Section 4.13 - Mars Development

< Space Transport and Engineering Methods

Entire books have been written about getting to and developing Mars. The ones I will refer to as Flags and Footprints exploration missions are similar the Apollo missions to the Moon. They go there, do a bit of exploring, and return, without a supporting infrastructure and leaving no improvements at the destination. These are particularly expensive and wasteful.

The combined systems discussed in the previous sections are a different route to getting to Mars. They build capabilities step by step, each one preparing for the next, and generally using machines to prepare the way for humans. In this section we will discuss the last steps to get to Mars, building on the prior technology from our combined system. Mars is the most nearly Earth-like planet we know of, so we will also mention some ideas for long term development. They would get done, if ever, much later, when Mars and the Solar System are more fully developed.

Early Development

The early development would start with the Martian moons Phobos and Deimos, since they are easiest to reach, and progress down from there.

Phobos and Deimos

Conveniently Mars already has two fairly large asteroids in orbit (Phobos and Deimos), giving us a ready supply of materials. Phobos alone has a mass of about 10,000 gigatons, which is more than we would need for a very long time. The first step in development is to set up a base of operations on one of them. This would do mining, processing, and construction of habitats. The composition of the Martian moons is not entirely clear yet^[1], but they appear to be similar to CM type Chondrite asteroids and meteorites. If any particular materials are not found in these satellites, they can be fetched from Near Mars Objects, otherwise known as the Main Asteroid Belt, which starts just outside Mars' orbit. The velocity to reach them from the Martian satellites is fairly low.

Chondrites are known to contain a large amount of carbon, so the main goal of the Phobos base would be to provide cable and other materials for a Mars Skyhook. Secondary goals are to provide fuel for landers and human control of surface operations before humans can be supported on the surface. The short distance of the Mars satellites relative to Earth allow for real-time control.

Mars Skyhooks

There are two sizes of Martian Skyhooks that make sense to look at. One set is capable of reaching from low Mars orbit (LMO) to Phobos orbit, and whatever suborbital velocity to Mars that produces. The other set would be capable of doing a full velocity transfer to the Martian surface. The first could grow into the second over time, but we will look at them as specific design points.

LMO to Phobos Skyhooks

Orbit Mechanics - The product of a planet's mass M and the universal gravitational constant G is called the Standard Gravitational Parameter or $\mu$ . For Mars the value is $\mu$ = GM = 42.837 x 10^12. This value is useful for calculating circular orbit velocities by the formula

v_{o}\approx {\sqrt {\frac {GM}{r}}}

Knowing the average radius of Phobos from the center of Mars is 9,377 km, putting that into the formula in meters (9,377,000 since we must use all SI units and not multiples thereof), we can determine the orbit velocity of Phobos is 2137 m/s. Since Mars has an equatorial radius of 3,396 km, then Phobos is 5,981 km from the surface. For elliptical transfer orbits, where r is the current radius from the body center, and a is the semi-major axis, or half the long axis of the ellipse, the velocity is

v^{2}=\mu \left({\frac {2}{r}}-{\frac {1}{a}}\right)

If we want to transfer from Phobos to 400 km above the Mars surface ( r = 3796 km ), then the velocities at the high and low points of the transfer orbit, and at the low orbit can be calculate as follows:

Transfer High point: r = 9,377,000 m ; a = half of high + low altitudes = 6,586,000 m ; from formula $v^{2}=$ 2,632,300 m^2/s^2, and so v = 1622 m/s.
Transfer Low point: r = 3,796,000 m ; a is the same as previous = 6,586,000 m ; therefore $v^{2}=$ = 16,065,000 m^2/s^2 and v = 4008 m/s.
400 km Circular: r = a = 3,796,000 m ; v = 3,359 m/s.

The velocity difference from Phobos to the transfer orbit is 515 m/s, and from the transfer orbit to 400 km circular is 649 m/s. These velocities are relatively small compared to the Lunar or Earth orbit Skyhooks we have looked at previously, so they would be relatively low mass ratio. Assuming the tips are at 1 gravity, the Phobos Skyhook would have a radius of 26.5 km, and the LMO one would be 42 km. Phobos is tidally locked to Mars, always keeping one face to the planet, but it is not locked in rotation about the Mars-pointing axis, and tidal variations from the sun and slight orbit eccentricity cause it to wobble. Thus the Phobos Skyhook should probably not be attached to Phobos directly, but placed nearby.

Both Skyhooks will have low mass ratios because of their low tip velocities. Therefore they would shift their own orbits by a large amount when transferring cargo. The solution is to anchor both of them with a sufficient amount of ballast mass from Phobos at their center points. The LMO Skyhook can drop cargo at 3,359 - 649 = 2,710 m/s. Mars' equatorial rotation is 241 m/s, so the relative velocity to the surface will be 2,469 m/s. To reach Mars escape from Phobos requires adding 884 m/s. Since our Phobos Skyhook can add 514 m/s, that leaves 370 m/s to be done by other means.

A "Flags and Footprints" type mission would require no propulsion in theory to land on Mars, if it uses all aerodynamic braking. It would need 5 km/s of propulsion to reach Mars escape velocity on the return trip. With a base at Phobos and the transfer Skyhooks, no propulsion is needed is needed to land either. The LMO Skyhook drops the vehicle about 20% below orbit velocity and re-entry will be automatic. The return propulsion would require about half as much velocity to reach orbit - 2.47 km/s. With a permanent habitat at Phobos, there is no need to escape from Mars, but if you choose to do so, it would require 2.84 km/s. Assuming a chemical rocket for both with an exhaust velocity of 3,500 m/s, the mass ratio will be 4.17 without the Skyhooks, giving a cargo of 14% if the vehicle hardware is 10%. With the Skyhooks, the mass ratio is 2.02, and landed cargo is 39.4%. So the Skyhooks allow 2.8 times as much payload per trip.

LMO to Surface Skyhook

A full orbit to ground Skyhook will likely not make economic sense until traffic grows to a higher level, but let us take a look at a possible design. Start by assuming a 1000 km high orbit. That will have a radius of 4,396,000 m. From the above formula we calculate the orbit velocity is 3122 m/s. Subtracting 241 m/s for the rotation of Mars gives a relative velocity of 2881 m/s. If the tip is at 1 gravity centrifugal acceleration, then the Skyhook radius will be 846 km, and the tip will become motionless when the Skyhook is vertical 154 km above Mars mean surface level. That should be high enough to avoid significant atmosphere friction. The Mars Global Surveyor spacecraft used a no-drag holding orbit at around 175 km lowest point, and active aerobraking between 120 and 135 km. It did so when moving between circular and escape orbit velocities of 3,500 to 5,000 m/s. So the Skyhook with near zero velocity at the lowest point should not see much drag.

Pavonis Mons is a 14 km tall mountain on the Martian Equator. So a vehicle wanting to reach the Skyhook from there would need to climb 140 km vertically. This requires a vertical velocity of 1,020 m/s plus about 12% gravity loss for a total velocity of 1145 m/s. This might be reduced if the Skyhook reached a lower altitude, but drag must be carefully looked at so the Skyhook is not in danger of de-orbiting itself. Without a Skyhook we found in the previous section we needed 5.4 units of fuel for each unit of payload returned from Mars surface to Mars escape. Escape velocity is 1.41 times circular orbit velocity, and this Skyhook has a release velocity of 1.92 times orbit velocity at the highest point, so well above escape velocity. Thus with this system the mass ratio becomes 1.387, and fuel used is 28% of takeoff mass, or 45% of cargo mass. Therefore it uses 12 times less fuel than without a Skyhook.

The working length we previously used for Carbon fiber is 126.4 g-km. Acceleration in the Skyhook varies smoothly from 0 at the center to 1.0 gravities at the tip. With an average of 0.5 gravities times a radius of 846 km the requirement is 423 g-km. Therefore the mass ratio of the cable is 28.4 times it's load per arm, or 56.8 for both arms. That is heavy enough that it may not need ballast mass at the center to keep from shifting it's orbit too much in operation. It will still need propulsion to maintain orbit when traffic going up and down are not in balance. Since the Skyhook saves 4.95 units of fuel on the Martian surface for each unit of return cargo, in theory it pays for itself in fuel savings in under 12 flights. In practice it will take a detailed design to find out what the system mass and payback times in terms of mass and cost will be.

Mars Surface Systems

Construction

Earth-moving equipment will be needed for a number of purposes. The Mars surface is not protected from radiation like the Earth is, so long term habitats would need to be protected by a layer of soil. Landing areas will need to be flattened, and protective berms built around them so exhaust plumes don't sandblast nearby equipment. Once cargoes are delivered to the surface, they will need to be moved, lifted, and assembled, so devices to do those tasks will be needed. Most of the site preparation will likely be done by remote controlled machines.

Power Supply

Solar panels are a viable power supply on the Martian surface. It is rarely cloudy, aside from dust storms, and the atmosphere is thin, which partially compensates for the greater distance from the Sun. When larger amounts of power are needed, then radio-isotope or reactor devices can be added.

On-Site Propellant Manufacture

Producing propellant on the surface of Mars has been studied extensively, since it lowers the mass brought from Earth for a "Flags and Footprints" mission. If we already have a robust orbital mining and processing capability and Skyhooks in place to deliver cargo, there may not be much benefit in early production of fuel locally vs delivery. The economics of doing so will need to be examined. For portable power, such as in moving vehicles, and for rocket propellant to reach a Skyhook on a return mission, an Oxygen/Methane fuel mix is a reasonable combination. Once sufficient need for fuel exists, producing it locally will make more sense

Linear Accelerator

Pavonis Mons, which is located on the Martian Equator, has a slope about 175 km long, which rises about 6.5 km. If large amounts of cargo need to be delivered from Mars, a gas or electromagnetic accelerator can be used here. If the full slope is used, orbital velocity can be reached with human-tolerable accelerations (3.6 gravities). This would not be an early system, since sufficient traffic is needed to justify such a large installation. Another option is a centrifugal catapult on top of the mountain for early cargo launch.

It is quite feasible to build a rotating space elevator (Rotovator) in orbit, coupled to a linear accelerator on Pavonis Mons ( http://upload.wikimedia.org/wikipedia/commons/1/12/Pavonis_mons_topo.jpg ). You have 60-120 km of ramp space, and no atmosphere to speak of, so at 3 g's and 60 km you can reach half of Mars orbit velocity, and the Rotovator provide the rest.

Long Term Development

Often the phrase "Terraforming Mars" has been used in the past. This is not a good phrase because it means "Make Mars like Earth". Because of orbit and mass differences, we cannot make Mars just like Earth, nor do I think that should be the goal. I prefer the word "humanize", meaning making it more suitable for humans. It may also mean modifying humans to better suit the Mars environment (like the lower gravity). Large scale changes to Mars should be delayed till after we have a firm idea if there is any native life on the planet, and even then done with due consideration and forethought. They should also be delayed until there are enough people on Mars to justify the large-scale projects. So what follows is more to answer what is possible from a technical point of view, and less to say "I urge you to do all these".

Magnetosphere

Mars lacks a strong magnetosphere - a magnetic field around the planet that traps and diverts charged particles from space. The Earth has one due to the magnetic field generated by our planet's core. A strong magnetosphere protects the atmosphere from being slowly stripped off as solar wind and other particles hit the upper atmosphere. Short of stirring up the planet's core, there may be some other ways to generate a field. The practicality of any of them is yet to be determined:

Run one or more superconducting cables around lines of latitude, which, like any current-carrying wires, will generate a field
Place some number of iron-nickel asteroids in Mars orbit and magnetize them, and point their fields in the same direction.
Mars is red because there is a lot of iron oxide on it's surface. Extract the iron, and magnetize it. You might be able to use the iron for other purposes at the same time as it being a magnet.

Magnets to make the magnetic field have fewer ways to break than superconductors, but if the superconductors work 99% of the time the other 1% doesn't make much difference to long term atmosphere loss. Some leakage of the atmosphere will still happen because Mars is a smaller planet than the Earth, so it is easier for atoms to escape.

Greenhouses

If you want to eliminate leakage, and bring up the pressure to breathable levels without importing a planet's worth of atmosphere, you can use greenhouse domes. If you really need the space, you can extend the domes to cover the entire planet bit by bit. To create Earth sea level pressure on Mars, a pressure balanced dome would consist of 10 meter thick quartz, glass, or equivalent, which you extract from Mars surface material. Lighter domes tend to float up, as the internal pressure is higher than the surrounding air. In that case they need to be tied down so they don't float away. A very large or planetary dome doesn't need much to hold it up, just some towers or cables to keep it from moving sideways.

You can design the clear material like armored glass to be resist damage, and ten meters of anything is pretty hard to break. But anything can be broken, so a lot of thought needs to go into how to deal with damage. As a greenhouse, you can take advantage of the "greenhouse effect", which is the trapping of infrared heat radiated back from the ground. You can specifically select the glass type or add coatings to trap infrared. You also want to block Solar UV radiation, which is not blocked by the Martian atmosphere. On Earth the greenhouse effect is a problem, since we don't want the planet warmer than it already is. On Mars it's a solution, since it's too cold for us there at the moment.

Full Atmosphere

If you find living under a dome objectionable, you would need to provide a full atmosphere at a breathable level. That is a very big job because planets are large. On Mars you need to provide 25 tons of atmosphere for each and every square meter of the planet, or 3.6 million billion tons total. That's to provide one Earth atmosphere pressure. If you are satisfied with less oxygen (similar to mountains on Earth), and a different mix for the rest of the air, you can get by with somewhat less. Despite it's distance, the easiest place to get enough nitrogen might be the Kuiper belt, which is outside Neptune's orbit and which Pluto is a part of. You could use a "reverse gravity assist" from Neptune to drop the material into the inner solar system. Nitrogen is rather scarce in the inner solar system, and getting it from anyplace with a deep gravity well (like Earth) takes a lot of work. Some of the outer moons might have enough ammonia (NH3).

References

↑ Rivkin et al MRO/CRISM Observations of Phobos and Deimos 2009

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