1990 – Hubble
Launched in 1990, the Hubble Space Telescope has produced some of the most startling and memorable pictures of galaxies and other features of the universe. However, for me the most interesting aspect of this amazing telescope is how it was a failure to begin with, and how that failure was turned around. It is a story about technology, determination, and patience.
The idea of a space telescope is an old one. To put a telescope above the Earth’s atmosphere was one of those tantalising possibilities that must have given many astronomers and cosmologists sleepless nights as they restlessly thought of ways to make it happen. They had good reason, as the atmosphere distorts what we see, and at the same time limits the range of observations. A space-based telescope would have greatly enhanced resolution, away from atmospheric turbulence. At the same time, it would be possible to observe objects using infrared and ultraviolet light, which are blocked (in fact absorbed) by the atmosphere. The prospect was exciting, enticing, and for a long time only to be imagined.
The first serious consideration of how a space telescope could be realised was made in 1923 by Hermann Oberth, who thought a telescope could be propelled into an orbit around the Earth by a rocket. Once suggested, interest grew, and just after the Second World War, Lyman Spitzer, an astronomer, reminded scientists and money men of the reasons to get a telescope into orbit, given those advantages of better resolution and the wider range of the electromagnetic spectrum that could be studied. Space-based astronomy had begun to take a step forward as scientists started to make use of wartime developments in rocket technology. The first ultraviolet spectrum of the Sun was collected in 1946, and Spitzer continued to push for further developments. Continuing pressure led to the development of a space telescope being introduced into the US space program in 1962. Three years later Spitzer was appointed head of a group to develop the scientific objectives a large space telescope might pursue.
To begin with, work was focussed on orbiting observatories, mainly set up to obtain ultraviolet, X-ray, and gamma-ray spectra. In 1966 NASA launched its first Orbiting Astronomical Observatory mission, only for the module’s batteries to fail after three days. Undeterred, they sent up a second Observatory, which carried out ultraviolet observations from 1968 until 1972, looking at stars and galaxies, operating well beyond its expected lifetime of one year. Those two ventures had shown the value of space-based observations and in 1968, NASA prepared plans for a space-based reflecting telescope with 3 metre mirror, variously known as the Large Orbiting Telescope (LOT) or Large Space Telescope (LST), with a launch proposed to take place in 1979. The plans envisaged crewed maintenance missions to the telescope to ensure it had a lengthy working life. Although that might have sounded unlikely, this proposal was being worked on at the same time as plans for the Space Shuttle were advancing. It seemed those dreams could become real.
There are several books on the history of NASA in the 20th Century, and the trials and tribulations of each stage of its development have been well documented. For every project, the critical issue over the years was always the same, money! In 1970, NASA had established two committees, one to plan the engineering side of the space telescope project, and the other to determine the scientific goals of the mission. However, the real hurdle was to obtain the necessary funding for the telescope, which would be far more costly than any Earth-based instrument. US Congress kept sniping at the budget, and in 1974 it cancelled the whole proposal, which was only resuscitated after further prolonged lobbying. However, a reduced budget saw the telescope’s mirror diameter reduced from 3 m to 2.4 m, and a smaller precursor to test the systems was dropped. All these challenges led to collaboration with the European Space Agency, and in 1978 enough funding was provided to get the project moving, now with a launch date in 1983. That was the year the telescope was named after Edwin Hubble, a scientist whose observations had confirmed the universe was expanding.
I suppose you must love astronomy to get excited by the story: I am, the result of having a father who was an amateur astronomer, with a telescope in the back garden! Telescopes work by focussing light from distant objects, and there are two basic types: refractors that use lenses to concentrate light along a tube, and reflectors that use curved mirrors to do the same. The Hubble is a reflector. In technical terms, for those who like such detailss, a Ritchey-Chrétien Cassegrain reflector, the design used in most large telescopes. Not surprisingly, it is the quality of the mirror that is critical. In the case of the Hubble, its mirror needed to be polished to an accuracy of 10 nanometres, or about 1/65 of the wavelength of red light. To put that in perspective, one nanometre is one-billionth of a meter. As a comparison, a typical sheet of paper is about 100,000 nanometres thick!
Building the Hubble faced various problems. Construction of the mirror began in 1979, using Corning ultra-low expansion glass, and after various delays was completed two years later, by which time it was well over budget. To save money, NASA halted work on a back-up mirror and delayed the telescope launch date to October 1984. The Hubble spacecraft was a second major engineering challenge, as it had to withstand major temperature changes as it moved in and out of the Earth’s shadow, while remaining sufficiently stable to allow extremely accurate pointing of the telescope. By the summer of 1985, construction of the spacecraft was well over budget too, and it was three months behind schedule.
Despite delays, in January 1986, the planned launch date of October looked feasible. Then the Challenger rocket blew up soon after launch, killing all the astronauts on board, bringing the U.S. space program to a halt. The space shuttle fleet was grounded, forcing the launch of Hubble to be postponed for several years. Not an easy requirement as the telescope had to be kept in a clean room, regularly powered up and purged with nitrogen, until a firm date for a launch could be scheduled. Costing around US$6 million per month, this pushed the overall expense of the project even higher. However, the delay did have some benefits, as it provided time for engineers to perform extensive tests, swap out a possibly failure-prone battery, and make other improvements. It was also evident that the ground operated software needed to control Hubble wasn’t ready in 1986; it was barely ready four years later.
Eventually, following the resumption of shuttle flights in 1988, the launch of the telescope was scheduled for 1990, and on April 24, 1990, space shuttle Discovery took the telescope and its spacecraft up as part of its mission load. From an original estimate of around US$400 million, the Hubble cost about US$4.7 billion by the time of its launch, and its cumulative costs was to rise to US$10 billion in 2010, twenty years after the launch.
So much expenditure, and yet within weeks of the launch of the telescope, the returned images indicated a serious problem with the optical system. Although the initial images appeared to be sharper than those of ground-based telescopes, Hubble failed to produce the eventual level of sharp focus expected, and the best image quality obtained was drastically lower than intended. Analysis of the flawed pictures revealed that the primary mirror had been polished to the wrong shape! Although it was believed to be one of the most precisely ground mirrors ever made, smooth to about 10 nanometres, the outer perimeter was too flat (by about 1⁄11000 of an inch. As a result, nearly all the Hubble’s programs were essentially doomed, since they depended on accurate observations of exceptionally faint objects. Some work could be done, and the telescope still carried out several useful studies for the next three years, but only ones where less exact measurements were required.
How did this happen? A review found that a testing device had been incorrectly assembled, with one lens out of position by 0.051 in. In the final phases of preparation, the mirror had been ground very precisely but to the wrong shape. A few final tests had correctly identified the problem, but these results were dismissed because the instrument used at this stage, the one with the misaligned lens, was regarded as more accurate than the testing devices. Given fears Hubble would be abandoned, the race was on to find solutions to the problem that could be applied at the first servicing mission, scheduled for 1993. It was impossible to replace the mirror, but the fact the mirror had been ground so precisely to the wrong shape led to a solution, which was to design telescope components with exactly the same error but in the opposite sense. In effect, the plan was to give the Hubble ‘spectacles’ to correct the mirror’s spherical aberration and allow it to see clearly. Now, that was clever!
Two major systems were required to deal with the problem. One was a camera system, where it was relatively easy to design a corrective mechanism, as the cameral used mirrors, and new mirrors with an appropriate correcting design could be made. More complex was the need to correct distorted light, and this led to the development of what is known as COSTAR (Corrective Optics Space Telescope Axial Replacement – you did want to know, didn’t you!). COSTAR could be placed in the Hubble by removing one of the research instruments, and, over the years, each of the individual components that used light rays were themselves replaced, until eventually COSTAR could be removed: it now lives in the Space Race Exhibition in the National Air and Space Museum, in Washington DC.
Hubble is the only telescope designed to be maintained in space by astronauts. The first Hubble servicing mission was scheduled for 1993 . An earlier space mission in 1992 had demonstrated the difficulties in space work. While the Sun’s heat had been a problem previously, Hubble needed to be repaired away from sunlight. Seven months before the mission, it was found spacesuit gloves gave insufficient protection against the cold of space. For this and other reasons, NASA had to change equipment, procedures, and flight plans: seven total mission simulations took plaace before launch, the most thorough preparation in shuttle history. Moreover, as there were no complete Hubble mock-ups, the astronauts studied many separate models (including one you can see at the Smithsonian) to make sense of the task. The preparation paid off, and on January 13, 1994, NASA declared that first mission a complete success and released the first sharper images from the telescope.
Further missions took place in 1997, 1999, and 2002. One final visit to Hubble had been scheduled for February 2005, but another disaster interrupted plans, the disintegration of the shuttle Columbia as it re-entered the atmosphere in February 2003. NASA decided no further missions could take place unless a shuttle could travel to the International Space Station if problems arose. No shuttles had the capability to reach both Hubble and the space station during the same mission, and any further service missions to the telescope were cancelled.
After an outcry, mainly from scientists, a detailed proposal for a robotic service mission was developed. However, in 2004 these plans were later cancelled, as the robotic mission was found “not feasible”. In 2005 a new NASA Administrator stated he would consider a crewed servicing mission. The need was becoming pressing. Hubble’s main data-handling unit failed in September 2008. Servicing Mission 4 (SM4), flown by Atlantis in May 2009, was the last scheduled shuttle mission to ensure the telescope remained fully functional. The telescope completed 30 years in operation in April 2020 and could last until 2030–2040.
What has the Hubble achieved? One of its primary goals was to measure distances more accurately than ever before, and thus define more closely the rate at which the universe is expanding, and its age. The estimated age is now about 13.7 billion years, but before the Hubble results, scientists predicted an age ranging from 10 to 20 billion years. Hubble has also cast doubt on theories about the universe’s future, by contributing to the evidence that, far from decelerating, the expansion of the universe may be increasing. Another issue where the telescope has helped is in relation to black holes. While theorists in the early 1960s had suggested black holes would be found at the centres of some galaxies, Hubble showed that black holes are probably common to the centres of all galaxies. The Hubble program also established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble observations on black holes in galaxies has been to confirm a deep connection between galaxies and those rather scary central black holes.
The Hubble space telescope has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto and Eris. In 2012, U.S. astronomers using Hubble discovered Styx, a tiny fifth moon orbiting Pluto. In March 2015, researchers announced that measurements of aurorae around one of Jupiter’s moons revealed that it has a subsurface ocean estimated to be 100 km deep, trapped beneath a 150 km ice crust.
In case all that sounds big, esoteric and rather specialised, the policy for the Hubble is that anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation, although funding for analysis is available only to U.S. institutions. As you can imagine competition for time is intense, with about one-fifth of the proposals submitted in each cycle successful. The first director of the Hubble announced in 1986 that he intended to devote some of his director discretionary time to allow amateur astronomers to use the telescope. The total time to be allocated was only a few hours per cycle but budget reductions eventually made the support of work by amateur astronomers untenable. Despite this, amateurs have discovered minor planets, and changes in the atmosphere of several of the major planets. Above all, there is a treasure trove of extraordinary images from Hubble for public access: they are available at https://hubblesite.org.
Is that the end of the story? There is no direct replacement planned for Hubble as an ultraviolet and visible light space telescope, but a next generation project have consolidated around the James Webb Space Telescope. Very different from Hubble, it is designed to operate farther away from the Earth, not is it intended to be fully serviceable, although there’s a docking ring to enable visits from another spacecraft. JWST is the result of international collaboration between NASA, the European Space Agency, and the Canadian Space Agency. There are some great video introductions to this amazing telescope on You Tube. It’s big. The reflector comprises 18 hexagonal mirror segments made from gold-plated beryllium, which will combine to create a 21 ft diameter mirror, compared to Hubble’s at 7 ft 10 in. The other key difference is that the JWST must be kept very cold, a requirement to observe in the infrared spectrum without interference. Given this, it will be deployed further out in space, about 1m miles beyond the Earth, and will be protected from the sun in order to keep the mirror and instruments below −223°C ( −370°F). The technology is exciting and deserves another blog, but as the launch is planned for 18 December 2021, we’ll be bombarded with opportunities to learn about JWST in the next few months. For now, let’s hope it’s launch and deployment prove easy and successful. In the meantime, long live Hubble!
