Selection of Mission Profile
The ARCHIMEDES project was initiated through a proposal of the Mars Society Germany in early 2001, following an invitation to a Mars Mission assessment workshop by the German Aerospace Centre (DLR). MSD proposed an aerial reconnaissance mission to planet Mars using an inflated balloon as carrier of scientific instruments, based on a publication by Robert Zubrin. As a follow-up to this initial short proposal a more extensive system study was performed.
Already early during this study it became clear that a very lightweight balloon of at least 10 to 16 m diameter would be required. Such a balloon could reach either a floating condition in the thin Mars atmosphere, or at least a relatively long descent time while traversing the Mars atmosphere down to the ground in the order of ½ to 1 hour. This minimum time span is required for being able to perform meaningful scientific measurements of the atmosphere and magnetic field at different altitudes. Such measurements are prevented by plasma effects when space vehicles enter and traverse the Mars atmosphere within minutes, which is the case with all Mars landing missions so far.
In any possible mission scenario the balloon would have, due to its size and vulnerability, to be transported to Mars in a folded and densely packed condition and be inflated and released only after arrival at Mars. Different technical solutions have been looked at:
- Balloon inflated on the ground and then ascending in the Mars atmosphere for its scientific mission
- Balloon inflated after entry in the Mars atmosphere
- Balloon inflated in free space and entering the Mars atmosphere by being slowed down in the thin upper atmosphere (“aero-breaking”) and finally entering into the atmosphere and descending slowly towards the Mars surface either to a free floating condition or until ground impact
In any of these cases either an entry vehicle (solutions 1 and 2) or the balloon itself (solution 3) must be able to withstand the heat load at entry into the Mars atmosphere.
In the cases 1 and 2 the balloon would only be inflated after entry and hence not be exposed to the high entry heat load. However, both solutions would be technically and operationally risky or not really practical. The entry vehicles would be heavy compared with solution 3, because they would have to contain the complete balloon storage, deployment, inflation and release system and require a sizeable heat protection due to its mass.
The solutions 1 and 2 were therefore ruled out early in the study as not being feasible and the ARCHIMEDES design concentrated thereafter on solution 3 (balloon inflated in space prior entry). For this mission profile different optional scenarios have been taken into account before deciding on the present ARCHIMEDES baseline.
Two different mission profiles have been finally retained:
Large ballute of up 16 m diameter* decelerating itself during atmospheric entry and descending slowly to the Mars surface with a mission duration of several hours from entry to touch-down while flying over a relatively extended Mars area
Ballute of appr. 10 m diameter* also decelerating itself during atmospheric entry and descending relatively fast to the Mars surface within ½ to 1 hour from entry to touch-down while flying over a relatively small Mars area.
* The ballute diameters are estimates, extrapolated from the balloon and avionics technologies developed for the MIRIAM-2 project
A third mission profile, where an even larger ballute than for option 1 would float over Mars for weeks, with the attractive perspective of the ballute being transported eventually by the wind over extended regions of Mars, has been ruled out very quickly. The large climate variations on Mars and the necessity to keep the ballute floating and operational at a given altitude for weeks would increase the overall ARCHIMEDES design complexity beyond the capabilities of an amateur project, and increase ARCHIMEDES volume and mass beyond the capabilities of the carrier satellite AMSAT P5-A.
In both mission profiles the scientific objectives are identical:
- Proving the suitability of a ballute for Mars exploration
- Proving the suitability of the ballute inflation and release systems design
- Providing for the first time scientific measurements of
- the ballute behaviour during atmospheric entry
- the aero-thermodynamic properties of a ballute during atmospheric entry of the Mars atmosphere
- the Mars atmosphere composition and profile from entry down to the Mars surface
- the profile of the low Mars magnetic field
- taking images of the Mars surface at altitudes between 60 km and the ground
In both scenarios the ballute would be inflated in space to a pre-calculated pressure without the capability to change its gas pressure after its release for entry and descent.
- The type of measurements taken are identical in both mission options with the difference of the longer mission duration in option 1 allowing to establish a profile of the measurements over an extended portion of the Mars surface
- In mission option 2 the measurement time is limited to ½ to 1 hour. No measurements over an extended portion of the Mars surface
- In mission option 2 the orbiter is, due to the short mission time, always visible from the ballute, allowing to transmit all measurements in realtime to the orbiter
- Mission option 1 leads to a more complicated design. The longer mission duration and larger balloon would lead to an overall mass increase, compared with option 2, due to the ballute diameter increase from 10 to 16 m, requiring 4x the amount of gas and 2,4x more ballute storage room and mass
- In mission option 1 the orbiter will become “out of sight” of the ballute long before the end of the ballute mission. This complicates the design in addition to the longer mission duration, because orbiter visibility and data transmission must be synchronized
Due to the relatively small differences between the expected scientific results of the two mission profile options and the much more complex design and higher mass and volume demands of option 1 it was decided to continue ARCHIMEDES studies and design on the basis of option2.