Euclid's Galactic Preview Jumpstarts NASA's Roman Telescope Survey

In March 2025, the European Space Agency's Euclid telescope did something it almost never does: it stopped what it was doing. For one day, it paused its methodical survey of the distant universe—work designed to map the cosmos and understand dark matter and dark energy—and pointed instead at the crowded heart of our own Milky Way galaxy. The reason was practical and ambitious: to give NASA's Nancy Grace Roman Space Telescope a head start on one of its most important missions.

Roman, scheduled to launch in the summer of 2026, will spend five years studying the galactic bulge—the dense central region of our galaxy where hundreds of millions of stars crowd together. But before Roman could even begin that work in spring 2027, Euclid had already collected preliminary data from the same patch of sky. That single day of observation, covering about five square degrees (roughly the area of 25 full moons), would effectively add two years to Roman's survey timeline, multiplying what both telescopes could discover together.

The partnership makes sense because the two missions see the universe differently. Euclid's snapshot of the galactic bulge lacks some of the color detail Roman will eventually capture, but it has comparable resolution and covers a wider area—useful groundwork for what comes next. "This takes a lot of work and planning, so it really has to be something with high impact for science," said Jason Rhodes, a senior research scientist at NASA's Jet Propulsion Laboratory who leads the U.S. side of Euclid science and serves as the Roman project scientist. "Adding Euclid's snapshot to Roman's future survey will help us map our galaxy better and identify hard-to-find cosmic treasures like isolated black holes and rogue planets more easily."

The real power of the combined data lies in what astronomers call microlensing—a phenomenon where gravity bends light. When a massive object like a star, planet, or black hole aligns with a distant star from our perspective, the nearer object's gravity warps spacetime, acting like a cosmic lens that magnifies and focuses light from the background star. Most often, the lensing object is another star, but Roman will detect planets orbiting those stars and stranger objects still: isolated black holes that wander the galaxy alone, invisible to conventional detection methods. Astronomers estimate roughly 100 million stellar-mass black holes exist in the Milky Way, yet nearly all confirmed detections come from black holes interacting with companion stars. The vast majority, they believe, are solitary wanderers.

Microlensing events created by planets typically last hours or days, but black holes, with their enormous mass, bend light over much larger regions of space, creating signals that can persist for years. That's where Euclid's two-year head start becomes crucial. "The extra two years provided by Euclid give astronomers more time to watch the lens and source star drift apart, making it easier to identify the lens and measure its mass," explained Himanshu Verma, a postdoctoral researcher at Louisiana State University analyzing Euclid images to predict the microlensing events Roman will observe. Matthew Penny, an assistant professor at LSU and co-lead of Euclid's exoplanet science working group, added that microlensing excels at finding planets in wide orbits—worlds farther from their stars than Neptune orbits the Sun, and even rogue planets ejected from their original systems, now drifting alone through the galaxy.

Beyond the hunt for hidden objects, the combined observations will help astronomers map the Milky Way itself with unprecedented precision. Roman will also conduct a separate Galactic Plane Survey covering an area 400 times larger than the bulge survey, revealing tens of billions of stars and previously unmapped structures across our home galaxy. The challenge is fundamental: studying our own galaxy is like trying to map the human body from inside a cell, with dust and gas obscuring the view. By watching how stars move across the sky over time, and knowing that stars in different galactic regions follow different paths, astronomers can determine which part of the galaxy each star inhabits. Euclid's observations provide a crucial test bed for refining those models.

The one-day detour represents something larger than a single scientific payoff. "We've shown that these two telescopes can work together to do science that surpasses what either was originally designed for," Rhodes said. "In doing so, we've established a model for future coordinated observations that can unlock far more discoveries than either mission could make alone." The partnership suggests a future where space telescopes, rather than working in isolation, are orchestrated to amplify each other's strengths—a template that could reshape how humanity explores the cosmos.