Mars science and telecommunications orbiter

Summary of Measurement Requirements

The following summarizes the required measurements:

Lower Atmosphere:

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  Infrared limb spectrum: 800 to 4000 cm^-1 x 0.02 cm^-1; all L_s; 3 or more Mars years.
  
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  Sub-mm radiometry: nadir to limb scan; selected frequencies from IR spectrum; same seasonal and duration coverage.
  
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  Winds: surface to 130 km
  

Upper Atmosphere and Exosphere

Measurements of the following parameters are required throughout the solar cycle and particularly at solar maximum and solar minimum:

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  Atomic and molecular neutral and ions listed in Error: Reference source not found
  
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  Vector magnetic field: crustal, upper atmosphere and solar wind.
  
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  Ion and electron 3-Dim. energy distribution in the upper atmosphere and solar wind.
  
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  Atmospheric winds above 130 km
  
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Instrument Payload

The SAG identified the following candidate instruments that would be required to make the recommended observations described above:

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  High Resolution IR Spectrometer. Measure the IR spectrum 800 to 5000 cm^-1. Limb measurements (solar occultation).of molecular composition to <10^-10 mrbv
  
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  Sub-mm Radiometer; channel frequencies by command; nadir to limb scan. Measure the spatial distribution selected species in lower atmosphere. Measure zonal and longitudinal components of lower atmosphere winds.
  
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  High Resolution Neutral Mass Spectrometer; mass range 1 to 100amu, mass resolution ~3000. Measure the neutral composition and structure of the upper atmosphere.
  
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  Retarding Potential Analyzer. Measure neutral and ion (thermal ions) species and fluxes in upper atmosphere; measure upper atmosphere winds.
  
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  Ion/Electron Detector. Measure the velocity distribution of solar wind ions and electrons, pickup ions, etc.
  
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  UV Spectrometer. Determine the properties of the hot corona and the aurora.
  
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  Langmuir probe. Measure the electron temperature and plasma density.
  
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  Vector Magnetometer. Characterize the solar wind, the bow shock, and the magnetic pile-up boundary. Characterize the magnetic properties of the ionosphere and exosphere, and map the crustal magnetic field.
  
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  Context camera; record atmospheric phenomena, hazes, clouds, dust storms, polar activity; synoptic nadir observations; limb observations w/<1 km vertical resolution
  

This information is also displayed in Table 4.

Instrument development needs

A major characteristic of the science activities on MSTO is to carry out measurements that span the relatively long time period of a solar cycle. The requirement that the instrument payload be capable of functioning in the Mars environment for this duration will necessitate a development program that adequately ensures their survival. Previous missions, such as Cassini, Rosetta, etc., have carried out such design-lifetime verification programs within acceptable financial resource allocations and a similar program will be necessary for MSTO.

Additionally, many of the proposed instrument types have technical issues that would require early attention. Even those with a high Technology Readiness Level (TRL) will benefit from developments resulting in lower mass and power requirements. Upper atmosphere neutral wind devices and mass spectrometers will need some further development resources to enable them to fit the MSTO mission profile, for example.

Instruments needed to make critical atmospheric measurements include Fourier Transform Infrared Spectrometers and Sub-millimeter Radiometers. Both of these are relatively mature instrument types. However, technology development needs arise from the constant evolution of mission and system architectures and the needs for new, more sensitive measurements.

Fourier Transform Infrared Spectrometers require detectors cooled to the temperature range of 80-100K in order to achieve the required sensitivity. Instruments with detectors cooled to this temperature range have been used in earth orbit, in sun synchronous orbits, and in a small number of planetary instruments. For many of these the low detector temperature was achieved by the use of radiative coolers, designed to minimize structural and environmental heat loads and maximize the view of a radiator to cold space. Minimizing heat loads implies very careful structural, thermal and electronic design to isolate the cold detectors or optics from warmer parts of the instrument and spacecraft. Maximizing and maintaining the view from the radiator to cold space places constraints on the spacecraft configuration, orbit design and spacecraft pointing, to prevent solar or planetary radiation from entering the radiator field of view.

Recently, mechanical refrigerators (cryocoolers) have been used in earth orbiting spacecraft to avoid these problems. They have been designed to achieve multi-year lifetime and to minimize vibration inputs to the detectors. The tradeoff is that these coolers have been heavy and expensive. Technology approaches have recently been identified which have the potential to provide long life, low cost, low mass cryocoolers. These designs have been produced in response to military requirements for cooling infrared detectors in harsh environments. They are pulse tube coolers using non-contacting, flexure bearing compressors that provide refrigeration in the range of 1W to 4W at 80K. The new mechanical designs reduced their mass to just a few kg. Similarly, recent improvements in cooler drive electronics have reduced electronics packaging mass and volume significantly over their predecessors. 

Consideration of cooler options in the overall spacecraft design would ensure that cooled instruments can be accommodated at reasonable cost without overly constraining the spacecraft design or placing too great a burden on instrument design. Provision of cooling as a spacecraft resource is one way to accomplish that objective.

Sub-millimeter spectrometers are in demand for multiple planetary science mission applications. These instruments have in the past used detector technology that was difficult to fabricate and provided low device yield. To meet future demands for instrument performance and cost, promising new detector technologies, e.g. membrane-based sub-millimeter receiver components, must be developed to a Technology Readiness Level appropriate to insertion into a flight project. Even though sub-millimeter receiver technology has been infused into flight instruments in the past (e.g. MIRO on the Rosetta Orbiter, EOS MLS, Herschel HIFI), the new membrane-based process offers several advantages over these previous device processing approaches, including better yield and greater spectral coverage. Development of this technology will provide lower cost, higher performance instruments that can be developed in the time frames required by flight projects.