The final flight of the SOFIA airborne observatory

Astronomers mourn the loss of a one-of-a-kind window into the universe as the SOFIA infrared observatory's mission winds down.

To study the infrared universe, SOFIA used a 2.5-meter telescope on a modified Boeing 747SP. Credit: NASA Dryden photo by Tony Landis

The vast majority of infrared light from space never reaches the ground. Along the way, the abundant water in Earth's atmosphere absorbs it. Therefore, astronomers will need to think outside the box in order to observe that far-off light.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) was that innovative solution for years, particularly between the launch of the James Webb Space Telescope (JWST) and the retirement of the Spitzer Space Telescope. On a modified Boeing 747SP, the airborne observatory carried a 2.5-meter telescope. However, SOFIA simply flew high enough to get above approximately 99 percent of the water in our atmosphere, reaching a height of 45,000 feet (13.7 kilometers).

However, NASA and the German Space Agency announced earlier this year that SOFIA would be shut down by September 30.The final SOFIA flight took place over night on Sept. 28 and 29, departing from Palmdale, California, and landing there. Those who flew on SOFIA say that it will be a devastating loss. However, its legacy is something to treasure.

Unlocking the infrared

Recently, JWST has appropriately stolen headlines. However, it is only one tool in astronomers' arsenal for observing the infrared universe.

The wavelengths of infrared light, which range from about 1 to 1,000 micrometers (m), are longer than those of visible light. When studying astronomical objects like cool stars, gas and dust, planets, and moons, observations at these wavelengths are especially helpful. Because infrared light is not absorbed by cool dust the way visible light is, the former often reveals what’s behind curtains of obscuring material that block our views in the visible portion of the spectrum. (Consider the infrared views of, for instance, the Pillars of Creation, which reveal the young stars normally concealed within sculpted dust and gas clouds.)

To get above the annoying water vapor in the atmosphere, it seems like the logical next step to send a telescope into space. There have been a number of telescopes built with instruments that can either see in the infrared or some longer wavelengths. However, building a space telescope is a costly, time-consuming, and challenging endeavor that may come to an end before or just after it is launched.

There is a third choice: an aircraft SOFIA carried researchers and instruments into the dry, high stratosphere, above nearly all water vapor, at altitudes of 37,000 to 45,000 feet. Additionally, SOFIA could be easily upgraded, repaired, and replaced with new instruments because it landed after each flight. If necessary, telescope-flying astronomers could even make adjustments and repairs during an observation.

This innovative solution combined the convenience and accessibility of a ground-based observatory with the capabilities of a space-based observatory, following in the footsteps of previous airborne observatories. Pete Ashton, a SOFIA instrument scientist, tells Astronomy that SOFIA "found its own niche... and used it to do science that isn't possible from anywhere else."

The FORCAST instrument, which is mounted on the tilting blue area in the background, is utilized by astronomers aboard SOFIA. Credit: Sarah Scoles

Greatest hits

SOFIA has achieved a number of impressive feats since its first light in 2010.The discovery of helium hydride, the earliest known molecule, within the planetary nebula NGC 7027 was one of the most significant and recent. In spite of the fact that astronomers had been looking for helium hydride since the 1970s, it took the capabilities of SOFIA until 2019 to locate it.

During the observatory's first ever look at our satellite, SOFIA also found evidence of water in the soil of Clavius Crater on the Moon.The presence of Clavius suggests that precious water deposits may not be restricted to permanently shadowed, cold lunar locations because it is located on the sunlit, Earth-facing side of Luna.

Additionally, SOFIA's mobile capabilities made it the ideal platform for observing occultations, which occur when a planet or asteroid in our solar system passes in front of a faraway star. Unlike total solar eclipse shadows, which are much easier to predict, occultations produce much smaller shadows. Therefore, it is essential to have a telescope that can quickly move to precisely where it is required to make a difficult observation.

SOFIA rose to the challenge, traveling at speeds of 53,000 mph (85,300 km/h) through remote areas of our planet while flying several times in the fast-moving shadow of Pluto. These observations provided researchers with an inside look at Pluto's thin atmosphere, revealing that it is full of short-lived haze that must be replenished frequently by the dwarf planet's processes.

During a 2013 flight, SOFIA sits in the sun just before takeoff. Credit: Sarah Scoles

Grounded for good

But despite its successes, the recently released decadal survey, also known as Astro2020, stated that SOFIA's annual operating cost of $86 million USD, which is comparable to that of the Hubble Space Telescope and the Chandra X-ray Observatory, could not be justified by its "modest" scientific output, which was measured by the number of papers that were published using data from the telescope.

According to the survey, 178 scientific papers were produced by SOFIA flights from 2014 to 2020.Over the same time period, more than 2,700 papers made use of the combined data from Chandra and Hubble. According to the survey, "The committee found no path by which SOFIA can significantly increase its scientific output to a degree that is proportional to its cost. "In the end, the committee approved NASA's current strategy to shut down SOFIA.

According to SOFIA Science Center senior astronomer Dario Fadda, "the loss of SOFIA simply means loss of access to the far-infrared window for at least the next ten years."

Both Fadda and Ashton are concerned that if SOFIA is shut down, students and early-career researchers will move away from research in the far infrared and into other wavelength regimes. Both are also concerned that institutional knowledge will be lost and need to be rebuilt as older scientists who have worked with the kind of data that SOFIA produced retire.

Not only was SOFIA crucial for the research that was carried out, but it also helped to preserve a core of people who were knowledgeable about infrared astronomy. Science, like any human achievement, requires continuity, according to Fadda.

However, there are still valuable lessons to be learned from SOFIA's success. The lesson of SOFIA, in my opinion, is that you can't get certain kinds of science by building a bigger version of something else with bigger mirrors, more detectors, etc. remarks Ashton. I hope it serves as a reminder that the best tool for the job may look completely different from anything you've ever seen.

Nancy McKown, principal systems engineer for SOFIA and the observatory's Mission Operations Manager and Director, flew the aircraft more than 100 times, including its first light flight in 2010.She admits that her final SOFIA flights were bittersweet, but she says, "I cherish the memories of the scientists' smiles and shouts of joy when a difficult observation was executed flawlessly."

The proposed Origins Space Telescope, which could launch in 2035, would reopen the far infrared to astronomers, but there are currently no plans for a flying observatory. Alternately, the Astro2020 committee recommended the creation of an infrared and X-ray space telescope. However, even if funding agencies accepted its proposal and adopted it, its launch would still be a long way off.

Despite this, McKown remains optimistic. She continues, "I'm confident that the thirst for knowledge will keep it in the long view of the big picture," despite the fact that there are currently some gaps. After all, we will still be able to obtain the data in the future.

However, the six instruments used by SOFIA covered the entire infrared spectrum, from 0.3 to 1,600 m.JWST is unable to replicate this range. JWST only observes between 0.6 and 28.3 m, despite having a mirror that is larger than 6.5 meters. Thus, SOFIA was able to see into the far infrared, revealing regions of the universe that JWST could never reach.