On our way to the Galilean system

TK is finishing up its work on providing hardware to go on ESA’s JUICE mission to the Galilean moons – specifically for the THz radiometer on board.

The SWI instrument (SWI) will look at the temperature structure, composition and dynamics of Jupiter’s stratosphere as well as the thin atmospheres and exospheres of three of four of the moons first seen by Galileo in the winter of 1610 using his newly invented telescope: Ganymede, Europa and Callisto – just missing out on Io, the most inner one.

The SWI – by looking at emissions from the rotation lines of a range of trace gasses including CO, CS, HCN and wayer – will be able to characterise these atmospheres in great detail – including determining the dominant isotopic ratios in the atmospheres of Jupiter and the thee moons, and therefore the origin and evolution of the entire Jupiter system.

The Institute of Applied Physics in Bern is tasked by the SWI team (Principal Investigator: Dr. P. Hartogh, Max-Planck-Institut für Sonnensystemforschung in Germany) to provided the Quasi-Optics sub-system of the SWI, routing signals from the main 300mm reflector to two receivers operating around 600 and 1200 GHz, and providing warm temperature calibration of the radiometers.

With fund provided by the ESA/Swiss PRODEX office, we have jointly designed and then manufactured in Billingshurst three vital components of this sub-system.

Mass is obviously a critical issue – as in any space mission – as well as the ability to withstand the rigours of launch and the cold of space but the components have to withstand the harsh radiation environment surrounding Jupiter – over the mission lifetime some 5 MRads (5 KRads will kill a human) and TK has provided sample to allow tests to be undertaken to ensure that the materials used will cope with this level of attack.

TK is supplying three parts:

Firstly a polarizing wire grid, which acts like a piece of Polaroid in sunglasses, but at much longer wavelength than we see with: The grid splits the incoming signal from thetrace gas’s rotation line emisions into two beams which are sent – via mirrors – to the two radiometers.

Secondly, the corrugated feed horn operating around 600 GHz which defines the beam entering the lower frequency receiver. The horn is unusual in that it contain a 90 degree waveguide twist required by the relative orientation of the polarizing wire grids and the 600 GHz receiver mixer. The complex internal structure of the horn is made by electroforming – copper plating – onto an aluminium former, or mandrel, and then dissolving out the aluminium using Sodium Hydroxide.

And a microwave target, which provided the warm temperature reference to calibrate the both radiometers:

The radiometers look into the internal form of the cone from time-to-time to calibrate the instrument with a precisely know temperature: In the same way you could check a thermometer by putting it into boiling water to check that it was reading 100C.

Source: http://www.terahertz.co.uk/

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