SOFIA: NASA’s flying infrared observatory prepares for its final lift-off

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You can follow the latest SOFIA flight in real time here. (Search “NASA747” on flight trackers such as flightaware.com)

For eight years, groups of astronomers have regularly embarked on a flight with the same destination as its origin. Flying in seemingly random loops and circles, they racked up thousands of air miles – to be redeemed later for scientific discoveries.

Technically they are stratonauts, flying in the stratosphere which is the layer just above the Earth’s atmosphere. And their airship is SOFIA, short for Stratospheric Observatory for Infrared Astronomy. SOFIA has a 2.7 meter infrared telescope that looks through a hole in the back of a Boeing 747 more than 40,000 feet in the air. From this unique vantage point, he unveiled the first molecule in the universe, mapped magnetic fields in distant galaxies, and found water on the Moon’s sunlit surface, among many other discoveries.

Today, SOFIA is about to take off for one last time.

SOFIA takes off from NASA’s Armstrong Flight Research Center in Palmdale, California. (NASA/Carla Thomas)

“We are all quite sad that this mission is coming to an end,” said Margaret Meixner, SOFIA’s director of science mission operations. Astrobites in a telephone interview. NASA decided to ground SOFIA based on Astronomy 2020 recommendation ten-year survey in November 2021, citing that its “scientific productivity does not justify its operating costs”. SOFIA requires around $85 million a year to operate, second only to Hubble in the agency’s annual budget.

Infrared image of the Horsehead Nebula from SOFIA. The glowing dust shows carbon monoxide molecules in the dense nebula in red, and carbon atoms and ions that have been affected by radiation from nearby stars in green. (NASA/SOFIA/J. Bally and. Al)

News of SOFIA’s cancellation came around the same time 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 wavelengths starting at a bit longer than our eyes can comprehend. The first images from the JWST recently took our breath away with several celestial gems captured in the near-infrared. The SOFIA, meanwhile, is sensitive to far infrared wavelengths ranging from 10 to 615 microns.

Turns out, taking your breath away is only part of the activity of infrared astronomy – you just can’t do it from where you can breathe. Earth’s atmosphere is a permanently frosted window to the infrared sky. Water molecules in the air scatter and obscure the infrared radiation trying to pass through. That’s why to see beyond, you have to send telescopes like JWST and Spitzer into space. The best option from Earth is through the crisp, clear skies above the driest place on earth, the high-altitude Atacama Desert in Chile where the ALMA Observatory can see through the infrared haze. .

There is, however, a third option: raise a telescope just above the atmosphere so that it passes 99% of the water vapour. Telescopes are reputedly heavy, bulky but delicate pieces of scientific equipment that depend on stability for their performance. Exactly none of these attributes make them ideal to put on a plane. But the promised rewards of observing space from the sky were so enticing 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 an aircraft, it has been made as light as possible by using a 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 upgrades and refurbishments. For added stability, it was mounted on what is essentially 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 it in real time, SOFIA’s secondary mirror was designed to alternate between its target and an empty field of view using a technique called chopping.

Regular science flight cycles began in 2014, and since then SOFIA has performed more than 100 flights per year from its home base at NASA Armstrong Flight Research Center in Palmdale, California.

Meixner, who joined SOFIA in 2020, had to wait until March of this year to earn his 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 watch. “So I’m waiting, it’s like an hour flight when I say, I don’t know, did we open the door?” The pilot replied, “Oh, that was half an hour ago.”

SOFIA discovered water molecules trapped inside rocks on the sunlit surface of the 2021 Moon (NASA/SOFIA)

The best thing about having a flying observatory is that you can take it wherever 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, greeted by the cooler and longer nights of the Southern Hemisphere winter. .

SOFIA may be the only telescope to work on an airplane to date, but it wasn’t the first of its kind. The Kuiper Airborne Observatory operated for over twenty years between 1974 and 1995 with a 36-inch telescope in a Lockheed P-3C Orion patrol aircraft. Kuiper’s titles of 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 major scientific results. On June 29, 2015, Pluto passed in front of a star, casting a tiny shadow on Earth near New Zealand. It was a time when everything aligned, as SOFIA was on a summer stay in Christchurch and had the ability to fly to the exact spot from which she could see the occultation. From there, SOFIA’s FPI+ photometer was able to calculate the precise dip in the star’s light curve and how Pluto’s atmosphere affected it.

The photometer is one of the 7 instruments that SOFIA can use to extract information from the light it captures from the stratosphere. Others include cameras for direct imaging, spectrographs for studying astrochemical compositions, and polarimeters for mapping magnetic fields dear to all astrophysicists.

Helium hydride, a molecule that contains the first known chemical bond to form in our universe, was discovered in space by SOFIA in 2019. (NASA/ESA/Hubble/Judy Schmidt)

SOFIA spectrometers have made two fundamental discoveries in recent years. In 2019, the GREAT spectrometer detected helium hydride, long thought to be the first molecule to form in the universe, in the Jewel Bug Nebula. Prior to this, the universe’s first molecular bond between ionized hydrogen and neutral helium had only been observed in the laboratory. In cosmic history, helium hydride has been the launching pad for making 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 Moon’s sunlit surface. This discovery is essential for understanding the role of water in planetary interiors and for telemetry and exploration of the lunar surface with the upcoming Artemis program.

Although he has several instruments to play with, Meixner noted that only one instrument can be fitted and carried on any given flight. And for Wednesday’s final flight, SOFIA will carry the HAWC+ polarimeter, an instrument that has made one of its favorite discoveries. “It’s basically our most requested instrument,” she said. “Investigators are using HAWC+ to map magnetic fields everywhere, and it just fills an information void.”

In 2019, HAWC+ provided important insights into star birth by recording the strength and direction of magnetic field lines along interstellar filaments of gas and dust in the southern Serpens cluster. This region of intense star formation showed a curious twist in magnetic fields from being parallel to the filaments at low gas densities, to perpendicular at higher densities, and back to parallel in the denser regions of the planet. matter. “We could see with HAWC+ that at the highest densities the magnetic field realigns and actually helps star formation,” Meixner said.

Hubble/SOFIA composite image of the starburst galaxy M82, also known as the Cigar galaxy. Magnetic fields mapped by SOFIA’s polarimeters show torsion geometries in different star forming zones. SOFIA also determined that these fields flow outward in a open rather than being closed or looped. (NASA/SOFIA/JPL-Caltech)

SOFIA’s final observation targets also consist of two similar filaments of interstellar gas and dust: Nebula L1157 in Cepheus and Perseus B. Next, it will examine magnetic fields in the Sculptor’s starburst galaxy.

According to Meixner, there is nothing particularly special about these targets, and it is a feature of how observation plans for a flight cycle are made. “We try to get the most important sightings in as early as possible because if you don’t get them, you can push them to the next flights and knock the last objects off the last flight,” she said. Nevertheless, they are now history as the swansong of SOFIA – the last things this unique observatory will watch from the highest clouds in the sky.

Astrobite Edited by Ali Crisp.

About Sumeet Kulkarni

I am a third-year doctoral student at the University of Mississippi. My research focuses on various aspects of gravitational wave astrophysics as well as the characterization of LIGO detector noise. It involves a lot of coding, and I like to keep typing on a keyboard even in my spare time, creating melodies instead of bugs. I run a science cafe offering monthly public lectures for the local community here in Oxford, MS, and I also enjoy writing popular science articles. My other interests include reading, cooking, cats and coffee.

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