Observing with SOFIA

SOFIA at Palmdale, CA (Jan 28, 2015)

Once we agree that the Moon is not made of swiss cheese, it becomes pretty clear that something violent must have happened in the lunar past. Many of the craters that pepper the surface of our satellite were formed when the Earth-Moon system was less than 800 million years old, in a cataclysmic event called the Late Heavy Bombardment. During that phase the celestial spheres skipped a beat and the sky, quite literally, fell: icy comets and asteroids were swung towards the rocky bodies orbiting the inner Solar System, bringing destruction but also drenching their parched surfaces with water. The same event that transformed the Moon into a block of Emmental cheese was the harbinger of life on Earth.

. . .

At first sight, it may not seem like a practical idea. Taking a big airplane, opening a huge hole in the back, bolting a large telescope into it, and then flying with the door open? Why would anybody dream anything like that?

The answer can be condensed in one word: water. Or, rather, the lack of it.

Water may be the elixir of life, but astronomers, as it turns out, are not very fond of it. And this is not because of the many squalls guilty of ruining countless observing nights. It’s because water molecules are naturally tuned to absorb infrared light, that part of the electromagnetic spectrum discovered by William Herschel beyond the deepest reds in the rainbow. And astronomers love infrared radiation, because it gives us the unique chance of studying the most elusive subjects in cutting-edge astrophysics research: planets and planetary debris surrounding the Sun and other stars, newborn protostars still hidden in their natal cocoons, dying stars enshrouded by their dusty winds. While these objects are often too dim to detect in visible light, they are copious emitters of thermal radiation, carried through space in the form of infrared light. It is to reveal the hidden secrets of these infrared sources that astronomers place their telescopes in the driest locations on Earth, blessed by unhindered access to the infrared photons coming from the heavens. The high deserts of Chile and the american Southwest, and the summit of volcanic mountains in the middle of Earth’s oceans, offer the best compromise between the needs of infrared astronomy and practical accessibility. The ice dome of Antarctica (9,000 ft thick at the South Pole) is even dryer (all water vapor is frozen), but is challenged by prohibitive environmental conditions. Deep space is the ideal location for parking infrared telescopes, but the size and weight of their mirrors, as well the possibility of repairs after launch, are limited by current technology and cost.

Inside SOFIA

Enters SOFIA, NASA’s Stratospheric Observatory For Infrared Astronomy. A wide-body Boeing 747 cargo aircraft, SOFIA serves as the flying platform for a 2.5-meters primary mirror telescope, designed to capture the infrared sky from the stratosphere. SOFIA’s cruising altitude tops at 45,000 ft, significantly higher than even long-haul transoceanic flights (typically flying at 35,000 ft, and only rarely climbing to 40,000 ft). At that height the air is very thin and, with a water vapor column reduced by 99% with respect to the ground, almost transparent to infrared radiation. This allows infrared observations at wavelengths that cannot be reached from the surface of Earth, or even from the icy deserts of Antarctica. From SOFIA’s stratospheric platform the “infrared window” is wide open up to a wavelength of 240 microns: ground-based telescopes rarely achieve observations beyond 20 microns. Its suite of 7 scientific instruments is designed to collect images and spectra in space-like conditions, but with the advantage of flying home at the end of each night, where they can be repaired and upgraded as necessary. This ability to continuously servicing the system is one of the main selling points of SOFIA with respect to space telescopes of equivalent capabilities. A human crew servicing SOFIA’s telescope and instrumentation has only to climb a ladder in the aircraft hangar. Compare that with servicing the Hubble Space Telescope that, when the Space Shuttle was still available, required the work of several astronauts in dangerous extravehicular activities, at costs approaching a billion dollar per mission. Hubble successor, the James Webb Space Telescope, will be not serviceable at all.

Despite being a plane, and not a spacecraft, SOFIA is managed by NASA with the feel of a mission to outer space. I got a taste of this in January 2015, when I briefly escaped the sub-freezing temperatures of Iowa’s winter to meet my University of Arizona collaborator Kate Su in Palmdale, at the border of the Mojave desert, where the plane is based. SOFIA is housed in the huge building 703 at the Neil A. Armstrong Flight Research Center (formerly known as “Dryden”), located inside the Edwards Air Force base. This is a legendary place in the human quest to conquer the heavens. It is from Edwards that “Chuck” Yeager became the first human to exceed the speed of sound with its Bullet-shaped Bell X-1 experimental aircraft. The base had also a prominent role during the moon race, leading the testing of essential technologies, including the Lunar Landing Prototype Vehicle that almost killed Neil Armstrong when it tipped over and crashed in a ball of fire (Armstrong ejected at the last minute). During the Shuttle era Edwards was the alternate landing site for the spacecraft, and building 703 in Dryden housed the two specially modified 747s used to ferry the orbiter back to its launching site at the Kennedy Space Center in Florida. Since the Shuttle retirement, one of the huge Shuttle Carrier Aircrafts has been moved in front of building 703, where it greets the visitors approaching the center from the south entrance. Building 703 is now shared by SOFIA and other NASA aircrafts, including two high altitude jets capable of flying at over 70,000 ft, instrumented to collect atmospheric data at the edge of space.

SOFIA's flight plan

Planning observations with SOFIA is a logistic ordeal. Since the observing chamber opens on the port side of the aircraft, the telescope can only point to the part of the sky directly on the left side of the plane. The only way to steer the telescope is to turn the whole aircraft. The typical 10-hours observing night looks then like a seemingly random-walk through the American and Pacific skies, with each leg chosen to match the orientation of one of the targets in the observing list. At the end of the night the plane must return to base, which further constrains the choice and order of the sources that can be observed. The flight plan for my trip brought us all the way down to Mexico, almost to the edge of the Intertropical Convergence Zone, where the moist air from the equator rises up to the stratosphere, an insuperable barrier in our quest for dry skies. All this was shown to us in the pre-flight briefing, where we (the two observers, the instrument scientists, telescope operators and flight support crew —— over 25 people in total) all assembled in preparation for the flight. This briefing is what you would expect for the typical NASA operation, with our mission director (Karina Leppik, a charismatic Antartica winter-over veteran) calling each sub-team for a “go”/“no go” status. At the end of the role-call we were a “go”, ready for our trip to the stratosphere.

Flying on SOFIA is nothing like flying commercial: we were reminded of that in the hour-long safety training we had to complete before boarding the plane. After all, SOFIA flies higher than a regular jet, and a sudden loss of pressurization would make you pass-out within less than 15 seconds. We were instructed to always carry with us our Emergency Portable Oxygen System (the EPOS, a sort of smoke hood with its own oxygen supply), when walking around in the plane. Stripped of all the furnishing and insulation of regular airplanes, the main cabin of SOFIA is cold and noisy, requiring to wear noise-canceling headphones connected to an internal PA system. A cumbersome system, but necessary to communicate with the telescope and camera operators during science time, and with the added bonus of allowing to overhear the cool chatter between pilots and ground traffic control during take-off and landing (pilots talk a lot, really). 

. . . 

Our program was scheduled in the second half of the night, during the northbound leg of the flight, as SOFIA returned to its base in California. The target of our observations was ε Eridani, one of the nearest neighbors of the Sun. The goal of our program was to probe the present of ε Eridani’s young planetary system, as a proxy to study the violent past of our own Solar System. Remember that dramatic event that bombarded the Moon into a maze of craters? It’s everyday news on ε Eridani, where the local “production” of the Late Heavy Bombardment “show”, is being re-enacted as you are reading these lines. By studying the drama happening today on the stage of ε Eridani, we can peer in the remote past of Earth’s history, at the age when life first appeared in the depths of its newly-formed oceans.

Specifically, we boarded SOFIA with the mission of resolving a long-standing controversy about the exact configuration of the swarming comets and asteroids that are circling ε Eridani and are responsible for its meteor bombardment. In a paper based on observations we performed a decade ago with NASA’s Spitzer space telescope (the infrared cousin of Hubble), we determined that this star possesses three separate circumstellar belts. Two inner rings are analogous to the Solar System’s asteroid belt. On the outside, a broader icy disk is instead similar to the Sun’s Kuiper belt, but on steroids: a massive version of the far-out belt that exists in the Solar System beyond the orbit of Neptune, the realm of comets and icy worlds ruled by Pluto and other dwarf planets. A key result in our work was to postulate the presence of clearly defined gaps between these three belts, a telltale sign that a whole family of planets are actively carving the humongous disk of ε Eridani into separate rings. Our conjecture, however, was not based on an actual image of the belts with their gaps, but rather on the spectral energy distribution of the infrared light emanated by the source. Spitzer lacked the visual acuity for imaging the details of ε Eridani’s disk, but possessed the sensitivity to map the thermal radiation of its emission, from which we estimated the rough distance of its components as they are heated by the central star. Our inference, however, was cast into doubts within a few years from publication, as an independent group derived a new fit of our data with a gapless (and planet-less) disk. A sure way to resolve the controversy was to obtain a true image of the gaps, in case they exist. SOFIA, with its much larger telescope, and its ability to observe infrared radiation at just the right wavelength where a gapless disk would be brightest, is the perfect tool to answer this puzzle. This was a convincing case for NASA, and we boarded the plane to find out.

. . .

Looking at ε Eridani images
(photo by B. J. Andersson)

In the stereotypical sci-fi Hollywood drama, discoveries follow experiments in real time. It takes just a few seconds for the scientists to look at the elaborate display on huge screens to declare triumphantly that the mystery is solved (and that humanity is inevitably saved by some impending cataclysm). In the real world, however, science works at a slower pace. As we huddled around the laptop of Andrew Helton, one of SOFIA's staff scientists, the image of ε Eridani captured by the FORCAST camera slowly came to life during the three hour-long exposure. Promisingly, it looked like a star, a featureless point in the sky without the extended ring of infrared emission to be expected if the gaps in the disk were filled-in, with no separation between the belts. Encouraging, but astronomy is a hard science, and as tantalizing as one single image could be, the answer for this puzzle will have to wait for the tremendous number-crunching that our computers will perform, once all the data are calibrated and merged with all other available evidence. In an age of data-intensive astrophysics, the quality and sophistication of the observations demand complex models for their full interpretation. Our few hours of telescope time, as glamorous as a trip to the stratosphere could be, will now be followed by months of gritty analysis work. The images we collected will be stacked against detailed physical models of the two competing hypothesis; hard numbers will tell us the results, and their statistical significance.

As we descended SOFIA’s ladder, heading back to building 703 in the twilight of a new day, we carried with us not just a night worth of data, but a valuable experience in how SOFIA is operated. This will be crucial for the work ahead, because it will allow us to fully appreciate the subtleties of our data, and push our analysis to its finer details. The prize at the end of this road is to understand the true structure of ε Eridani’s out-of-this-world disk, and its interactions with the cohort of planets likely inhabiting its system. SOFIA, by its unique ability of capturing infrared light in the dry stratospheric sky, is the closest we have to a time machine, revealing a glimpse of Earth’s ancient past by observing the present of a nearby young sun.

This story was first published on the Iowa State University Physics and Astronomy Department 'Quanta and Cosmos" Newsletter, June 2015 issue.