Over the past eight years, a modified Boeing 747 jetliner has flown hundreds of flights on a unique mission: carrying a 19-ton, 2.5-meter telescope known as Sofia, or the Stratospheric Observatory for Infrared Astronomy. Flying a telescope on a jumbo jet offered a way to peer into the heavens at wavelengths that could not be glimpsed from the ground—but the ticket was expensive. So yesterday, NASA and the German space agency grounded the mission. Its final flight landed early Thursday morning at NASA’s Armstrong Flight Research Center in the desert near Los Angeles.

Sofia was an innovative way to gaze at the infrared universe. Infrared light is essentially heat radiation—but astronomers can’t probe cosmic objects like dust-enshrouded stars and galaxies without the water vapor in Earth’s atmosphere absorbing that light. That confounds attempts to observe those objects with telescopes built on mountaintops, like the observatories in Hawaii and Chile. But by soaring through the stratosphere, at an elevation of 40,000 feet or higher, Sofia could fly above that water vapor and get a much better view.

“Almost 50 percent of the energy of the universe comes out in the mid- to far infrared. Sofia has played an important and unique role for its lifetime, probing that entire wavelength range, and we’ve been able to observe all manner of phenomena that were otherwise invisible to other facilities,” says Jim De Buizer, Sofia senior scientist at NASA’s Ames Research Center in Mountain View, California.

De Buizer and the Sofia team have made a number of significant astronomical discoveries, including measuring cosmic magnetic fields permeating nearby galaxies, charting the growth of massive stars, observing Pluto’s faint shadow as it passed in front of a distant star, and even discovering water on the sunlit surface of the moon’s southern hemisphere. The data from Sofia’s final flight will map stellar nebulas and help scientists study the magnetic fields of the Sculptor starburst galaxy.

But while flying a telescope in a jet is much less expensive than launching one aboard a spacecraft, like NASA’s Spitzer and Webb space telescopes and the European Space Agency’s Herschel Space Observatory, it’s still not cheap. There are costs for the pilots, staff, engineers, and mechanics—plus a round of repairs to the aircraft that had to be made in 2018. Sofia costs NASA about $85 million per year—a significant fraction of its astrophysics budget. And that’s actually only 80 percent of the funding it needs; NASA’s German counterparts provided the rest. It was ultimately the mission’s high operating costs, relative to its scientific output, that took Sofia down.

“At the end of the day, the project itself just wasn’t productive. You’re talking about almost a Hubble cost for operations, but with a fraction of the scientific productivity,” says Casey Dreier, senior space policy adviser for the Planetary Society, a nonprofit research organization based in Pasadena, California.

This wasn’t the first time its budget came into question. In 2014, following debates about budget constraints and austerity measures, the Obama administration threatened to cut Sofia’s funding—just 11 days after the telescope and plane became operational. But the US Congress opted to continue funding it. In 2019, after Sofia completed its main mission, advancing projects that studied nebulas, stars, and galaxies in the infrared, Congress extended the project for three more years, with the possibility of additional extensions. Citing budget concerns, NASA proposed canceling the program in the 2021 fiscal year and again each of the two following years. (The 2023 fiscal year begins tomorrow.)