Caltech Leads NASA Mission to Make the First Deep Ultraviolet All-Sky Maps

To the casual observer, the starry sky offers a picture of tranquility. In reality, stars flare, explode, and occasionally crash into each other in cataclysmic collisions that bathe the universe in heavy elements and light. Today, NASA has announced its commitment to launch, by 2031, the Caltech-led Ultraviolet Explorer (UVEX) mission—a new space observatory that will provide unprecedented views of our dynamic universe. The mission will also, for the first time, identify millions of tiny galaxies and make the very first deep all-sky maps in ultraviolet light. With NASA's commitment in place, the UVEX team will begin building and executing the mission.

"UVEX is a bold leap forward in our exploration of the ultraviolet universe," says Caltech President Ray Jayawardhana, the Sonja and William Davidow Presidential Chair and professor of astronomy.

UVEX will join a growing number of sky-mapping surveys that have come online in the past decade. Among them are the Caltech-led NASA SPHEREx mission, a space-based observatory that scans the entire sky in infrared wavelengths, and the Vera C. Rubin Observatory in Chile, which captures a dizzying array of primarily visible-light changes over the Southern Hemisphere.

The space telescope stands out in its ability to capture the dynamic ultraviolet skies, and to make very deep pictures of large areas of the sky. Ultraviolet light, which most people know as the cause of sunburns, is invisible to human eyes; its energies are up to several dozen times higher than those of visible light.

With its superb ultraviolet vision, UVEX will detect stars, galaxies, and other explosive events too faint to be seen before. It will deepen our understanding of the evolution of stars and galaxies and provide new insights into the mechanisms behind some of the most powerful blasts in the cosmos.

The space telescope will also be able to rapidly swing across the sky to watch dramatic events as they unfold, such as collisions between dense stellar remnants called neutron stars and the demise of massive stars in supernovae. These events hurl the stuff of stars and planets into the universe, which is then recycled into new celestial bodies.

UVEX is led by Caltech's Fiona Harrison, the mission's principal investigator and the Harold A. Rosen Professor of Physics, and it is managed by the UC Berkeley Space Sciences Laboratory. Both Caltech and UC Berkeley will play leading roles in developing the UVEX instrument, while Northrop Grumman will build the spacecraft. In addition, NASA's Jet Propulsion Laboratory (JPL), which is managed by Caltech for NASA, will provide key aspects of the detector technology. Caltech's IPAC, a data center for astrophysics and planetary science, will process and archive the mission's data.

"The first major ultraviolet space mission in two decades, this Caltech-led effort brings together our distinctive strengths in scientific and instrumentation leadership, JPL's advanced detector technology, and IPAC's expertise in transforming mission data into a resource for astronomers around the world," Jayawardhana says. "It's a powerful expression of how Caltech pursues fundamental questions about the cosmos with ingenuity and audacity."

The mission will "advance fundamental frontiers in astrophysics, from understanding the smallest galaxies to diagnosing the aftermaths of neutron-star mergers and other cosmic explosions," says Harrison, who uniquely wears a dual hat as the principal investigator for another NASA space observatory, NuSTAR (the Nuclear Spectroscopic Telescope Array), launched in 2012, which is currently studying the universe in high-energy X-rays.

A Powerful Set of Ultraviolet Eyes

In addition to Rubin and SPHEREx, UVEX will join a fleet of telescopes that are mapping the turbulent skies, a field known as time-domain astronomy. These include NASA's newly launched Nancy Grace Roman Space Telescope, which seeks to study dark energy, image exoplanets, and more, and for which Caltech and JPL play key roles; and the Zwicky Transient Facility (ZTF), which maps the skies from Caltech's Palomar Observatory and has confirmed more than 10,000 new supernovae.

UVEX will be the first-ever telescope to make deep maps of the entire sky in ultraviolet light, and it will do so with unprecedented sensitivity while producing detailed spectra that break up the light from cosmic objects into its basic wavelengths, revealing the elements they contain and other characteristics.

The mission follows in the legacy of NASA's Galaxy Evolution Explorer (GALEX), also led by Caltech, which mapped the ultraviolet skies between 2003 and 2013. UVEX, however, adds a dedicated time-domain capability and will be up to 100 times more sensitive than its predecessor.

The leap in sensitivity comes in part from innovative ultraviolet detectors to be built jointly by Caltech, SRI International, and JPL. The silicon-based detectors were derived from the CMOS (complementary metal–oxide–semiconductor) imaging technology, which was invented at JPL in the 1990s and is now found in billions of smartphone cameras. Standard CMOS sensors measure visible light, while the UVEX detectors will be modified—using a technique developed through decades of research at JPL—to sense ultraviolet light.

UVEX science-team member Shouleh Nikzad (PhD '90), the chief technologist at JPL, a JPL Fellow, and a 2026 Caltech Distinguished Alumna, has led the decades-long effort to invent, develop, and refine these ultraviolet detectors and instrument technologies. Over the years, Nikzad and her team, who perform research in JPL's Microdevices Lab, have created specialized filters and coatings to tailor the sensors to the desired wavelength range of ultraviolet light.

"We are so proud that this partnership with Caltech has come to fruition to enable the exciting science that will be achieved by UVEX," says Nikzad, who is also a senior faculty associate in physics at Caltech. She notes that similar ultraviolet detectors are being considered for potential use on NASA's next flagship astrophysics mission, the Habitable Worlds Observatory (HWO). Still in the concept stage, HWO aims to directly image exoplanets around Sun-like stars.

Bright with Starlight

Harrison, the UVEX principal investigator, has spent her career innovating and advising on new ways to see the cosmos. In addition to leading NuSTAR, she was co-chair of the Decadal Survey on Astronomy and Astrophysics 2020, a National Academies-sponsored effort that set the priorities for astronomy for the current decade. When considering whether to lead another NASA mission, she says she could not resist "the untapped opportunity in the ultraviolet." For example, the mission has been optimized to spot millions of never-before-seen small nearby galaxies that are teeming with new stars and difficult to observe in visible-light images.

"In optical light, it's very hard to distinguish these nearby galaxies from faraway ones that look similar," Harrison says. "Ultraviolet light will make them easier to find."

Front and Center Views of Light Shows

Astronomers are particularly thrilled by the potential to view the smashups of neutron star pairs in ultraviolet light. One such collision, a landmark event in 2017, produced gold and other heavy elements needed for star and planet formation.

That collision was first detected in gravitational waves, ripples in space and time, by the Laser Interferometer Gravitational-wave Observatory, (LIGO, which was founded by and continues to be managed by Caltech and MIT with funding from the National Science Foundation) and Virgo, its European partner observatory. As the neutron stars spiraled together and merged, they sent gravitational waves speeding through the cosmos. LIGO-Virgo then alerted the astronomical community to the unfolding drama, and dozens of telescopes soon observed the event's afterglow across wavelengths ranging from gamma rays to radio waves.

"With UVEX, we will now be able to follow up on gravitational-wave events in ultraviolet light," explains UVEX team member Mansi Kasliwal (PhD '11), a professor of astronomy at Caltech and director of the Palomar Observatory.

UVEX will be able to image these and other transient events like supernovae within hours of an alert to the astronomical community. Perhaps more exciting, UVEX will obtain detailed spectra that reveal the ingredients of the explosions and track the speed of outflowing matter.

"I can count on two hands how many spectra we have of transient events in their infancy phase with the Hubble Space Telescope," Kasliwal says. "Hubble is, of course, amazing and does have some ultraviolet capabilities, but it cannot rapidly pivot to observe real-time events."

A New Look at Blue Stripped Stars

Another UVEX focus will be the study of massive blue "stripped" stars whose outer hydrogen-rich layers have been torn off by orbiting stellar companions.

"We will be able to see what's inside the stars," explains Kareem El-Badry, associate professor of astronomy at Caltech and a member of the UVEX team.

Because UVEX will continually scan large portions of the sky, it will be able to discover new stripped stars by looking for characteristic changes to their ultraviolet light. "We only know of a small number of stripped stars of this type now," El-Badry says. "UVEX will find hundreds."

In addition to these and other mission goals, the astronomers say they are looking forward to what they have notanticipated. "Every time you look at the skies in a new wavelength," El-Badry says, "you learn something new and unexpected."