At an altitude of 40 kilometers, the sky is crystal clear. There is hardly any water vapor, hardly anything disturbing at all: the edge of space is a perfect environment for doing astronomical observations. NASA uses super pressure balloons to lift observatories to that height. TU Delft is developing a terahertz receiver for such a balloon observatory.

In 2012, the Stratospheric Terahertz Observatory (STO) circled Antarctica in 14 days, using the stable polar wind. On board was a 0.8 meter telescope for observing the interstellar medium – the material between the stars – and the life cycle of interstellar clouds. Terahertz measurements are hard to do from the ground because of the water vapor in the atmosphere.

Missing pieces

The mission was a success, so NASA decided to fund its successor STO2. As leading experts in the field of terahertz receivers, Delft University and SRON are being asked to deliver a new 4.7 terahertz receiver. No one has ever flown such a receiver at this frequency, so this new channel on STO2 is a proof-of-concept. “The 4.7 terahertz receiver can map neutral atomic oxygen, a longstandig dream of astronomers. Missions with terahertz receivers will provide them with missing pieces in the puzzle of the life cycle of galaxies”, says team leader Jian-Rong Gao. The project is a collaboration between TU Delft and SRON. Gao regards the project as an important stepping stone mission to space.

40 days

The detector is based on a so called single pixel superconducting hot bolometer (HEB) mixer, the most sensitive heterodyne detector available in the terahertz domain. A quantum cascade laser, developed through a collaboration with MIT, is needed to provide a reference frequency for the incoming signals from space. The balloon will be equipped with a 90 liter liquid-helium tank and a 60 Kelvin cryocooler to keep the system at the right temperature. The hold time of the cryostat will limit the observation time to a maximum of 40 days.