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\medskip
\centerline {\Large \bf PROJECT SUMMARY }
\bigskip
\centerline {\large \bf The Stratospheric Terahertz Observatory}
\smallskip
\centerline {\bf Solicitation: NNH06ZDA001N-APRA }

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This lead proposal for the Stratospheric Terahertz Observatory (STO)
describes a Long Duration Balloon (LDB) experiment designed to address
a key problem in modern astrophysics: understanding the Life Cycle of
the Interstellar Medium (ISM). Rapid, high resolution spectral line
imaging allows STO to survey roughly one-fourth of the Milky Way's ISM
to probe uniquely the pivotal formative and disruptive stages in the
life cycle of interstellar clouds and the relationship between global
star formation rates and the properties of the ISM. Combined with
previous HI and CO surveys, STO will create 3D maps of the structure,
dynamics, turbulence, energy balance, and pressure of the Milky Way's
ISM, as well as the star formation rate. Once we gain an understanding
of the relationship between ISM properties and star formation in the
Milky Way, we can also better understand the observations of nearby
galaxies and the distant universe. STO will first survey a section of
the Galactic plane in the dominant interstellar cooling line [C~II]
(158~$\mu$m) and the important star formation tracer [N~II] (205~$\mu$m) at
1 arcminute angular resolution, sufficient to spatially resolve
atomic, ionic and molecular clouds at 10 kpc. Our mission goals for this survey are to: 

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\item Determine the life cycle of Galactic interstellar gas.
\item Study the creation and disruption of star-forming clouds in the
  Galaxy.
\item Determine the parameters that affect the star formation rate in the galaxy.
\item Provide templates for star formation and stellar/interstellar feedback in other galaxies.   
\end{enumerate}

To achieve the angular resolution requirement STO will have a 80~cm
aperture. In order to discriminate clouds in a given beam and
determine their distance from Galactic rotation, STO will utilize a
heterodyne receiver system with a resolving power, $R  > 10^6$ . The
first flight receiver will consist of eight, phonon-cooled HEB mixers;
four optimized for the [C~II] line and four for the [N~II] line. The STO
spectrometer will have sufficient bandwidth to detect all clouds
participating in Galactic rotation in each of the 8 pixels. STO is
capable of detecting every giant molecular cloud in the Galaxy, every
HII region of significance, and every diffuse HI cloud with $A_V \ge
0.4$. Once the [C~II] and [N~II] surveys are completed, we will propose
to use STO to perform complementary surveys in emission lines of [O~I],
HD and the [NII] (122~$\mu$m) line.  


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