronomers was negligible, he concluded. “Its chief effect,” he wrote, “was on me.”

Convinced that a large space telescope was feasible, he championed the concept. Congress approved $36 million to fund its development in 1978. Spitzer recounted in his postscript that one of the crucial technologies that convinced Congress of the success of a large space telescope was the vidicon. Developed at RCA in the 1950s, the vidicon is a type of video camera tube. Focused photons strike and excite a photoconductor, whose charge distribution—the image—is read out by low-energy electrons and converted to a telemeterable electrical signal.

Four UV-adapted vidicons made by Westinghouse flew aboard Orbiting Astronomical Observatory 2 (OAO-2), which was launched in 1968 and is recognized as the first successful optical space telescope.

That same year Spitzer wrote a paper for Science that updated his 1946 report.2 One of the biggest challenges that remained was how to hold the telescope’s gaze. For the four 12-inch telescopes that focused light onto OAO-2’s vidicons, a pointing accuracy of 0.1 arcseconds sufficed. For the 120-inch Large Space Telescope that Spitzer envisioned, reaping the gain in resolution from observing in space would require far greater steadiness. He calculated that a pointing accuracy of 0.004 arcseconds would be needed when observing at 500 nanometers, the middle of the visible spectrum. Hubble ended up with a primary mirror of 2.4 meters, or 95 inches. The spacecraft’s attitude control achieved a pointing precision of 0.007 arcseconds over an entire day.

Among the observations Spitzer anticipated in 1968 were of Cepheid variable stars. Distances to that class of variable stars can be derived thanks to a relation between their absolute luminosity and the period of their brightness fluctuations. Galactic and extragalactic Cepheids constitute the first rungs on the extragalactic distance ladder. The Large Space Telescope, Spitzer predicted, could extend the range of Cepheid observations by an order of magnitude.

At the time of Hubble’s launch, the current value of the Hubble constant H0 was so uncertain that cosmologists used a dimensionless variable, h, to represent the factor by which H0 differed from an upper limit of 100 km/s/Mpc. Before Hubble’s advent, h could conceivably be as small as 0.5. By the time Hubble had completed its first set of Cepheid observations in 2005, H0 was determined to be 72 ± 8 km/s/Mpc. Last year, a team using Hubble images of gravitationally lensed quasars derived a value of 73.3 + 1.7 − 1.8 km/s/Mpc.

In print, this article accompanies a pullout visualization of Hubble’s history of observations. That visualization and additional articles celebrating Hubble’s 30th anniversary can be found here.

1.
L.
Spitzer
Jr
, “
Astronomical Advantages of an Extra-terrestrial Observatory
,” originally printed as appendix V in a report for Project RAND,
Douglas Aircraft Co
(
1946
);
reprinted in
L.
Spitzer
 Jr
, “
Astron. Q.
7
,
131
(
1990
).

Charles Day is Physics Today’s editor-in-chief.