The Case for Mars (27 page)

This reaction is mildly exothermic, and, if cycled together with reaction (2), would produce oxygen and methane in a mixture ratio of 4:1, which would give the optimum propellant mass leveraging of 18:1 with a large extra quantity of oxygen also produced that could function as a massive backup to the life-support system. In addition, salvageable carbon monoxide would also be produced that could conceivably be used in various combustion devices or fuel cells. If all the carbon monoxide and oxygen produced is included, the total propellant mass leveraging obtained could be as high as 34:1!

Another method of obtaining the required extra oxygen is simply to take some of the methane produced in reaction (1) and p
yrolyze it into carbon and hydrogen.

The hydrogen so produced would then be cycled back to attack more Martian carbon dioxide via reaction (1). After a while, a graphite deposit would build up in the chamber in which reaction (6) was being carried on. (This reaction is actually the most common method used in industry today to produce pyrolytic graphite.) At such a time, the methane input to the reactor would be shut off, and instead the chamber would be flushed with hot carbon dioxide gas. The hot carbon dioxide would react with the graphite to form CO, which would then be vented, cleaning out the chamber.

Such a plan, incorporating two chambers, with one carrying out pyrolysis while the other is being cleaned, has been suggested to me as the simplest solution to the extra oxygen problem by Jim McElroy and his group of researchers at Hamilton Standard.

Now, it is sometimes the case that it is very easy to write down a chemical synthesis system as a series of equations on paper, but a very different matter to build a unit that can perform it as a matter of practice. That, however, is not the case here. I know, because I’ve led a project that built a Mars ISPP unit from scratch. In the fall of 1993, David Kaplan and David Weaver, representing NASA’s Johnson Space Center, approached me and asked if Martin Marietta could demonstrate a working model of the kind of Mars ISPP system I had been promoting at conferences and in papers. There was a catch, however. NASA could provide only $47,000 in funds to support the project, a very small budget to develop and demonstrate a new aerospace technology, and the work would have to be completed by January 1994. This was quite a challenge—at Martin Marietta $47,000 will ordinarily buy you a report containing a couple dozen viewgraphs. I was convinced, however, that the technology involved was fundamentally simple, and that the project, whose accomplishment seemed farfetched on the budget and schedule proposed, was fundamentally feasible. After much discussion with Martin Marietta management, th
e challenge was accepted. In October 1993 Martin Marietta was put on contract to undertake the work, with David Kaplan serving as the JSC program manager, Steve Price as the project manager at Martin Marietta, and me serving as principal investigator and lead engineer.

The design of the system was done during October 1993, and most of November was spent waiting for parts to arrive in the mail. By the end of November, all the required components were in hand, and construction, sized full scale to the requirements of a Mars Sample Return mission, was begun in earnest.

The Sabatier reactor was built from scratch, filling a metal pipe 36 centimeters long and 5 centimeters in diameter with a Ruthenium catalyst obtained from a chemical supply company. (We found out later that this was ten times the volume we needed for our system, but we were on a tight schedule that would not allow us to build anything twice. So, overdesigning seemed like the way to go.) The electrolyser, standing just 25 centimeters tall and weighing only 3 kilograms, water included, was ripped from a Packard Instrument laboratory hydrogen supply unit. Nichrome heaters, used to warm the Sabatier reactor up to its operating temperature (after which the heat from chemical reactions would keep it hot without electricity) were obtained and wrapped around the Sabatier reactor. A condensing system was built to separate the methane product from the water product, and the whole system plumbed into a cycle, with pressure and temperature sensors and gas flow meters inserted at strategic points and wired to a computer data display to allow system monitoring and control. By the end of the second week of December, the system was complete and ready for operation. (See plates.)

On December 15, the system was turned on for the first time, with just the Sabatier reactor running. By the end of the second hour of operation, the water level in the condenser vessel had risen noticeably, indicating that the system was working. Subsequent laboratory analysis of the effluent gas from the Sabatier reactor showed that it was operating at 68 percent efficiency in converting input hydrogen and carbon dioxide into methane and water.

On subsequent days, adjustments were made to the system to improve performance. On the 22nd of December, 85 percent conver
sion efficiency was achieved, with hydrogen for the Sabatier reactor being supplied by the electrolyser. On January 5, running for the first time with full system integrated operation, we reached 92 percent efficiency. Finally, on January 6, 1994, the full system ran in its fully integrated mode all day, achieving a 94 percent conversion efficiency in the process.

With the conclusion of the January 6 run, all test objectives had been achieved, and there was still enough money left in the kitty to write the report.
¹⁸

Since that time, additio
nal small chunks of money from first JSC, then the Jet Propulsion Laboratory, have allowed further improvements and elaborations of the system. Sorption beds were added, allowing the unit to acquire its carbon dioxide from a Mars atmospheric reservoir held at Martian pressures.e Sabatier reactor had its efficiency increased to 96 percent and was miniaturized by a factor of ten, and a 2-kilogram Stirling cycle refrigerator was added, allowing us to liquefy all the product oxygen and store it in a cryogenic dewar. Automated control systems were also added, allowing the system to run 10 days at a time without operator intervention. The total mass of all working components in this system, which is sized to produce 400 kilograms of propellant to support a Mars Sample Return mission, is about 20 kilograms, and the total power needed is less than 300 watts.
¹⁹
Studies indicate that when scaled up to the size needed to accomplish the Mars Dire
ct mission, the mass leverage of the unit would be much more pronounced, as the percent of system mass represented by parasitic elements such as flow meters and pressure sensors would fall into the noise.

We can make rocket fuel and oxygen on Mars.

STAYING IN TOUCH WITH THE BASE

Using combustion-powered ground vehicles, the first Mars explorers will be able to wander far from their base, but if they do, how will they maintain communication? Mars is after all a little more than half the diameter of the Earth, so the horizon is correspondingly closer If the terrain on Mars were as flat as Kansas, the horizon would only be about 40 kilometers (25 miles) away—and Mars is most definitely
not like Kansas. So, if the excursion team wants to go anywhere on Mars, they’re definitely going over the horizon. That rules out line-of-sight radio transmissions. How will they manage to stay in touch with the base?

One answer is to have a communication relay satellite stationed in Mars orbit, 17,065 kilometers above the equator. At that altitude, the satellite will be flying at a velocity of 1.45 km/s, and will take 24.6 hours to orbit Mars. Since this is the length of a Martian day, the satellite will keep pace with the planet as it turns, and to an observer on the ground will not appear to move at all. Such an “aereosynchronous” satellite is the exact analog on Mars of the geosynchronous satellites currently used extensively to support communication on Earth. If our Mars expedition lands at the equator, the satellite will hang directly overhead all day and all night, supporting communication from the base to anyone or anything within a region with a radius about the base of nearly 5,000 kilometers, covering nearly half the surface of the planet.

But communications satellites cost money, and, more importantly, are subject to failure. If the satellite should go on the blink while the exploration team is 400 kilometers out from the base, what then?

The backup plan is to use ham radio. You see, Mars has an ionosphere, a layer of charged particles in the high upper reaches of its atmosphere, that can be used to reflect radio signals, enabling global surface-to-surface communication in the short-wave radio bands, just as on Earth. We know a lot about the properties of the Martian ionosphere from measurements taken by
Mariner
9 and the
Viking
orbiters and landers. It extends upwards from an altitude of about 120 kilometers, and is composed of an ion population consisting of 90 percent O
₂
+ and 10 percent CO
₂
+, matched by an equal number of free electrons produced via photo-ionization. During the day the electron density reaches a peak concentration of about 200,000/cm
³
at an altitude of about 135 kilometers. During the night, this density falls off to a peak value of about 5,000/cm
³
at an altitude of about 120 kilometers. These numbers are lower than the electron density in Earth’s ionosphere by about a factor of 25. Howevr, since the maximum usable frequency for shortwave radio goes as the square root of the electron density, the maximum usable frequency available on Mars is only lower than that obtainable on Earth by about a factor
of five. So, while on Earth, hams can talk to each other with frequencies as high as 20 MHz, the best on you can do on Mars is about 4 MHz during the day and 700 kHz at night. The latter figure is kind of low if you want to transmit images or engage in other kinds of high data rate transmission, but it is more than adequate for engineering telemetry or voice communication. In fact on Earth this frequency band—AM radio—is favored for the commercial transmission of pop music, talk radio, and other forms of communication.

Furthermore, while Mars’ shortwave communication bands are positioned at a somewhat lower frequency than those of the Earth, this disadvantage (using higher frequencies enables a higher data rate) is more than counterbalanced by the fact the Martian ionosphere is much less afflicted with radio noise. On Earth, shortwave radio transmission power requirements are driven up by radio noise caused by distant thunderstorms and large numbers of other hams, military users, and pop radio stations. All of these problems would be absent on Mars.

Some ham setups in current use may conjure up images of heavy, unwieldy equipment unsuitable for mobile communications. However, advanced shortwave technology highly suitable for use by Mars explorers has been developed on Earth for military purposes. One such example is the Advanced Miniature High Frequency System (AMHFS) developed by Defense Systems Inc. The AMHFS is a two-way transmitter/receiver system, with each unit having a mass of 0.8 kilograms and a volume of 0.7 liters (smaller than a quart-sized container)—small enough to be carried not only in rovers, but by individual astronauts during EVA. Based upon its terrestrial performance, this system could transmit globally over the sunlit side of Mars at a rate of 2.4 kb/s using 10W of radiated power, or 30 W electric. This speed of transmission is adequate for engineering telemetry, e-mail, real-time low-quality voice, or high-quality voice packet transmission. To achieve high-quality realtime voice (as in terrestrial telephones) twenty times this data rate would be required, and thus 600 W of power, which could be easily generated on the rover. However, these power requirements may be sharply reduced if Mars’ ionosphere is really as quiet as theory predicts. The Russian
Mars 96
orbiter will car
ry a top-side ionosphere radar sounder that should give us the data we need to quantify Martian shortwave power requirements more accurately. In any case, the AMHFS uses an adaptive sounding technique that automatically searches the radio spectrum to find the maximum usable frequency in real time, and then causes the two units in communication to perform a hand shake acknowledging the link and verifying that data has been transmitted correctly. Thus, even if ionospheric conditions are unpredictable or variable during the transmission, the AMHFS can adapt to find and maintain the best communication channel. The AMHFS uses its electronics to compensate its antennae size for the wavelength chosen for communication. Thus the same 6-meter whip antenna can be used for transmission at 0.5 MHz as at 5 MHz. The antennae used are very lightweight and are typically simply helical springs, or “stacers,” that can be released to pop into place for deployment.