Information about Great Observatories

NASA's series of Great Observatories satellites are four large, powerful space-based telescopes. Each of the Great Observatories has had a similar size and cost at program outset, and each has made a substantial contribution to astronomy. The four missions each examined a region of the electromagnetic spectrum to which it was particularly suited.

Great Observatories

• Hubble Space Telescope (HST) was called the Space Telescope (ST) before being named. It primarily observes visible light and near-ultraviolet. A 1997 servicing mission added capability in the near-infrared range. Launched in 1990 aboard the Space Shuttle Discovery STS-31
• Compton Gamma Ray Observatory (CGRO) was called the Gamma Ray Observatory (GRO), before being named. It primarily observed gamma rays, though it extended into hard x-rays as well. Launched in 1991 aboard the Space Shuttle Atlantis STS-37
• Chandra X-ray Observatory (CXO) was called the Advanced X-ray Astronomical Facility (AXAF), before being named. It primarily observes soft x-rays. Launched in 1999 aboard the Space Shuttle Columbia STS-93
• Spitzer Space Telescope (SST) was called the Space Infrared Telescope Facility (SIRTF), before being named. It observes infrared. Launched in 2003 aboard a Delta II rocket.

The concept of a Great Observatory program was developed in the mid-1980's by Charles Pellerin, NASA's Director of Astrophysics, working with Astronomer Martin Harwitt. This perspective enabled the yet-approved X-ray and InfraRed observatories to be appropriately seen as a continuation of the astronomical program begun with Hubble and CGRO rather than competitors or replacements.

Of these satellites, only the Compton is not operating; one of its gyroscopes failed, and NASA ordered it to be de-orbited on June 4, 2000. Parts which survived reentry splashed into the Pacific Ocean. Hubble was intended to be retrieved and returned to Earth by the Space Shuttle in 2010. Although this retrieval plan has been officially abandoned, NASA officials are reconsidering due to public pressure.

Spitzer was the only one of the Great Observatories not launched by the Space Shuttle. It was originally intended to be, but after the Challenger disaster, the Centaur LH2/LOX upper stage that would have been required to push it into a heliocentric orbit was banned from Shuttle use. Titan and Atlas rockets were cancelled for cost reasons. After redesign and lightening, it was launched by a Delta II rocket instead.

Strengths

Each observatory was designed to push the state of technology in its intended wavelength region. As x-rays, gamma-rays and far-infrared radiation do not pass through the Earth's atmosphere, space missions were essential for the Compton, Chandra and Spitzer observatories.

Hubble also benefits from being above the atmosphere, as the atmosphere blurs ground-based observations of very faint objects, decreasing spatial resolution (however brighter objects can be imaged in much higher resolution than Hubble from the ground using astronomical interferometers). Larger, ground-based telescopes have also recently matched Hubble in resolution for near-infrared wavelengths of faint objects. Being above the atmosphere eliminates the problem of airglow, allowing Hubble to make observations of ultrafaint objects. Ground-based telescopes cannot compensate for airglow on ultrafaint objects, and so very faint objects require unwieldy and inefficient exposure times. Hubble can also observe at ultraviolet wavelengths which do not penetrate the atmosphere.

Compton observed in gamma rays, which do not penetrate the atmosphere. It was dramatically larger than previous gamma-ray observatories, opening entirely new areas of observation. It also had four instruments, which complemented each other's sensitivities, resolutions, and fields of view. Gamma rays are emitted by ultrapowerful energy sources, such as black holes and supernovae.

Chandra, similarly, had no ground predecessors, and small orbital predecessors. Its spatial resolution was an order of magnitude better than previous missions (becoming comparable to some optical telescopes), and its large size, high orbit, and sensitive CCDs allowed observations of faint x-ray sources. These are also powerful objects, but with more visible details than in gamma rays.

Spitzer is quite difficult or impossible to replicate with ground telescopes, and had few orbiting predecessors. Spitzer was not an order of magnitude larger than its latest predecessor, ISO (the Infrared Space Observatory). However, Spitzer's instruments took advantage of the rapid advances in infrared detector technology at the time. Combined with its slightly larger aperture, favorable fields of view, and longer life, science return will be unprecedented. Infrared observations are useful for distant astronomical objects where all the visible light is redshifted to infrared wavelengths, cool objects which do not emit much visible light, or objects obscured by dust at visible light wavelengths.

Impact

All four telescopes have had a substantial impact on astronomy. The opening up of new wavebands to high resolution, high sensitivity observations by the Compton, Chandra and Spitzer has revolutionized our understanding of a wide range of astronomical objects, and has led to the detection of thousands of new, interesting objects. In comparison, at optical wavelengths Hubble has provided a more modest improvement in sensitivity and resolution over existing instruments. Hubble's capability for uniform high-quality imaging of any astronomical object at any time has allowed accurate surveys and comparisons of large numbers of astronomical objects. The Hubble Deep Field observations have been very important for studies of distant galaxies, as they provide rest-frame ultraviolet images of these objects with a similar number of pixels across the galaxies as previous ultraviolet images of closer galaxies, allowing direct comparison. The James Webb Space Telescope will provide an even greater step forward, providing rest-frame visible light images of even more distant galaxies which can be directly compared with images of nearby galaxies at (more familiar) visible light wavelengths.

Hubble has had a much larger public and media impact, partly because it operates at wavelengths which are familiar to us, and partly because of a lack of public appreciation of the importance of other wavebands in modern astronomy.

Synergies

Aside from inherent mission capabilities (particularly sensitivities, which cannot be replicated by ground observatories), the Great Observatories program allows missions to interact for greater science return. Different objects shine in different wavelengths, but training two or more observatories on an object allows a deeper understanding.

High-energy studies (in x-rays and gamma rays) have had only moderate imaging resolutions so far. Studying x-ray and gamma-ray objects with Hubble, as well as Chandra and Compton, gives accurate size and positional data. In particular, Hubble's resolution can often discern whether the target is a standalone object, or part of a parent galaxy, and if a bright object is in the nucleus, arms, or halo of a spiral galaxy. Similarly, the smaller aperture of Spitzer means that Hubble can add finer spatial information to a Spitzer image.

Ultraviolet studies with Hubble also reveal the temporal states of high-energy objects. X-rays and gamma rays are harder to detect with current technologies than visible and ultraviolet. Therefore, Chandra and Compton needed long integration times to gather enough photons. However, objects which shine in x-rays and gamma rays can be small, and can vary on timescales of minutes or seconds. Such objects then call for followup with Hubble or the Rossi X-ray Timing Explorer, which can measure details in seconds or fractions of a second, due to different designs.

The ability of Spitzer to see through dust and thick gases is good for galactic nuclei observations. Massive objects at the hearts of galaxies shine in x-rays, gamma rays, and radio waves, but infrared studies into these clouded regions can reveal the number and positions of objects.

Hubble, meanwhile, has neither the field of view nor the available time to study all interesting objects. Worthwhile targets are often found with ground telescopes, which are cheaper, or with smaller space observatories, which are sometimes expressly designed to cover large areas of the sky. Also, the other three Great Observatories have found interesting new objects, which merit diversion of Hubble.

One example of observatory synergy is solar system and asteroid studies. Small bodies, such as small moons and asteroids, are too small and/or distant to be directly resolved even by Hubble; their image appears as a diffraction pattern determined by brightness, not size. However, the minimum size can be deduced by Hubble through knowledge of the body's albedo. The maximum size can be determined by Spitzer through knowledge of the body's temperature, which is largely known from its orbit. Thus, the body's true size is bracketed. Further spectroscopy by Spitzer can determine the chemical composition of the object's surface, which limits its possible albedos, and therefore sharpens the low size estimate.

• Late 1991: Operation of both Hubble and Compton
• Late 1999: Operation of Hubble, Compton, and Chandra
• Mid 2000: Operation of both Hubble and Chandra
• Late 2003-2008 (projected): Operation of Hubble, Chandra, and Spitzer

Successors to the original GO program

• James Webb Space Telescope (JWST) - the JWST, previously known as the NGST (Next Generation Space Telescope) is projected to replace Hubble (HST) around 2013. Its segmented, deployable mirror will be over twice as large, increasing angular resolution noticeably, and sensitivity dramatically. Unlike Hubble, JWST will observe in the infrared, in order to penetrate dust at cosmological distances. This means it will continue some Spitzer capabilities, while some Hubble capabilities will be lost (as currently planned). New advances in ground telescopes will take over some visible observations, but fewer in ultraviolet.
• Constellation-X - A mission to perform extremely sensitive x-ray observations, beginning around 2016. This is not a direct replacement for Chandra; Chandra is optimized for high angular resolution. Constellation-X is more of a follow-on to the XMM-Newton mission, which trades resolution for sensitivity. Constellation-X may be several times to several dozen times more sensitive than Compton. It will also extend further into the hard x-ray regions, giving it some abilities of Compton.
• GLAST, the Gamma Ray Large Area Space Telescope, is a follow-on to Compton scheduled for launch on January 31, 2008. GLAST will be more narrowly defined, and much smaller; it will carry only one main instrument and a secondary experiment. Other missions, such as HETE-2, launched in 2000, and Swift, launched in 2004, will complement GLAST. The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), launched in 2002, observes in some Compton and Chandra wavelengths, but is pointed at the Sun at all times. Occasionally it observes high-energy objects which happen to be in the view around the Sun.
• Another large, high-energy observatory is INTEGRAL, Europe's INTErnational Gamma Ray Astrophysics Laboratory, launched in 2002. It observes in similar frequencies to Compton. But INTEGRAL uses a fundamentally different telescope technology, coded-aperture masks. Thus, its capabilities are complementary to Compton and GLAST, not a direct replacement.
• Spitzer has no direct successor planned. However, JWST will exceed its performance in near-infrared, and the European Space Agency's Herschel Space Observatory will exceed it in the far-infrared when launched around February 2008. The SOFIA (Stratospheric Observatory For Infrared Astronomy) airborne platform will observe in near- and mid-infrared. SOFIA will have a larger aperture than Spitzer, but at lower relative sensitivities in restricted duty cycles. Also, smaller space missions will perform specialized infrared observations.

Note that none of these missions are designed for Shuttle launch, or manned servicing. Most are in orbits beyond the Shuttle's capability, to allow new observing modes. GLAST will have no appreciable instrument upgrades after launch.

See also

• Space observatory
• Terrestrial Planet Finder