There can also be a bit of a delay in gratification when the telescope in the back of NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747SP that carries a 106-inch diameter telescope where passengers once sat, probes the universe. Analyzing the images takes time, often a matter of months or years.
“We don’t often get that ah-ha moment,” said Elizabeth Ruth, one of 14 NASA pilots who fly SOFIA, in a videoconference with Chief Pilot Andrew Barry and AOPA, a few days after the discovery of water on the sunlit side of the moon was announced, a little more than two years after the August 2018 flight. “We didn’t know about the effects of this until Monday as well, when we heard the announcement, and since then I’ve heard from friends around the world.”
Confirming that significant amounts of water await future lunar explorers is significant, even if the quantity is relatively small. Measuring reflected light, scientists calculated that roughly 12 ounces of water are trapped in a cubic meter of lunar soil in the Clavius Crater. It remains to be seen if there’s enough water waiting on the moon to support NASA’s plan to return there in 2024, but it may well help. The results and analysis of the 2018 telescope run were published October 26 in Nature Astronomy, and propelled the flying telescope back into the news, though it was hardly the only significant discovery SOFIA has made. Barry and Ruth have flown many missions, no two of them exactly alike.
“It’s quite an adventure every night,” Barry said.
SOFIA climbs quickly and nearly to its altitude limit on a typical mission to offer a clear view of the sky, then traces precise, arcing tracks above 99 percent of the Earth’s atmospheric water vapor to capture the infrared light from distant galaxies, and more recently the not-so-distant moon. While SOFIA has logged many important discoveries, rarely has there been an ear-splitting “whoop” of celebration from the team of scientists sequestered downstairs from the flight deck.
Barry, who manages SOFIA’s flight operations at NASA’s Armstrong Flight Research Center in Palmdale, California, said the crews take turns flying the huge telescope, and random chance explains why he was on the flight deck that night in August 2018: “That was just the luck of the draw for Liz and I. Now, it’s history, apparently.”
Turning a Boeing 747 into a flying observatory was a team effort that began in 1997, when the Universities Space Research Association (USRA) acquired a retired airplane from United Airlines that had a storied history even before it became the largest flying observatory ever built. According to USRA, the aircraft that became SOFIA was christened Pan Am’s Clipper Lindbergh by Anne Morrow Lindbergh and placed in service on May 6, 1977, the fiftieth anniversary of Charles Lindbergh’s most famous flight.
United purchased the well-traveled Boeing 747SP (“SP” stands for “special performance,” a short-body variant of the Boeing 747 Classics, as the -100, -200, and -300 series models are known) in 1986 and kept it in service until 1995.
NASA, which acquired the aircraft from and collaborated with USRA and the German Aerospace Center (DLR), had previous experience mounting telescopes in aircraft, the better to see through haze with, though none nearly as large. (The first iteration of the airborne observatory was a Learjet with a 12-inch telescope.) NASA worked with engineers from L-3 Communications Integrated Systems, the prime contractor for the telescope modifications, and sought out expertise from United alumni.
“We kind of arose from the dust of United’s unloading of their classic-74 line in the late nineties as this project was getting traction as an idea,” Barry said. “We literally bought their sim from them.”
Along with buying the equipment, NASA hired many former United pilots, technical writers, and other staff familiar with the fine points of running this particular airplane. Then, they set about making a very typical airliner quite extraordinary, installing a door in the aft fuselage that is 18 feet tall and nearly 14 feet wide, designed to roll open in flight to reveal the 106-inch (2.5-meter) business end of the most powerful flying telescope on the planet. SOFIA’s first observation from that telescope was made in May 2010. Many notable astronomical discoveries have followed, if none quite so high-profile as the lunar exploration news delivered October 26 that pushed discussion of future visits to the moon and Mars back into the public awareness.
Ruth said the modifications, including the Raytheon-designed door that can be opened once the aircraft reaches 35,000 feet (much of the telescope is outside of the aircraft’s modified pressure vessel), have not noticeably affected SOFIA’s handling characteristics.
“With that big hole in the side of the airplane when we open the door once we’re above 35,000 feet, you would think that would cause some problems, but it was so well-designed that we cannot tell if the door is open or closed, we can’t even tell if it’s in transit,” Ruth said. “We had to put a light up in the cockpit just so that we know that the door is open, because we don’t want to go below 35,000 feet with that door open.”
The massive opening is framed by curved borders that create laminar flow over the opening when the telescope is in use. “It was an amazing piece of engineering.”
Climb performance is prized by SOFIA crews, because while the missions last for hours, every second the telescope spends on target is precious. Once they reach their initial observation altitude, typically 41,000 feet when the fuel tanks are still nearly full, climbing to 43,000 about halfway through the flight, much of the mission is spent making tiny turns, one degree at a time, to create long arcs through the sky that allow the telescope to steadily track distant objects.
“The mission director gives us exact, precise headings,” Ruth explained. “Once we get on a leg, they will tell us when they want us to change heading, by 1-degree increments. They’ll say, ‘one left,’ or ‘one right.’ So that’s pretty much what we’re listening to all night, is ‘one left, one right, OK you can do your turn now.’”
While the autopilot maintains the selected heading, the engines require attention. Telescope observation missions require precise timing to account for the distant object’s transit across the sky as the Earth rotates, and flights are planned with waypoints that must be crossed within two minutes of the prescribed time, sometimes much less. For some missions, waypoint passage precision is measured in seconds, Barry said.
“We have no autothrottles, so managing .85 Mach … which the plane really wasn’t exactly designed, that’s not its real happy place all the time, so it’s really very engaging,” Barry said. “That’s one way to stay awake all night is manipulating the throttles continuously to keep that speed exactly where we need it to be.”
The telescope built to study objects across mind-boggling distances had to be used in a new way to spot water on Earth’s nearest celestial neighbor. The object of the science team’s attention is usually tracked with a guide camera that helps operators keep the telescope locked on that distant target by tracking multiple stars. (Lindbergh managed a few star sightings to navigate across the Atlantic Ocean, in another little historical twist.) Celestial navigation of aircraft or telescopes is not particularly feasible when the moon fills the guide camera’s entire field of view, so they were not sure it would work, and the lunar observation was tacked onto the end of an unrelated mission that had taken SOFIA west off the coast of Southern California, crossing thousands of miles of empty Pacific Ocean before turning east toward home.
“They timed the last leg… to wrap up on the flight home over Nevada and California looking at the full moon that was available that night,” Barry said.
Both Barry and Ruth have built long and distinguished careers as aviators in military and civilian service. Barry is also NASA’s chief Gulfstream pilot and a U.S. Naval Academy graduate who held several military posts, and later flew for JetBlue. Ruth, who was the first woman hired to fly SOFIA, previously served in the U.S. Air Force flying various aircraft including the T–43, a Boeing 737-200 configured for airborne navigation training. She later flew for United Airlines. Both pilots chuckled when an interviewer pointed out that their SOFIA mission flights, which can cover 4,000 nautical miles or more, are technically considered local flights under FAR 61.1, since they begin and end at the same airport, though such flights would count as cross-country time toward experience required for the airline transport pilot certificates they already hold.
“A little cross-country time, but you never actually land and get a burger,” Barry said with a smile.
Ruth takes pride in every SOFIA mission she flies. “We’re contributing to understanding of the universe.”
Both pilots noted that it requires a large team of people to not only get the telescope in the air, but harvest knowledge from the images it captures.
“We’re one part of that bigger machine, providing a bigger picture to these lunar scientists,” Barry said. “They are trying to determine what we can do to aid in that broader human exploration side of this… hopefully, this is a good start in that direction. Water is life. If we can reduce that cargo load, whatever it might be to not have to carry water along with you to the moon… we’re part of the machine and helping to produce this.”
Barry noted that it’s an additional and gratifying perk to fly “such a classic aircraft with a storied history.” This pilot prefers the classics, and the Boeing 747 is certainly such a specimen. “I’ve never flown anything new, I can say that.”