ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 76 of 422
• The global dust storm season on Mars. This occurs for approximately 3 months on either
side of the Martian perihelion passage.
• Superior conjunction (i.e. the Earth and Mars are on opposite sides of the Sun). In this
situation there is a communications difficulty that can last for up to 55 days (an angle of
10° either side of the Sun).
• Martian winter at the landing site if power is supplied by solar arrays. The power
subsystem provides a cut-off associated with the size of the solar arrays. In winter the
solar flux reaching the Martian surface is reduced so the lander would have insufficient
power. The exact length of the excluded time period depends on the landing site latitude.
There is a trade-off to ensure that the size of the solar arrays does not impact too greatly
on other subsystems by providing more opportunities for surface stays. In the case of a
fuel cell power source, the landing site and hence the Martian season are immaterial.
In addition there are several mission operations during which the surface stay cannot take place.
These are:
• Aerobraking
• Final Martian orbit acquisition
• System check. These are assumed to last 1 week after arrival in the final Martian orbit
and 2 weeks before departure on the return journey to Earth.
• MAV-TV rendezvous. This can take several days at the end of the surface stay.
The analysis took into account:
• Selection of the power source used, whether solar arrays or fuel cells.
• Selection of one of four landing site latitudes: 21°N, 12.5°N, 12.5°S and 22.5°S.
However, in the case of using fuel cells, the landing site is immaterial.
• Selection of whether or not to include dust deposition on the solar arrays when
considering the power constraints.
Figure 2-38 shows how the opportunities vary when the landing site and the aerobraking
manoeuvre are taken into account. Note that the fuel cell case is the same as the best case of
each of the solar cell launch window scenarios. Aerobraking manoeuvres are considered to last
six months in this case also.