As the Hubble Space Telescope marks 25 years in service, the James Webb Space Telescope is on its way.
Before the Space Age, professional astronomers were largely confined to mountain tops far from the light pollution of towns and cities. Although their instruments often replaced them at the eyepiece, the quality of the 'seeing', as astronomers call it, was very much dependent on the Earth's atmosphere. Even without cloud, the atmosphere is a seething cauldron of turbulence as air masses of differing temperatures and refractive indices move between the telescope and its objective. There is no better demonstration of this effect than the twinkling of the stars themselves.
Although spacecraft have obvious limitations, the best way to avoid the deleterious effects of the Earth's atmosphere is to rise above it. The first astronomical observatory in space was Nasa's Orbiting Solar Observatory, OSO-1. It was launched on 7 March 1962, just two weeks after John Glenn had become the first American to orbit the Earth. So it was no surprise that few outside the astronomical community noticed the event. The first satellite one would recognise as a 'real' space telescope - it carried four 31cm-diameter telescopes and four photometers - was the Orbiting Astronomical Observatory (OAO), launched in December 1968. Again, its advent was eclipsed by a manned mission, which saw the Apollo 8 astronauts orbiting the Moon, reading from the Bible and photographing Earthrise.
Space astronomy - astronomy conducted from space itself - finally made headlines with the Hubble Space Telescope (HST), which was launched on 24 April 1990. In fact, it's arguable that most folks didn't really notice the payload of the 35th Space Shuttle mission until reports came in that all its images were blurred. As a result of a measurement error during the final grinding and polishing process, the telescope's main mirror was given a precise but incorrect shape that resulted in an optical flaw known as spherical aberration.
The telescope squinted at the stars until December 1993, when another Shuttle mission was launched to fit some corrective optics. Like a blind man given sight, the effect was a revelation: suddenly the Universe sprang into sharp focus and Hubble images found their way into newspapers, coffee-table books and calendars. 'The Hubble' developed a persona that no other spacecraft had engendered since the little Sputnik had beeped its way across the sky.
The 25th anniversary of the HST's launch is a good time to take stock how the telescope has revolutionised our view of the universe. Dr Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, USA, puts it succinctly: "HST has significantly contributed to almost all areas of research in astronomy and astrophysics: from the composition of the atmospheres of extrasolar planets to the Dark Energy that is pushing the cosmic expansion to accelerate, and from supermassive black holes to the cosmic history of the rate of birth of new stars".
Professor Martin Barstow, president of the Royal Astronomical Society, adds a human perspective: "Of course, understanding the universe is what astronomy is all about, but I think the greatest contribution has been to the accessibility of astronomy to the population as a whole. During its 25 years, Hubble has become an iconic instrument and its spectacular images have had an important impact on the public perception of science, and astronomy in particular."
Since the 1993 repair mission, Hubble has been visited four more times by Shuttle astronauts (in 1997, 1999, 2002 and 2009) to replace solar arrays and update several observing instruments. Although astronomers agree that the HST is now a much better instrument than when it was launched, it is way beyond its 15-year design lifetime and, with the Space Shuttle fleet residing in museums across the US, there is no way to update or repair the telescope.
So how does Nasa follow the iconic Hubble? The answer lies with the James Webb Space Telescope (JWST), which is planned for launch in 2018. Although we may consider JWST to be the son, or possibly younger brother of the Hubble, it is interesting to contrast the two telescopes in a technical sense.
"While JWST is the scientific successor of HST, it is a very different type of telescope," explains Livio. "JWST will observe exclusively in the infrared, while HST was mostly an optical and ultraviolet telescope, with some capability in the near infrared."
The fact that JWST is essentially an infrared telescope means that, in the most simplistic sense, it is designed to detect heat. This makes the thermal engineering completely different; any heat generated by the telescope optics and detectors themselves manifests itself as thermal noise, which could easily drown the diminutive thermal signal from the objects under observation. Because of this, the spacecraft is protected from the heat of the Sun by a large five-layer sunshield, and there is a helium-based cryogenic system to cool the instruments.
Barstow explains that JWST is optimised for the infrared "to study distant galaxies and the earliest phases of the universe to understand how galaxies emerged." Its much larger mirror allows study of fainter and more distant objects than Hubble, he adds.
Livio says another task for JWST will be to "characterise the atmospheres of a few extrasolar planets in the habitable zone around their host stars" in the hope of finding an Earth-like planet with liquid water.
JWST's four main payload instruments were delivered and installed on their support module in April 2014 at Nasa's Goddard Space Flight Centre in Maryland, where thermal-vacuum tests were conducted. The module is due to be integrated with the Optical Telescope Assembly in early 2016 at Nasa's Johnson Space Centre in Texas. After further T-V tests the system will be sent to Northrop Grumman for integration with the service module, which supplies power and other services to the payload.
Another obvious difference between the HST and JWST is overall size. The Hubble is a large spacecraft, at 13m long and 4.25m in diameter, with a solar array span of about 10m, but the JWST is really huge because of its sunshield, which Nasa likes to call 'tennis court sized' (it's more than 21m by 14m). The successive layers of the shield work by absorbing solar energy and reradiating it towards the next layer and out into space from between the layers; that next layer receives a lower amount of energy, because some has been radiated away, and so on. According to design predictions, spacecraft temperatures on the hot side will be around 85°C, the cold side will be at minus 233°C (which is a heck of a temperature gradient).
Also important is the size of the collecting area provided by the primary mirror, since that determines photon collection capability and resolving power. Where Hubble's mirror is a commendable 2.4m in diameter, the overall diameter of JWST's 18 gold-coated beryllium mirror segments is 6.5m. To enable them to operate as a single monolithic mirror, the telescope uses a 'wavefront sensing and control subsystem' to sense and correct any alignment errors. Although the process is complex, it is essentially a matter of finding a single bright star with each of the segments and then driving them to a position where the star images coincide.
A final key difference between HST and JWST is their respective positions in space. While HST operates in low Earth orbit (about 550km above the planet), JWST will be stationed at the L2 Lagrange Point, a point in space where gravitational forces are sufficiently balanced to station a spacecraft. L2 is 1.5 million kilometres from Earth, so for JWST prime contractor Northrop Grumman, equipment failure is not an option.
Cold and headaches
It is thermal issues that are causing the biggest headaches on the JWST programme today. A US government report notes that Northrop Grumman "continues to face major technical challenges building the cryocooler" (a cryogenic cooling system for the telescope's Mid-Infrared Instrument) that have made it "the driver of the project's overall schedule". Northrop had been expected to deliver the cryocooler to Nasa in January 2014, but has not yet done so. The original contract, which was signed in 2006, called for delivery by 2010.
But things don't always go to plan at the cutting edge of technology. Development work on the original cryocooler showed that it would not be able to keep MIRI cold enough, so a new design was approved in 2008. Unfortunately, manufacturing issues (from leaky valves to brazed seals) have subsequently delayed the device. Since 2006, Northrop Grumman has tripled the head-count committed to the cryocooler and secured a seven-fold budget increase (from $22m to $150m).
As if this wasn't enough, the deployable carbon-composite framework that protects the large sunshield from vibration damage during launch has also eaten into funding reserves, as the first set did not meet Nasa's specifications and had to be replaced.
Last year JWST's deputy programme manager Eric Smith said: "Nasa is unhappy with performance and expects everyone on the team to do better". Considering the money involved, this is hardly surprising: Northrop Grumman's prime contract with is worth $3.5bn of a total of almost $9bn, which covers the manufacturing and launch of JWST and five years of operation.
Ask a professional astronomer about their hopes for the JWST, and the importance of engineering becomes clear. Both Barstow and Livio cite the complexity of the mission and hope for correct functioning of equipment: from the unfolding of the mirror to the deployment of the sunshield. Barstow adds: "My first hope is that it will be launched successfully". Beyond that, Livio has the simple wish that JWST will "give us a clear view of the very first galaxies [and] of galaxy evolution".
Ask a person in the street to name a spacecraft, and they might struggle, but ask them to name a space telescope and they are likely to say "Hubble". It remains to be seen whether the JWST will have the same impact: will James Webb become the new Hubble?
The Hubble space telescope: 25 years of watching the skies
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