Here's What Sets The Nancy Grace Roman Space Telescope Apart From The James Webb

NASA's Nancy Grace Roman Space Telescope is currently on its way to its service station, 930,000 miles away from the planet it was made on. Once there, this triumph of engineering will explore our universe with groundbreaking technology far beyond what's come before it. However, when it arrives, it will find that it's not alone. That's because the James Webb Space Telescope is already there, and it is also a triumph of engineering, and it is already exploring our universe with groundbreaking technology far beyond what's come before it. So why is NASA bothering with two space telescopes at the same location in space? Isn't that a little redundant? What exactly sets these two apart from one another?

The answer has to do with how their respective cameras are designed, which in turn means they have very different capabilities and missions. In brief, Roman has a very wide field of view, allowing it to capture huge patches of the sky at once. By contrast, the Webb has a much narrower field of view and much higher resolution, meaning it can take incredibly detailed photos of distant objects. Think of it as the difference between glancing around (Roman) and peering really closely at something (Webb). That makes Webb great for detailed studies of things that scientists already know about, but it's narrow vision makes it bad at discovering new things. Roman will survey the whole sky, and it is expected to find billions of never-before-seen stars. In fact, if Roman does find something new and exciting, you know who NASA will send in to check it out? Webb.

As any terrestrial photographer will know, the picture a camera can take comes down to two main components: the mirrors (lenses) and the detectors. The mirrors are what actually capture the dim, distant lights of faraway stars; the more light you catch, the better the image resolution. This is Webb's specialty: Its primary mirror is composed of 18 hexagonal segments combined into a roughly circular shape, reaching 21.3 feet in diameter. It is the largest mirror ever put into space. This is so sensitive that it needs to stay cooled to -364 degrees Fahrenheit, because otherwise the minuscule infrared light from the segments' warmth would interfere with the incoming light. This astonishing mirror is connected to several different scientific instruments, but as an example, the near-infrared NIRCam features eight 2,048 x 2,048 pixel sensors with a 260x130 arcsecond field of view. All in all, this gives Webb incredible detail (because of all the captured light) in a very narrow field of view.

Roman flips all of that around. Its mirror, which is only one giant segment, is just 7.9 feet in diameter. That's exactly the same as Hubble's, although only one-fourth the weight. So compared to Webb, Roman captures far less light, meaning much less detail. This also means it can be warmer, at -288 F. Roman's muscles are in its detectors: its Wide-Field Instrument (WFI) has 18 sensors at 4,096 x 4,096 pixels each with a 2700x1280 arcsecond field of view. In other words, it's taking much larger pictures across a much wider field of view, but with less light (resolution) than Webb.

Of course, part of getting just the right photo is getting rid of light that you don't want. Both Roman and Webb will live and work at Earth's Lagrange Point 2, or L2, for exactly this reason. This is a point in space where the centrifugal forces of the space telescopes' orbit will be exactly countered by the combined gravitational pulls of the Earth and the Sun behind it. That has two advantages: The telescopes can stay "in position" for comparatively little fuel spend, and it's as far from the Sun's glare as they can get. Still, both telescopes have intensive shielding to keep solar glint out.

What sets Roman apart is that it also has a way to block the light of the stars that it's observing. That might seem really counterintuitive, but the onboard Coronagraph Instrument is a marvel constructed for just this purpose. Using a series of masks and prisms, the Coronagraph can essentially turn down an observed star's light. Why? To see the planets around it in greater detail, of course. To do this, the Coronagraph's mirrors are flexible, or "deformable." It can even reshape itself with enough precision to offset an error the width of a strand of DNA.

So once Roman arrives at its destination, sometime late this year or early next year, expect it to find not just new stars, but new planets in great detail. And if it finds something truly interesting? Then NASA can send in Webb to take a very detailed look. Roman and Webb are partners in the most advanced astronomical work ever done.