The Advanced Technology Large-Aperture Space Telescope (ATLAST) is an 8– to 16.8–meter UV-optical-NIR space telescope proposed by the Space Telescope Science Institute (STScI), the science operations center for the Hubble Space Telescope (HST). If launched, ATLAST would be a replacement and successor for the HST, with the ability to obtain spectroscopic and imaging observations of astronomical objects in the ultraviolet, optical, and infrared wavelengths, but with substantially better resolution than either HST or the planned James Webb Space Telescope (JWST). Like JWST, ATLAST would be launched to the Sun-Earth L2 Lagrange point. ATLAST is envisioned as a flagship mission of the 2025–2035 period, designed to determine whether there is life elsewhere in the galaxy. It would attempt to accomplish this by searching for "biosignatures" (such as molecular oxygen, ozone, water, and methane) in the spectra of terrestrial exoplanets. The backronym that the project currently uses, 'ATLAST', is in fact a pun. It refers to the time taken to decide on a true, visible-light, successor for the Hubble Space Telescope. However, it is expected that, as the project progresses, a new name would be chosen for the mission.
ATLAST proposals have a primary mirror diameter in the 8 to 16.8 meters (26 to 55 ft) range. Two different telescope architectures have been identified for ATLAST, but with similar optical designs, that span the range in technologies. The architectures are a telescope with a monolithic primary mirror and two variations of a telescope with a large segmented primary mirror. The concepts invoke heritage from the HST and JWST designs, but also take significant departures from these designs to minimize complexity, mass, or both. ATLAST would have an angular resolution that is 5–10 times better than JWST and a sensitivity limit that is up to 2,000 times better than HST.
Two of the concepts, the 8 meters (26 ft) monolithic mirror telescope and the 16.8 meters (55 ft) segmented mirror telescope, span the range of UVOIR (ultraviolet, optical, infrared) observatories that are enabled by NASA's Space Launch System (SLS) vehicle and the satellite delivery version of SpaceX's BFR vehicle. The third concept, a 9.2 meters (30 ft) segmented mirror telescope is compatible with the existing Evolved Expendable Launch Vehicle (EELV).
The 8 m ATLAST offers the inherent advantages of a monolithic aperture telescope in terms of high-contrast imaging and superb wavefront control. The 16 m segmented mirror ATLAST represents a pathway to truly large apertures in space and uses the largest extrapolation of a JWST-like chord-fold primary mirror packaging. The 9.2 m segmented mirror ATLAST adopts JWST design heritage, essentially being an incrementally larger variant of the JWST, which has a 6.5 m segmented main mirror.
ATLAST would be able to be serviced, much like the HST has been. Using either a robotic ferry (the currently proposed method), or an astronaut crew flying in an Orion spacecraft (which would allow NASA to gain experience for future manned Solar System missions), instruments such as cameras would be replaced and returned to Earth for analysis and future upgrades. Like the HST and JWST, ATLAST would be powered by solar panels.
The ATLAST technology development plan is supported with funding from NASA's Astrophysics Strategic Mission Concept Studies program, the Goddard Space Flight Center, the Marshall Space Flight Center, the Jet Propulsion Laboratory and related programs at Northrop Grumman Aerospace Systems and Ball Aerospace.
ATLAST was originally intended to be launched from Kennedy Space Center's Launch Complex 39A atop of the Ares V. Following the cancellation of that vehicle, ATLAST would launch aboard the Space Launch System (SLS). If the 9.2 m design is adopted, launch would take place from NASA facilities capable of launching EELVs. Much like the proposed Orion/Altair flights to the Moon, the SLS would place ATLAST and the Exploration Upper Stage (EUS) into a "parking" orbit while engineers check out the systems of both the EUS and ATLAST. Once cleared, the EUS will fire again and ATLAST would begin a three-month journey to the Sun-Earth L2 point, entering a halo orbit around the Lagrange point. The segmented versions of the telescope would deploy their optics while en route.
Servicing missions, launched every 5 to 7 years, would allow astronomers to upgrade ATLAST with new instruments and technologies. Like the HST, ATLAST should achieve or exceed a 20-year lifespan.
ATLAST, using an internal coronagraph or an external occulter, can characterize the atmosphere and surface of an Earth-sized exoplanet in the habitable zone of long-lived stars at distances up to 140 light-years (43 pc), including its rotation rate, climate, and habitability. ATLAST would also allow researchers to glean information on the nature of the dominant surface features, changes in cloud cover and climate, and, potentially, seasonal variations in surface vegetation.