Telescope Reviews: Mercury Encounter on Jan 14th

It's all about the cost per kilo of payload delivered to destination. Mars is a bit of a unique destination in that it has an atmosphere, so, aero-breaking can be used to apply a significant amount of the delta-V to a vehicle headed there, which translates directly to 'less fuel carried'. The lifting capacity of the launch vehicle is fixed, so, every kilo of fuel you can get off the thing, means another kilo of payload that can be delivered to destination. Mars landers are yet a more special case, since they are going to do all the breaking in one quick go, they 'arrive' quickly. Mars orbiters using aero-breaking tend to arrive in the vicinity just as fast as a lander, but, then spend up to a year fussing with various passes thru the upper atmosphere lowering an orbit before the final orbit is achieved. Again, it's all about the most cost effective way to apply a required delta-V to the vehicle.

It is interesting to compare a mars lander with a lunar lander when doing the math. If you assume the same launch vehicle in both cases, then you have an all up launch weight maximum value, and an empty weight of the launcher. Load the launcher with enough fuel to achieve a Hollmann trajectory for mars, and call that X1. Conversly, load it with enough for a lunar trajectory, and call it X2. Max launch weight minus X = the payload enroute. For the Mars case, figure out the weight of a heat shield and a parachute, call that Y1. For the lunar vehicle, figure out how much fuel is required for a soft land, plus the weight of the rocket it'll be run thru during the process, all that Y2. Max - X1 - Y1 = Z1 where Z1 is the resultant final payload delivered to destination in the mars case. Max - X2 - Y2 = Z2, where Z2 is the resultant final payload delivered to the soft landing on the lunar surface. When you grind all the numbers, the surprising result is, Z1 is MUCH larger than Z2, an the conclusion is, for a given cost of launch using the specified vehicle, a far larger scientific payload can be soft landed on mars than can be soft landed on the moon.

For an orbiter, the math gets a little more complex, it's going to require some sort of propulsion for final orbit adjustments, but again, it can get a lot of the delta V reductions thru aerobraking, and in the end, the result is similar. For a given launch vehicle, a larger payload can be placed in a mars orbit than lunar orbit, by making use of aerobraking for a big chunk of the delta V adjustments. All of this assumes the use of chemical reaction rockets. Ion propulsion enroute changes the math some, but, as we see farther down, it dramatically increases the time component, which has other effects.

The tradeoff is ofc now time, doing multiple aerobraking passes will take time, and to maximize the payload, very large eliptical orbits will be achieved on the first pass, with a lowering of apogee on subsequent passes. When the target orbit is 'almost' achieved, then a firing of the motors to raise perigee back out of the atmosphere will finalize orbit insertion. This whole process can take a very long time.

This is a big part of the reason why the one-way robotic mission to mars has such a following in the scientific community today. The cost of such a mission is actually within the reach of the handouts, errr, I mean grants achievable today.

The economics change entirely if you start to include silliness such as return trips into the equation. If you do the math, and include 'sample return' into the initial specification, everything changes, and a lunar mission gets signficantly less expensive. If you add in 'life support consumables' enroute, then the time factor is no longer 'free', and costs run up exponentially yet again. Such missions quickly reach cost projections such that the grants can no longer absorb them, so they remain interesting diversions on the drawing board, but, not gonna happen anytime soon.

In the case of the mission in and around mercury, it's a one way robotic probe. There is no allowance for return fuel, and, time is not the big expense. The mission uses gravitational forces 'enroute' to modify trajectory for the most part, minimizing the fuel requirement, therefor allowing a maximum scientific payload. It's a tradeoff, will take longer to get the data, but, there will be more data once it all arrives. If there was a compelling reason to go look 'right now', more fuel could be loaded onto the probe, at the expense of scientific instruments, and it could take a less optimum, and much quicker, trajectory to 'go look'.