STARGAZING FROM THE STRATOSPHERE.
The James Webb Space Telescope, set to launch in 2018, may be the talk of the town in infrared astronomy circles, and for a bunch of reasons; for its staggering capabilities to look further and deeper into the universe than ever before possible, or for its eye-watering price tag of $8.8bn ... But there is another infrared observatory much closer to home that can see things the Webb's powerful spyglass will never gaze upon--and it keeps making scientific discoveries. The Clipper Lindbergh, now better known as SOFIA (Stratospheric Observatory for Infrared Astronomy), is a 40-year-old 747SP jumbo jet. SP stands for Special Performance and in this case the acronym is particularly apt, because the aircraft has been modified to become the world's only flying observatory.
Water is the enemy
It may seem counter-intuitive to put a telescope in an aircraft when we have so many ground-based observatories all over the world, but since 2010 SOFIA--an 80/20 partnership between NASA and the German space agency DLR--has flown above the troposphere for the same reason that the James Webb Space Telescope (JWST) will float in space: to get above the Earth's water vapour.
"Water in the Earth's atmosphere is the enemy of infrared astronomy," says SOFIA programme manager Eddie Zavala. "It blocks infrared light from coming in from the universe to the ground." German SOFIA E/PO lead Dorte Mehlert puts this into perspective: "If human beings had only far-infrared sensitive eyes, we would not know that there are stars in the sky." To combat this, SOFIA flies in the stratosphere, typically between 38,000 and 45,000ft, positioning the observatory above 99% of the Earth's water vapour.
And, because SOFIA is an aeroplane, unlike ground-based telescopes it can be wherever it needs to be to catch spectacular events. This happened in July 2015 when scientists calculated that observers in a specific zone in the southern hemisphere would be able to see Pluto pass in front of a star in Sagittarius. Known as an occultation, this seemingly mundane event is actually quite rare, and requires observers to be on a specific line of sight in the shadow path for the distant star to be seen exactly behind the centre of the planet.
Secrets in the starlight
SOFIA was flying on precisely this path at the right time and saw a ring of refracted starlight, uncovering key information about the planet's atmosphere. "Capturing the Pluto occultation really stands out to me," says Ryan Lau, a postdoctoral scholar at NASA's Jet Propulsion Laboratory. "Not only does it provide interesting information about Pluto's atmosphere, but also it's a really incredible technical achievement to be able to fly SOFIA to a precise location on the Earth at the right time."
That SOFIA can see anything is an astounding achievement. Battling turbulence, vibrations, temperature changes and an 800km/h stream of air caused by the plane's motion, the SOFIA team had to overcome major obstacles.
Take the mount as an example. Every amateur astronomer knows that a stable telescope mount is essential for good viewing. So how do you create one in a moving, vibrating aircraft? While it may look like NASA simply cut a hole out of the aircraft's fuselage and bolted a telescope to the floor, in reality painstaking work was required.
The open-port cavity is the result of numerous wind-tunnel tests and computational fluid dynamics and detailed finite element structural modelling--all ensuring that no aerodynamic flow is ingested into the cavity. "In flight, no one can tell if the door is open or closed," says Zavala. "There's no perceivable change in the aircraft motion or sound whatsoever."
In addition, the telescope is equipped with a vibration isolation system comprised of 24 air bladders that are inflated to offer soft support. In combination with 3D dampers, this provides good isolation. Meanwhile, turbulence is addressed by a telescope control system that includes gyrostabilisation and a central spherical bearing that floats on 14 microns of pressurised hydraulic oil.
"All of this gives the required pointing and tracking stability, so that SOFIA can even make observations in moderate to high in-flight turbulent conditions," adds Zavala.
Another key challenge is temperature changes. "The telescope is exposed to the environment at an altitude of 12-14km with temperatures around -50[degrees]C," explains Mehlert. "Meanwhile, in California where SOFIA is based, temperatures could easily reach about +50[degrees]C during the daytime."
These wild temperature changes would warp any normal mirror, so the German SOFIA team turned to glass innovator Schott to solve the problem. Schott made SOFIA's main mirror out of Zerodur, a glass-ceramic with a key property: it has a coefficient of thermal expansion that is almost zero, so the shape doesn't change with temperature.
Innovations don't stop at the aircraft and telescope. The five US and three German science instruments aboard SOFIA are cutting-edge cameras, spectrometers, photometers and imagers (see box on page 54). Combined, they cover the optical, near-, mid- and far-infrared spectrum, and offer unprecedented spectral resolution.
This power has given astronomers unique insights into our home, the Milky Way--from detecting atomic oxygen in Mars' atmosphere for the first time in 40 years to creating a 3D map of the whirlpool galaxy M51 that allows experts to explore the roles spiral arms play in the formation of giant molecular clouds, or observing supernova remnant Sagittarius A East to prove that supernovae are capable of producing a substantial amount of the material from which planets such as Earth can form.
"We have also selected a new instrument called Hirmes, which is a high-resolution imaging spectrometer under construction now," explains Matt Greenhouse, James Webb Space Telescope project scientist and SOFIA user group chair.
"One of the exciting things that it will be able to do is image with high-resolution debris discs that exist around stars, and from which solar systems and planets are formed, allowing us to map and observe the so-called snow line (the distance from a star beyond which water only exists in solid form)."
Despite these achievements over the past seven years, it would be easy to assume that SOFIA will become surplus to requirements in 2018 with the launch of the JWST. The Webb has a huge 6.5m mirror that dwarfs SOFIA's 2.5m version and will be able to see further back in time to the formation of the first galaxies, deeper inside distant dust clouds where stars and planetary systems are forming. It will also offer the chance to analyse the atmospheres of distant, potentially habitable worlds that may harbour life.
Capable and versatile
"SOFIA has performed valuable studies of our Galactic centre, but lacks the sensitivity to probe the centres of other galaxies," says Lau. "JWST will be sensitive enough to accomplish these observations in distant galaxies." However, what might be surprising is that in many ways SOFIA is a much more capable and versatile observatory than JWST.
With a lifecycle cost of $3bn, SOFIA is expensive but considerably cheaper than JWST or the Hubble Space Telescope. It has more electric power and more space to house big, power-thirsty instruments, and those instruments can be maintained and upgraded easily. Perhaps most importantly, though, SOFIA covers the full infrared spectrum.
"The Webb telescope can observe over a wavelength range from about 0.6 to 28[micro]m; the red end of the visible to the mid-infrared. It does that with imaging and low-resolution spectroscopy," says Greenhouse. "But SOFIA is geared towards longer wavelengths (up to 300[micro]m) than the Webb can observe and emphasises high-resolution spectroscopy."
And SOFIA has eight instruments available for operation right now and will have more and newer instruments in the future. "The James Webb Space Telescope only has three near-infrared and one mid-infrared instrument, which cuts down the range of scientific questions you may ask," says Mehlert.
"And onboard SOFIA we can teach and train the next generation of scientists, engineers and technicians during operation."
If anything then, instead of threatening SOFIA'S existence, JWST strengthens the case for continuing the SOFIA mission. "JWST is going to look much deeper into the early universe and its sensitivity is going to be unprecedented. Having said that, it will have limited capability to observe bright objects--and that's where SOFIA can complement JWST," says Zavala. "The missions and capabilities of JWST and SOFIA in my opinion complement one another, and they can certainly co-exist and serve the broad interest of the infrared science community."
So there may still be plenty of life left in the 40-year-old jumbo jet.
SOFIA'S INSTRUMENT SUITE
Echelon-Cross-Echelle Spectrograph (EXES): mid-infrared(4.5-28[micro]m). "The unique capability of EXES is that it has high spectral resolution--it can spread out the spectrum of infrared light to a very large degree," says Eddie Zavala. "With that high spectral resolution we can detect the presence of water and other constituents and separate them from the other water sources from the Earth's atmosphere."
Far-Infrared Field-Imaging Line Spectrometer (FIFI-LS): far-infrared (51-203[micro]m). The Orion Nebula was mapped by FIFI-LS to study the physical conditions of its atomic and molecular gas. "Only with such a field-imaging instrument like FIFI-LS is it possible to study the evolution of matter in between the stars in our galaxy as well as in other galaxies," says Dorte Mehlert.
First Light Infrared Test Camera (Flitecam): near-infrared(1.0-5.5[micro]m). "There's a big question in astronomy on where polycyclic aromatic hydrocarbons (PAHs) in space come from," says Ryan Lau. Using a PAH filter, Flitecam studied the photochemical evolution of PAHs in the reflection nebula NGC 7023.
Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST): mid-infrared(5-40[micro]m). "One of the workhorses of the NASA complement of instruments is FORCAST," explains Zavala. "It's an imager that really provides good general access to the universe for the infrared astronomical community."
Focal Plane Imager (FPI+): visible (0.36-1.1[micro]m). FPI+ is the main acquisition and tracking camera for the SOFIA telescope. "Due to its high sensitivity and data quality it is now also used as the third German science instrument for the observation of stellar occultations by bodies of the solar system," says Mehlert.
German REceiver for Astronomy at Terahertz Frequencies (GREAT): far-infrared (60-240[micro]m). "The observed far-infrared signal is mixed with a well-known laser signal and hence shifted to longer wavelengths (THz), where more precise and sensitive detectors are available," explains Mehlert. Using this heterodyne technique, GREAT is the perfect instrument for high-resolution far-infrared spectroscopy.
High-resolution Airborne Wideband Camera-plus (HAWC+): far-infrared (40-300[micro]m). "This instrument provides polarimetry with an angular resolution that's about 35 times greater than what Herschel had--a whole new regime of infrared polarimetry," says Matt Greenhouse. "That allows us to map and study the presence of magnetic fields in space."
High-Speed Imaging Photometer for Occultations (HIPO): visible (0.3-1.1[micro]m). Primarily designed for observing stellar occultations, HIPO can be co-mounted with Flitecam to offer simultaneous optical and near-infrared imaging.
Caption: Set to launch in October 2018, the James Webb Space Telescope sits in chamber A at NASA's Johnson Space Center in Houston at the start of a cryogenic test
Caption: Required to fly at 41,000+ feet for 10- to 12-hour missions, a 747SP was the natural choice to house the SOFIA observatory
Caption: Illustration of a dust ring near a black hole of an active galactic nucleus. SOFIA studies suggest the dust distribution is 30% smaller than previously thought
Caption: SOFIA oxygen spectrum superimposed on a picture of Mars. The abundance of atomic oxygen computed from the data is less than expected
Caption: SOFIA (left) and Hubble Space Telescope's images of the Milky Way's nucleus. SOFIA reveals gas and dust clouds orbiting a black hole; Hubble highlights star clustering
Caption: SOFIA's primary telescope mirror, seen under yellow protective panels inside the open telescope bay, is shown at its first public viewing in 2010
SOFIA VERSUS JWST Sofia JWST Lifetime cost $3bn $8.8bn Wavelength 0.36- 0.6- coverage 300[micro]m 28[micro]m Diameter of 2.7m 6.5m light- collecting mirror Number of 8 4 science instruments Resolution 1-2 arc 0.1 arc seconds seconds Spectral 100,000 3,600 resolution Size 56 x 60m 22 x 12m Distance from 12km 1.5 ground million km Mission 20 years 5-10 lifetime years Telescope 19 tonnes 7.15 weight tonnes The HAWC+ infrared camera (top) has been added to SOFIA; and JWST's main mirror
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|Publication:||Professional Engineering Magazine|
|Date:||Oct 1, 2017|
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