Hubble Space Telescope - Future - Successors

Successors

Further information: James Webb Space Telescope

Visible spectrum range
Color	Wavelength
violet	380–450 nm
blue	450–475 nm
cyan	476–495 nm
green	495–570 nm
yellow	570–590 nm
orange	590–620 nm
red	620–750 nm

There is no direct successor to the Hubble as an ultraviolet and visible-light space telescope, as near-term space telescopes do not duplicate Hubble's wavelength coverage (near-ultraviolet to near-infrared wavelengths), instead concentrating on the farther infrared bands. These bands are preferred for studying high redshift and low-temperature objects, objects generally older and farther away in the universe. These wavelengths are also difficult or impossible to study from the ground, justifying the expense of a space-based telescope. Large ground-based telescopes can image some of the same wavelengths as Hubble, sometimes challenge HST in terms of resolution (via adaptive optics), have much larger light-gathering power, and can be upgraded more easily, but cannot yet match the Hubble's excellent resolution over a wide field of view with the very dark background of space.

Plans for a Hubble successor materialized as the Next Generation Space Telescope project, which culminated in plans for the James Webb Space Telescope (JWST), the formal successor of Hubble. Very different from a scaled-up Hubble, it is designed to operate colder and farther away from the Earth at the L2 Lagrangian point, where thermal and optical interference from the Earth and Moon are lessened. It is not engineered to be fully serviceable (such as replaceable instruments), but the design includes a docking ring to enable visits from other spacecraft. A main scientific goal of JWST is to observe the most distant objects in the universe, beyond the reach of existing instruments. It is expected to detect stars in the early Universe approximately 280 million years older than stars HST now detects. The telescope is an international collaboration between NASA, the European Space Agency, and the Canadian Space Agency since 1996, and is planned for launch on an Ariane 5 rocket. Although JWST is primarily an infrared instrument, its coverage extends down to 600 nm wavelength light, or roughly orange in the visible spectrum. A typical human eye can see to about 750 nm wavelength light, so there is some overlap with the longest visible wavelength bands, including orange and red light.

A complementary telescope, looking at even longer wavelengths than Hubble or JWST, is the European Space Agency's Herschel Space Observatory, launched on May 14, 2009. Like JWST, Herschel is not designed to be serviced after launch, and has a mirror substantially larger than Hubble's, but observes only in the far infrared.

Further concepts for advanced 21st-century space telescopes include the Advanced Technology Large-Aperture Space Telescope., a conceptualized 8- to 16-meter (320- to 640-inch) optical space telescope that if realized could be a more direct successor to HST, with the ability to observe and photograph astronomical objects in the visible, ultraviolet, and infrared wavelengths, with substantially better resolution than Hubble or the Spitzer Space telescope. This effort is being planned for the 2025–2035 time frame.

Selected space telescopes & instruments
Name	Year	Wavelength	Aperture
Human eye	—	0.39–0.75 μm	0.01 m
Spitzer	2003	3–180 μm	0.85 m
Hubble STIS	1997	0.115–1.03 μm	2.4 m
Hubble WFC3	2009	0.2–1.7 μm	2.4 m
Herschel	2009	60–672 μm	3.5 m
JWST	Planned	0.6–28.5 μm	6.5 m

Existing ground-based telescopes, and various proposed Extremely Large Telescopes, can exceed the HST in terms of sheer light-gathering power and diffraction limit due to larger mirrors, but other factors affect telescopes. In some cases, they may be able to match or beat Hubble in resolution by using adaptive optics (AO). However, AO on large ground-based reflectors will not make Hubble and other space telescopes obsolete. Most AO systems sharpen the view over a very narrow field—Lucky Cam, for example, produces crisp images just 10" to 20" wide, whereas Hubble's cameras are super sharp across a 2½' (150") field. Furthermore, space telescopes can study the heavens across the entire electromagnetic spectrum, most of which is blocked by Earth's atmosphere. Finally, the background sky is darker in space than on the ground, because air absorbs solar energy during the day and then releases it at night, producing a faint—but nevertheless discernible—airglow that washes out low-contrast astronomical objects.