Mars science and telecommunications orbiter

Mission Characteristics

The investigations described above can be accomplished with a high performance orbiter such as MRO, the most recent of the current Mars orbiters. A long duration orbital mission, as is projected for the decade-long telecommunication function of MSTO, is especially well suited to the particular combination of the primary science goals discussed here. Investigation of the solar-planetary environment at the top of the Mars atmosphere and measurement of the escape fluxes of key atmospheric species requires access to a range of altitudes, to allow sampling from below the peak region of the ionosphere to above the bow shock, in order to understand and quantify the different processes by which the solar wind has eroded the Mars atmosphere over time. Thus an elliptical orbit that samples this range of distances (i.e., from ~100 to 6000 km) from the surface of the planet is necessary. A second, vital aspect of the compatibility between MSTO and investigations concerned with the evolution of the atmosphere is the non-linear nature of the dependence of escape processes on the intensity of the solar input. In order to model these processes correctly, therefore, it is essential that measurements be made at a time close to the maximum of solar activity as well as at times between maxima. Against this, remote sensing measurements of the detailed composition of the neutral lower atmosphere favor near-circular orbits, with precession periods that provide global coverage on seasonal time scales; investigation of spatial and secular variability of the atmospheric composition again points to the need for extended observations made at intervals spanning several Mars years. Thus, a mission design characterized by orbital flexibility, and an overall duration which ensures inclusion of a solar maximum, provides the ideal opportunity for carrying out the atmospheric investigations that are the focus of this report. Further detailed discussion of orbit design considerations is given below.

Nine instrument types were identified by the SAG as being necessary to make the required measurements. These are described briefly in the preceding sections and are summarized in Table 4, which includes examples of instruments having characteristics and performance capability similar to the present requirements. A rough estimate of the expected mass of this instrument complement indicates that a mass allocation of about 100 kg would be required for the science payload. Again, an MRO-class orbiter is appropriately suited to this purpose.

Launch Date

The MSTO mission is proposed for the 2011 or 2013 launch opportunities. Among the many factors that should enter into consideration in choosing between these dates is the expected timing with respect to the maxima of solar activity. The approximately 11-year solar cycle has nominal dates of 2011 and 2022 for the next two maxima. Figure 4 shows the expected variation of the solar output through this period. It can be seen that a 2011 launch, which would have an arrival date at Mars of September, 2012, may be too late to commence science activities in time to capture the effects of the 2011 maximum. This would put the only opportunity to observe a true maximum beyond the end of the nominal mission, with the attendant increase in risk from component failure or depletion of fuel margins. The 2013 launch, however, would permit a slower build up of the science activities and give some 5 or 6 years for initial aeronomy and lower atmosphere spectroscopy before the crucial measurements at solar maximum are made.

MSTO will arrive at Mars (in 2012 or 2014) at a time when other relevant measurements will have been made and will be ongoing. The Mars Science Laboratory (MSL) will be completing two years of operations on the surface, providing a wealth of data on atmospheric composition, including isotope ratios. Instruments such as the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) will continue to map water vapor, dust and aerosol, and Phoenix will have completed its high latitude observations on the surface.

Scientific investigations from MSTO will be made independently and in context with data from MSL and MRO that can be used for validation of remote sensing instruments. For example, measurements from the SAM suite on MSL will include the localized abundance of methane, its ¹³C/¹²C ratio with respect to that of CO₂, whether hydrogen peroxide is present in the atmosphere and soil, the presence of organics, and several other gases. For the Martian atmosphere, MSTO will take advantage of ground-truth opportunities with MSL, but then expand the localized in situ measurements of MSL’s targeted species to full global maps that will become a subset of the full atmospheric inventory capability of MSTO. Measurements of dust and water from MRO and initial isotopic water abundances from MSL will be dovetailed with the MSTO inventory to provide unprecedented capability for linking photochemical and heterogeneous chemical models of Mars that will be vital for assessing human habitability.

Orbit Design Considerations

The MSTO orbit design must accommodate the requirements of three separate functions: lower atmosphere science, upper atmosphere science, and command and data relay. At the outset of the SAG deliberations it was suggested that perhaps the "ideal" orbits (i.e., those that would best satisfy the individual needs of these three separate functions) are not sufficiently advantageous, in terms of the respective returned science value or communications coverage, to warrant the risk and expenditure of fuel to make the frequent orbit changes required in order to meet these idealized needs.

The preferred orbit characteristics for the individual functions are as follows: For the remote spectroscopic observations of the lower atmosphere, a high inclination, near-circular orbit at an altitude of a few hundred km would provide the best spatial and seasonal coverage, together with the required vertical resolution. The orbital motion should provide global coverage over a period short compared to a quarter of the Mars year and should be maintained, or at least repeated, over two or more Mars years to allow for the observation of inter-annual and other secular variability.

For the investigation of atmospheric escape processes at Mars the ideal orbit is one with an eccentricity sufficient to probe, on every orbit, regions from below the exobase (i.e., down to the ionospheric peak region at ~110 km altitude) to above the bow shock (to, say, 5000 km average altitude at local noon). Using the Mars-centric Solar Ecliptic (MSE) coordinate system (X-axis pointing from Mars towards the sun, Z-axis towards the ecliptic north pole), the line of apsides for the ideal, high inclination orbit would rotate through 360˚ several times per Mars year. The rotation should take place primarily in the X-Y plane, although some rotation in the X-Z plane also has benefits. A rapid orbit walk will allow the mapping of Mars’ near-space volume on the day-side, through the dawn and dusk regions and into the tail, over a short enough period to allow for the detection of seasonal variations. It is essential that the orbit configuration allow variations in escape fluxes throughout the solar cycle to be determined (solar maxima will occur at around 2011 +/-1 and 2022+/-1).

For the telecommunications functions it is understood that a 400 x 2000 km orbit would be ideal.

The SAG studied the spatial, seasonal and solar cycle coverage achievable with several different orbit configurations and combinations. These included the initial design suggested by the MSTO mission design study team, viz. a 3-phase design (Figure 5) consisting of:

Phase 1. Post aerobraking science phase: 150 km x 6500 km; 1 (earth) year

Phase 2. Solar occultation phase: 400 x 400 km; 1earth year

Phase 3. Telecomm. plus science phase: 400 km x 2000 km; 8 years,

as well as several other elliptical orbits which might provide good, albeit not ideal, coverage for all three orbiter functions simultaneously.

The SAG also took into consideration the relative advantages and disadvantages for the lower atmosphere occultation and down-looking observations provided by the "ideal" circular orbit, maintained for a duration of one year, versus the more limited coverage of the low altitude portions of an elliptical orbit which could be utilized over a period of several Mars years to acquire data pertaining to inter-annual variability and other effects, such as the occurrence of dust storms and clouds and hazes. The long duration of the mission - essential to the upper atmosphere and exosphere observations - is also of importance to the scientific goals of the lower atmosphere investigations, and the correct weighting of the many factors involved in the choice of the "best" orbit, or sequence of orbits, is difficult to balance. Figure 6 illustrates, for example, the coverage achieved, for solar occultation observations, with the 150 x 6500 km orbit over one Mars year.

The most important outcome of these considerations was the realization that there are, in fact, many orbit strategies that will satisfy to a large extent the key requirements of the three functions of the orbiter, provided the activities for the science investigations can be maintained at appropriate intervals throughout the duration of the mission, as opposed to attempting to accommodate them within fixed "one time only" science phases. This applies particularly to the measurements of solar-planetary interactions. Further, in the context of the requirement that in situ observations of the atmosphere be made as low as possible (100 km being a desired minimum altitude), the SAG assumed that opportunities to acquire science data at the lowest altitudes encountered during the aerobraking maneuver can be exploited for this purpose. The SAG concluded that orbit design and strategy should not be an outstanding problem in the accomplishment of the science goals of the mission.