Changing of the Guard: The Hubble Telescope Yields to the Next Generation
For 11 years, NASA's Hubble Space Telescope has amazed us with incredible pictures of stars, planets, and black holes. Eleven years is an eternity on the timescale of most technologies. For comparison, imagine a computer from 1991 still being used for the most advanced research, and it becomes obvious just how long-lived the Hubble has been. By 2010, a new telescope that promises to delve into the unknown reaches of space will replace the aged Hubble.
You've seen Hubble images in science textbooks and maybe even have caught the live-action chronicle of the fragmented comet Shoemaker-Levy smashing into Jupiter in 1994. But did you know that NASA's long-lived telescope is even today transforming humanity's view of the heavens? On its 58,000 turns around the earth, it has made 271,000 observations and generated more than 2,650 scientific papers (Sawyer 2000). To keep the Hubble Space Telescope (Figure 1) attuned to the needs of astronomy researchers, it has been continually improved by the frequent in-space addition of new instruments and the dismantling of non-essential equipment. The Hubble is now slated for use until at least 2010, and the extraordinarily powerful successor to the Hubble, the Next Generation Space Telescope, or NGST (Figure 2), will then replace it. With the information made available by the Hubble, scientists are making detailed studies of the birth and evolution of normal galaxies such as the Milky Way. They are also studying planets around other stars and searching for evidence of life.
Much of our knowledge of the universe comes from light. Starlight is composed of visible light. Visible light only makes up a small portion of the electromagnetic spectrum (Figure 3), which ranges from radio waves billions of times longer than those of visible light, to gamma rays, with wavelengths millions of times shorter than those of visible light. The wavelengths most important to astronomers in the electromagnetic spectrum lie between 100 and 1,000,000 nm (0.1 and 1000 µm) in the ultraviolet, optical, and infrared (UVOIR) regions (Next Generation 2001). Researchers are greatly interested in this region because stars and planets emit most of their radiation in this energy range. Additionally, the electronic and the vibration-rotation transitions of molecules, atoms, and ions all fall within the UVIOR - this range includes all the chemistry important to life (McKee 2000).
Land-based astronomical studies in the UVOIR part of the electromagnetic spectrum are problematic because of the effect of Earth's atmosphere. The earth's atmosphere absorbs a great deal of ultraviolet and infrared radiation, and distorts images in the visible part of the spectrum. Any starlight that reaches land-based telescopes is smeared by the atmosphere, causing the starlight to overwhelm the light from planets -- light that is needed for imaging purposes. Space telescopes, such as the Hubble and the NGST, fly in the upper reaches of the atmosphere so they can capture images and spectra from distant stars that would be difficult or impossible to obtain from the ground (The Hubble Project 2001).
Space-based measurements are not easy to obtain, however. Scientists have stretched the capabilities of the Hubble to be able to see galaxies as they were in the remote past, within a few billion years of the Big Bang itself. Yet, we need to look back even further. Distances in the universe are so vast that it takes light billions of years to cross it, and the distance shifts light from distant galaxies of the universe farther into the infrared part of the spectrum than the Hubble can image. This shift in wavelength, or redshift, is due to the expansion of space. As light traverses the expanding universe, the wavelength of the light increases according to the amount that space has expanded during the crossing time. In order to look back in time even farther and directly witness the events of galaxy formation, the NGST is being designed with instruments that will process the near- to mid-infrared radiation ( = 1-10 µm) from distant galaxies (Dressler 1996).
A comparison of the capabilities of the Hubble telescope and the proposed capabilities of the NGST reveals a great deal about the direction that space-based telescope science is taking. The Hubble was designed to be a work in progress - it was expected that space shuttle missions would periodically upgrade its instruments as needed. The current instruments on board include a Space Telescope Imaging Spectrograph (STIS) a Near Infrared Camera/Multi-Object Spectrometer (NICMOS), and a Wide Field Camera (WFC2). An Advanced Camera for Surveys (ACS), a Cosmic Origins Spectrograph (COS), and a next generation Wide Field Camera (WFC3) are slated for installation during missions in February 2002 and in 2003 (The HST Project 2001).
The Hubble telescope works by collecting light using an Optical Telescope Assembly (OTA). The light collected is directed through an aperture in the primary mirror and brought to a focus at a surface called the "focal plane." Light from this plane is then sent to a suite of scientific instruments for measurement and analysis (The HST Project 2001). The primary mirror of the Hubble is eight feet in diameter and provides image resolution from 120 nm (near-ultraviolet) to 2500 nm (near-infrared). In contrast, the planned diameter of the NGST is 20-23 feet, three times that of the Hubble, and would provide the NGST with unprecedented resolution. This large mirror must be assembled in space because of the difficulty of sending a pre-assembled mirror into orbit. Once the mirror is deployed, control and image analysis systems must be present to align the primary mirror segments and to keep them aligned in the changing temperatures to which they will be exposed (McKee 2000).
The STIS, a camera that covers the wavelength range from 115-1000 nm (ultraviolet and optical wavelengths), is one of the instruments that detects light reflected from the mirror. The special ability of the STIS is to simultaneously obtain spectra from many different points along a target (Woodgate 1998).
The NICMOS is Hubble's only near-infrared instrument. Because of the high background level of near-infrared radiation produced by a "warm" (near room temperature) telescope, NICMOS has inadequate sensitivity at wavelengths of greater than 2 m, where the radiation from distant galaxies is detected (Dressler 1996). Sophisticated coolers have been installed to cool the instrument as much as possible. To alleviate these difficulties altogether, plans for the NGST include a radiatively-cooled space telescope of large aperture. The instrument would function well in the near- to mid-infrared region, particularly at wavelengths of 1 to 5 m, from which much of the light from forming galaxies is expected.
The Wide-Field Planetary Camera 2 (WFC2) currently on board the Hubble can obtain high resolution images of astronomical objects over wavelengths ranging from 115 to 1100 nm (ultraviolet to near-infrared). This camera will be replaced in 2003 by the WFC3, which will operate over the wavelength range from 200 nm to 1800 nm (near-ultraviolet to near-infrared) and which is intended to guarantee the imaging capability of the Hubble until the new NGST is ready for launch (The HST Project 2001). Figure 4 is an example of an image that has been taken by the WFC instruments, depicting an unusual edge-on galaxy that shows how colliding galaxies spawn the formation of generations of stars. Another example of the capabilities of the Hubble's current instruments is shown in Figure 5, which shows the difference between an optical image taken by the WFC2 and an infrared picture taken by NICMOS.
The ACS, which will be installed in February, 2002, will consist of three electronic cameras for detecting light of 120 to 1000 nm in wavelength (ultraviolet to near-infrared). Each camera has specialized capabilities. A high-resolution camera will take extremely detailed pictures of the inner regions of galaxies and will search neighboring stars for planets and planets-to-be. A solar blind camera will block visible light to enhance ultraviolet sensitivity and will be used to study weather on planets in our own solar system, among other things. A wide-field camera will have a field of view twice the size of the Hubble's current camera. The new camera will conduct new surveys of the universe and will be used to study the nature and distribution of galaxies to improve our understanding of how the universe evolved (Ford 1998).
An additional camera that will be added during a 2003 mission is the Cosmic Origins Spectrograph (COS). COS is a near- to mid-ultraviolet (115-300 nm) spectrograph for observing faint point sources with moderate spectral resolution. The ultraviolet region is particularly useful for observing high-energy activities such as those found in new, hot stars. It is also a good region for viewing the composition and character of the Interstellar Medium (ISM) (The HST Project 2001).
Aided by continual improvements to the Hubble, scientists have made ground-shaking discoveries over the past 11 years. The Hubble's pictures have provided direct, unambiguous evidence for the central idea of modern astronomy - that the universe has evolved from a very different state, a hot and dense plasma left by the Big Bang, to the cooler world of galaxies and stars that is present today. The Hubble has also made what may prove to be the most important find in cosmology in the last 50 years. By isolating and studying individual stars in galaxies 50 million light years away, the Hubble is providing convincing evidence that the age of the universe deduced from the conventional cosmological model is measurably younger than the oldest stars in the galaxies themselves (McKee 2000)!
Despite these discoveries, Hubble's instruments are all limited in their ability to provide the detailed pictures and spectroscopy needed to understand the earliest steps in assembling a galaxy. This limitation is primarily due to the fact that the Hubble is mainly an optical, rather than an infrared telescope. Hubble sees deeply into the universe, but not deeply enough to see the "edge of light" where the first stars and galaxies came into existence after the Big Bang (McKee 2000).
The new NGST will have spectral imaging capability in the thermal infrared (5 to 20 m) and will image targets that were inaccessible to the Hubble (Next Generation 2001). This capability will result from increased efforts to protect the NGST from heat, which gives off radiation in the infrared region. Because the NGST must maintain a weight of about 12,000 lbs. to enable launch on a medium-sized rocket, it can't carry enough of its own coolant to reduce the interference from unwanted infrared radiation.
To solve this problem, the telescope will take a more distant orbit around the sun - the Hubble has an orbit of 600 km from the earth, while the NGST will orbit at a distance of 1.5 million km from the earth. This great distance will minimize the effect that sunlight reflecting from the earth has on the telescope. A second solution will be the use of a deployable lightweight sunshield to further block heat and stray light. The resulting decreased infrared noise will lead to increased imaging capability in the infrared.
Scientists will, for the first time, investigate in detail how matter accretes around a star, forms a disk, creates planets, and eventually disperses after the planet formation phase. Observations of stars at different ages, including older stars with "debris disks" akin to the remnant material in the solar system, will show how the disks evolve throughout their histories. With the NGST, scientists hope to detect the light from the first epoch of star formation in the universe and to trace the evolution of galaxies to the present time. They also hope to understand the pattern and history of element production, beginning with the first generation of stars and leading to the current time (Dressler 1996).
The plans for the NGST are grand, but the telescope still faces hurdles. Aside from technology development issues, the project is continually fighting for funding from Congress and other investors. The scientific community is lobbying hard to convey the urgent need to build this state-of-the-art replacement for the aging Hubble and, so far, money is still trickling in. As long as we continue research without the NGST, we are searching for our keys under the lamppost because the light is better there, even though the keys were lost in a dimly lit park. With the infrared capabilities of the NGST, astronomers will probe this dimly lit park of outer space, stretching our knowledge of the universe into realms previously only known in science fiction novels.
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Ford, H.C. "The HST Advanced Camera for Surveys in Space Telescopes and Instruments" V. Proc. SPIE. 3356 (1998): 234
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