What the Nancy Grace Roman Space Telescope Is and what it will do

Roman combines a 2.4 metre primary mirror—the same diameter as Hubble's—with a very large field of view. That wide-field camera will image an area of sky about 100 times larger than Hubble can cover at comparable image quality. The result is a sky-mapping instrument rather than a single-target, high-detail observatory like Webb. Roman will survey large swaths of sky quickly and detect rare or transient phenomena that narrower-field telescopes can miss.

The telescope's optics began life in a U.S. reconnaissance programme after 2001. When surplus 2.4 m space telescope systems became available, NASA acquired one in 2012 and repurposed it for astrophysics. The agency redesigned the payload around that optical system to create a wide-field, space-based observatory for the 21st century.

• Dark energy: Roman will observe billions of galaxies and thousands of supernovae to build a precise reconstruction of the Universe's expansion history. Those measurements aim to improve constraints on whatever is driving cosmic acceleration.

• Dark matter: By mapping distortions in the shapes of distant galaxies (gravitational lensing), Roman will produce high-precision maps of dark matter distribution, revealing how matter clusters over cosmic time.

• Exoplanets: Roman will use gravitational microlensing and wide-area surveys to detect thousands of exoplanets, including planets with masses and orbital separations that are hard to find with other methods. The mission is expected to expand the catalogue of known planets and find Earth-like worlds inaccessible to current techniques.

Hubble is a high-resolution imager with a relatively narrow field. Webb is designed for extreme sensitivity at infrared wavelengths, probing the earliest galaxies and detailed atmospheres. Roman sits between those capabilities: it matches Hubble's mirror size but trades narrow-field detail for a much larger field of view. Think of Roman as a sky scout that finds targets and builds statistical maps, while Webb and Hubble provide targeted follow-up at higher sensitivity or resolution.

Operational details and capabilities

The telescope weighs about 10.5 tonnes and will operate at the stable L2 location, the same region used by Webb. NASA is commissioning the Wide Field Instrument and other systems during the cruise to L2. Roman's combination of mirror size and wide-field imaging will enable observations of hundreds of millions of galaxies and the discovery of thousands of planets.

The mission fills a gap in space-based survey capability: a large, space-quality mirror with a very wide field. That combination supports large cosmological surveys and exoplanet demographics that ground-based facilities or narrow-field space telescopes cannot deliver at the same scale.

Roman's commissioning phase will continue during the transit to L2. The first public images and initial science observations are expected in early 2027, after instrument calibration and performance validation are complete.