Dust in the wind
Balloon-borne telescope surveys violent, star-making clouds of Milky Way Galaxy
Posted February 3, 2012
“All we are is dust in the wind.”
So sang the 1970s rock band Kansas for its peaceful, philosophical ballad about mortality.
But the place from where the clouds of cosmic dust and gas blow that eventually forms stars and planets — and, by extension, us — is far less idyllic.
“These clouds of dust and gas aren’t gently moving around. This is a very violent, nasty place in the interstellar medium. Terrible things are happening. Things are being ripped apart; gravity is shoving stuff together,” said Christopher Walker , about six weeks before his team launched a giant balloon from Antarctica high into the Earth’s atmosphere carrying a very special type of high-frequency radio.
The Stratospheric Terahertz Observatory (STO) won’t be tuning into Kansas’ greatest hits. Instead, it will pick up the faint, high-frequency radio signals emitted by carbon atoms within those violent interstellar clouds of gas and dust that are found in the Milky Way Galaxy.
“These clouds are actually made out of the debris of supernovas that went off long before the sun ever existed,” Walker said. In turn, remnants from those explosions eventually coalesced — or collapsed — to form the sun. The planets were byproducts of the sun’s formation.
It’s an interstellar evolution — birth, death and rebirth — that still goes on today in this and other galaxies. The clouds in what astronomers call the interstellar medium, the matter that exists in the space between the star systems in a galaxy, play a key role in the process.
“In order to understand the lifecycle of all of this gas and dust in the Milky Way that goes from gas and dust to stars, back to gas and dust, back to stars, the formation of planets, we need to understand how these clouds of gas and dust form — how long are they in this phase,” Walker explained. “No one really knows. … We can learn a lot just by listening to the faint radio signals coming from these atoms and molecules about what is going on out there.”
A professor of astronomy at the University of Arizona , Walker is the principal investigator on the NASA -funded project, which is also supported by the National Science Foundation (NSF) through the U.S. Antarctic Program . The project includes a number of investigators and institutions, including the University of Arizona, Johns Hopkins University Applied Physics Laboratory (APL) , NASA’s Jet Propulsion Lab , the University of Cologne in Germany, Arizona State University and Caltech .
STO is a balloon-borne observatory, using the same gondola and telescope that Johns Hopkins APL investigators had previously used for solar astronomy. It was launched Jan. 15 from the Long Duration Balloon (LDB) facility located on an ice shelf near McMurdo Station . It spent two weeks circling the Antarctic, thanks to a summertime vortex that moves over the continent during this time of year. STO flew in the stratosphere at around 125,000 feet or more — about three times as high as commercial aircraft fly.
“We’re halfway to space,” noted Tony Stark, a co-principal investigator on the project and an astronomer at the Smithsonian Astrophysical Observatory . He also pioneered radio astronomy at the South Pole Station , particularly as the lead investigator on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) , a 1.7-meter diameter telescope that operated at the Pole for more than a decade.
“We were the first really successful winter experiment at the Pole,” said Stark while sitting in the crowded mezzanine of the balloon hangar. “When we started, people said it couldn’t be done. It was almost true. It was really difficult.”
The science goals were similar — investigations into the nature of the interstellar medium lifecycle. More than 100 papers were based on data from AST/RO, including the discovery that most galaxies experience sudden star-forming periods, or starburst, every 20 million years, based on observations of dust clouds in the center of the Milky Way Galaxy.
Eventually, AST/RO maxed out in terms of the frequencies it could detect through the high and dry atmosphere at the South Pole.
“It’s water vapor that’s our enemy. It absorbs the light we’re trying to detect. The South Pole is good up to a certain frequency, and after that you need to get above everything,” explained Walker, who made eight trips to Antarctica for the AST/RO experiment.
Enter STO, which sports the “most complicated high-frequency radios on Earth right now,” according to Walker. The long-duration balloon carried the roughly two tons of gondola, telescope, radio receivers and associated gadgetry to the fringes of the Earth’s atmosphere where interference from water vapor is far less of a problem.
The researchers believe the balloon platform will give them plenty of bang for the buck. A space-based mission would have cost at least $120 million when the team first proposed the project in 2007, according to Walker. The STO experiment will cost about one-twentieth of that.
The gondola carries two star cameras so the team knows which part of the Milky Way the telescope is pointing. It also boasts three gyroscopes to provide an inertial guidance system so the astronomers can point it to the places in the galaxy they want to map. Large solar panels provide the kilowatt of power needed to run the small, robotic observatory.
An onboard cryogenic system uses liquid helium to cool the ultra-high frequency radio receivers down to just a few degrees above absolute zero. There is also a receiver that can operate at atmospheric temperatures.
“This thing has all of the features and subsystems of an orbiting spacecraft,” Walker said. “No one has yet put these kinds of detectors on the back of a balloon-borne telescope.”
The team already has a proposal into NASA to build a new version of the gondola and telescope to probe even deeper into the Milky Way and other nearby galaxies, such as the Large Magellanic Cloud.
Dubbed GUSSTO, for Galactic/extragalactic Ultra/LDB Spectroscopic/Stratospheric Terahertz Observatory (GUSSTO), the one-meter telescope would fly on one of NASA’s newly designed super-pressure balloons that can stay aloft for 100 days or more.
“STO is unique in what it can do, and it’s also a precursor to GUSSTO, which we hope to start next year,” Walker said. “It fills in a big gap in our knowledge of the lifecycle of galaxies like our Milky Way.”
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