This paper presents a low-cost Mars sample return mission based on In Situ Resource Utilization, or ISRU. A trade study investigating three different ISRU options for Mars, as well as a conventional terrestrial propellant option, is presented within a si milar mission architecture. The options examined are:

1. Bringing liquid hydrogen from Earth, reacting it with Martian CO₂, and making methane and oxygen propellant;

2. Bringing liquid methane from Earth and making oxygen directly from the Martian atmosphere;

3. Collecting water vapor from the Martian atmosphere, electrolyzing it to produce H₂ and O₂, and then combining the hydrogen with ambient CO₂ to make methane and additional O₂;

4. Utilizing imported propellants (mono-methylhydrazine [MMH] and nitrogen tetroxide [NTO]), a traditional mission which serves as a baseline comparison.

The future of Mars exploration hinges on the development of In Situ Resource Utilization (ISRU) technology. ISRU is the use of indigenous materials at the site of an interplanetary mission for the production of rocket propellant or life support products, and can result in dramatic decreases in Earth launch mass and overall mission costs. ISRU is possible on the moon, Mars (including Phobos and Deimos), in interplanetary space via asteroids and comets, and on other planetary bodies.
The first proponents for the use of extraterrestrial resources came from science fiction authors such as Heinlein and Clarke in the 1940’s and 1950’s. A proposal for the use of Martian resources with ISRU technology was first published by Ash et al. of JPL in 1978. They proposed many different processes, including a Sabatier/electrolysis cycle for methane/oxygen propellant, transporting methane and producing oxygen with zirconia cells, and using carbon monoxide/oxygen propellant, again produced with zirconia cells. Zirconia cell technology was further investigated through the 1980’s by Frisbee at JPL and continues through the 1990’s with Ramohalli et al. and Nolan at the University of Arizona, as well as Linne at NASA LeRC. French and Zubrin e t al., have investigated the production of methane/oxygen propellant on Mars and have also been strong advocates of the importance of ISRU in the future of Mars exploration. In addition, Kaplan, Connolly, and co-workers at NASA JSC have been focusing on the use of extraterrestrial resources to enable lower-cost robotic missions. Mars sample return and precursor technology demonstration missions are currently under study at JSC under Kaplan.

In addition to the groups listed above, design teams at the University of Washington (UW) under A.P. Bruckner have conducted ISRU Mars mission analyses. For the past several years the UW teams have developed both manned and unmanned Mars missions incorporating the advantages of ISRU with robust and economically sound designs. In 1992, the UW team developed a $55 billion manned mission to Mars incorporating ISRU.12 A $1 billion ISRU Mars Sample Return Mission (MSRM) was developed in 1993,13 and a $ 200 million ISRU technology demonstrator mission to Mars was developed in 1994.14 The present work, Ares Acquire, is a trade study for an unmanned MSRM using ISRU technology to produce propellants for the return trip under a cost cap of $200 million (excluding launch vehicle).

The mission requirements for a low-cost MSRM with ISRU technology as detailed by NASA are as follows:

• Collect and return 2500 grams of rock, soil and atmospheric samples from Martian surface.
• Select samples uncontaminated by lander exhaust.
• Sample multiple sites.
• Return samples in ambient Martian conditions to an Earth-based facility.
• Utilize ISRU propellant and/or oxidizer for Mars ascent and Earth return.
• Produce propellant within 500-day surface stay.
• Use atmospheric CO₂ for feedstock.
• Use non-nuclear power systems.
• Prevent fore- and back-contamination.
• Launch between years 2000-2005.
• Use Delta II or smaller launch vehicle.
• Total mission cost excluding launch vehicle $200 million.

The most highly characterized and readily accessible resource on Mars is the atmosphere; therefore the available options for the production of rocket propellant on the surface of Mars make use of components of this resource. The composition of the Martian atmosphere is shown in Table 1. The most abundant component of the atmosphere is carbon dioxide. This CO₂ can be utilized in a number of ways. It can be used to produce oxygen, which would then be used with an imported fuel, or with the carbon monoxide by-product, for rocket propellant. It can also be used to produce methane/oxygen propellant after being processed through a Sabatier reactor.

The trace amount of water vapor in the Martian atmosphere is also available for utilization. It can be used for human consumption during manned missions, or can be electrolyzed and its components further processed. The hydrogen and oxygen can be used dir exactly as rocket propellants or processed through a Sabatier reactor to produce methane and oxygen for the same purpose. Any of these ISRU options alone or in combination can provide significant decreases in Earth launch mass.

In this trade study, three different ISRU mission options are presented and are compared to a traditional non-ISRU mission. These options are differentiated by the way the return trip propellant is produced. The options investigated are:

1. Bringing liquid hydrogen from Earth, reacting it with Martian CO₂ and making methane and oxygen.

2. Bringing liquid methane from Earth and making oxygen directly from the Martian atmosphere.

3. Collecting water vapor from the Martian atmosphere, electrolyzing it to produce H₂ and O₂ and then combining the H₂ with ambient CO₂ to make methane.

4. Utilizing imported propellants (mono-methylhydrazine [MMH] and nitrogen tetroxide [NTO]) .

Seed Hydrogen - Methane Produced Option

The Propellant Production Plant (PPP) for the Seed Hydrogen - Methane Produced (SHMP) option uses liquid hydrogen imported from Earth and processed through a Sabatier reactor with Martian carbon dioxide to produce methane and oxygen for propellant, in a manner similar to that developed by Zubrin.

The Sabatier-electrolysis reaction produces methane and oxygen by first using the Sabatier reaction

CO₂ + 4H₂ -> CH₄ + 2H₂O

and then processing the water through the water electro lysis reaction

2H₂O -> 2H₂ + O₂

The hydrogen obtained from the electrolyzer is reprocessed through the Sabatier reactor. These two reactions combine to produce a natural O/F ratio of 2, which is far below the optimum 3.5. To compensate for this, the oxygen requirement for an optimum O /F ratio determines the amount of hydrogen that must be taken to Mars. The extra methane produced is simply vented.

The PPP process diagram for the SHMP option is shown in Fig. 1. The Martian atmosphere is brought into the system through a filter by the zeolite sorption pump.15 The zeolite adsorbs the CO2 from the atmosphere and pressurizes it when desorbing. The CO 2 and H2 are combined in the Sabatier reactor where the ruthenium-on-alumina catalyst converts them into methane, water and a small excess of H2, which is removed by the electrochemical fuel separator and returned to the reactor. The methane and water are separated by condensing the water in a storage tank, and the methane is then compressed and stored cryogenically. The water is piped to an electrolyzer. The resulting oxygen is sent to storage and the hydrogen is returned to the Sabatier reactor. The SHMP PPP produces 0.36 kg of methane and 0.73 kg oxygen from 1.0 kg atmospheric carbon dioxide and 0.09 kg hydrogen from the storage tanks every sol. The propellants are stored at 31 bar and 125 K in cryogenic tanks on the Mars Ascent Vehicle (MAV).

Once the methane tanks are full, the plant continues to run to produce more oxygen to get the 3.5 O/F ratio, and the additional methane produced is vented. The production rates given above are averages. The actual production rate varies throughout the surface stay as the PPP throttles to follow the available power curve. A schematic of the SHMP PPP hardware is shown in Fig. 2. The mass of the PPP is 30 kg, with a daytime power requirement of 230 W and a night-time power requirement of 85 W. A full de scription of this PPP and process can be found in the complete report (approx. 600 pages) available from the authors.

In order to reduce the amount of power needed at night (thereby reducing the number of energy storage batteries needed), the power-hungry electrolyzer (~100 W) is run only during the day, when the solar arrays can provide power. The Sabatier reactor continues to operate during the night, with the methane sent to the MAV storage tanks and the water stored for daytime electrolysis.

Water-Sabatier

The problems associated with long-term cryogenic hydrogen storage prompted the search for alternative methods for getting hydrogen to Mars, including the transport of water. While the mass penalty for taking water from Earth to Mars turned out to be too great, it was thought that if a reliable source of water on Mars could be accessed, a completely ISRU mission could be undertaken. This idea led to the development of the Water Vapor Adsorption Reactor (WAVAR) concept for the separation of the trace amounts of water vapor from the Martian atmosphere. The WAVAR unit combined with a Sabatier reactor would be able to produce methane/oxygen propellant entirely from indigenous resources. The Water-Sabatier (WATS) option uses the WAVAR to extract water vapor from the Martian atmosphere. The water is then electrolyzed, with the O₂ stored for oxidizer and the H₂ processed in a Sabatier reactor to produce CH₄ and more O₂.

The Martian atmosphere is brought in through a bag-type filter by a high-speed axial-flow fan similar in design to a turbomolecular pump. The disc-shaped zeolite bed situated in the flow behind the fan is divided radially into eight sectors (Fig. 4). These sectors are insulated to prevent lateral heat and mass flow. The bed rotates saturated sectors into the microwave regeneration unit (Fig. 5), which recovers the adsorbed water vapor and passes it on to a condenser. From there the water is stored until processed through an electrolyzer. The hydrogen is then sent to the Sabatier reactor and the oxygen is sent to cryogenic storage.

The remainder of the WATS PPP works identically to the SHMP case (Fig. 6).

As with the SHMP case, to avoid the high power draw of the electrolyzer, water electrolysis is only done during the day. However, since this scenario relies on the electrolyzed water for the hydrogen which feeds the Sabatier reactor, the Sabatier part of the PPP must also be shut down during the night. The WAVAR unit is run continuously through both day and night, with the extracted water stored for electrolysis during the day. The WAVAR device needs to produce an average of 0.5 kg of water per day to maintain the necessary propellant production rate.

The mass of the WATS PPP is 57 kg. The daytime PPP power requirement is 420 W and the nighttime 150 W.

Methane Transported-Oxygen Produced Option

The Methane Transported - Oxygen Produced (MTOP) scenario utilizes the carbon dioxide found in the Martian atmosphere to produce oxygen. This O2 is used as an oxidizer, in conjunction with methane imported from Earth.

The MTOP PPP is based on work done by Frisbee. Kaloupis et al., Ramohalli, and Linne. The PPP needs to produce O₂ at an average rate of 0.73 kg per daylight period. Since one major design restriction of the PPP is the power required to run t he zirconia electrolyzer, it is operated during daylight hours only. This significantly reduces the required battery mass. However, to avoid thermal stress cycles on the zirconia electrolyzer membranes, the zirconia module is kept at its operating temperature (1070 K) at all times once the plant has been activated.

The heart of the PPP is the zirconia electrolyzer, which thermally dissociates the carbon dioxide in the Martian atmosphere and produces carbon monoxide and oxygen via the reaction:

2CO₂ -> 2CO + O₂

Figure 7 shows a process diagram of the MTOP PPP. The Martian atmosphere enters the PPP through a pleated filter. It is then compressed and fed into a heat-recovery system in order to recover the energy of the gas-streams exiting the electrolyzer. The CO₂ then enters the zirconia electrolyzer, where it is thermally decomposed into CO and O₂, which are then separated by an electro-chemical process.

After having run through the aforementioned heat recovery system, the exiting stream of CO is vented to the Martian environment. The O₂ is also channeled through the heat-exchanger system and then cooled further by running it through a length of coiled tubing, exposed to the Martian ambient conditions. The O₂ is then compressed to the storage pressure of 31 bar, cooled with the help of another coiled tubing heat rejector and refrigerator, and then finally sent to the MAV tanks for storage in liquid form at 125 K.

Figure 8 shows the geometry of the MTOP PPP hardware. Due to the extremely low mass-flow of oxygen (1.69 x 10-5 kg/s) a simple stretch of coiled tubing is sufficient to provide the desirable heat-rejection from the oxygen stream to the environment, through a combination of convection and radiation.
The total power required for the daytime operation of the MTOP PPP is 395 W. During the nighttime standby mode, the plant does not actively produce any propellants, but merely keeps the already stored propellant at the desired conditions. In addition, the operating temperature of the highly insulated zirconia electrolyzer is maintained. The power requirement for nighttime therefore is only 40 W, including thermal control for the MAV tanks. The MTOP PPP has a mass of 40 kg.

Other ISRU Propellant Options

All of the above ISRU mission alternatives use methane/oxygen propellant for the return trip. While other ISRU propellants were considered, methane/oxygen offered the best trade-offs in terms of Isp and storability. With an O/F ratio of 3.5, the Isp for methane/oxygen is 368 sec and both the methane and oxygen can be stored at the same temperature (125 K at 31 bar).

Among the other options considered was carbon monoxide/oxygen, which was quickly shown to be unacceptable for large missions as it has a very poor Isp (~290 sec), requiring a large amount of propellant for a given lift-off mass. Another propellant combination considered was propane/oxygen. Propane, while liquid at ambient temperatures, results in a lower Isp than methane. The mass savings from the reduction in refrigeration equipment is quickly out paced by the increase in propellant mass needed due to the lower Isp of propane.

With the long-term storage of hydrogen being such a problem, a number of different media for getting the necessary hydrogen to Mars were considered for the SHMP option. Among those considered were benzene, acetylene, water, and all phases of hydrogen. A ll, save liquid and slush hydrogen, were quickly eliminated due to mass, volume, and structural considerations. Slush hydrogen, though thermally desirable, was felt to be immature technologically.

Non-ISRU Propellant Options

The traditional, Apollo-style mission, in which all the propellant for the return trip is imported from Earth, uses mono-methylhydrazine and nitrogen tetroxide (MMH/NTO) as the propellants. Also considered for the TERP scenario were a number of storable mono- and bi-propellants. However, MMH/NTO provided the safest handling and exhaust residue characteristics. In addition, the associated technology is very well developed. The transporting of all propellants from Earth results not only in an increased Earth launch mass, but also in increased Mars landed mass. These increases scale up the structural component of the spacecraft, resulting in a DV requirement of 6300 m/s for Mars ascent. This translates into a propellant requirement of 710 kg MMH/NTO.