Editing Great Observatories program (section)

== Strengths ==
[[File:Crab Nebula NGC 1952 (composite from Chandra, Hubble and Spitzer).jpg|thumb|upright=1.0|right|Chandra, Hubble, and Spitzer composite image of the [[Crab Nebula]] (2009)]]

Since the Earth's atmosphere prevents [[X-ray astronomy|X-rays]], [[gamma-ray astronomy|gamma-rays]]<ref>Note: Gamma-rays from space can be detected indirectly from the ground by a technique known as [[IACT|Imaging Air Cherenkov Technique]] or IACT for short. It was pioneered by the [[Fred Lawrence Whipple Observatory|Whipple Observatory]] in 1968 and several newer telescopes have been built in various countries since then.</ref> and [[far infrared astronomy|far-infrared]] [[radiation]] from reaching the ground, 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 by Hubble from the ground using [[astronomical interferometer]]s or [[adaptive optics]]). Larger, ground-based telescopes have only 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.

Each observatory was designed to push the state of technology in its region of the electromagnetic spectrum. Compton was much larger than any gamma-ray instruments flown on the previous [[HEAO Program|HEAO]] missions, opening entirely new areas of observation. It had four instruments covering the 20 [[keV]] to 30 [[GeV]] energy range, which complemented each other's sensitivities, resolutions, and fields of view. Gamma rays are emitted by various high-energy and high-temperature sources, such as [[black hole]]s, [[pulsar]]s, and [[supernovae]].

Chandra similarly had no ground predecessors. It followed the three NASA [[HEAO Program]] satellites, notably the highly successful [[Einstein Observatory]], which was the first to demonstrate the power of [[Wolter telescope|grazing-incidence, focusing X-ray optics]], giving spatial resolution an order of magnitude better than [[collimator|collimated]] instruments (comparable to optical telescopes), with an enormous improvement in sensitivity. Chandra's large size, high orbit, and sensitive [[Charge-coupled device|CCDs]] allowed observations of very faint X-ray sources.

Spitzer also observes at wavelength largely inaccessible to ground telescopes. It was preceded in space by NASA's smaller [[IRAS]] mission and [[European Space Agency]] (ESA)'s large [[Infrared Space Observatory|ISO]] telescope. Spitzer's instruments took advantage of the rapid advances in infrared detector technology since IRAS, combined with its large aperture, favorable fields of view, and long life. Science returns were accordingly outstanding.{{Citation needed|date=April 2024}} Infrared observations are necessary for very distant astronomical objects where all the visible light is [[redshift]]ed to infrared wavelengths, for cool objects which emit little visible light, and for regions optically obscured by dust.