QUESTION:  
When and why did you decide to make the KAO?

ANSWER from Scott Sandford on May 6:
The idea of the Kuiper Airborne Observatory (KAO) evolved over time.  
Astronomers who study things in the infrared part of the spectrum have a 
big problem, namely that many molecules in the Earth's atmosphere, 
especially water (H2O), absorb infrared light.  Most of the infrared photons 
that come to us from space get blocked by the atmosphere before they reach 
our telescopes on the ground.  Only light in a few select wavelength regions 
can get through.  We call these regions "windows."

So, unless an infrared astronomer is observing in one of these infrared 
"windows", he or she might as well point the telescope at the ground 
instead of space for all the good it will do.  The atmosphere acts like a 
'brick wall' that cannot be penetrated from the ground.  [This problem is not 
unique to the infrared.  Astronomers that want to look at gamma rays, 
X-rays, and ultraviolet light have a similar problem.]

In order to study astronomical objects at wavelengths other than the infrared 
"windows", you need to get above the clouds and water vapor in the Earth's 
atmosphere.  You can do this by putting your telescope in space, which is 
very expensive and difficult, or by putting your telescope into the upper 
atmosphere where it will be above most of the absorbing water vapor.  This 
could be done by putting the telescope into a high-altitude airplane.  This is 
also difficult, but not nearly as hard as putting the telescope into space.

Before the KAO was built, scientists and engineers first tested out the 
concept of airborne astronomy (telescopes in airplanes) on a smaller scale.  
As early as 1968 and 1969, NASA had put a small telescope into a Lear jet 
and proven it could be used to usefully study astronomical objects.  The 
Lear jet was a sort of mini-KAO.  The plane was only big enough to hold a 
few people and its telescope was actually smaller than many of the 
telescopes owned by amateur astronomers.  

The scientists and engineers were then able to use the experience they had 
gained from the Lear jet to design and build the KAO, which is much 
bigger.  The KAO is a modified C-141 transport plane that can carry over 
25 people and has a telescope that is 90 centimeters (about 3 feet) across.  
The KAO was completed and began making its first test flights in 1972.  
These were completed and the KAO started being used for serious 
astronomy in 1974.  It has been in use ever since.

The jump from the Lear jet to the KAO made a big difference in what 
astronomy we could do because we had a much larger telescope.  NASA is 
now considering making a jump to an even bigger plane and telescope.  The 
plan would be to use a modified Boeing 747 that would carry a mirror a 
whopping 2.5 meters (over 8 feet) across!  This would allow us to carry out 
many studies that are beyond the capabilities of the KAO.  This plane would 
be called the Stratospheric Observatory for Infrared Astronomy (SOFIA).  
We hope to be able to start the SOFIA project in 1996.

Scott Sandford
Astrophysics Branch
NASA/Ames Research Center
(5/6/94)