NASA’s new flagship space observatory, the Nancy Grace Roman Space Telescope, is now attached to its Falcon Heavy rocket and ready to set out on its journey to uncover the secrets of the universe. From its high perch at the L2 Lagrange Point, it will join the James Webb Space Telescope and the European Space Agency’s Euclid observatory in using Infrared light to peer through dust and see back in time.

History

Like most flagship NASA science missions, Roman finds its origins with one of the National Academies’ Decadal Surveys. In 2010, the focus was on dark energy and the expansion history of the universe, so the survey recommended that NASA build the Wide-Field InfraRed Survey Telescope (WFIRST). Once NASA began work, they decided that WFIRST would be a small telescope, with a 1.1 or 1.3-meter mirror, as opposed to Euclid’s 1.2-meter mirror. Depending on the design option, the mission duration could be as short as three years. While it would avoid Webb’s issues, calling it a flagship mission was questionable when Euclid itself was only medium-class.

In 2012, the National Reconnaissance Office informed NASA that they had two 2.4-meter optical systems that they no longer needed and that they would be willing to give them to NASA for free (not counting transportation and storage) if NASA wanted them. One of these was given to WFIRST to enhance its capabilities and make it a true flagship-class mission. This version would have images nearly twice as sharp and could stare deeper into its survey fields. In 2015, this version of the mission entered full-scale development and was renamed the Nancy Grace Roman Space Telescope in 2020.

The mission passed its critical design review in September 2021, allowing flight hardware production, assembly and testing to proceed. Major spacecraft integration was completed across 2024 and 2025, followed by acoustic, thermal, and vibration testing before being shipped to the Kennedy Space Center in July 2026 for launch.

Science Program

Roman is a survey telescope, meaning that by and large, astronomers will not write proposals to request observing time, but rather will request funding to analyze and make discoveries from existing data. Only in relatively rare cases will individual proposals result in new observations. This is not uncommon in astronomy, and is the way survey telescopes generally operate, but is different from space telescopes like Hubble and Webb.

Roman’s observing schedule during its 5-year primary mission will instead be dominated by surveys that were pre-defined by the astronomical community at large in the years leading to its launch. 75% of the observing time in those five years will be dedicated to three Core Community Surveys, with the remaining 25% dedicated to additional community-defined General Astrophysics Surveys.

The three Core Community Surveys that make up the bread and butter of Roman’s first five years are the Galactic Bulge Time Domain Survey, High Latitude Time Domain Survey, and High Latitude Wide Area Survey. Community-led definition committees were formed to define the observing strategies and areas, and their results were reviewed by the Roman Observations Time Allocation Committee, or ROTAC. The ROTAC report’s recommendations will guide the Roman project and the Space Telescope Science Institute in the detailed scheduling of observations over the first five years. 

Core Surveys

The Galactic Bulge Time Domain Survey is by far the smallest of the surveys, but is no less consequential. It consists of a mosaic of five Roman fields of view stacked end-to-end near the galactic center, and an additional sixth over the exact center of the Milky Way. Due to the need to keep the telescope behind the solar arrays and out of direct sunlight, it is only able to observe the galactic bulge during two seasons per year, each around 72 days long.

The main purpose of this survey is to detect and categorize exoplanets via microlensing, including freefloating Rogue Planets. It will also be able to detect microlensed stars and black holes, measure star masses via astroseismology, and detect more than 100,000 exoplanets via the transit method. Microlensing occurs when one star passes in front of another one from our perspective, warping the light with its gravity and causing the background star to appear to brighten. If the lensing star happens to have planetary companions, they can also influence the light of the background star, causing distortions in its light curve that can be detected. Microlensing is sensitive to more distant planets than transit surveys such as the Kepler and TESS missions, as well as being able to detect lower mass planets than transits from 1-100AU from the host star.

The “split” nature of the survey area is due to the galactic plane’s high dust content causing severe “extinction”, or light scattering/reddening, reducing the visibility of microlensing signatures. Moving the survey south of the galactic plane fixes this, but one field is kept at the galactic center to observe the surroundings of Sagittarius A*, the supermassive black hole at the center of the galaxy.

The High Latitude Time Domain Survey will take place over the course of two years in the middle of Roman’s prime mission. It will consist of both imaging and spectroscopic observations every 5 days during the survey period, over both wide and deep tiers. Each tier includes areas above and below the celestial equator, in Roman’s continuous viewing zones (areas that Roman can point at safely no matter the time of year). The main purpose of this survey is to detect type 1a supernovae, the main “standard candle” used in cosmology. By mapping their brightness and redshift, cosmologists can improve the precision and certainty of their models of how the universe has expanded over time, which helps determine what Dark Energy could be. Roman can observe type 1a supernovae not easily seen from the ground, enabling it to significantly improve our catalogs of them.

For a deeper dive on how Roman will study Dark Energy, see this video by NASA’s Goddard Space Flight Center:

The need to observe supernovae and map out how their brightness changes over time also makes the survey an excellent tool for spotting other transient events such as kilonovae, core-collapse supernovae, tidal disruption events, and variable sources such as active galactic nuclei.

The High Latitude Wide Area Survey also probes the expansion of the universe, but additionally studies the origin and evolution of the large scale structure of the universe. By imaging and measuring the distance to millions of galaxies over a wide range of ages, the relationship between normal matter and dark matter over the life of the universe can be charted. The survey’s medium tier can see objects around 158 million times dimmer than can be seen with the naked eye in areas with no light pollution. The wide and medium tiers cover part of the area imaged by the Rubin Observatory’s Legacy Survey of Space and Time as well as partial overlapping with the DESI dark energy survey, greatly augmenting the science value of all involved telescopes.

General Surveys

Those Core Community Surveys are collectively allocated approximately 75% of the mission’s observing time, with the remaining 25% reserved for General Astrophysics Surveys, selected via periodic calls for proposals. This process is similar to the General Observer system used by Hubble, Webb, and most general-purpose telescopes around the world. This corresponds to 389 days, to be divided among up to 30 additional survey programs, proposed and selected between 2026 and 2029.

Some of the surveys selected from the first set of proposals include mapping both the Andromeda and Triangulum galaxies multiple times, re-observation of the full Kepler field of view, an extreme deep field, and multiple surveys of the milky way to better understand its history. One Milky Way survey is the Galactic Plane Survey, which will study essentially every major astronomical category including transient events, the stellar lifecycle, exoplanets, and black holes.

The Instruments

Roman’s main camera is the Wide Field Instrument. It will perform every survey observation, be it imagery or spectroscopy. The sensor module consists of 18 4096×4096-pixel H4RG detectors, for a total of over 300 million pixels. Like most science cameras, the WFI detectors cannot themselves discriminate different colors. The instrument is equipped with an optical element wheel that will allow it to take pictures of the same scene through different filters, which can then be colored and overlaid to produce a finished image. The wheel is populated with seven broad filters (meaning they allow a relatively broad range of wavelengths through), one wide filter (allowing basically every wavelength the detectors can see through), one opaque element for noise calibration, and two dispersive elements (a prism and grism, or combined grating and prism) for spectroscopy.

While not a primary science instrument, Roman also carries the Coronagraph Instrument (CGI) as a technology demonstration. A coronagraph is a special type of camera that blocks the light from a star to make fainter structures or objects close to the star visible, in this case to enable imaging of potential exoplanets or disks of material orbiting them. Because virtually all stars are pointlike light sources in any telescope, no matter how large, simply placing an opaque disk in the center of the camera field of view will not suffice. 

Roman’s coronagraph must be able to image objects at least 10 million times dimmer than their parent star. This is only possible with several technological enhancements, such as large deformable mirrors and active wavefront sensing/control like are used in Adaptive Optics at large ground-based observatories. For a fuller demonstration of how these technologies help, see the following video produced by NASA:

In addition to the deformable mirrors, masks, and stops the observatory itself can be used to increase image quality. By rolling the telescope while staying pointed at the same target, much of the sensor noise can be eliminated, known as Angular Differential Imaging. This technique, especially when combined with first viewing a planetless reference star to allow ground teams to subtract away most of the sensor noise, significantly increases the clarity of the final result.

Roman is also designed to be serviceable. It can be refueled and the spacecraft avionics (computers, reaction wheels, communications, etc.) have been designed to be removable and replaceable by a robotic servicing spacecraft, perhaps not unlike Northrop Grumman’s Mission Robotic Vehicle. While less clear, it is also possible that the instruments themselves are able to be replaced. It will likely be a while before any servicing mission though, as Roman has enough fuel to last for a decade.

What next?

Even as Roman prepares to launch, preparations for what comes next are ongoing. Work has been underway since 2023 on the next flagship space telescope, the Habitable Worlds Observatory (HWO). While it is intended to be useful for a wide variety of astronomical topics, the primary driving objective is to image and characterize 25 Earth-like exoplanets around Sun-like stars. To image Earth from 10 parsecs away would require a coronagraph contrast 1-2 orders of magnitude better than what Roman’s coronagraph is theoretically capable of. The architecture and technology of Roman’s coronagraph are theoretically capable of reaching that performance, with the primary limiting factor being the telescope itself and the fact that much of the field of view is obscured by the secondary mirror and supports. An off-axis secondary mirror, which HWO is currently very strongly leaning towards, will significantly increase its discovery potential, alongside the 6-8 meter wide primary mirror. 

Significant work remains, as a full instrument will require more science modes, a wider wavelength range, and have much greater margins and efficiency to be useful. This is one of the core focuses of the technology development plan and is regarded as critical to the mission, with maturation and testbeds currently in progress.

All in all, just like when Webb launched in 2021, we are on the verge of a surge of new discoveries and awe-inspiring images. Roman is scheduled to launch at 7:26 A.M. EDT on Sunday, August 31st, and can be watched live on the NASA YouTube Channel (link).