SOFIA’s final flight (NASA747) departed at 03:44 UTC on 09/29/2022. You can track it here in real-time.
For the past eight years, groups of astronomers have regularly boarded a flight with the same destination as its origin. Flying in seemingly random loops and circles, they’ve racked up thousands of air miles–to redeem later for scientific discoveries.
Technically, they’re stratonauts, flying in the stratosphere which the second layer of the Earth’s atmosphere. And their airship is SOFIA, short for the Stratospheric Observatory for Infrared Astronomy. SOFIA has a 2.7 meter infrared telescope that peers out of a hole at the rear side of a Boeing 747 from over 40,000 feet in the air. From this unique vantage point, it has unveiled the first molecule in the universe, charted magnetic fields in distant galaxies, and found water on the sunlit surface of the Moon, among many other discoveries.
Today, SOFIA is preparing to take off for one final time.
“We’re all quite sad that this mission is coming to an end,” Margaret Meixner, the director of SOFIA’s science mission operations told Astrobites in a phone interview. NASA decided to ground SOFIA based on the recommendation of the Astronomy 2020 decadal survey in November 2021, citing that its “science productivity does not justify its operating costs.” SOFIA takes around 85 million USD per year to operate, second only to Hubble in the agency’s annual budget.
News of SOFIA’s cancellation came around the same time as another one-of-a-kind infrared telescope, NASA’s JWST, was preparing to launch into space. JWST observes in the near-infrared spectrum, picking up on wavelengths starting at just longer than what our eyes can make sense of. The first images from JWST recently took our breath away with several celestial jewels captured in the near-infrared. SOFIA, on the other hand, is sensitive to the far-infrared wavelengths ranging from 10 to 615 microns.
Turns out, taking your breath away is just part of the business of infrared astronomy–one simply cannot do it from where one can breathe. The earth’s atmosphere is a permanently frosted window for the infrared sky. Water molecules in air scatter and obfuscate infrared radiation that tries to pass through. That is why to see beyond, one needs to send telescopes like JWST and Spitzer into space. The best option from Earth is through the clear and crisp sky above the driest place on the planet, the high-altitude Atacama desert in Chile where the ALMA observatory can see through the infrared mist.
There is however a third option: to carry a telescope just above the troposphere (the first layer of our atmosphere) which contains 99% of the water vapor. Telescopes are famously heavy, clunky, yet delicate pieces of scientific equipment which rely on stability for their performance. Exactly none of these attributes make them ideal to put on an airplane. But the promised rewards of observing space from above the sky were so appealing that engineers made it possible.
One of SOFIA’s engineering marvels, according to Meixner, is the 2.7 meter telescope itself, which was built by the German Space Agency (DLR). The largest telescope ever put on a plane, it was made as light as possible by using polymer composite material in its structure. By the time the telescope made its way to the United States, a former United Airlines Boeing 747SP aircraft had already undergone extensive retrofitting and refurbishment. For stability, it was mounted on what is basically a giant ball bearing suspended in pressurized oil. Infrared telescopes, unless cryogenically cooled, face immense background noise due to their own heat. To get rid of this in real-time, SOFIA’s secondary mirror was designed to alternate between its target and a blank field of view using a technique called chopping.
Regular science flight cycles started in 2014, and since then SOFIA has flown over 100 flights per year from its home base at the NASA Armstrong Flight Research Center in Palmdale, CA.
Meixner, who joined SOFIA in 2020, had to wait until March this year to get her so-called stratonaut badge because of the pandemic. The thing that impressed her the most was the smooth opening of the telescope hatch, which the pilot had asked her to look out for. “So I’m waiting around, it’s like an hour into flight when I say, I don’t know, have we opened the door?” The pilot replied, “Oh, that was a half hour ago.”
The best thing about having a flying observatory is that you can take it anywhere you want without literally moving mountains. As summer hits the hot desert of Palmdale, SOFIA astronomers simply fly to a second base in Christchurch, New Zealand, welcomed by the cooler and longer nights of the Southern Hemisphere winter.
SOFIA may be the only telescope to operate in an airplane as of now, but it wasn’t the first of its kind. The Kuiper Airborne Observatory operated for over twenty years between 1974-1995 with a 36-inch telescope in a Lockheed P-3C Orion patrol aircraft. Kuiper’s claims to fame include discovering rings around Uranus, and confirming the presence of an atmosphere on Pluto.
The characterization of Pluto’s atmosphere was one of SOFIA’s first big scientific results. On 29th June 2015, Pluto passed in front of a star, casting a tiny shadow of it on Earth near New Zealand. It was a time when all things lined up, since SOFIA was on its summer sojourn in Christchurch and had the ability to fly to the exact place from where it could see the occultation. From here, SOFIA’s FPI+ photometer could calculate the precise dip in the star’s lightcurve and how Pluto’s atmosphere affected it.
The photometer is one among 7 instruments that SOFIA can use to extract information from the light it scoops up from the stratosphere. Others include cameras for direct imaging, spectrographs to study astrochemical compositions, and polarimeters to map out every astrophysicists’ dearly beloved magnetic fields.
SOFIA’s spectrometers have made two fundamental discoveries in recent years. In 2019, the GREAT spectrometer detected helium hydride, long hypothesized to be the first ever molecule to have formed in the universe, in the Jewel Bug nebula. Before this, the universe’s first molecular bond between ionized hydrogen and neutral helium had only been seen in a lab. In cosmic history, helium hydride was the launchpad to make molecular hydrogen, the most abundant molecule in the universe.
In 2021, another SOFIA spectrometer called FORCAST detected the presence of normally volatile water molecules trapped inside rocks on the sunlit surface of the Moon. This finding is critical to understanding the role of water within planetary interiors, and for lunar surface ranging and exploration with the upcoming Artemis program.
Despite having several instruments to play with, Meixner noted that only one instrument can be mounted and carried on any given flight. And for Wednesday’s final flight, SOFIA is going to take the HAWC+ polarimeter, an instrument that made one of her favorite discoveries. “It’s been basically our highest demand instrument,” she said. “Investigators are using HAWC+ to map magnetic fields everywhere, and it’s just filling a vacuum of information.”
In 2019, HAWC+ provided important insights into how stars are born by recording the strength and direction of magnetic field lines along interstellar filaments of gas and dust in the Serpens south cluster. This region of intense star formation showed a curious twisting of magnetic fields from being parallel to the filaments at low gas densities, to perpendicular at higher densities, and back to parallel in the densest regions of matter. “We were able to see with HAWC+ that at the highest densities, the magnetic field realigns and actually helps star formation,” Meixner said.
The targets for SOFIA’s final observations also consist of two similar filaments of interstellar gas and dust: the L1157 nebula in Cepheus, and Perseus B. It will then look at magnetic fields in the Sculptor starburst galaxy.
According to Meixner, there is nothing particularly special about these targets, and that is a feature of how observing plans for a flight cycle are made. “We try to make the most important observations done as early as possible, because if you don’t get them, you can push it to succeeding flights, and bump the last objects off the last flight,” she said. Nevertheless, they are now a part of history as SOFIA’s swan song–the last things this unique observatory will look at from above the highest clouds in the sky.
SOFIA’s final flight departed at 03:44 UTC on 09/29/2022, and flew for 7h 57m along the following flight path:
Astrobite Edited by Ali Crisp.