Balloon Astronomy Takes Flight with FIREBall-2

Twenty miles above the New Mexico desert, the FIREBall-2 Mission seeks to map the most diffuse gas in the universe and its role in the formation and evolution of galaxies

To lift an SUV-sized telescope into the stratosphere with a balloon, the weather needs to be just right: barely a breath of breeze, low humidity. High in the stratosphere, slow currents of air need to circulate gently—too strong a wind, and the flight could end up across international borders, or adrift over the Pacific Ocean. The balloon takes flight at dawn, slowly rising 120,000 feet through layers of atmosphere, until the air thins enough for faint signals from UV light to transmit, arriving in invisible wavelengths in the dark desert air at the end of a 9 billion light-year journey. For a single night only, the telescope swims through the high atmosphere. Twenty miles below, astronomers follow in a flatbed truck, tracking the course of the flight, waiting to receive the "payload” as it sinks down toward some quiet field, by parachute, at dawn.

So it began at dawn on September 24th, when the FIREBall-2 mission took flight in the dry air over New Mexico just outside of Fort Sumner. On this particular flight, unforeseen challenges prevented the balloon from reaching maximum altitude. However, the launch itself—a dawn pageant of trucks piping helium into the enormous balloon in the orange light of sunrise—marked a profound landmark achievement for the University of Arizona team of astronomers, whose fine-tuned systems clicked flawlessly into action as the telescope lifted off.

An experiment in flying an enormous multi-slit spectrograph using an equally enormous balloon—and a careful dance with the whims of wind and weather—the mission aimed to map the most diffuse gas in the universe.

When the FIREBall project began over 15 years ago, astronomers suspected that galaxies were surrounded by faint clouds of gas that could condense to form new stars. Without this gas, they theorized, it seemed that galaxies already should have run out of star-making material—but instead, it appeared that they drew their energy from some unseen source, likely hydrogen.

Lyman-alpha radiation from hydrogen shows up on a spectrograph as one of the brightest lines of all the elements. What makes such a bright signature so challenging to observe? Hydrogen from these gas clouds has wavelengths just beyond the edge of the visible light spectrum, in the UV. Very little UV light penetrates the earth’s atmosphere, making it impossible to detect it from earth-based telescopes.

As the universe expands, light signatures from very distant objects get stretched out—an effect called redshift, which helps us pinpoint how far away celestial objects are. In recent years, this effect has allowed ground-based telescopes to capture the first evidence of diffuse galactic clouds. Arriving from billions of light years away, the tight wavelengths of UV light from remote galaxies has had time to stretch into lengths that are visible, affirming that as galaxies were forming in the early universe, they were surrounded by faint, swirling clouds of star-making gas, called the intergalactic medium.

Pulling from this diffuse web of material, galaxies snowballed toward an intense star-making frenzy that climaxed between 10 and 11 billion years ago. Then, star creation began to slow down. Astronomers refer to this peak of star birthing activity as Cosmic Noon, as if the dial on some unseen clock reached its zenith and began to tick slowly downward.

The FIREBall team aims to understand what happened after this—what galaxies look like in the “Cosmic Afternoon.” In order to witness this period of declining star formation, where UV light hasn’t been stretched into visible wavelengths, a telescope needs to be above most of the atmosphere. “That’s where the balloon comes in,” Hamden says. Capable of rising to heights above 120,000 feet that can allow it to “see” in UV, FIREBall-2 seeks light signatures that have been theorized but never before detected. What FIREBall finds might ultimately help us better understand the story of our own galaxy, too.

A point of pride for the FIREBall team is that it’s “a limited resource project,” graduate student Aafaque Raza Khan says. “It’s not a multi-million dollar space mission, but it has all the features of one.” UV light from the faint intergalactic medium could be seen from a space telescope targeting that wavelength, but gathering the same data from within the earth’s atmosphere makes the preliminary research more affordable and achievable. For astronomers, this unusual stratospheric telescope is a creative challenge: “it requires you to solve really complicated problems in the fastest and simplest way,” Khan says.

Erika Hamden, Project Scientist, thinks of the program and its novel technology as “proof of concept,” after which a space mission could perform similar observations “across larger wavelengths and more comprehensively.”

Aboard FIREBall, a large amount of University of Arizona instrumentation took flight. In response to technical difficulties that impeded FIREBall 2 from collecting a full set of data in 2018, a team of UArizona scientists set out to solve an optical alignment problem using computer-generated holograms—a unique technique developed at UArizona College of Optical Sciences. The team fabricated much of this interferometer in-house, more quickly and for less money than it would have cost to outsource it: a point of pride in a scrappy project that seeks to capture space-telescope quality data on a small budget, says Khan.

Precise calibration is key for these optics to work. Undergraduate Nazende Ipek Kerkeser had the opportunity to work on the new calibration system, connecting colorful skeins of wire between zinc lamps, control boards, and fans. When it was sent to Caltech to test with the spectrograph, the system worked on the first try.

Kerkeser’s involvement in the project is a testament to the unique nature of the FIREBall mission, engaging researchers across all levels of education. “Not many groups are open to having undergrads involved in their projects, and I'm very happy to be on a team that allowed me to do all,” she says. The FIREBall project is a joint collaboration between NASA and CNES,

with collaborators from Caltech, University of Arizona, Columbia, Jet Propulsion Laboratory (JPL), Laboratoire d’Astrophysique de Marseille and CNES. “As a graduate student,” says Khan, “it's an incredible experience to be able to be in a mission of the scale.”

The on-board calibration system that Kerkeser worked on, as well as the optical alignment the Khan’s team tackled “worked perfectly in flight,” Hamden says. “Both systems have never been better.” The larger team of astronomers are still analyzing the challenges that prevented a full flight for FIREBall-2, but the launch shone a bright spotlight for these UArizona technologies in action. It was a milestone for these teams to see their instrumentation excel after years of careful development.

For Hamden, September’s launch represented 15 years of involvement in the project. She was Principal Investigator of the program as a postdoc at Caltech, and now she gets to take the wide view as Chris Martin takes on the PI role for FIREBall-2. In the time since the first launch, Hamden has become a TED Fellow, received numerous awards from NASA, and stepped into a role as Director of the University of Arizona Space Institute. In the long evolution of this project, each new FIREBall development—the successes, the learnings—has strengthened the lineage of other young astronomers building their careers with FIREBall as their launching pad. “I’m incredibly proud of them and the work they’ve done,” says Hamden. “We will be ready to do it again for the next flight!”

As for that perfect scenario for a balloon launch? The team will wait for another ideal weather window, some other dawn, some other September. It is a long-game venture that will offer even more students and early-career scientists to tackle large-scape optical and calibration projects. One thing is certain: in wide sky above the New Mexico desert, another balloon carrying UArizona innovations will fly. It will be, Khan says, “a first giant step into the unknowns of the cosmic web.”