We present the results of an instrument concept study for a low cost terahertz sounder of the mesosphere and lower
thermosphere (MLT). Recent advances in the development of Quantum Cascade Laser (QCL) technology to be used for
Local Oscillators (LOs) mean that it has now become viable for the first time to build compact, low weight heterodyne
receivers in the terahertz (THz) frequency range . Some of the most important atmospheric constituents of the MLT
region, e.g. atomic oxygen (O) and the hydroxyl radical (OH), can only realistically be measured at THz frequencies.
The technical challenges of THz remote sensing result in a large uncertainly of the global distribution of these species.
Recent research indicates that the MLT region exhibits links to processes associated with climate change. From this
follows a strong need to measure the composition and dynamic of the MLT region more accurately and more
The various demands on funding agencies make it difficult to sustain the level of expenditure required to provide the
broad range of space astronomy missions that the research community would like to have available. Multi-billion
pound/dollar observatories such Chandra, XMM-Newton and HST have been enormously successful, but JWST has been
delayed and plans for an equivalent large X-ray mission seem to be on-hold. Furthermore, the medium size ESA and
NASA missions provide only a small number of opportunities over the next decade. Much exciting and important
science, by default, will not be done. If satellite mission costs could be reduced significantly, by a factor of 5-10, we
would open up a new parameter space of opportunity that is not currently offered by any agency. Significant
improvement in instrument technology coupled with simplification of optical systems and the development of efficient,
high performance small satellite platforms and ground systems has led to the prospect of the development of some low-cost
opportunities. In this paper, we outline one such possible mission, based on a successful sounding rocket-borne
payload. This comprises a high throughput normal incidence extreme ultraviolet spectrometer, with the design adapted
for accommodation on the SSTL 300 platform. We make use of a segmented diffraction grating to provide an overall
wavelength coverage from ~170-260Å by tuning the multi-layers of the individual elements to different, overlapping
ranges. We outline the capability and science goals of the mission, and how they influence the design and operation of
the satellite platform. We conclude with a discussion of how missions of this type operating both as constellations and
as formation flying sparse apertures, could offer a scientifically viable alternative to monolothic 'great observatory'
missions in the future.