NASA's Stratospheric Observatory For Infrared Astronomy (SOFIA) is a stargazing platform unlike any other.
SOFIA observes nebulae and galaxies in a variety of "colors" of infrared light. It may not boast as large a mirror as some of its ground-based relatives, and it doesn't enjoy the complete freedom from Earth's atmosphere that the Spitzer Space Telescope does, but SOFIA's ability to capture a wide range of wavelengths and distinguish between fine shades of colors make it an observatory unrivaled in the astronomical world. The fact that SOFIA lives on an airplane also makes it pretty remarkable, as it has made observations from above a dozen countries spanning both hemispheres.
"This observatory allows us access to a part of the universe that otherwise we cannot study from any other facility," said Naseem Rangwala, the NASA SOFIA Project Scientist.
SOFIA in the sky
Taking over from the Kuiper Airborne Observatory, NASA's previous high-flying infrared eye, SOFIA has been watching the skies since 2010 and is scheduled to operate until the early 2030s. The observatory takes the form of a compact Boeing 747, retrofitted specifically for this purpose. The aircraft makes about four flights each week, cruising for 10 hours at a time between 40,000 and 44,000 feet (12,000 and 13,000 meters), putting it above more than 99% of the infrared-scattering water vapor in Earth's atmosphere. For most of the year SOFIA operates from California, but it also makes trips to New Zealand for Southern Hemisphere stargazing, as well as to Germany, whose space agency developed three of the platform's eight instruments.
A large door toward the rear of the craft opens to reveal a 8.9-foot (2.7 m), nearly 20-ton mirror, which swivels nimbly to maintain a fixed lock on its celestial marks while the plane bobs and vibrates.
"One of the things I like best is just watching the telescope," said Michael Person, a planetary scientist at the Massachusetts Institute of Technology who uses SOFIA to study planetary atmospheres. "Eventually you realize the telescope is perfectly still as it must be to be pointing at the target, and it's the plane and you and everything else that's jostling and moving around."
Seats have been stripped from the main cabin to transform it into a control room, with table-mounted consoles for instrument operators, data analysts and visiting scientists. The flight crew and navigators hang out on an upper level, and the front of the plane retains its seats for takeoff, landing and enjoying the view. "In the Southern Hemisphere, you get to see the lights of the aurora," Rangwala said. "It's an amazing experience."
What makes SOFIA special
Portable, cutting-edge observatories don't come cheap. SOFIA cost $85.2 million to run in 2017, putting it close to the Hubble Space Telescope as one of NASA's priciest programs (although DLR, the German space agency, shoulders 20% of SOFIA’s cost). But the missions the telescopes work on couldn't be more different.
Once a telescope arrives in space, that's typically the end of its development. SOFIA, however, which returns to the ground every day, can add new instruments and upgrade old ones without launching a single rocket.
In 2015, the German Aerospace Center upgraded its German Receiver for Astronomy at Terahertz Frequencies (GREAT) instrument aboard SOFIA. With the new hardware, researchers were able to identify in deep space molecules of helium hydride — the type of molecules long thought to have participated in the universe's earliest chemical reactions. "This molecule was predicted by theorists for decades," Rangwala said. "We finally found it."
Then last year SOFIA’s High-Resolution Airborne Wideband Camera Plus (HAWC+) came online, allowing researchers to image magnetic fields and study the role they play in star creation.
Another unique characteristic of SOFIA is its range. Some telescopes specialize in a few particular colors of infrared light. Others, like the upcoming James Webb Space Telescope, are powerful but narrowly focused on a small spot of space. SOFIA, however, can do it all. Its instruments span much of the infrared spectrum from a few microns to hundreds. Stars burn brightly enough to emit visible light, but in this other swath of the spectrum SOFIA can pick out dimmer, cooler objects from galaxies to nebulae to dust clouds, similar to how infrared goggles can discern people and animals at night. The telescope can also tell one shade from another with rare precision — an important ability for spotting the fingerprints of individual molecules.
SOFIA's scientific contributions
The astronomical community has fully embraced the platform's unique résumé of skills. For instance, Michael Person, a research scientist at MIT, used SOFIA to observe Pluto in the summer of 2015. He and his colleagues have been studying the dwarf planet's atmosphere for 20 years through an eclipse-like phenomenon called occultation — when Pluto moves in front of a star, casting a shadow out into space. At that moment, starlight passes through Pluto's atmosphere, and any telescope that finds itself in Pluto's diminutive shadow can extract some information about the gases that surround the dwarf planet.
Most occultation shadows fall over the ocean, though, and even if they don't, their path across the Earth is tough to predict. But SOFIA can overcome both of those challenges. In June of 2015, Person found himself on board the aircraft, fielding calls from MIT with final predictions and updating the navigators, who tweaked the flight plan in real time to chase Pluto's shadow across the Pacific Ocean. "At the last minute we can reposition [SOFIA] in a way you can't just quickly move a telescope on the ground," Person said.
The team's improvising paid off. By observing Pluto's atmosphere in two colors, they were able to help settle a long-standing debate about whether the dwarf planet's fuzziness indicated haze or heat. Two weeks later, the New Horizons probe flew by Pluto and confirmed their findings: Pluto was hazy. "It was basically the ideal experiment," Person said.
Recently, SOFIA has embarked on two legacy programs — both require observations spanning many hours. One aims to study groups of stars of different sizes to determine whether their bubbles and shockwaves make it easier or harder for other stars to form nearby.
The other is targeting a large tract of the center of the Milky Way about the size of four full moons. Despite an abundance of star ingredients, something seems to be stopping stellar birth in this region, and researchers hope more detailed images will help them figure out what.
Even as the more powerful James Webb Space Telescope comes online, Rangwala emphasized that SOFIA's complementary nature will make it an even more valuable part of NASA's fleet of astronomical hardware. Such sweeping maps of the Milky Way will be essential for helping the much more narrowly focused space telescope get its bearings, she said. "If the [JWST] wants to know where to point, [SOFIA] can provide some of the most precise pointing information."
- Read more about SOFIA's results at the instrument's home page.
- See what it's like onboard SOFIA with a NASA video crew.
- Read a first hand account of SOFIA chasing Pluto's shadow.
Editor's note: This article was updated on Nov. 12, 2019 to correct Naseem Rangwala's title as NASA SOFIA Project Scientist, and to add clarification to her quote in the last line.