TELIS-SIR channel

Technology STO2

The detector of STO2 is based on a superconducting hot electron bolometer (HEB) mixer, using nanotechnology developed at TU Delft. This detector is the most sensitive heterodyne detector now available in the terahertz domain. A heterodyne receiver can convert a high-frequency spectral line signal from space into a spectral line at a microwave frequency without losing any information. Similar to FM radio, this mixing of frequencies makes the reception clearer and the signal from space can be amplified better. It can deliver an unparalleled high spectral resolution.

A quantum cascade laser at 4.7 terahertz, actually a tiny semiconductor chip developed through a collaboration between TU Delft, SRON and MIT, will be used as a so-called local oscillator, providing a reference frequency for the incoming signals from space.

STO2: balloon observatory ride at the edge of space

Stars and planets are born in molecular clouds that form and finally get disrupted again in the interstellar medium, the matter that exists in the space between the star systems in a galaxy. Astronomers still don't fully understand how this life cycle works in our Milky Way. The Stratospheric Terahertz Observatory (STO2), a NASA balloon-borne mission, led by the University of Arizona and with significant contributions from SRON, will travel to the edge of space above Antarctica in December 2015 to provide a missing piece of the puzzle.

At an altitude of 40 kilometers above Antarctica the sky is crystal clear. There is hardly any water vapor which can block the far-infrared radiation (also called terahertz radiation) from space. The edge of space is therefore a perfect environment for doing astronomical observations in the terahertz range. NASA uses super pressure balloons to lift observatories to that altitude. STO2, which will fly over Antarctica in December 2015, is one of them.

STO2 will circle Antarctica at an altitude of 40 kilometers, taking advantage of the stable polar wind. The mission plan is to complete two circular trips, with a duration of roughly 14 days each, during which STO2 will map a part of our own galactic plane. STO2 wil look at astronomic fine structure carbon [CII] and nitrogen [NII] lines at 1.9 terahertz and 1.4 terahertz respectively. STO2 will also observe the electrically neutral atomic oxygen [OI] line to demonstrate the new 4.7 terahertz technology (1 THz is equivalent to 300 micrometers in wavelength).

After a successful hang test in Palestine, Texas, in August 2015, the STO2 Gondola and subsystems are now on their way to McMurdo in Antarctica.

As leading experts in the field of terahertz receivers, SRON and the Delft University of Technology( TU Delft) have been asked to deliver the STO2-receivers for the three different channels (4.7, 1.9, and 1.4 terahertz) and the 4.7 terahertz local oscillator unit. The receivers are based on superconducting hot electron bolometers, which were fabricated at TU Delft. The 4.7 THz local oscillator was built in collaboration with a MIT group, USA (for the quantum cascade laser) and with two groups in Nizhny Novgorod, Russia (for the frequency stabilization technology).

SRON scientists
Two SRON instrument scientists (Darren Hayton and Wouter Laauwen) will travel to McMurdo to perform the final integration and test. Furthermore, the teams of Prof. Alexander Tielens (Leiden University) and Prof. Floris van der Tak (SRON/RUG) will contribute to the data analysis and the science.

More facts & figures
In 2012 the first Stratospheric Terahertz Observatory (STO) circled Antarctica in 14 days, using the stable polar wind. On board was a 0,8 m telescope. STO2 will re-fly STO, equipped with two new two-pixel receivers (at 1.4 terahertz and 1.9 terahertz), and a state of the art 4.7 terahertz single pixel receiver.

This project was co-supported by NWO, NASA and SRON.

Jian-Rong Gao, senior instrument scientist, SRON/TU Delft, This email address is being protected from spambots. You need JavaScript enabled to view it. / This email address is being protected from spambots. You need JavaScript enabled to view it. , or Frank Helmich, head of the Astro programme, This email address is being protected from spambots. You need JavaScript enabled to view it. .

Science STO2

Especially the 4.7 terahertz receiver is important because no one has ever flown such a receiver at this frequency, which means that this new channel on STO2 is a proof-of-concept. "Missions with such a terahertz receiver will provide missing pieces in the puzzle of the life cycle of stars and planets" says team leader Jian-Rong Gao. "The 4.7 terahertz receiver will for instance map neutral atomic oxygen, a longstanding dream of astronomers. That is why STO2 is an important stepping stone for future terahertz missions in space. The outcome of this study will enable us to make a Milky Way template which we can compare to other galaxies. "

The fine-structure line of electrically neutral atomic oxygen (OI) at 4.7 terahertz is the dominant cooling line of warm, dense, and neutral atomic gas. In strongly UV irradiated photodissociation regions (PDRs), the OI line flux is generally larger than that of the carbon CII line, making it an ideal diagnostic for probing the physical conditions in regions of massive star formation and galactic centers. The spectrally resolved OI line is necessary to untangle the complexities of the interstellar medium.

The teams of Prof. Alexander Tielens (Leiden University) and Prof. Floris van der Tak (SRON/RUG) will contribute to the data analysis and the science.

Links and websites Ariel/Echo

Ariel (ESA)