Date: June 24, 2010
Title: Observing Between the Ground and Orbit
Podcaster: Rob Berthiaume
Links: www.yorkobservatory.com
www.youdontfreezeinspace.com
Description: You can build really a whole bunch of big telescopes on the ground, or put really fancy ones with great results in orbit, but there’s another option, in between – observing from aircraft.
Bio: Robert Berthiaume is working towards an MSc in atomic physics at York University in Toronto, Canada. When he can get away from making measurements of local gravitational acceleration, he rides his motorcycle when the sun is up, and shares the stars with the public at the observatory when it’s not.
Today’s sponsor: “Between the Hayabusa homecoming from Itokawa and the Rosetta flyby of asteroid Lutetia, 13 June until 10 July 2010, this episode of 365 Days of Astronomy is sponsored anonymously and dedicated to the memory of Annie Cameron, designer of the Tryphena Sun Wheel, Great Barrier Island, New Zealand, a project that remains to be started.”
Transcript:
Hi there. I’m Robert Berthiaume bringing you the June 24th edition of the 365 Days of Astronomy Podcast from the York University Observatory in Toronto, Canada. Today I’ll be talking about a really neat way of making astronomical observations that’s probably not widely known. It’s called airborne astronomy, and as the name implies, the observations collected come from telescopes and instruments mounted on aircraft, such as airplanes and balloons, instead of the more widely known observations from ground based observatories and space based observatories. It combines some of the advantages of both of these while at the same time eliminating some of their disadvantages. When all is said and done, it allows astronomers the ability to do some pretty unique science that can’t be done with either.
Most of the observations of galaxies and star-forming regions and planets and all the other stuff out in space are collected using ground based observatories. There are hundreds of these telescopes all around the world, of all different shapes and sizes, outfitted with cameras and spectrographs and a suite of different sensors. Ground based telescopes have two really big advantages: economy and accessibility. Once someone has a piece of land and enough money in their bank account, they can get to work building a nice big telescope. Once it’s all set up, they can go out to the telescope on any clear night and make observations, until something breaks. But no worries, since they’re sitting right there, they can grab their duct tape and WD-40, fix the glitch, and get back to observing. And if a new, faster, more sensitive, higher resolution camera than the one they already have installed comes out on the market 6 months later? Just go out and buy it, switch the cameras, and upgrade the observatory.
You can’t say the same about space telescopes, like Hubble, Kepler, SOHO, and the like. When these telescopes are built, there isn’t a crew of construction workers that arrives on site every day…I mean, the site is out in space. These telescopes are built here on the ground and then delivered to orbit with rockets. So you can’t build an arbitrarily large telescope to go to space, because it has to fit inside the rocket. Not only that, but they are incredibly complex machines that must be able to work non-stop while withstanding the harsh space environment. This means they take years to build and test, which makes them very expensive, and by the time they are launched they will always have cameras and antennae and computers that are definitely outdated and not state-of-the-art. Once they’re in space, that’s it; there’s no options to tighten any bolts or replace any sensors, because they’re so remote and it would be much too expensive (Hubble is an exception to this…we decided to spend the billions of dollars every few years to upgrade it as time went on).
Space based telescopes of course, have advantages, or else we wouldn’t launch them. They can be put in orbits that allow them to see the entire sky, at any point in time, unlike ground based telescopes that are locked into one particular geographic location. Also, they don’t have to deal with cloudy nights nor do they have to look though any of the air or water vapour or dust that is present in the atmosphere. This gives them non-stop, crystal clear views of anything they want to look at. Observatories on the ground must wait for storms to pass, and astronomers have to work very hard to get past the blurriness the atmosphere imposes on their images. The biggest advantage that space based telescopes have, arguably, is that they are able to see certain types of light that are permanently blocked by the Earth’s atmosphere.
So… so far we like ground based observatories because they are relatively cheap and we have easy access to them. But we’d like to be able to get around bad weather and above the atmosphere that keeps us from seeing detail and some wavelengths of light altogether. Can we have our cake and eat it too? Certainly, airborne observatories fill the gap between space and the ground.
The first instances of astronomers using aircraft came very early, back in the 20’s and 30’s. Observers used cameras and visual observations in small planes built of wood and canvas to observe eclipses. They could fly around and above any clouds that would otherwise block their views. In the coming decades, several expeditions were led by the National Geographic Society during subsequent eclipses. This allowed astronomers to view the eclipses without the worry of clouds, and by following the Moon’s moving shadow, they could actually lengthen the duration of the eclipse. But by the 1960’s, when larger, faster, higher flying jets came on the scene, airborne astronomy offered something much more valuable.
The stars seem to twinkle because the starlight must travel through the turbulent air in the atmosphere before it reaches us. This makes for blurry images that lack detail, something astronomers call bad “seeing”. Moreover, it is impossible to see some wavelengths of infrared light coming from space, because water vapour in the atmosphere absorbs this kind of light. Now, over three quarters of the air and over 90 percent of the water vapour in the atmosphere is contained in the first 10 km above the ground. So by flying at or above this altitude, the ‘seeing’ improves dramatically, and the absorption of those infrared wavelengths almost disappears. And so this is a perfect place to put a telescope!
Well, not perfect. There are definite limitations and challenges, but given the advantages I mentioned earlier, they can be worth the time, money, and effort to overcome. The biggest problem facing putting telescopes on planes is stabilizing the telescopes. Imagine pointing a telescope out of a hole in the side of a plane that is flying hundreds of kilometers an hour and encountering turbulence along the way and trying to take a nice, clear picture at hundreds of times magnification. It requires very sophisticated reaction control and guidance systems on which to mount the telescopes. But technology has come a long way in the past 50 years. NASA has been the leader in the business of putting telescopes in high flying airplanes, since the beginning when telescopes only 10’s of centimeters in diameter flew up on modified DC-8s and Learjets.
By the 1970’s, NASA was using the dedicated Kuiper Airborne Observatory, based on a C-141 Cargo plane with a 90cm telescope which would make discoveries impossible on the ground: the discovery of the rings of Uranus and the atmosphere of Pluto to name a couple. These were only possible due to the fact that scientists were able to use up to date instruments on a telescope that could travel to a specific location at a specific time, something that neither ground-based telescopes nor spacecraft were capable of.
The Kuiper Airborne Observatory was retired in 1995, and SOFIA, the stratospheric observatory for infrared astronomy will be soon taking it’s place. SOFIA is the most sophisticated and capable of any airborne observatory yet, with a 2.5 m telescope flown on a Boeing 747. I’m going to avoid going into the details of SOFIA; there are a few great 365 days of astronomy podcasts on the subject if you’re interested.
Airborne observatories aren’t limited to airplanes, though. There have been several missions that use high altitude balloons in order to ascend above the atmosphere for observations. The idea here is to suspend a telescope and instrumentation under a large balloon that will then rise up to an altitude of about 30km. Then, the prevailing winds carry the balloon and it’s instruments for a ride usually a few days to a few weeks long. Then, the balloon is vented, and it returns to the surface. This usually means you need to launch in one spot and retrieve in another, but in the arctic and Antarctic, the winds circle around the poles, and so if the timing is right, the balloon will ascend and descend around the same spot. Two major missions in recent history took advantage of this. BLAST, or the Balloon-borne Large Aperture Submillimeter Telescope, observes the darkness of space in the far infrared with a 2m telescope, and has flown around both poles, with a third science mission planned for late 2010. Meanwhile, the Sunrise mission observes the sun in much the same way, but uses a 1m telescope to collect UV light, which is also accessible at these altitudes, but blocked further down in the atmosphere.
What does the future hold for airborne astronomy? At this time I’m not aware of any specific plans for SOFIA’s next-generation successor, but it is designed to operate for at least 2 decades, so there’s no rush to replace this observatory, which only saw first light last month. The technology for stabilizing and pointing telescopes on balloon-based platforms continues to develop, so we can definitely expect to see more telescopes floating around in circles in the arctic and Antarctic in the near future. And maybe in the coming years, amateurs will find a way to start doing some real astronomy with small-scale balloon-borne equipment. Certainly in the past decade some privateers and university students have used little weather balloons, a tank of helium, and some off-the-shelf cameras and GPS receivers to launch little missions for only hundreds of dollars. Maybe soon some will graduate from taking pictures looking down towards Earth to something more challenging, and more rewarding; taking pictures looking up.
And with that, this podcast concludes. I hope you learned something and had a little fun. Thanks for listening; until next time, this is Robert Berthiaume wishing you all clear skies and good times.
End of podcast:
365 Days of Astronomy
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