SPHEREx launches this week to conduct a cosmic census

An infrared space telescope launches this week to map the sky not once but four times, and with unrivaled clarity. NASA’s Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) will tackle the thorniest mysteries of physics, from what happened right after the Big Bang to how the first galaxies formed and where the building blocks of life come from.

Shaped for success

Built by Ball Aerospace with a payload provided by Caltech and NASA’s Jet Propulsion Laboratory (JPL) and test facilities from the Korean Astronomy and Space Science Institute (KASI), the 1,100-pound (500 kilograms) SPHEREx weighs about as much as a grand piano and runs on less power than a kitchen refrigerator. But it promises to pack a scientific punch well above its meager dimensions.

Shaped like an oversized bullhorn, it stands 8.5 feet (2.6 meters) tall and spans 10.5 feet (3.2 m) at its widest — three nested photon shields that surround and protect its delicate optics from the heat and light of Earth and the Sun, as well as from the warmth of the spacecraft’s own making, generated by its computers and electronics.

“The shields are actually quite light and made with layers of material like a sandwich,” said Sara Susca, SPHEREx deputy payload manager and payload systems engineer, in a press release. “The outside has aluminum sheets, and inside is an aluminum honeycomb structure that looks like cardboard — light but sturdy.”

Gaps between the shields and specialized conical mirrors (called V-groove radiators) will expel heat into space, keeping SPHEREx’s temperature below –350 degrees Fahrenheit (–210 degrees Celsius). That helps prevent its own infrared glow from overwhelming the faint light emitted by distant celestial objects. 

“We’re not just concerned with how cold SPHEREx is, but also that its temperature stays the same,” said Konstantin Penanen, the mission payload manager, in JPL coverage of the mission. “If the temperature varies, it could change the sensitivity of the detector, which could translate as a false signal.”

The shields guard SPHEREx’s triple-mirror telescope, a 164-pound (74.5 kg) spectrophotometer that will scan the sky across 102 color bands with better resolution than earlier all-sky maps. With an 8-inch (20 centimeters) aperture, a 3.5° by 11.3° field of view, and two focal-plane assemblies housing six photodetector arrays, its lack of moving parts minimizes the risk of failures — but meant its focus had to be precisely configured on Earth to withstand the rigors of launch. 

A long road

That launch has inexorably slid out of reach for years. SPHEREx was proposed for NASA’s Small Explorer (SMEX) program in 2014, but was not selected. It was resubmitted in 2016 as a Medium-Class Explorer (MIDEX) mission, capped at $250 million (not including launch vehicle costs) and was picked by NASA in 2019 for launch in 2023.

But the project — led by Principal Investigator Jamie Bock of Caltech — battled multiple unforeseen events. The worldwide march of COVID-19 hit supply chains, affected vendors, and imposed limitations on lab space, forcing project staff to build engineering models of the spacecraft in their home workshops during the pandemic. As delays mounted, SPHEREx’s launch slipped to June 2024, then early 2025. 

“The team is very cohesive and it’s almost like a family,” said project systems engineer Jennifer Rocca in a live Q&A, speaking of SPHEREx’s resilient workforce, which maxed out at about 150 staff at its peak. “We’ve been in the trenches together. Our development team was together through COVID. We survived that. We recently had a bunch of our team members affected by the LA fires. And we’ve survived that together to continue our launch campaign.”

Sharing space

In 2021, SpaceX’s Falcon 9 rocket was selected to launch SPHEREx at a cost of $98.8 million. And the spacecraft’s low mass meant another NASA science mission was added to the launch in 2022 as a rideshare payload. “It just turns out that there was another satellite that was being developed that needed a ride to a very similar location in space,” said Cesar Marin, SPHEREx integration engineer with NASA’s Launch Services Program, in a Jan. 31 news conference panel.

SPHEREx’s carpooling buddy is the Polarimeter to Unify the Corona and Heliosphere (PUNCH) — four suitcase-sized satellites each weighing 88 pounds (40 kg) that will explore the Sun’s corona and solar wind. PUNCH may afford insights into space weather, including coronal mass ejections that can disable spacecraft and disrupt terrestrial electrical grids.

SPHEREx will enter a near-polar orbit of 430 miles (700 kilometers) above Earth, with PUNCH targeting a 350-mile (560 km) drop-off point. Liftoff will occur from Space Launch Complex 4 East (SLC-4E) at California’s Vandenberg Space Force Base during a 30-second launch window at 7:09 P.M. PST on Feb. 27. Weather permitting, the post-sunset launch should be visible along the California coast.

In 2023, SPHEREx underwent environmental trials in KASI’s vacuum chamber, an SUV-sized structure shipped from South Korea to Caltech for acoustic, thermal, and vibration tests. The spacecraft was cooled to cryogenic temperatures and engineers verified its optics were aligned accurately to within 0.0003 inch (7.5 micrometers) — one-tenth the width of a human hair.

“It’s absolutely essential that we get this thing sharply into focus before we fly,” said Phil Korngut, SPHEREx’s instrument scientist, in JPL coverage of the mission. “And the only way to accomplish that is through specific cryogenic optical testing in the environment provided by the KASI chamber.”

Mission goals

Once in space, SPHEREx will occupy a dawn-dusk Sun-synchronous orbit. “That means our orbit plane … is always facing the Sun,” said SPHEREx deputy project manager Beth Fabinsky in the news conference. “The telescope can point out and away from the Earth and at the same time point normal to the Sun line, and it can prevent us from getting Sun and Earth on our cold, dark detectors.”

The mission’s 25-month science phase will scan more than 99 percent of the sky every six months, completing four all-sky maps. Unlike most space telescopes, SPHEREx will quickly observe large swaths of the sky and rapidly survey multiple celestial objects.

That sets it apart from point-and-shoot missions like the Hubble Space Telescope, the James Webb Space Telescope (JWST), and NASA’s forthcoming Nancy Grace Roman Space Telescope. SPHEREx’s global perspective will help answer broad questions about the universe’s evolution, zeroing in on features of scientific interest for detailed inspection by Hubble, JWST, and Roman.

SPHEREx has three science goals. It will explore inflation — a brief yet powerful cosmic event when space-time expanded in size a trillion-trillionfold a fraction of a second after the Big Bang. Although that event occurred nearly 14 billion years ago, SPHEREx maps of the relative locations of 450 million galaxies could reveal clues about the physics behind inflation and how it affected the large-scale distribution of matter in the universe. 

Second, it will measure the collective glow from galaxies — including those that are too small, too diffuse, or too distant for other telescopes to see — and create a more complete picture of radiating objects in the universe. SPHEREx will tease out the processes by which the earliest galaxies took shape and how their first stars evolved.

And third, it will scour the Milky Way for icy granules of water, carbon dioxide, and other essential ingredients for life in stellar nurseries and protoplanetary disks to understand their relative abundances and locations across our galaxy. This promises greater insights into how likely icy compounds are incorporated into newly forming planets. 

A cosmic census

Past infrared space telescopes typically carried large dewars of cryogenic fluid to cool their optics, but these reservoirs were rapidly exhausted, limiting the missions’ operational lifetimes. SPHEREx will be passively cooled via its photon shields and radiators, potentially allowing a mission extension beyond the 25-month baseline.

Like the bullhorn whose shape it closely mimics, SPHEREx’s discoveries promise to echo across the astronomical community, unmasking the universe’s most mysterious events and identifying areas of interest for focused study. Fabinsky considers SPHEREx nothing less than a census of the cosmos.

“It’s the difference between getting to know a few individual people, and doing a census and learning about the population as a whole,” she said in JPL coverage of the mission. “Both types of studies are important, and they complement each other. But there are some questions that can only be answered through that census.”