Although the concept of something being “there” but being hidden from our eyes under the cover of darkness is one that has disturbed usually one or other of us once upon a time, when it comes to space it’s hard to find an idea that thrills scientists and astronomers more. That is perhaps other than them suddenly gaining the ability to “see” these previously inconspicuous cosmic objects, and the James Webb Space Telescope is being heralded as the astronomical tool that will help them do just that.
To understand what the James Webb Space Telescope is and what is hoped of it, we first need to have some idea of the current astronomical context of discovery in which it will find itself. We know the dimmest celestial objects that we can see in the night sky tend to be those that lie farthest from us and the visible light emanating from those objects in some cases has taken millions of years to travel across the measureless void of space to reach our eyes. However what about beyond? Do these heavenly bodies mark the edge of the Universe because they are the last deep-space objects to be seen? Most of us are aware that they do not. What then lies behind them, and more importantly why can these even deeper space objects presently not be seen? Well there are a couple of reasons, firstly, atmosphere. Whether it be you or I standing outside on a dark night looking out into space with the naked eye, or whether it be professional astronomers using the world’s largest land-based optical telescopes in light-pollution-free deserts or mountain ranges, in either case ground-based astronomy necessarily requires that we peer up into the heavens through that obscuring blanket of gases that make up Earth’s atmosphere. After centuries of stargazing though it comes as little surprise that astronomers have found a way of remedying this problem, by adjusting their optical technology slightly to account for the predictable distortion of space imagery as seen from terra firma.
Despite the adjustment we can imagine that doing astronomy through the sky’s ‘dirty pane of glass’ is not the preferred observational process. Enter the space telescope, or telescope that lives in space. Before looking at the James Webb Space Telescope for purely comparative purposes however let’s briefly consider another eminent representative from this space-based genre of observatories, the HST or Hubble Space Telescope. In the vacuum of space with little standing between the nominated object and the mirror of HST, it’s of little surprise that NASA ‘s current flagship optical instrument is credited with capturing many of the sharpest and most revealing space images ever captured, not least the Hubble Ultra Deep Field of view.
However despite its wonderful vision even Hubble has its limitations, for in certain directions HST can also only see blackness. There are thought to be two primary reasons for this. Firstly, interstellar dust. Although the Universe is essentially a great empty void, separating and engulfing certain deep space objects are great clouds of dust and gas. While the gas itself may not be visually impenetrable in terms of visible light, opaque dust and fragments of rock will be. So where cosmic dust abides is it game over as far as astronomical study is concerned? Far from it. While a significant portion of stellar and galactic radiation is in the naked-eye-perceptible white or visible light band of the electromagnetic spectrum, differing but very real levels of ‘other’ radiation simultaneously emanate from these sources along with the ‘white’ light. For skywatchers however to ‘see’ these other wavelengths of the EM spectrum radiating from black holes, quasars, nebulae, exoplanets, supernovae, galaxies, Gamma-ray bursts and the like, we would need superhuman vision or some sort of “super eye” in the sky viewing things on our behalf. Once again we re-examine the Hubble Space Telescope, because not only is this a powerful optical instrument in its own right, but it is equipped with a technological alter-ego, a ‘super’ side that enables it to see, photograph on our behalf, and relay to Earth images of these dust-obscured objects in infra-red light.
This remarkable feat is possible because while opaque dust and rock fragments may block visible rays, infra-red waves also emitted from this range of deep space sources penetrate these dust barriers and travel toward Hubble and its solely infra-red-programmed sister observatory, Spitzer, unhindered. However there are thought to be numerous other extreme deep space objects not hidden, emitting white light but still somehow exceeding Hubble’s optical sensitivities. One phenomenon thought to be responsible for their visual elusiveness is called Red Shift. It was that famous astronomer Edwin Hubble that first discovered a correlation between particularly distant celestial objects (such as galaxies) and their corresponding light signatures which seemed to have longer wavelengths. In other words where the bright stars and brighter naked-eye galaxies closer to Earth gave off plenty of ‘white’ or visible light, emitting radiation with predictably average wavelengths and frequencies, it was noted that much more distant objects in space that appeared far dimmer gave out mostly orange or red visible light often with considerable emissions of infra-red radiation, all characterised by lower wave frequencies and longer wavelengths. From this data Edwin Hubble proposed that the entire Universe was expanding and the incidentally ever increasing distance therefore between deep space objects and the observer on Earth meant that light waves trying to travel to our planet but getting “pulled” as their source moved in the opposite direction into space meant that the waves were getting “stretched” into the longer wavelengths on the EM spectrum of red and infra-red light. It is this transformative process that changes or “shifts” normal visible light into infra-red light that has earned it the name ‘Red Shift’. Besides the red shift factor though, near invisibility of some cosmic objects can also be attributed to age.
If we consider for a moment that galaxies are great cities containing billions of stars and we can imagine that the oldest of these cities are the ones in which the ‘stellar street lights’ are most likely to be dwindling or in some cases have dwindled away completely, then we can understand how the heat signatures from these galactic old-timers will also be the coolest. As a lower frequency and longer wavelength are also the characteristics associated with coolness (and antithesis of heat), the very oldest and coolest of still-existent celestial bodies will therefore exhibit primarily infra-red radiation and no white light, hence the need for the best infra-red detecting technologies to discern their presence. If HST and SST are already armed with Infra-red sensitive technologies surely there are no other hardware-induced limitations to NASA’s deep space observations? Well except one. This is a principle with which any telescope or binocular owner will be familiar. Another criterion on which our ability to see far into space depends is how big the ‘eye’, (namely lens or mirror) in our optical aid actually is.
Effectively the bigger the eye or “optical net” in their telescope the more rays of starlight an amateur astronomer can capture to improve the image he or she is viewing, and so make that distant celestial object less faint. However, as we progress farther out in space there will logically come a point at which even Hubble and Spitzer with their 2.4 metre and 0.85 metre mirrors respectively will catch so few rays of light from these most distant of heavenly bodies that the targeted objects remain effectively invisible when it comes to imaging them, that is, until now. It’s fair to say that NASA has had plenty of time since HST’s launch in 1990 to think about what the next rung in the cosmic observational ladder should be, and with recently released plans revealing what that bigger space telescope dream is and what it will look like, the administration’s great brains appear not to have been idol in the interim. Initially called NGST (or Next Generation Space Telescope) somewhere over the course of the project’s long developmental journey it was renamed, and with a year having now been officially announced, the entire international astronomical community eagerly awaits 2018, the year that the multi-space-agency venture, the James Webb Space Telescope is at last due to be borne into space.
(Part Two will follow in June 2014)
(Article by Nick Parke, Education Support Officer)