NASA’s Nancy Grace Roman Space Telescope is being built to do a job that sounds rude to stars: remove them from the picture. When the observatory launches, as early as the end of next month, it will carry what NASA’s Jet Propulsion Laboratory engineers describe as the first active coronagraph flown in space, a system meant to measure and suppress stray starlight rather than merely block it.
That matters because stars are obnoxiously bright. A planet beside one can be buried under leaked light from the telescope’s own optics, including scattering from tiny flaws in mirrors, coatings and mechanical edges. Existing space coronagraphs on Hubble and the James Webb Space Telescope use fixed masks to cut down that glare. Roman’s instrument adds active wavefront control, meaning it tries to measure the remaining glare before an observation and reshape the light path to cancel it.
Brandon Creager, the coronagraph’s lead mechanical engineer at JPL, said he hopes the instrument is remembered as “that critical stepping stone” toward finding an Earth-like world. That is the long game. The near-term test is whether Roman can directly image colder, more mature gas giants that look more like Jupiter than the inflated young exoplanets astronomers have mostly photographed so far.
How Roman makes a dark hole around a star
The key hardware is a pair of deformable mirrors. Each mirror sits over a 48-by-48 grid of actuators, tiny pistons that tug on a thin glass surface when voltage is applied. JPL project systems engineer Ilya Poberezhskiy said each patch can move by up to 0.5 micrometers, with adjustments as small as about 10 picometers, roughly a tenth the diameter of a hydrogen atom.
Those tiny surface changes let the coronagraph sculpt the incoming light so unwanted starlight interferes with itself. Poberezhskiy compared the effect to noise-canceling headphones, except the cancellation is applied to light waves. The goal is a dim, doughnut-shaped zone around the star where a planet’s reflected light can survive.
Roman will still use masks, the old-fashioned part of the trick. Some configurations include “silicon grass,” microscopic spikes that trap light by bouncing it deeper into the structure instead of letting it ricochet toward the detector. Poberezhskiy said light that enters that structure does not get back out.
Engineers expect the active system to make Roman up to 1,000 times more sensitive to exoplanets near bright stars than current space-based coronagraphs. That is a claim Roman will have to prove on orbit, where the spacecraft will shift temperature and position while engineers try to keep a target star centered and the mirrors shaped correctly.
Why astronomers want the pixels
Most directly imaged exoplanets are easy targets by exoplanet standards: very large, very young, still hot from formation and orbiting far from their stars. Roman could instead catch reflected light from a Jupiter analogue, a mature gas giant with a Jupiter-like mass orbiting a sunlike star at a distance more like those in our solar system.
Astronomers have already inferred such planets from the gravitational wobble they induce in their stars. Meredith MacGregor, a Johns Hopkins astronomy professor with an observing program on Roman, said direct imaging changes the measurement: “We are actually looking at the planet, and that is super powerful.”
The pictures will not be glossy planet portraits. MacGregor said a planet may appear as only a few pixels. Even that can be scientifically useful because Roman can split the planet’s light by wavelength, giving astronomers clues about atmospheric chemistry.
Roman also carries a roughly 300-megapixel wide-field camera, intended to take images about 100 times larger than Hubble’s widest views at similar resolution. NASA has said Roman could help study dark matter and dark energy and detect around 100,000 exoplanets through the way foreground systems distort light from more distant stars.
The coronagraph’s results are also expected to feed into NASA’s proposed Habitable Worlds Observatory, a future mission concept aimed at separating the light of an Earth-like planet from a sunlike star billions of times brighter. Roman’s first coronagraph images may be dots. The engineering story is the darkness carved around them.
This story draws on original reporting from MIT Technology Review.
