Telescope Reviews: Robots vs Humans

Many many years ago, I worked on a project where we did a preliminary study of a bunch of problems in this area. The question to be answered was, what gives the most 'bang for the buck' in terms of choosing a destination for various scientific missions. The comparison was, mars vs moon as destination. The specific problem was, utilizing a given launcher, what scientific payload could be soft landed on the surface.

So, the first thing we did was to start by analyzing trajectories, and delta-V requirements for various options. We chose to use a low orbit kick off point, rather than a direct launch 'on course'. The rationale, it adds a little to the fuel requirements for a burn to stabilize the low orbit kick off, but, it removes a lot of the tight launch window constraints of a direct on course launch. The rationale was similar to that used for the apollo missions. In this particular scenario, the launch mass for both missions was deemed identical, so, it follows that you end up with identical mass in the parking orbit prior to kick off. Fuel consumed from the pad to the parking orbit is called X.

For the next stage, we have two options. First option, fuel required for a kick off to a trajectory that _just_ barly goes over the top and into a lunar descent. Call this Y1. On the second side of the equation, fuel required to kick out of the parking orbit, and onto a hallman trajectory for mars. Call this Y2.

For the third part of the analysis, we looked at the various landing requirements. We made a rather large simplifying assumption at this point, but, it's close enough to be valid for a first approximation. For the mars lander, a given amount of mass would be required for the landing system, heat shields, parachutes, etc. For the lunar lander, mass would be required for reaction engines, and the landing gear. The simplifying assumption, by the time all was said and done, these would be similar quantities of mass. The martian lander would use aerobraking for decelaration, the lunar lander needed to fire the engine.

Fuel requirements for the lunar lander were pretty strait forward to calculate, and, this amount of mass is designated Z1. For the martian lander, deceleration was mostly via aerobraking, so the fuel requirements for this stage would be designated Z2, but, Z2=0.

So, now the final big number that matters, how much scientific payload is delivered 'on station' for similar launchers. All up launch mass is designated as A, and for both missions, A is the same number. For the lunar payload, calculation is now P1=A-X-Y1-Z1. For the martian payload, P2=A-X-Y2-Z2. Y2 is a little larger than Y1, but, surprisingly, it's not a huge factor larger. Z1 is HUGE in comparison to Z2, which is zero. Y2+Z2 turns out to be much smaller than Y1+Z1.

After crunching all the numbers, and interesting conclusion comes out of it all. P2 > P1 for pretty much every scenario, and not just a little bit larger, we are talking factors of 2 and 3. Bottom line, with a given launcher sitting on the pad, a larger scientific payload can be delivered to the surface of mars, than the surface of the moon. Trajectory analysis will tell you, it takes a lot longer to get there, but, with aerobraking on the other end, and no aerobraking available on the moon, you will need a lot more fuel on the pad if destination is moon, leaving less mass available for payload, because the lifting ability of the launcher is fixed. The lunar soft landing requires a higher fuel/payload ratio than the martian soft lander.

This was a very interesting conclusion for me at the time, because I found it counter intuitive. For a one way robotic mission, mars is a more cost effective destination than the moon, in terms of dollars spent per kilo of payload on station after the soft landing.

It was 20+ years ago when we did this, so, my mind is surely fuzzy on some of the specific details, but, the conclusion I do remember very clearly. For one way robotic missions, using current propulsion technology, mars is the more interesting destination, a larger payload can be soft landed on mars than the moon, for similar launch costs.

Keep also in mind, we were working with the assumption that payloads would be relatively inert enroute, so, there was no consideration for enroute consumeables, which means enroute time doesn't make a material difference in mass budget allocation. If you add in a factor for consumeables, then each equation gets another factor of dc(T) (rate of consumption multiplied by enroute time), and the time factor of the enroute leg to mars suddenly makes it not only an unappealing target, depending on the value for dc (rate of consumption), it likely becomes an unnatainable target.

But the real thing I realize typing this all in from memory, all of these numbers essentially boil down to the efficiency of propulsion as it was back then. To me, the sad part is, that hasn't changed much since then, and aside from a few incremental adjustments in efficiency factors here and there, those results are just as valid today as they were then. We can refine chemical rockets forever, but, they are limited by capability.

Until there is a breakthru in propulsion technology, which provides an order of magnitude of improvement in mass efficiency for propulsion, we aren't going anywhere, but, we can send out some one way probes. The real benefit of that, when the propulsion breakthru comes, we will likely know where we want to go, and why. Sail did eventually give way to steam, and, that was ultimately replaced by even more efficient propulsion. The chemical rockets will eventually end up in the museums, but for now, it's all there is.

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