The Nancy Grace Roman Space Telescope launched aboard a Falcon Heavy on August 30 and is settling into orbit around the second Sun–Earth Lagrange point (L2), about a million miles from Earth. Its instruments are undergoing commissioning, with the first science images expected by early 2027. The guidance system passed tests of its ability to hold the observatory steady, and the separate Coronagraph Instrument had observed stars in the Large Magellanic Cloud.
By October 14, NASA reported that the team had sharpened Roman’s initially blurry vision by moving its secondary mirror about one-sixteenth of the thickness of a human hair. That tiny adjustment was the fix Roman needed in order to bring the universe into focus.
The Mother of Hubble
Nancy Grace Roman, born in 1925, grew up watching the sky with her mother and, at 11, organized an astronomy club with classmates in Reno, Nevada. She graduated from Baltimore’s Western High School, studied at Swarthmore, and earned her doctorate at the University of Chicago. After researching stars at Yerkes Observatory and teaching at Chicago, she left academia when she saw little prospect of obtaining tenure as a woman.
Roman was working at the Naval Research Laboratory when someone from the newly created NASA asked whether she knew anybody interested in establishing its astronomy program. She put herself forward, and the agency hired her in 1959. She became NASA’s first chief of astronomy.
Among the early proposals for a large space telescope were designs that would send an astronaut along with it. Roman pointed out that even an astronaut trying to sit perfectly still would eventually move, and that movement would disturb a long exposure. As she later put it, “the man would wiggle.” She brought astronomers and engineers together to work out an observatory the engineers could build and astronomers could use, then helped secure support for the telescope eventually named Hubble.
Hubble reached orbit in 1990 with a wrongly shaped primary mirror. Astronauts installed a new camera and corrective optics in December 1993 to compensate for the flaw. At the televised press event celebrating the repair’s success in 1994, Senator Barbara Mikulski announced that Hubble’s trouble was over. Roman, by then known as the “Mother of Hubble,” sat in the audience and knitted.
In December 1995, Robert Williams, director of the Space Telescope Science Institute in Baltimore, devoted ten days of Hubble observations to a seemingly empty patch near the Big Dipper. His team had deliberately avoided bright nearby objects that would obscure fainter ones behind them. The resulting Deep Field revealed roughly 3,000 distant galaxies in an area about the size of a pinhead held at arm’s length. Light from the most distant galaxies had traveled for billions of years, so astronomers saw them at much earlier stages of development.
Roman’s namesake space telescope’s camera contains 18 infrared detectors with roughly 300 million pixels, and each exposure covers about 0.28 square degrees of sky, more area than the full Moon. That’s approximately 200 times the field of the infrared channel of Hubble’s Wide Field Camera 3, with comparable detail at near-infrared wavelengths. Its wide-area survey is expected to record more than a billion galaxies. Hubble retains its ultraviolet and visible-light capabilities, while Webb’s larger mirror supports deeper infrared investigations of selected targets.
Roman and Hubble both use 2.4-meter primary mirrors. In 2012, NASA inherited telescope optics from the National Reconnaissance Office, including the primary and secondary mirrors and supporting hardware. L3Harris reshaped the primary mirror and developed the optical assembly for Roman’s scientific requirements.
The Accelerating Universe
When Roman (the telescope) finds an exploding white dwarf in a distant galaxy, astronomers will follow its changing brightness and color. These Type Ia supernovae have a relationship between how bright they become and how their light fades, which allows researchers to calibrate the explosions and estimate their distances. The expansion of space also stretches the light on its journey toward us, shifting its spectrum toward longer, redder wavelengths. Taken together, the brightness and redshift record how the universe expanded while that light was traveling.
In 1998, two competing teams using ground telescopes and Hubble found that distant supernovae were fainter than expected at their measured redshifts. The distances inferred from their brightness favored a history of accelerating expansion over models in which matter’s gravity slowed it. The discovery eventually brought Saul Perlmutter, Brian Schmidt, and Adam Riess the Nobel Prize, and left astronomers with an unknown cause they called dark energy.
In a model with Einstein’s cosmological constant, empty space carries an energy density that stays constant as the universe expands. As matter spreads more thinly, that constant energy density becomes more important to the universe’s evolution. Roman’s supernova observations will test how well that model fits the expansion at different times. An energy field that changes over cosmic history could leave a different pattern in those measurements.
Roman’s galaxy images will contain distortions caused by matter between us and the galaxies. Gravity bends the path of light, so a distant galaxy can appear slightly stretched or misshapen. Its natural shape makes a small distortion difficult to recognize on its own. Across hundreds of millions of galaxies, shared patterns in those distortions reveal where the intervening mass lies, including dark matter that emits no detectable light.
Gravity draws matter into galaxies and clusters, while accelerated cosmic expansion inhibits the growth of structure on large scales. Roman’s mass maps will show structures at different stages in cosmic history because light from more distant galaxies has taken longer to reach us. Astronomers can compare that record of growth with the expansion history reconstructed from supernovae.
Pressure waves traveled through the hot plasma of the young universe, and their imprint survives as a preferred separation scale in the distribution of matter. That scale stretched as the universe expanded. Measuring its apparent size at different distances gives astronomers a cosmic ruler to check against the distances inferred from supernovae.
Planets Without Suns
Toward the crowded center of the Milky Way, Roman will monitor six fields during its Galactic Bulge Time-Domain Survey. In the intensive observing seasons, it will return to the full area about every 12 minutes for roughly 70 days at a stretch. Infrared light penetrates much of the dust that obscures these stars in visible light.
A star in those images may gradually brighten and fade during an event astronomers call gravitational microlensing. A nearer star passes almost directly in front of it, and the foreground star’s gravity bends the background light and temporarily magnifies it. A planet orbiting the foreground star can briefly disturb that otherwise smooth brightening, sometimes increasing the magnification and sometimes reducing it relative to the star acting alone. Astronomers look for these short deviations in the record of the background star’s brightness.
The planet itself can remain invisible throughout the event. It doesn’t have to cross its own star’s face, and a wide orbit that takes years to complete can still leave a detectable gravitational signature during the alignment.
A planet without a star can magnify background light too. As it passes through the alignment, the distant star briefly brightens and then returns toward its usual appearance. For a low-mass rogue planet, the whole event may last only hours or a few days. At a 12-minute cadence, Roman can record the rise and fall of a signal that a single nightly observation could miss.
Close gravitational encounters in a young planetary system can eject a world into interstellar space. Some isolated objects of planetary mass may instead form independently, and the abundance of different masses would help distinguish these origins. A 2023 analysis of ground observations suggested Roman could detect about 400 rogue planets with masses similar to Earth’s.
Roman’s Coronagraph Instrument uses masks and deformable mirrors to suppress the glare of nearby stars so it can observe faint planets and dusty disks. Its September star observations were early commissioning checks. Engineers are still preparing the instrument for its full starlight-suppression demonstration.
The Sky Between Visits
Away from the galactic bulge, Roman’s High-Latitude Time-Domain Survey will revisit its fields about every five days during its main two-year observing campaign, provisionally scheduled to begin in 2028. Repeated images will follow supernovae as they brighten and fade, as well as variable stars and rarer events such as a black hole tearing apart a star.
In the 1960s, the American Vela satellites carried detectors intended to help identify nuclear explosions in space. Their records also contained gamma-ray flashes that didn’t resemble nuclear detonations. Los Alamos researchers compared the arrival times at different satellites and ruled out Earth and the Sun as their sources. The team announced the discovery of cosmic gamma-ray bursts in 1973.
Roman’s survey data will be public without an exclusive period reserved for the teams that designed the observations. Researchers searching those images for distant supernovae will share them with colleagues looking for faint bodies in our solar system. The bulge survey’s record of stellar brightness can reveal planets or the gravitational passage of an isolated black hole.
Asked which Hubble discovery interested her most, Nancy Grace Roman chose dark energy. She had helped secure support for the telescope decades before astronomers found that cosmic expansion was accelerating. The observatory bearing her name will test the simplest explanation for that acceleration: empty space carrying the same energy density now as it did billions of years ago.